1 5-/. 3 BOOK 15 1.3.AV3 1 c 1 Zfn'^r.L^^"^^^ INSTINCTS AND I^J^llll m.,^^^ OF ANIMALS WITH SPEC 3 ilS3 oooomsfl M /. >/ THE INTERNATIONAL SCIENTIFIC SERIES, Each Book Complete in One Volume. Crown 8vo. cloth, os. unless otherwise described. I. FORMS of 'WATER: in Clouds and Rivers, Ice and Glaciers. By J. Tyndau., LL.i>., F.R.S. With 25 Illustrations. Is'iutti Eaition. II. PHYSICS and POLITICS; or, Thoughts on the Application of the Prmciples of *2 Newport, on the contrary, maintained that the anteuDse possess no sense of smell. He experimented on a water-beetle, Hi/daticus cinereus, which, he says, " I had purposely confined for three days without food in a cup about half filled with water, and, at the expiration of that time, attached a small piece of raw flesh to the end of a wire, and carried it several times along the sides of the insect, particularly near the spiracles, where it was suffered to remain for a short time. The insect, however, did not appear to perceive it, but during the EXPERIMENTS WITH SILPHA. 41 whole time remained in the water perfectly undisturbed. The flesh was then carried very near to one of the antennae, but without exciting the slightest motion in that organ, while the insect began to move its palpi very briskly, as if it detected the presence of something; but continued, in other respects, motionless as before. The flesh was then brought in direct contact with the antennae, and the insect immediately withdrew them as if annoyed, as in the experiment with the Silpha. It was then carried exactly in front, and at about the distance of an inch. The palpi were instantly in rapid motion, and the creature, darting forward, seized the flesh, and began to devour it most voraciously. The following day the experiment was repeated several times, and with precisely the same result; but on this occasion the antennae were so repeatedly touched with the flesh, that the annoyed insect kept them at last beneath the sides of the thorax. Hence I think it must appear that, from there being no alterations in the motions of the insect when the food was held near the sides of its body, the sense of smelling does not reside in the spiracles, nor, for like reasons, in the antennae; while, from the motion of the palpi and the avidity with which the insect darted upon the food when held in front of it, it seems but fair to con- clude that the sense of smelling must certainly reside in the head, as above suggested." * Agaiu, he took a Silpha (one of the carrion-eating beetles), and, *' placing it in a glass, attached a small piece of flesh within half an inch of it. The antennae, as is usual with these insects, continued to * Newport, " On the Antenuse of Insects," Transactions of the Ento- mclogical Socidy, 1837-1810. 42 EXPERIMENTS WITH SILPHA. be moved about on either side, but with nothing remarkable in their motions, while the head of the insect was a little elevated and carried forwards, as if it perceived the flesh, and the palpi were in rapid vibra- tory motion. It soon approached very near to the food, and at length touched it three or four times with the antennae, but each time suddenly withdrew them as if they had fallen unexpectedly on something obnoxious, the palpi during the wdiole time continuing their motion. The insect at length reached the food, and, after having touched it once or twice with the extremities of the palpi, their motion ceased, and it commenced feeding, while the antennae were occasionally in motion as before." It would certainly seem, therefore, that in these insects, at any rate, the sense of smell resides principally in the palpi. Newport made certain other experiments on the powers of hearing of insects, which I shall mention in the next chapter, and he concludes, "These facts, connected, wdth the previous experiments, have con- vinced me that the antennae in all insects are the auditory organs, whatever may be their particular structure, and that, however this is varied, it is appro- priated to the perception and transmission of sound." Newport was an excellent observer and profound entomologist, and I see no reason to doubt the correct- ness of his observations ; nor, indeed, of his inferences, so long as we confine them to the species on which the observations were made. They may prove that some insects possess no sense of smell, or that, at any rate, it does not reside in the antennae. On the other hand, they cannot disprove the positive results obtained by other observers, that in other species the opposite is EXPERIMENTS WITH STAG-BEETLE, ANTS. 43 the case, and that in them the sense of smell does reside in the antennae. That the stag-beetle can smell seems clearly proved, but Landois found * that, after the removal of the terminal plates of the antennae, the insect still possessed this faculty, whence he concluded that the sense of smell must reside in some other part of the body, and that the antennae probably serve as organs of hearing. This does not, however, prove that the sense of smell does not reside partly in the antennae. Forel removed the palpi and mouth parts of a wasp, and she appeared to perceive the presence of honey as well as before. I myself took a large ant {Formica ligniperda), and tethered her on a board by a thread. When she was quite quiet, I tried her with tuning-forks ; but they did not disturb her in the least. I then approached the feather of a pen very quietly, so as almost to touch first one and then the other of the antennae, which, however, did not move. I then dipped the pen in essence of musk and did the same ; the antenna was slowly retracted and drawn quite back. I then repeated the same with the other antenna. I was, of course, careful not to touch the antennae. I have repeated this experiment with other substances with several ants, and with the same results. Ferris also made the same experiments with the palpi, and with the same result ; but if the palpi were removed, the rest of the mouth gave no indications of perceiving odours. Graber f also has made a number of experiments, and * "Das Gehororgan des Ilirschkafers," Arch. filr. Mic. Anat, 1868. t "Vergl. Grundversuche iiber die Wirkung und die Aufnahme- stcllen chemischer Reize bei den Thieren," Biol. Centralblatt, 1885. 44 SEAT OF THE SENSE OF SMELL found that in some cases (though by no means in all), insects which had been deprived of their antennae still appeared to possess the sense of smell. But if, as we have, I think, good reason to suppose, the power of smell resides partly in the palpi, this would naturally be the case. He also tested a beetle, Silplia tJioraeica, with oil of rosemary and assafoetida. It showed its perception by a movement in half a second to a second in the case of the .oil of rosemary, and rather longer — one second to two seconds — in the case of the assafoetida. He then deprived it of its antennae, after which it showed its perception of the oil of rosemary in three seconds on an average of eleven trials ; while in no case did it show any indication of perceiving the assafoetida even in sixty seconds. This would seem to indicate a further complication — not only that both the antennge and the palpi may possess the sense of smell, but also that certain odours may be perceived by the former, and others by the latter. Graber questions some of the experiments which seemed to me * to demonstrate the existence of a sense of smell in ants.f * *' Ants, Bees, and Wasps." t He says, "Da Lubbock noch hinzufiigt, dass keiner, der das Benebmen der Ameisen unter dieseu Umstandeu beobacbteu wiirde, den geringsten Zweifel an ibrem Gerucbsvermogen baben konnte, wablte icb aucb diese Metbcde, um zu erforscbeu, wie sicb etwa der Fubler beraubte Ameisen verbalten wiirden. Icb war nicbt weuig iiberrascbt zu finden, dass aucb diese (es bandelt sicb um Formica rufa) vor dem Riecbobjekt umkehrten. Um ganz sicber zu geben, versucbte icb's aber nocb mit dem gleicben Arrangement aber mit Weglassung des Biechstoffes, uud sicbe da ! sie kebrten aucb jetzt nocb um ! Bei genauerer Beobacbtung der von einer Ameise vom Aufang an auf dem Papiersteg zuriickgelegtcn Strecke stellte sicb aucb bald PARTLY IN THE PALPI. 45 I fastened a strip of paper in the air by means of two pins, suspended over it a camel's-liair brush containing scent, and then put an ant at one end. She ran forward, but stopped dead short when she came to the scented brush. Graber suggests that she did so from giddiness, but I am satisfied that this is not so. Ants which habitually climb trees are not likely to be affected by any such sensation. In my experi- ments, whether the bridge was high or low, broad or narrow, made no difference to them. Moreover, in each case they stopped exactly when they came to the scented pencil. Again, Graber has not observed that I expressly stated that " after passing two or three times, they took no further notice of the scent ; " nor did they notice the caniel's-hair pencil unless it was scented. As regards flies (Musca), Forel removed the wings from some bluebottle flies and placed them near a decaying mole. They immediately walked to it, and began licking it and laying eggs. He then took them away and removed the antennse, after which, even when placed close to the mole, they did not appear to perceive it. Plateau also * put some food of which cockroaches are fond, on^ a table, and surrounded it with a low heraus dass es sich bei dem gewissen Umkelii-en lediglich um ein versuchsweises Absclireiten oder Ausprobiren des unbekannteu Weges handelte, oder das sich die Ameisen alinlich benehmen wie wir selbst, ■wenn wir etwa auf einem schwanken Brette eine tiefe Gebirgskluft iiberschreiten sollen." Graber's observation is, I doubt not, quite correct ; but his inference is not, I think, well founded, nor was his experiment the same as mine. * Bull, de la Soc. Ent. Belgique, 1870. 46 SEAT OF SMELL PARTLY IN ANTENNA.. circular wall of cardboard. He then put some cock- roaches on the table : they evidently scented the food, and made straight for it. He then removed their antennae, after which, as long as they could not see the food, they failed to find it, even though they wandered about quite close to it. On the whole, then, the experiments which have been made seem clearly to prove that in insects the sense of smell resides partly in the antennge and partly in the palpi. This distribution would be manifestly advantageous. The palpi are more suited for the ex- amination of food; while the antennae are more con- veniently situated for the perception of more distant objects. We will now glance at the antennae and palpi themselves, and consider briefly the structures which are supposed to give the sensation of smell. For this three conditions are requisite : (1) an appropriate nerve; (2) free access to air; and perhaps, though this is not so clear, (3) a fluid which can dissolve the odoriferous substance. The olfactory organ in Vertebrata consists, as already mentioned, of a mucous membrane containing (1) cylindrical epithelial cells, with a broad, flat termination at the free end ; and (2) of rod-like filaments which, some little distance below the surface, swell out into a nut-shaped expansion, and then contract again into a fine thread, which is probably continuous with the fibrils of the olfactory nerve. In Insects and Crustacea the conditions are different. The cellular "underskiu," or hypoderm, secretes a hard, horny envelope, and the terminations of the olfac- tory nerves are enclosed in a horny tube with a THE ORGANS OF SMELL. 47 terminal perforation, or project as free threads. They differ, again, between themselves. Insects being as a general rule aerial, and Crustacea aquatic. Erichson* has the merit of having been the first to support this theory by anatomical examination. Newport had previously mentioned the existence in many insects of certain pits, or *' pores," closed by a delicate membrane, and which he regarded as the seat of hearing. Erichson extended his observations, and suggested that the pits were rather to be regarded as organs of smell. His descriptions were confirmed by Fig. 34.— Antenna of Pontella Bairdii (Lubbock). Burmeister, who, moreover, detected in some of these " pits " the presence of a small knob, or liair. In 1853 I called special attention to the antennae of certain Crustacea, distinguishing five kinds of hairs — (1) short, downy hairs; (2) plumose hairs; (3) cylindrical, tapering hairs; (4) flattened, lanceolate hairs ; (5) wrinkled hairs — and pointed out that they were by no means scattered indiscriminately, but arranged in definite situations, indicating special functions. The two last I was disposed to regard as sense-organs. The above is a figure of the right male antenna of Pontella Bairdii, one of the Cope- * " De Fabrica et usu Antennarum in Insectis." 1847. 48 LEYDIG'S OLFACTORY CONES. poda, from one of my memoirs in that group,* and shows the curious clasping organ. ^ Leydig, in his beautiful work on the Daphnidse, and more fully in a special memoir on the subject,! de- scribed certain organs which had been also mentioned by La Vallette. I give below his figure of the terminal segments of one of the smaller antenn£e of the water-woodlouse {AseUiis aquatieus) magnified 500 times. It will be seen that there are three kinds of appendages — 1. Ordinary stiff, cylindrical, tapering, pointed hairs, which are not connected with any nerve. 2. Pale, cylindrical hairs, with a blunt termination and a tuft of fine setse. These hairs are connected with a nerve, and Leydig regards them as organs of touch. 3. Peculiar cylinders, of which there is one to each segment. They are com- posed of three parts, the middle one somewhat wider than the others. The lower third is strongly chiti- nized, like the ordinary hairs ; the other two are more delicate. At the free end he observed, in some cases, a group of very fine, short hairs. At the base of Fig. 35. — Terminal segments of one of the smaller antennas of the water-woodlouse (^Asellus aquations), x 500 (after Lej-dig). a, Ordinary hairs (not connected with a nerve) ; b, sensitive hairs (with a nerve at the base) ; c, special cylinders (olfactory cylinders). * Ann: and Mag. of Natural History, 1853. t "Ueber Geruchs imd Geburorgaue der Krebse und Insekten," Muller's Ar., 1860. ORGANS OF SMELL IN CRUSTACEA— CENTIPEDES. 49 each cylinder is a nerve, wliicli apparently swells into a ganglion. Leydig described similar organs on the antennae and palpi of various other Crustacea. They have obviously some special function, and he suggests that they are olfactory organs. It is interest- ing that, in certain species which live in subter- ranean waters and have lost their eyes, these olfactory cones are unusually developed. They are much larger, for instance, in Asellus eavaticus and Gammarus Fig. 36.— Tip of the autuana of a ceiitip<:Je (Julus teri-tstria), X COO (after Leydig"). At the apex are four olfactory cylinders, a few of which are also seen on the fol- lowing segment, among the ordinary hairs. puteanus, which live in the dark and are blind, than in. Asellus aqiicdieiis and Gammarus pulex or G.fluviatilis. Fig. 36 represents the end of the antenna of a centi- pede {Julus terrestris). There are four olfactory cylinders at the tip, and several are also seen on the following segment among the ordinary hairs. In this species the cuticle of the cylinder appeared sometimes as if wrinkled, and Leydig believes that the end is open.* Similar cylinders occur in Scolopendra, Glo- * Loc. cit, p. 286. 50 OLFACTORY CONES IN INSECTS. meris, and other centipedes. He also described similar cones in certain iosects. Further details with reference to the structure and arrangement of these bodies have been given by Glaus, Sars, Weissman, Rouge mont, Gamroth, Heller, Hensen, Hauser, and others, who have also ascribed to them this function. In Claus's opinion, the nerve itself enters these bodies. On this point, however, there is Fig. 37.^End of a palpus of Staphy- linus erythropterus, x COO (after Leydig). a, Olfactory pit. Fig. 38.— Part of antenna of CaUinnassa suJ)- terranea. b, Olfactory hairs ; g, peculiar curved hairs. still much difference of opinion. At any rate, it seems to be established, by the most recent observations, that even if the cones are in some cases closed at the end, they certainly remain open in others. Similar organs also occur in the palpi (see Fig. 37). Kraepelin describes other peculiar forms of hairs to which he ascribes the perception of smell, as occurring in all the stalk-eyed Crustacea (Podophthalmata). OLFACTOEY HAIKS. 51 These olfactory liairs are partly round (Pontonia), partly flat (Pagurus) ; the end is described as being sometimes simply open (Fig. 39, a, 5), sometimes provided with a small cone (Fig. 39, e, d, e). The number of these hairs is often very considerable. Moreover, they them- selves sometimes bear, near the base, a number of very fine bristles (Pagurus). There can, I think, be no doubt that these hairs are organs of sense, and it is probable that they are olfactory. The antenna of Callianassa (Fig. 38) also bears another remarkable series of long, Fig. 39.— Terminations of olfactory hairs of Crustacea, a. Of larva of a Paloemon ; &, of a Pagurus ; c, of a Pinnotheres ; (i, of a Squilla ; e, of a Pontonia. thin, movable, but stiff and hooked hairs (Fig. 38, g), which also stand in direct connection with the nerve, and have probably some sense-function. In many cases the sense of smell is connected with minute depressions in the integument, iln spiders Dahl has described a structure in the maxilla which he believes to be olfactory. The skin presents a number of minute orifices, under which lie elongated cells, each terminating in a nervous fibril.* Leydig also mentions t the existence of small pits on * " Das Gelior-und Geruchsorgan der Spiunen," Arch, filr Mic. Anat., 1885. t "Ueber Geruchs uud Gehororgauc der Krebse imd Insekten," Muller's Arch., 1860. 62 OLFACTORY PITS. the antennae and mandibular palpi of the crayfish (Astacus fluviatilis) but I do not find any further description of them. On the other hand, in insects they play a more important part, and it will be convenient to describe here very briefly the various structures occurring on and in the antennae of insects, although it is not to be supposed that they all serve for the sense of smell. Newport * alludes to the " pits " ; but they were first described by Erichson f ; while Burmeister { suggested that there are two classes — those containing a hair, and those in which there is none. The pits are only found in certain regions, and have certainly some specific function. In the stag-beetle {Lucanus eervus) the terminal plate of the antenna shows two large pits, one on each side, and nearly opposite one another. In other Lamellicorn beetles, as, for instance, in the cock- chafer {Melolontha vulgaris), they are very numerous. Lespes§ supposed them to be closed sacs, each containing an otolithe. They certainly do present this appearance, but the existence of any otolithe has been conclusively disproved by Claparede,|| Glaus, Hicks, and others. Graber thought 1[ that he had discovered an organ of hearing containing an otolithe in the antennae of certain Diptera. Mayer,** however, has since examined * Transactions of the Entomological Society of London, vol. ii. t " De Fabrica et usu antennarum in Insectis." 1847. X "Beob. iiber den feineren Bau der Fiililerfachur der Lainelli- cornier." 1848. § "Mem. sur I'appareil auditif des Insectes," A^m. Sci. Nat , 1858. 11 "Sur les pretendus organes auditifs des Anteunes chez les Coleopteres," Ann. Sci. Nat., 1858. ^ " Ueber neue otocystenartige Sinuesorgane der Insekten," Arch, fur Mic. Anat., 1879. ** " Sopra certi organi di Senso nelle Autenne dei Ditteri," Jleale Ace. dei Lincei, 1878-79. OLFACTORY ORGANS OF FLY. 53 them, and it appears to be really a sac lined with sense- hairs. Hicks * described the structure of the antennae in a Fig. 40.— Antenna of blowfly (after Hicks), a. Enlarged third segment, showing pits ; c, base of the antenna. considerable number of insects. On the antenna of the blowfly (Musca ; Fig. 40) he found no less than 17,000 perforations, each leading into a small sac, besides which ♦ Transactions of the Linnean Society, 1857-1859. 54 ANTENNA OF ICHNEUMON. there are larger orifices leading into more complex de- pressions, apparently arising from the confluence of a number of the sim pie sacs. At th e base of these large sacs are a number of papillae, or small hairs. In the dragon- fly, each segment of the antenna contains a large con- voluted sac. The sacs, in fact, vary much in number, size, and form, but Hicks considered that "they all possess the same elements, and are formed on the same principle." In many cases he traced a nerve to the base of the pits. He considered that they were generally, if not always, closed in by a deli- cate membrane, which, indeed, sometimes projected in a hemispherical, conical, or even hair-like form. The minute structure of the pits was further studied by Leydig in 1860. He describes them as parts of the integument in which the chitine is very thin, and more or less depressed, centre. This hair may be even Fig. 41.— One segment of the an- tenna of an Ichneumon (after Hicks). with a hair in the reduced to a mere ring. Hicks also called attention to a remarkable speciality in the antennae of the Ichneumons, the true nature of which he did not, however, correctly ascertain. He describes the appearance presented as that of a great number of narrow inverted canoes, with a keel-like ridge, and each inverted over an oval perforation. He regarded these as consisting of a thin transparent membrane. Subsequent observations, however, have shown that each supposed canoe-shaped membrane is, OLFACTORY ORGAXS OF WASP. 55 in fact, a fine hair, inverted over one of the usual pits. In 1880 Hauser published an excellent memoir * on the olfactory organs of insects, from which I have taken Fig. 42, representing a section through part Fic^. 42.— Section through part of the antenna of a wasp (after Hanser), x 430, CE, Cbitinous skin ; Z, olfactory cone ; G, olfactory pit ; TB, tactile hairs ; E, hypodermic cells ; M, the membrane surrounding them ; K, nuclei of the olfac- tory cells ; K„ remains of the earlier upper nucleus ; SK, lower circle of rods ; RS, olfactory rod ; GZ, Geisselzelle ; J/Z, membrane forming cell ; M, membrane closing the pit. of the antenna of a wasp, showing two of the olfactory cones, one projecting beyond the general surface. They terminate above in a fine rod, below in a nerve-thread, and present a double series of ridges. * "Phys. und Hist. Unt. u. die Geruchsorgane der Insekten," Zeii. fur Wiss. ZooU 1880. 56 ANTENNAL ORGANS OF INSECTS. Kraepelin* and Sazepinf liave also publislied valuable memoirs containing many interesting details. The hairs of the antennse, then, serve some for touch and some for smell, while there is, as we shall presently see, strong reason for supposing that the sense of hearing is also in some insects seated in the antennae. The greatest variety of antennal organs, so far as we yet know, occurs in the Hymenoptera (ants, bees, and wasps). Of these I give a diagrammatic figure. There are at least nine different structures. 1. Ordinary hairs (Fig. 43, c). y\-V'-\^J-— Fig. 43.— Diagram showing structures on the terminal segments of the antenna of insects, a, Chitinous cuticle ; Z>, hypodermic layer ; c, ordinary hair ; d, tactile hair ; e, cone ; f, depressed hair, lying over g, cup, with rudimentary hair at the base ; h, simple cup ; i, champagne-cork-like organ of Forel ; k, flask-like organ ; 1, papilla, with a rudimentary hair at the apex. 2. Hairs of touch (Fig. 43, d). * " Phys. und Hist. Uut. ii. die Geruchsorgane der Iiisekten," Zeit, fur TFz'ss. Zool.^ 1880; and " Ueber die Geruchsorgane der Glieder- thiere," 1883. t " Ueber den histol. Ban und die Vert, der nervosen Endorgane auf den Fiihleru der Myriopoden," M€m. de VAcad. Imiie'r. de Sc. de St. Petersburg, 1885. ANTENNAL ORGANS OF INSECTS. 57 3. Flattened hairs (Fig. 43, e). 4. Depressed hairs (Fig. 43, /). 5. Pits with a minute hair at the base (Fig. 43, g). 6. Pits without a hair at the base (Fig. 43, h). 7. Cones containing a nerve (Fig. 43, I). 8. The champagne-cork-like organs of Forel (Fig. 43, i). These consist of a pit, with a constriction about halfway up. They differ, in fact, from the second sort mainly in the presence of this constriction. 9. The curious flasks (Fig. 43, h) first observed and described by Hicks.* " They consist," he says, " of a small pit leading to a long delicate tube, which, bending towards the base, dilates into an elongated sac having its end inverted." f Of these remarkable organs there are about twelve in the terminal segment, and one or rarely two in the others. Similar structures have since been found in other Hymenoptera ; but not, I believe, as yet in any other order of insects. I have ventured to suggest that they may serve as microscopic stethoscopes. Kraepelin was disposed to regard them as glands, but I agree with Forel that there is no suffi- cient reason for doing so. There may, moreover, be a distinctly characterized sense-organ without any alteration of the actual surface, as shown in some of the figures given by Kraepelin, and also by that from Hauser given above (Fig. 42). These are, perhaps, the principal types, but there * Transactions of the Linnean Society, vol. xxii. p. 39. Kraepelin attributes the observation to Forel, but this is an error. Forel had overlooked Hieks's description and figure. t Hicks, " On the Organs of the Antennce of Insects," Transactions of the Linnean Society, vol. xxii. 58 COMPLEX STRUCTURE OF THE ANTENNA. are many modifications; for instance, complex pits often arise from the confluence of several small ones. The structure of the antennaG is then very complex, and increases with the importance of the antennae in the life of the insect. Among the Hymenoptera, Lyda has about 600 pits; Tenthredo, 1200; Sirex,2000; Pompilus, 3000; Paniscus, 4000; Ichneumon, 5000; Hyloeus, 6000; the wasp (Vespa), about 13,000 pits and 700 cones ; the blowfly, 17,000 ; the hive bee, according to Hicks, about 20,000 pits and 200 cones. Among beetles (Coleoptera) the numbers are generally small, but the cockchafer (Melolontha) possesses, according to Hauser, on each antenna as many as 35,000 in the female, and 39,000 in the male. Moreover, it is significant that in those species where the females are quiescent and are actively sought out by the males, the antennae are much less highly developed in the female sex than in the male. As already mentioned, the antenna? probably serve partly as organs of touch, and in some cases for smell. On the other hand, I do not believe that touch and smell are the only two senses possessed by the antennae. Forel and I have shown that in the bee the sense of smell is by no means very highly developed. Yet their antenna is one of those most highly organized. It possesses, as I have just mentioned, besides 200 cones, which may probably serve for smell, as many as 20,000 pits; and it would certainly seem unlikely that an organization so exceptionally rich should solely serve for a sense so slightly developed. Much as these antennal structures differ from one another in form, arrangement, and structure, they are all reducible to one type — to a hair — more or less de- VARIOUS USES OF ANTENN.E. 59 veloped, more or less deeply seated, standing in con- nection with the ganglionic cells, and so with the cerebral ganglia. Even the long-necked " bottles " (Fig. 43, h) may be regarded as an extreme form of this type, especially if the inversion at the end can be, as seems probable, regarded as a hair. All entomologists are agreed that some of the anten- nal hairs serve as organs of protection, and others as organs of touch. The evidence is, as we have seen, very strong, that some of them serve as organs of smell. They fulfil, therefore, at least three different functions, and when we consider their manifold variety, there is not only no a priori improbability, but, on the contrary it seems very probable that some of them, at least, perform some other function in the animal economy. There is, indeed, strong reason, as we shall see in the next chapter, to believe that, in some cases at any rate, the antennse act also as ears; while some of these peculiar antenual organs, though obviously organs of sense, seem to have no special adaptation to any sense of which we are cognisant. ( CO ) CHAPTEK ly. THE SENSE OF HEARING. The sensation of sound is due to vibrations of the air striking on the drum of our ear. The intensity of the sound depends on the extent or amplitude of the sound- wave ; while the pitch of the tone depends on the fre- quence of vibration, and consequently on the number of waves which strike the ear during a given interval. The fewer the number of vibrations in a second, the deeper the sound ; the more numerous, the shriller it becomes. Our pianos generally begin with the of 32 vibrations in a second, and extend to K"" of 3520 vibrations. The number of vibrations for the tone A', which is that of the hum of a bee, is about 440 in a second. If the vibrations are fewer than 30 in a second, they produce only a buzzing and groaning sound, while the shrillest sound we can hear is produced by about 35,000 vibrations in a second. It may seem curious that there should be any dif- ficulty in ascertaining whether an animal can hear. But, in the first place, in order to experiment on them, we are often obliged to place them in situa- tions very unlike those to which they are accustomed ; and, secondly, it is by no means always easy to say ORGANS OF SOUND— MOLLUSCA— CRUSTACEA. 61 whether they are affected by a real noise, or whether they are merely conscious of a concussion or vibration. As regards the lower animals, it appears to me, I con- fess, that many organs have been described as auditory, on grounds which are anything but satisfactory. At the same time, it cannot be doubted that many of the lower animals do possess the power of hearing, especially as some have elaborate organs for the production of sound. Among the lowest groups, none of the Protozoa or Ccelenterata are known to produce sounds, and in the Mollusca, also, the power is very rare. The Pectens, which are the most lively of bivalves, moving actively by the sudden opening and closing of their valves — as Pliny says, " Saliunt Pectines et extra volitant seque ipsi carinant " — also produce in the same way a certain sound, which Aristotle * gives as an exceptional case among the Mollusca. Nor is the production of sound much more frequent among the Crustacea. In one genus of crabs (Ocypoda), the claw bears a rasp, or file, which can be rubbed against a ridge on the basal segment of the limb, and thus produces a harsh, jarring sound. Some of the lobsters also (Palinurus) make a noise by rubbing one segment of the antennae against another ; but, considering that the ear is well developed in this group, it is rather remarkable how few of them are known to possess the power of producing sounds. Passing on to the insects, the song of the Cicada has been celebrated from time immemorial ; the chirping of the crickets and grasshoppers is also familiar to us all. For the reasons, however, already alluded to in the * " Historia Animalium." 62 INSECTS— LOCUSTS. preceding chapter, no insect possesses a true voice. The sounds they make are produced in various ways — for instance, by the wings or the spiracles, by rubbing one part of the body against another, etc. The power of producing sounds audible to us is pos- sessed by many insects scattered sporadically through all the great groups. In many of these cases, the power of producing sound is confined to the males. Their sounds are really love- songs.* In Locusts, as Westwood says,t "The stridulating powers of these insects must have attracted the notice of every one who has walked through the fields in the autumn. Unlike the insects of the two preceding families, it is owing to the motion of the hind femora, either con- jointly or alternately rubbed against the sides of the wing- covers, that the sound is pro- duced, the insects resting on their four anterior legs during the operation ; the veins of the wing- Fig. 44.— Leg of stendbothrus covors beinfif considcrablv ele- _297-afor«m (after Landois). *-" *' vated, so as to be easily acted upon by the rugose inner edge of the thigh. Some species, according to Goreau, may be observed to exe- cute this movement without producing any sound per- ceptible to our ears, but which he thinks may be per- ceived by their companions." * The females are not, however, invariably dumb. In Ephippigera both sexes are able to produce a sound, which, however, is not very loud. t Westwood, "Modern Classification of Insects." GRASSHOPPERS— CRICKETS. 63 Fig. 44 represents the leg of a grasshopper (Steno- hothrus ])ratorum). On the inner side of the thigh, at s, is a file, consisting of a row of fine teeth (Fig. 45, ;s), which rub against the wing-covers, and thus produce the well-known sounds. Lehmann states that Brunelli " kept and fed several males of Grijllus viridissimus in a closet, which were very merry, and continued singing all the day ; but a rap at the door would stojD them instantly. By practice he learned to imitate their chirping ; when he did this at the door, at first a few would answer him in a low note, and then the whole party would take up the tune and sing with all their might. He once shut up a male of the species in his garden, and gave a female her liberty ; but when she heard the male chirp, ^ ^ ^ ^ i\ is. Js^ ^. — -^ she flew to him im- mediately." * In the males of the house and field crickets, the source of the sound is different. On the inner margin of the left wing-cover, about one-third of its length from the base, a thickened point is observed, from which several strong veins diverge. The strongest of these veins, that running towards the base of the wing- cover, is regularly notched on the under side trans- versely, like a file. When the wing-covers are closed, this oblique bar of the wing-cover lies upon the upper surface of the corresponding part of the right wing- * " De Sensibus externis Animaliiim exsanguinium." Gottingen : 1798. I give Kirby and Spence's translation. Fig. 45. — Sound-bow of Stenobotlirus (after Landois). s, Surface of the skin ; z, teeth. 64 CICADAS — BEETLES. cover, and wlien a tremulous motion is imparted to the wing-covers, this bar rubs against the corresponding bar of the right wing-cover, and thus produces the familiar chirping sound. The soug of the Cicadas is produced, again, in a dif- ferent manner. The musical organs are internal, are placed "at the base of the abdomen beneatb, and are covered by two large flat plates attached behind the place of insertion of the hind legs, varying in form in the different species, being, in fact, the dilated sides of the metasternum. . . . The sound issues out of two holes beneath the above-mentioned plates, in a manner somewhat analogous to the action of a violin." * Many beetles have special organs for the production of sounds. A remarkable case is that of the so-called " bombardier beetles," which, when attacked, discharge at the enemy, from the hinder part of their body, an acrid fluid which, as soon as it comes in contact with air, explodes with a sound resembling a miniature gun. Westwood mentions, on the authority of Burchell, that on one occasion, " whilst resting for the night on the banks of one of the large South American rivers, he went out with a lantern to make an astronomical ob- servation, accompanied by one of his black servant boys ; and as they were proceeding, their attention was directed to numerous beetles running about upon the shore, which, when captured, proved to be specimens of a large species of Brachinus. On being seized, they immediately began to play off their artillery, burning and staining the flesh to such a degree that only a few specimens could be captured withthenaked hand, leaving a mark which remained a considerable time. Upon ob- * Westwood, "Modern Classification of Insects," vol. ii. p. 42, THE BOMBARDIER BEETLE — PAUSSUS. 65 serving the whitish vapour with which the explosions were accompanied, the negro exclaimed in his broken English, with evident surprise, *Ah, massa, they make smoke ! '" * A similar means of defence is possessed by beetles belonging to a very different family — the Paiissidse. Captain Boyes mentions f that on one occasion, having captured a Faiissus Fichtelii " it immediately emitted two loud and very distinct crepitations, accompanied with a sensation of heat, and attended by a strong acidulous scent. It left a dark-coloured stain on the fingers resembling that produced by caustic, and which had a strong odour something like nitric acid. A circumstance so remarkable induced me to determine its truth, for which purpose I kept it alive till the next morning, and, in order to certify myself of the fact, the following experiments were resorted to. Having prepared some test-paper by colouring it with a few petals of a deep red oleander, I gently turned the Paussus over it, and immediately placed my finger on the insect, at which time I distinctly heard a crepi- tation, which was repeated in a few seconds on the pressure being renewed, and each discharge was ac- companied by a vapour-like steam, which was emitted to the distance of half an inch, and attended by a very strong and penetrating odour of nitric acid." I do not, how^ever, refer to these cases as affording any evidence that the insects themselves possess the power of hearing, but merely on account of their * Westwood, " Modern Classification of Insects," vol. i. p. 76. t " Tiie Economy of the Paussidse," Ann. and Magazine of Natural History, vol. xviii. ; see also Peringuay's "Notes on Three Paiissi," Transactions of the Entomological Society, 1883, p. 133. F 66 DEATH-WATCH — BUKYING BEETLES. intrinsic interest. The following instances, however, do seem to imply a powder of hearing. A well-known case is that of the death-watch, associated with so many superstitions, and sup]30sed in old days to be a certain indication of approaching death. In this case the insect produces the sound by tapping with its head or abdomen, or, according to Doubleday, with its thorax. If a male death-watch ticks, and there be a female even within several yards, she returns the tap, and they approach one another slowly, tapping at intervals, until they meet. The male Ateuches stridulates to encourage the female in her work, and also, according to Darwin, " from distress when she is removed." * It has long been known that among the Longicorn beetles many of the species, when alarmed, " produce a slight but acute sound by the friction of the narrowed anterior part of the mesothorax, or rather a polished part of the scutellum, against the edge of the protho- racic cavity, by which motion the head is alternately elevated and depressed. It has been generally stated that it was by the friction of the hind margin of the thorax against the base of the elytra that this sound was produced, but this is not the case."t The burying beetles (Necrophorus) produce a sound by rubbing the abdomen against the hinder edges of the wing-cases. Wollaston, in a short paper on certain musical Curculionidae,t describes a species of Acalles, which he found in Teneriffe. A number of specimens were in a hollow stem, and when it was shaken " the whole plant * ''Descent of Man," vol. i. t Westwood, "Modern Classification of Insects," vol. i. X Ann. and Magazine of Natural History, 1860. WEEVILS— COCKCHAFERS. 67 appeared musical." In this genus the sound is produced by rubbing the tip of the abdomen, so rapidly that the movements were scarcely visible to the eye, against the under surface of the ends of the elytra, or wing-cases. The tip of the abdomen, though roughened, is not con- spicuously so, the ends of the elytra are shagreened, though very finely, and Wollaston expresses his surprise that so small an instrument could produce so loud a noise. He describes a similar structure in other species of the group. The cockchafers (Melolontha), besides the humming of the wings, produce a sound which may almost be called a voice. In the large trachea, immediately behind each spiracle, is a chitinous process, or tongue, which is thrown into vibration by the air during respi- ration, and thus produces a humming noise. In the beetles, then, the sounds produced may be divided into three classes : 1. Incidental, such as those produced during flight. 2. Defensive. 3. For signals, as in Longicorn beetles, Ateuches, Anobium, etc. Laudois gives the following summary of the different modes in which sounds are produced by the Cole- optera : — 1. Tapping sounds (Bostrychidse, Anobium). 2. Grating sounds (Elaterida). 3. Friction without rasping organs (EucJilrus Ion- gimanus), 4. Easping sounds produced by friction, viz. — (1) Pronotum on Mesonotum (Cerambycida, with the exception of Spondylis and Prionus). 68 VARIETY OF ORGANS OF SOUND AMONG BEETLES. (2) Prosternum on Mesosternum {Omaloplia hrunnea). (3) Elytra with rasp at the end (Curculionida ; Dytiscida, Pelobius). (4) Cox8e with rasp (Geotrupes, Ceratophyus). (5) Cover-margin rasp rubbing against the thigh ( Ghiasognath us Gra ntii) . (6) Pygidium with two rasps in the middle (Crioceris, Lema, Copris, Oryctes, Necro- phorus, Tenebrionida). (7) Abdomen with a grating-ridge and four grating-plates (Trox sabulosiis), (8) Abdomen with two toothed ridges rubbing on cover-margin rasp (Elaphrus, Blethisa, Cychrus). (9) Elyti-a rubbing with under-wing rasp {PeloUus Herrmanni), (10) Wings rubbing against abdominal ringlets (Melolontha fullo) „ 5. Exploding sounds from the tail (Brachinus). 6. Sounds produced by the spiracles (Melolontha). Graber, moreover, has shown by a number of interesting experiments * that the power of hearing is by no means confined to those beetles which are known to produce sounds themselves. Passing on to other groups of insects, flies and gnats, besides the humming of the wings, produce sounds, like the cockchafer, through the spiracles, some of which are especially arranged for this purpose. If a fly be caught and held between the fingers, it will generally make a loud and peculiar sound. The hum of the mosquito is only too familiar to most of us. * "Die Chordotoual Siuuesorgaue der lusekteu," Arch, fiir Mic. Anal, 1882. DIPTERA — HYMENOPTERA. 69 Landois mentions that he has heard species of Eristalis and Syrphus sing while they have been sitting quietly. The dragon-flies (Libellulina) also produce a sound by means of their spiracles. Among Hymenoptera, the hum of an angry bee is proverbial. Nor must I omit to mention the piping noise made by young queen bees. It is well known that there is only one queen in a hive, and that working bees never turn their back on her; as she moves among the combs, they all turn towards her. If there has been a swarm led by the old queen, the young queen who has succeeded often makes a piping noise, first noticed by Huber, whose statements are generally recognized as correct.* While "singing" the queen assumes a particular attitude, and the other bees all lower their heads and remain motionless until she begins to move again. In the mean while, if there are any other young queens which have not yet left the cells, they answer the old one, and their notes seem to be sounds of challenge and defiance. Other bees also produce a sound by means of their spiracles quite different from the humming of their wings. MutlUa Eiirojysea, a wingless species, related to and not unlike the ants, makes, when alarmed, a rather sharp noise by rubbing one of the abdominal rings against the other. Under these circumstances, Landois asked himself whether other genera allied to Mutilla might not possess a similar organ, and also have the power of producing sound. He first examined the genus Ponera, which, in the structure of its abdomen, nearly resembles * Huber, '' Obs. sur les Abeilles; '^ Bcvan, " On the Honey Bee ;" Langstroth, <• On the Honey Bee." 70 ANTS — BEES. Milt ilia, and here also he found a fully developed stridulating apparatus. He then turned to the true ants, and here also he found a similar rasp-like organ in the same situation. It is indeed true that ants produce no sounds which are audible by us ; still, when we find that certain allied insects do produce sounds appreciable to us by rubbing the abdominal segments one over the other, and when we find, in smaller species, an entirely similar structure, it certainly seems reasonable to conclude that these latter also do produce sounds, even though we cannot hear them. Landois describes the structure in the workers of Lasius fuliginosus as having twenty ribs in a breadth of '13 of a millimeter. In Lasius flavus I found about ten well-marked ribs, occupying a length of jJpg- of an inch. Similar ridges also occur between the following segments. In the flies (Diptera) and dragon-flies (Libellulina), the four thoracic spiracles produce sounds ; while in Hymenoptera, as, for instance, in the humble bee (Bombus), the abdominal spiracles are also musical. The sounds produced by the wings are constant in each species, excepting where there are (as in Bombus) individuals of very different sizes. In these the larger specimens give generally a higher note. Thus the comparatively small male of Bomhus terrestris hums on A', while the large female hums a whole octave higher. There are, however, small species which give a deeper note than larger ones, on account of the wing-vibrations not being of the same number in a given time. Moreover, a tired insect produces a somewhat different note from one that is fresh, on account of the vibrations being slower. SOUNDS PRODUCED IN FLIGHT. 71 Indeed, from the note produced we can calculate the rapidity of the vibration. The slow flapping of a butterfly's wing- produces no sound, but when the move- ments are rapid a noise is produced, which increases in shrillness with the number of vibrations. Thus the house-fly, which produces the sound of F, vibrates its wings 21,120 times in a minute, or 335 times in a second ; and the bee, which makes a sound of A', as many as 26,400 times, or 440 times in a second. On the contrary, a tired bee hums on E', and therefore, according to theory, vibrates its wings only 330 times in a second. Marey has succeeded in confirming these numbers graphically. He fixed a fly so that the tip of the wing just touched a cylinder which was moved by clockwork. Each stroke of the wing caused a mark, of course very slight, but still quite perceptible, and he thus showed that there were actually 330 strokes in a second, agreeing almost exactly with the number inferred from the note produced. The sound emitted from the spiracles bears no re- lation to that produced by the wings. Thus, according to Landois, the wing-tone of the hive bee is A'; its " voice," if we may call it so, on the contrary, is an octave higher, and often goes to B" and C". In one of the solitary bees, Anthidium manicatum, the difference is still greater ; the wing-tone is G', and the " voice " nearly two octaves higher, reaching to F'". The wing-tone is constant, at least with the excep- tions just alluded to. The " voice," on the contrary, appears to be to some extent under the control of the will, and thus offers another point of similarity to a true " voice." Thus a bee in the pursuit of honey hums 72 POWER OF VARYING SOUND. continually and contentedly on A', but if it is excited or angry it produces a very different note. Thus, then, the sounds of insects do not merely serve to bring the sexes together ; they are not merely " love-songs," but also probably serve, like any true language, to express the feelings. Landois also describes the muscles by means of which the form of the organ, the tension of the drum, etc., is altered, and the tone thus, no doubt voluntarily, affected.* We can, indeed, only in few cases distinguish the differences thus produced; but as even we, far removed as we are in organization, habits, and senti- ments, from a fly or a bee, can yet feel the difference between a contented hum and an angry buzz, it is highly improbable that their power of expressing their feelings should stop there. One can scarcely doubt but that they have thus the means of conveying other sentiments and ideas to one another. Butterflies and moths do not habitually produce any sound in flight. The texture of their wings is com- paratively soft, and they are generally moved slowly. Still, they are not altogether silent. The death's-head moth {S])hinx atroioos) emits a mournful cry, first noticed by Eeaumur. This moth, he says, " dans le temps qu'il marche, a un cri qui a paru funebre ; au moins est-il le cri d'une bonne ame de papillon, s'il gemit des malheurs qu'il annonce. *' Le cri de notre papillon est asses fort et aigu ; il a quelque ressemblance avec celui des souris, mais il est plus plaintif ; il a quelque chose de plus lamentable. C'est surtout lorsque le papillon marche, ou qu'il se * "Die Ton and Stimm Apparate der Insekten," Zeit. fur Wissi. Zool, 3866. BUTTERFLIES — MOTHS. 73 trouve mal a son aise, qu'il crie ; il crie dans les poudries, dans les boistes ou on le tient renferme ; ses cris redoiiblent lorsqu'on le prend, et il ne cesse de crier tant qu'on le tient entre les doigts. En general il fait grand usage de la faculte de crier, que la nature lui a accordee." * There Las been much doubt how the sound arises, but it appears to be ascertained that the moth produces it by rubbing the palpi against the base of the proboscis.f Huber thought, and subsequent writers — as, for instance, Kirby and Spence, and Bevan — have con- curred in the opinion, that the sound " operates on the bees like the voice of their queen, and thus enables the moth to commit the greatest ravages in the hives with perfect immunity," J On the other hand, Huber ascertained by experiment that it exercises no such charm over humble bees. Several other species of the genus Sphinx also pro- duce a sound, and a few other moths, for instance, Noctua fovea. Darwin also mentions § a Brazilian butterfly, Ageronia feronia, as making " a noise like that produced by a toothed wheel passing under a spring catch, which could be heard at the distance of several yards." The peacock butterfly ( Vanessa io) \\ is also said to possess the same power. For further details with reference to the sounds produced by insects, and, indeed, by animals generally, * " Mem. p. servir a I'Histoire des Insectes." t Landois, "Die Ton uud Stimm Apparate der Insekten," Zeit. fur Wiss. Zool., vol. xvii. X Bevan, " On the Honey Bee." § " Descent of IMan," vol. i. II "Die Ton and Stimm Apparate der Insekten," Zeit. fur Wiss. 74 CENTIPEDES— SPIDEES. I may refer to Landois's interesting work, " Thier- stimmen." From the fact that the power of producing sounds audible to us is scattered among so many groups, and that the sounds themselves are often so shrill, I am disposed to suspect that many insects usually regarded as dumb really produce sounds, which, however, are beyond our range of hearing. Among centipedes Gerstacker has described* a sound-producing organ in Eucoryhar crotijlus. The posterior legs have the fourth segment much enlarged and leaf-like, with the edges raised and formed of very hard chitine. The legs are rubbed against one another, and thus produce a rasping sound. Bourne also has recently described f a stridulating organ in another genus (Sphaerotherium). It is situated just behind the tw^enty-first pair of legs, and consists of a hood-like process bearing a number of parallel ridges. There is a very general impression that spiders hear well, and even enjoy music ! There seems, however, very little evidence of any value on the subject. No doubt they are extremely sensitive to vibrations. The presence of even a very small insect on their web is at once perceived. Mr. Boys has shown that the vibrations of a tuning-fork affect them strongly. J This sensitiveness to vibrations is, however, not neces- sarily the same as a true sense of hearing. Kraepelin says § that he knows only one observation which seems to him to possess sufficient exactness to justify the conclusion that spiders possess any sense of hearing — namely, that of Lehmann. * Gerstacker, " Stettin Ent. ZeiL, 1854. t Bourne, Linnean Journal, 1885. % Nature, vol. xxiii. § " Ueber die Geruchsorgane der Gliederthiere." POWER OF HEAKING IN INSECTS. 75 It would be, on the other hand, most unsafe to conclude that spiders are incapable of hearing. Dahl * has given reasons for believing that some of their hairs serve as auditory organs. Westring has discovered, in certain species of Theridium (T. serratipes, oculatum, castaneicm, etc.), a stridulating organ, consisting of a sort of raised bow attached to the upper part of the abdomen, which rubs against the under and hinder part of the cephalothorax, producing a whirring sound. Lebert t naturally observes that this appears to indicate a power of hearing on their part. As regards insects, it would be easy to multiply such evidence almost indefinitely ; I have given more illus- trations than I should probably have otherwise thought necessary, because so excellent an observer as Forel, whose opinion I should value on such a point as much as that of any authority, expresses doubt whether insects really hear at all. '' Ce qu'on semble," he says, in his last memoir on the subject, " considerer comme preuve de I'ouie me parait comme a Duges reposer a peu d'excep- tions pres sur des ebranlements mecaniques de I'air ou du sol qui sent simplement perfus comme tels par les organes tactiles des insectes. Cela correspond a peu pres a la derniere opinion de Graber sur " I'ouie " de la Periplaneta. Mais on n'a pas le droit de nommer ouie de pareilles sensations." J Graber, however, has endeavoured to meet this objection by an ingenious experiment.§ He placed some water-boatmen (Corixa) in a deep jar full of * " Das Gelior-und Geriichsorgane der Spinnen," Arch, filr Mie. Anat, 1885. t " Die Spinnen der Schweiz." X A. Forel, " Sensations des Insectes," Becueil Zool. Suisse, t. iv. 1887, § Arch, fur Mic. Anat, 1882. 76 SENSE OF HEARING IN INSECTS. water, at the bottom of which was a layer of mud. He dropped a stone on the mud, but the beetles, which were reposing quietly on some weeds, took no notice. He then put a piece of glass on the mud, and dropped the stone on to it, thus making a noise, though the disturbance of the water was the same. The water-boatmen, however, then at once took flight. In face of all the evidence, then, I do not think there can reasonably be any doubt on the subject, and it seems to be clearly established that insects do possess the sense of hearing. ( 77 ) CHAPTER V. THE OEGANS OF HEARING. That many of the lower animals have special organs for the production of sound, and possess the sense of hearing, has been shown in the preceding chapter. I now proceed to consider the mechanism by which sounds are perceived. In our own ear we have, first of all, the external ear, much less important in man than in many other animals, as in the horse, for instance, where it may be seen moving continually, and almost automatically assuming the position most favour- able for conveying the waves of sound down the outer passage (Fig. 46, D) to the tympanum, or drum. This is a membrane stretched between the outer air on the one hand, and the drum on the other, which also contains air, transmitted through the mouth by means of the Eustachian tube (Fig. 46, E). The drum is separated from the brain by a hard, bony partition in which are two orifices, one oval and the other round. Across the drum stretches a chain of little bones (Fig. 47) ; first the "hammer," secondly the "anvil," and lastly the " stirrup." The flat plate of the stirrup, again, lies against the oval orifice, or fenestra ovalis, as it is techni- cally called, of the drum. Thus the sounds are intensi- 78 STRUCTURE OF THE HUMAN EAR. fied by being conveyed from the tympanic membrane to one which is twenty times smaller. Behind the Fig. 46, —Diagram of human ear (after Berusteiu). I), Auditory canal; E, mouth of Eustachian tube ; cc, tympanic membrane ; B, tympanic cavity ; o, fenestra ovalis ; r, fenestra rotunda ; s, semicircular canals ; A, cochlea. fenestra ovalis is the labyrinth, which is filled with fluid, and on which the final filaments of the auditory nerve are distributed. This fluid is thrown into vibrations by those of the stirrup, but as it is en- closed in a bony case, the vibrations would be greatly curtailed if it were not for the second meaibrane, or fenestra rotunda. This round membrane, there- fore, acts as a counter opening, for if the fluid is compressed in one place, it must claim more room in another. The labyrinth consists mainly of two parts, Fig. 47.— Ossicles of the ear. H, Hammer ; Am, anvil ; Am. k, shorter process of the anvil; Am. I, longer process of the anvil ; S, stirrup ; St, long process of the hammer. STRUCTURE OF THE HUMAN EAR. tlie cochlea and the semicircular canals. The semi- circular canals are three in number, and stand at right angles to one another. No satisfactory explanation of their function has yet been given ; but there is some evidence that, in addition to, or apart from, hearing, they are affected by the position of the head, and thus serve as organs for maintaining the equilibrium of the body. Each of the canals commences Avith an oval dilatation, or ampulla. In the ampulla is a projecting ridge, on which are long, stiff, delicate, hair-like pro- cesses, the vibrations of which probably give certain sound-sensa- tions. In the canals certain parts bear shorter hairs, over which are minute ear- stones, or otolithes, consisting of carbonate of lime, embedded in a gelatinous substance. The cochlea contains, moreover, a compli- cated and wonderful organ, discovered by Count Corti. This appears to be, in fact, a microscopic musical instru- ment, composed of some four thousand complex arches, increasing regularly in length and diminishing in height from the base to the summit of the cochlea. The waves of sound have been supposed to play on this organ, almost like the fingers of a performer on the keys of a musical instrument. Fig. 48. — Section through the ampulla (after Bernstein). JV, Nei've ; z, terminal cells; h, auditory hairs. 80 THE ORGAN OF CORTI. Fig. 49. — Tympanal wall of the ductus cochlearis, from the dog. Surface view from the side of the scala vestibuli, after the removal of Reissner's membrane, S^Q. I. Zona denticulata Corti. II. Zona pectinata Todd-Bowman : 1, Habenula sulcata Corti ; 2, Habenula denticulata Corti; 3, Habenula perforata Kolliker. III. Organ of Corti : a, portion of tlie lamina spiralis ossea (the epithelium is wanting) ; h and c, periosteal blood-vessels ; d, line of attachment of Keissner's membrane ; e and e^, epithelium of the crista spiralis ;/, auditory teeth, with the interdental furrows ; g, g^, large-celled (swollen) epithelium of the sulcus spiralis internus, over a certain extent shining through the auditory teeth ; from the left side of the preparation they have been removed ; h, smaller epithelial cells near the inner slope of the organ of Corti ; k, openings through which the nerves pass ; i, inner hair cells ; I, inner pillars; hi, their heads; o, outer pillars; n, their heads; p, lamina reticularis ; q, a few mutilated outer hair cells ; r, outer epithelium of the ductus cochlearis (Claudius's cells of the author's) ; removed at s in order to show the points of attachment of the cuter hair cells. After Waldeyer, in Strieker's " .Manual of Histology." MODE OF ACTION OF AUDITORY ORGANS. 81 The fibres of Corti, according to Helmholtz, may be distributed among the seven octaves which are in general use, so that there will be 33 J fibres to every semitone, and 400 to each octave. Weber has esti- mated that a skilful ear can perceive a difference even of the g^ of a tone, or nearly four thousand sounds, and this would agree fairly well with the number of fibres. But why, it may be asked, should a given musical sound act more on one of these " keys " than another ? If several tuniDg-forks which, sound different notes are placed on a table, and another in vibration be brought near them, the one sounding the same note is thrown into vibration, while the others are unaffected. A second tuning-fork would affect its owa fellow, but no other, and so on.* A very slight change in the tuning-fork, such, for instance, as would be made by fastening a piece of wax to one of the prongs, is sufficient to destroy the sympathetic vibrations. The sound of the human voice has been known to break a bell-shaped glass by the agitation thus caused. The difficulty is to hit the pitch with sufficient precision, and retain the tone long enough. It is probable, therefore, that each of Corti's arches is set for a particular sound, and sensitive to it alone. This suggestion derives additional probability from the observations of Hensen (see p. 93) on the auditory hairs of Crustacea. We thus obtain a glimpse, though but a glimpse, of the manner in which the arches of Corti may possibly act. There are many problems still to be solved, but it is at least easy to see that so complex an organ may be capable of conveying very complex sensations. * Helmholtz, " Sensations of Tone." G 82 ORGANS OF HEAEING IN THE LOWER ANIMALS. On the Organs of Hearing in the Lower Animals. The semicircular canals in the liuman ear (see p. 79) have been supposed by some, in addition to, or apart from, their functions as organs of hearing, to assist in maintaining the equilibrium of the body ; at all events, when they are injured, the movements frequently be- come disorderly, and the otolithic organs of the lower animals appear, at any rate in certain cases, to perform a similar function.* Otolithes, as we have seen, are present in our own ears, but they play a mucli more important part in those of the lower animals. In the lowest, the sound- waves may be considered to produce a certain effect upon the general tissues. The soft parts of the body are, however, not w^ell calculated to receive such impressions. Their effect would be heightened by the presence of any solid structures, whether spicules, as in spouges, etc., or solid hairs projecting from the general surface, as in a great many of the lower animals. The Medusae (jelly-fishes. Fig. 50) present round the edge of the umbrella certain "marginal bodies," w-ith reference to which there have been great difterences of opinion. 0. F. Miiller, by whom they were discovered, regarded them as orifices for the exclusion of digested food, Eosenthal and Escholtz considered them to be glands, Milne Edwards as ovaries; but it seems now clearly established that some are organs of hearing, Exp., 1887. Eiigelmaun, " Ueber d. Function der Otolitlien," Zool. Anz., 1887. MEDUSA. 83 Fig. 50. — Eutima gigas (after Haeckel). 84 MEDUSA. aud otliers of sight. Some species possess both, but, as a general rule, among Medusae, where organs of hearing are present, those of sight are wanting, and vice versa. It may seem extraordinary that there should be such differences of opinion as to these organs. The earlier naturalists, however, had but imperfect microscopes, and probably often examined specimens in a bad state of preparation. As regards the alternative between the view that they served as eyes and that which regarded them as ears, it must, moreover, be remem- bered that as long as we merely know that there was a capsule containing a transparent body, the function might well be doubtful The auditory organs of the jelly-fishes were first recognized as such by Kolliker.* They are ranged round the umbrella, and vary considerably iu number, ranging up to sixty in Cunina, eighty in Mitrocoma, and as many as six hundred in (Equorea. There are three types. In the first, the auditory organ is an open pit, lined with cells. The majority of those on the outer side contain an otolithe, while a row on the opposite side are strap- shaped, their free ends termi- nating in auditory hairs, which reach to the cells containiiifv the otolithes, while their inner ends are continuous with fibres from the inner nerve-ring. In such an auditory organ as that of Ontorchis (Fig. 51), the otolithes present a very deceptive resemblance to the lenses of an eye. * "Ueber die Raudkorper der Quallen," Frorieps Neue Not., 1843. Fig. 51. — Auditory organ of Ontor- chis Gegenhauri. MEDUSA. 85 Fig. 52 represents the somewhat more complex auditory organ of Phialidium. d^ Fig. 52.— Auditory organ of Pliialidium (after Hertwig). d', Epitlielium of tlie upper surface of tlie velum ; d-, epithelium of the under surface of the velum ; hh, auditory hairs ; /i, auditory cells ; 7?p, nervous cushion ; nr', nerve-ring ; r, circular canal at the edge of the velum. The second type is more advanced, the vesicle being closed, and the otolithes fewer in number, the Eucopidse, indeed, having only one.* In the third type, that of the Trachymedusse, the Fig. 53.— Auditory organ of Rhopalonema, still showing a small orifice (after Hertwig). hkf Modified tentacle ; o, auditory organ. auditory organs are modified tentacles. They form a club-shaped body, with a central endodermal axis, and * Hertwig considers that the supposed hairs shown by Hensen in his figure of the ear of Eucope are really the edges of auditory canals. S6 MEDUSA. bearing at the apex one or more sometimes spherical, sometimes prismatic, otolithes. In some cases the organ becomes enclosed in a cup, which in Geryonia closes at the top. In another family of the Hydromedusse, the Oceanidae, these organs are absent, and appear to be replaced by certain pigment spots at the base of the tentacles, which, however, from their structure are considered to be rudimentary organs of vision, and will be described in the chapter on eyes. ^ Some species have, in addi- tion, other organs, obviously of sense, but the function of which is still far from clear. Fig. 54 represents one of these curious sense-organs in Pelagia, after Hertwig. It is in the form of a somewhat bent finger, is situated in a deep fold of the umbrella, contains a branch of the gastrovascular canal, and is filled at the tip with a group of solid, shining, rod-like crystals. The auditory organ in worms and molluscs consists of a closed vesicle, containing one or more otolithes, and lined with nerve-cells, which are, in the higher groups, connected at their base with the auditory nerve, and bear setae at the other end. De Quatrefages was the first who established clearly the existence of auditory organs in worms. In the Mollusca, the existence of an organ of hearing in some Gasteropods was justly inferred by Grant from Fig. 54.— Sense-organ of Pelagia (after Hertwig). o, Group of crystals ; sk, sense-organ ; sf, fold of the skin ; ga, gastro-vas- cular channel. MOLLUSCA— ANNELIDES. 87 the fact that one species, Tritonia arhorescenSy emits certain sounds, doubtless intended to be heard by its fellows. The ciliae contained in the auditory vesicle are some- times short, and scattered over -^^^ the general surface, as in Unio (Fig. 55) ; sometimes long and borne on papillary projections, as in Carinaria and Pterotrachea* (Fig. 56), where also there are certain special cells, supposed to act as buffers or dampers. The otolithe is sometimes single, and nearly spherical, as in Acephala and Heteropoda, and consists of calcareous matter with an organic base ; in the Gasteropods, Pteropods, and 5R V Fig. 55. organ of Auditory Unio (after Leydig). a. Nerve ; h, cells ; c, cilite ; d, otolithe. Fig. 56.— Auditory organ of Pterotrachea Friderici. (after Claus). Na, Auditory nerve ; c, central cells ; d, supporting plate ; b, outer circle of auditory cells ; a, ciliated cells. some Annelid es (Arenicola, Amphicora) they are * Claus., " Ueber den Acoust. App. im Gehororgane der Hetero- podeD," Arch, fur Mic. Anat., 1878. 88 ANNELIDES—CRUSTACEA. numerous, and sometimes, as in Cymbulia, collected into a mulberry-like group. In many cases the auditory sac rests directly on the gauglion. The actual mode of termination of the nerves is still uncertain. I have already mentioned that vibrations, if fewer than thirty in a second, do not produce on us the effect of sound. But it is possible that these organs in the lower animals are intended quite as much to record movements in the water as for hearing properly so called. The Organs of Hearing in Crustacea. Fig. 57. — Base right antenuule of lobster {Astacus marinus) ; after Farre. a, Orifice ; ' '' ■ Fig. 58. — Interior of auditory sac of lobster (after Farre). a, Orifice ; b, auditory hairs. It was long supposed that the auditory organ of the Crustacea was situated in the basal segment of the outer antenna. The true auditory organ was, indeed, discovered by Eosenthal in 1811,* who, however, re- * HeiVs Arch, fur Phys.,lSn. CKUSTACEA. 89 garded it as an olfactory organ, as did also Treviranus, Fabricius, Scarpa, Brandt, Milne Edwards, and, in fact, the older naturalists generally. The discovery of its true nature is due to Farre,* was confirmed, by Huxley f and Leuckart, and is now generally admitted. It is a sac situated in the base, or first segment, of the lesser pair of antenna?, which is slightly dilated. In some species the sac communicates freely with the Fig. 59.— Part of wall of auditory sac of lobster (^Astacus marinus) ; after Hensen. a. Thickened bars in the membrane of the sac ; >), first row of auditory hairs; jj', second row of auditory hairs ; »", third row of auditory hairs; >/", fourth row of auditory hairs ; e, grains of sand, serving as otolithes. water by means of an orifice situated towards the inner and anterior margin, and guarded by rows of fine hairs. In others the orifice is closed, but its position is always marked, as the auditory sac is at this point connected with the skin. Both contain otolithes. Those of the closed sacs are generally rounded ; while, on the contrary, those of * Pliilosophical Transactions, 1843. t Ann. and Mag. of Natural History, 1851. 90 USE OF GRAINS OF SAND AS OTOLITHES. the open sacs are simply grains of sand, and are so numerous as sometimes to occupy one-fourth, or even one-third, of the sac. Farre stated that the otolithes in the auditory sacs of Crustacea were simply grains of sand, selected by the Crustacea, and put into their own sacs to serve as otolithes. It seemed, however, so improbable that Crustacea should pick up suitable particles of sand and place them in their ears, that the statement was not unnaturally received with incredulity. The obser- vation of Hensen appears, however, to leave no doubt on the subject. The sac, whether open or closed, is an extension of the outer skin, and is cast with it at each moult. Hensen examined them shortly after moultiDg, and found that the sacs contained no stones ; he saw the shrimps carefully selecting particles of sand, but could never detect one in the very act of placing one in the auditory sac. He therefore placed some shrimps in a vessel of filtered sea- water, and streued over the bottom some crystals of uric acid. Soon afterwards one of the shrimps moulted, and the auditory sac was found on examination to contain a few grains of sand, but no crystals of uric acid. Three hours later, however, Hensen found that the new sac con- tained numerous crystals of uric acid, but none re- sembling common sand. Evidently, therefore, the Crustacea pick up grains of sand, and actually intro- duce them into their own ears to serve as otolithes. Otolithes are not, however, universally present. In the true crabs (Brachyura) they appear to be always wanting, so that the auditory hairs (which present very nearly the same character as those of the lobsters, etc.) are capable of being thrown into vibrations without the mediation of otolitbes. AUDITORY HAIRS. 91 The interior of the sac is thus described by Farre : "Aloug the lower surface of the vestibuhir sac is seen running a semicircuhir line, broader at its upper than its lower extremity (Fig. dS, I). This part is more easily examined after the sand has been washed away by agitation under water. It is then seen, with -a power of 18-linear, to consist of several rows of ciliated pro- cesses, of which one row is more regular and prominent than the rest, and crests the entire margin of the ridge. The processes diminish in size and number on either side, and are in some places seen in groups, but always assume the general form represented in" Fig. 58. ^ In Astacus there are four rows of hairs. The first are somewhat scattered, and above the otolithes; the second consists of larger hairs, arranged close together ; the third and fourth are smaller again, and more scat- tered. These three rows of hairs are covered by the otolithes. They stand in connection with the terminal fibrils of the acoustic nerve, and through their vibra- tions the sense of sound is supposed to be conveyed. In the lobster Hensen counted 548 auditory hairs. He divides auditory hairs of Crustacea into three classes : otolithe hairs ; free hairs, enclosed in the audi- tory sac ; and auditory hairs on the outer body surface. These latter auditory hairs (Fig. 59) are situated over an orifice in the chitinous integument, and stand in direct communication with a fibril from the nerve ; the stem of the hair does not rest directly on the chitinous integument, but is supported by a delicate membrane, which is sometimes dilated at the base ; the edge of the chitine at one side of the hair is raised into a tooth ; lastly, according to Hensen, each auditory hair 92 EAR IN TAIL OF MYSIS. possesses a sort of appendage, or languette, to which the nerve is attached. As far as details are concerned — the form of the sac, the number, form, and arrangement of the hairs, etc. — the auditory organs of the Crustacea offer endless variations in the different species, while very constant in each. In the higher groups the auditory sac is always at the base of the small antennae. In one of the lower forms, however — the curious genus Mysis — the ear is situated in the tail. The genus Mysis (Fig. 61) is a group of Crustaceans, in outward appearance very like shrimps, but differing in the absence of external gills, and in the structure of the legs and other par- ticulars, so that it is placed in a different family. Frey and Leuckart, moreover, made the interesting discovery that it possesses two ears in its fail. Fig. 60. — Auditory hair of tlie crab {Carcinus mcenus), X 500. a, Skiu ; C, nerve; h, delicate intermediary mem- brane or hinge (after Heusen). Fig. 61.— Mysis (after Frey and Leuckart). The tail, like that of a lobster, consists of five flaps. In each of the two smaller flaps is an oval sac (Fig. 62) containiug a single, lens-shaped otolithe, consisting of MODE OF HEARING. 93 a calcareous matter embedded in an organic substance. That Crustacea do, as a matter of fact, possess the power of perceiving sounds, there can be no doubt. Hensen himself has made various experiments on the subjecti Moreover, strychnine possesses the pecub'ar property of augmenting the reflex power of the nervous centres. Taking advantage of this, Hensen placed some shrimps in sea-water containing strychnine. He then found that they became ex- tremely sensitive to even very slight noises. Further than this, Hensen availed himself of Helmholtz's re- searches on the perception of sound, and, suspecting that the different hairs might be affected by dif- erent notes, found that was actually the case. The vibration of the hairs is mechanical, 'not depend- ing on the life of the animal. Hensen took a Mysis, and fixed it in such a position that he could watch particular hairs with a microscope. He then sounded a scale ; to most of the notes the hair remained entirely passive, but to some one it responded so violently and vibrated so rapidly as to become invisible. When the note ceased, the hair became quiet ; as soon as it was resounded, the hair at once began to vibrate again. Other hairs in the same way responded to other notes. The relation of the hairs to particular notes is probably determined by various conditions ; for instance, by its length, thickness, etc. Fig. 92.— Tsd\ of Mysis vulgaris, show* ing the auditory organ. 94 OKGANS OF HEAEING IN INSECTS. That these plumose hairs, then, really serve for hear- ing may be inferred, not only from their structure and position, but also from the observed fact that they respond to sound-vibrations. Hensen's observations* have been repeated and verified by Helmholtz. The Organs of Hearing in Insects. I now pass on to insects. There has been great difference of opinion as to the seat of the organ of hearing in this group. The antennae have, as already mentioned, been re- garded as ears by many distinguished authorities, including Sulzer, Scarpa, Schneider, Bolk-Hausen, Bonsdorff, Cams, Strauss-Diirkheim, Oken, Burmeister, Kirby and Spence, Newport, Landois, Hicks, Wolff and Graber, who have supported their opinion by numerous observations. Kirby states that once " a little moth was reposing upon my window ; I made a quiet, not loud, but distinct noise : the antenna3 nearest to me immediately moved towards me. I repeated the noise at least a dozen times, and it was followed every time by the same motion of that organ, till at length the insect, being alarmed, became more agitated and violent in its motions." And again: "I was once observing the motions of an Apion (a small weevil) under a pocket microscope ; on seeing me it receded. Upon my making a slight but distinct noise, its antennae started. I repeated tie noise several times, and invariably with the same effect." t ♦ " Sensations of Tone." t Introduction to " Entomology," Kirby and Spence, vol. iv. i BEETLES. 95 Among beetles, the genus Copris, "particularly," says Newport, " Copris molossus, in which I first remarked it, have the antennae composed of ten joints, the last three of which form tlie knob or club wdth which it is surmounted. " When the insect is in motion, these plates or audi- tory organs, if we may be allowed so to call them, are extended as wide as possible, as if to direct the insect in its course ; but upon the occurrence of any loud but sudden noise are instantly closed, and the antennae retracted as if injured by the percussion, while the insect itself stops and assumes the appearance of death. A similar use of the antennse is made by another family, Geotrupidse, which also act in the same manner under like circumstances. ***** "These facts, connected with the previous experi- ments, have convinced me," he says, " that the antennae in all insects are the auditory organs, whatever may be their particular structure ; and that, however this is varied, it is appropriated to the perception and transmission of sound." t Will has made some interesting observations on some of the Longicorn beetles (Cerambyx), which tend to confirm this view. These insects produce a low shrill sound by rubbing together the prothorax and the mesothorax. The posterior edge of the prothorax bears a toothed ridge, and the anterior end of the mesothorax a roughened surface, and when these are rubbed together, a sound is produced something like that made by rubbing a quill on a fine file. * Newport, " On the Antennse of Insects," Transactions of the Entomological Society, 1836-40, vol. ii. 96 BEETLES. Will took a pair of Cerambyx (beetles), put tlie female in a box, and the male on a table at a distance of about fifteen centimetres (four inches). They were at first a little restless, but are naturally calm insects, and soon became quiet, resting' as usual with the antennae half extended. The male evidently was not conscious of the presence of the female. Will then touched the female with a long needle, and she began to stridulate. At the first sound the male became restless, extended bis antennae, moving them round and round as if to determine from which direction the sound came, and then marched straight towards the female. Will repeated this experiment many times, and with dif- ferent individuals, but always with the same result. As the male took no notice of the female until she began to stridulate, it is evident that he was not guided by smell. From the manner in which the Cerambyx was obviously made aware of the presence of the female by the sound. Will considered it clearly proved that in this case he was guided by the sense of hearing. Will has also repeated with these insects the experi- ments I made with ants, bees, and wasps, and found that they took no notice whatever of ordinary noises ; but when he imitated their own sounds with a quill and a fine file, their attention was excited — they extended their antennae as before, but evidently per- ceived the difference, for they appeared alarmed, and endeavoured to escape.* Hicks in 1859 justly observed that, "Whoever has observed a tranquilly proceeding Capricorn beetle which is suddenly surprised by a loud sound, will have seen * Will, "Das Geschmacksorgan der lusekteu," Zeit. fur. Wiss. Zool.t 1885. SEAT OF THE SENSE OF HEARING. 97 how immovably outward it spread its antennte, and holds them porrect, as it were with great attention, as long as it listens, and how carefully the insect proceeds in its course when it conceives that no clanger threatens it from the unusual noise." * Other similar observations might be quoted, but these sufficiently indicate that in some insects, at any rate, the organs of hearing are situated in the antennse. On the other hand, Lehmann long ago observed that the house cricket (Achefa domestica), when deprived of its antennae, remained as sensitive to sounds as previously. This is quite correct ; and yet, if a cricket be decapitated, and a shrill noise be made near the head, the antennae are thrown into vibration by each sound. In fact, not only do the highest authorities differ, but the observations themselves appear at first sight to be contradictory. The explanation seems to be that the sense of hearing is not confined to one spot. That the antennae do serve as ears, at least in some insects, the evidence leaves, I think, no room for doubt. But there is no reason, in the nature of things, why the sense of hearing should be confined to one part of the body. Taste, indeed, would be useless except in or near the mouth, and almost the same may be said of smell. But the sense of touch is spread, in greater or less perfection, over the whole skin. Indeed, there is among the lower animals a great tendency to repeti- tion, and not least so amongst insects. The body con- sists normally of a number of segments, each with a pair of appendages and a ganglion. There are three pairs of legs ; two pairs of jaws, opening, not vertically, * Transactions of the Linnean Society, vol. xxii. H 98 DIFFERENT SEATS OF ORGANS OF SENSE. as ours do, but laterally; several pairs of breathing- holes arranged along the sides of the body ; and two kinds of eyes. Moreover, unquestionable organs of sense occur in very different parts of the body. The Crustacean genus My sis, as already mentioned, has ears in its tail; one group of sea-woians (the Polyoph- thalmata) have a pair of eyes on each segment of the body. Of Amphicorine, a small worm of our coast?, M. de Quatrefages says that often,* "C'est la queue qui marche la premiere, explorant evidemment le terrain avec une grande activite et donnant autant de signes d'intelli- gence et de spontaneite que pourrait le faire la partie anterieure du corps. . . . Cette queue porte a son extremite un disque elargi sur lequel sont places deux points rouges. . . . Je ne mets nullement en doute que ces points ne soient en effet des organes de vision." He was not able, indeed, to make out their finer structure. On the other hand, the lateral eyes of the Polyophthalmata possess a well-formed lens. We need not, then, assume that the organs of hearing in insects must necessarily be in the that they trated in body. It had long been known that grasshoppers and Fig. 63.— Part of leg of Grasshopper (Gryllus) ; after Graber. o, t, n, b, Tympanum. head, or, indeed, need be concen- part of the one * Ann. des Sci. Nat., 1850. CRICKETS HAVE EARS IN THEIR LEGS. 99 crickets have on their anterior legs two peculiar, glassy, generally more or less oval, drumlike structures ; but these were supposed by the older entomologists to serve as resonators, and to reinforce or intensify the well-known chirping sounds which they produce. Johannes Miiller was the first who suggested that these drums, or tympana, act like the tympanum of our own ears, and that they are really the external parts of a true auditory apparatus. That any animal should have its ears in its legs sounds, no doubt, a pnori very unlikely, and hence probably the true function of this organ was so long unsuspected. That it is, how- ever, a true ear the following particulars, taken especially from the memoirs of Miiller,* Siebold,t Leydig,^ Hensen,§ Graber,|| and Schmidt, IT conclusively prove. The Leaping Orthoptera fall into three well-marked groups : the locusts (Locustidse), which have short antennae; the crickets (Achetidse), which have long antennas, and the wings flat on the back; and, thirdly, the Gryllidse, or grasshoppers (as I may perhaps call them), which have also long antennae, but in which the wings are sloping. This is the nomenclature adopted by English authorities, such as Westwood ; but unfortunately many foreign . entomologists call the * " Zur vergleichenden Physiol ogie des Gesichtsinnes." 1826. t "Ueber die Stimm und Gehororgane der Orthopteren," Arch, fur Natur geschichte, 1844. X "Ueber Geruchs-imd Gehororgane der Krebse und Insekten," Eeicherts' Arch, fur Anat., 1860. § "Ueber das Gehororgan von Locusta," Zelt. fiir Wiss. Zool., 1866. II " Die Tympanalen Sinnesapparate der Orthopteren," Arch, fiir Mic. Anat., vol. xx., 1875. 1 "Die Gehororgane der Heuschrecken," Arch, fiir Mic. Anat., vol. xi. 100 EAR OF GRASSHOPPERS. crickets GryllidaD, the grasshoppers Locustidse, and the locusts Acridiidse.* In grasshoppers (Gryllidoe) and crickets (Achetidse) the auditory organ lies in the tibia of the anterior leg, on both sides of which there is a disc (Fig. 63), generally more or less oval in form, and differing from the rest of the surface in consisting of a thin, tense, shining membrane, surrounded wholly or partially by a sort of frame or ridge. In some species the two tympana are similar in form ; in others they differ. For instance, in the field cricket, the hinder tympanum is elliptic, the front one nearly circular in outline. In many of the Gryllidse, the tympana are protected by a fold of the skin, which projects more or less over them. The corresponding spiracle is also specially modified in the stridulating locusts, while in those which are dumb it is formed in the same manner as the others. The tympana are not always present, and it is an additional reason for regarding them as auditory organs, that both among the Achetidse and the Gryllidae, in those species which possess no stridulating organs, the tympana are also wanting.f * The destructive "locust" of the East, which is so numerous that in one year our Government^ in Cyprus destroyed no less than 150,000,000,000 of eggs, and whose ravages are used in Eastern poetry as types of destructiveness, has short antennae, and belongs to the first division ; to which, therefore, English entomologists apply the name Locusta, while our foreign friends, on the contrary, apply the name to a totally different insect. However, I merely refer to this now, to explain why the terms I have used do not in all cases agree with those adopted by the observers to whom I am referring. t This rule seems, however, not to be entirely without exceptions. At least, Aspidonotus and Hetrodes are said to possess tympana, but * Report on the Locust Campaign, Pari. Paper, 5250 of 1888. STEUCTUEE OF EAE. 101 Graber regards the covered tympana as a develop- ment from the open ones, and suggests that in time to come the species in which the tympana are now exposed may develop a covering fold. If now we examine the interior of the leg, the trachea or air-tube will be found to be remarkably modified. Upon entering the tibia it immediately enlarges and divides into two branches, which reunite lower down. To supply air to this wide trachea the corresponding spiracle, or breathing-hole, is considerably enlarged, while in the dumb species it is only of the usual size. An idea of the form of the trachea will be given by Fig. 69, which, however, represents the anterior tibia of an ant, where these tracheae are less considerably enlarged, and where one of the branches is much smaller than the other, while in locusts they are nearly equal in width, and one lies against each tympanum. The enlarged trachea occupies a considerable part of the tibia, and its wall is closely applied to the tympanum, which thus has air on both sides of it ; the open air on the outer, the air of the trachea on its inner surface. In fact, the trachea acts like the Eustachian tube in our own ear ; it maintains an equilibrium of pressure no stridulating apparatus. For instance, in the following forms, both the stridulating apparatus and the tympana are absent, viz. : — Among the (Ecanthidse : Phalangopsis and Gryllomorpha (both are wingless). » Platydactylidse : Metrypa and Parametrypa (both wing- less). „ Tettigonidie : Trigonidiura. „ Gryllidffi : Gryllus apterus, Parahrachytrupes Ausfralis, and Apiotarsus (all wingless). „ Gryllotalpidge : Tridactylus apicalis. „ Mogoplistidse : Mogoplistes, Myrmecophila, Physoblemraa (all wingless), and Cacoplistes. 102 STRUCTUEE OF EAH. on each side of the tympanum, and enables it freely to transmit the atmospheric vibrations. These tracheae, though formed on a similar plan, present many variations, corresponding to those of the tympana, and showing that the tympana and the trachege stand in intimate connection with one another. For instance, in those species wdiere the tympana are equal, the tracheae are so likewise; in Gryllotalpa, where the front tympanum only is de- veloped, though both tracheal branches are present, the front one is much larger than the other; and where there is no tympanum, the trachea remains compara- tively small, and even in some cases, according to Graber, undivided. The tibia is thus divided into three parts, as shown in the diagram (Fig. 64), the central X5\ portion being occupied by the two /|'' ^ tracheae (Fig. 64, tr, tr). \i '!) _M_ ^^ "^^^ other two spaces, one (the IrX^l lower one in the figure) is occupied ^^ IT^I ^^ ^^ *^® muscles, nerves, etc., while K % the other is mostly filled wdth blood, V ' 1 / which thus surrounds and bathes the ^^ auditory vesicles and rods {ar). Fig. 6i.-section through Thc acoiistic ucrvc — which, next Mecon'i '^^?'' a^bout to the optic, is the thickest in the trachei? ar, tTie\udu body— dividcs soou after entering toryrod. ^-^^ t\\)\ii into two brauchcs ; the one forming almost immediately a ganglion, the supra- tympanal ganglion, to which I shall refer again pre- sently; the other passing down to the tympanum, where it expands into an elongated flat ganglion, known after its discoverer as the organ of Siebold (Fig. Qb), and closely applied to the anterior tracheae. STRUCTURE OF EAR. 103 It is well shown in Fig. 65, taken from Graber. At the upper part of the ganglion is a group terminating below in a single row of vesicles, the first few of which Fig. 65.— The trachete and nerve-end organs from the tibia (\cg) of a grasshopper (Ephippjgera vitium) ; after Graber. EBI, Terminal vesicles of Siebold's organ ; hT, hinder tympanum ; Sp, space between the tracheae ; hTi\ hinder branch of the trachea; SN, nerves of the organ of Siebold; go, supra-tympanal ganglion; Gr, group of vesicles of the organ of Siebold ; vN, connecting nerve-fibrils between the ganglionic cells and the terminal vesicles ; So, nerve terminations of the organ of Siebold ; vT, front tympanum ; vTr, front branch of the trachea. are approximately equal, but which subsequently diminish regularly in size. Each of these vesicles is connected with the nerve by a fibril (Fig. Qb, vN), and contains an auditory rod (Fig. Q6). 104 AUDITORY RODS. One of these auditory rods is shown in Fig. 66, and the general arrangement is shown in the subjoined diagrammatic figure (Fig. 67). The rods were first described by Siebold, who con- sidered them to be auditory from their association with the stridulating organs. l....fa They have since been discovered in many other insects, and may be re- garded as specially characteristic of the acoustic organs of insects. They are brightly refractive, more or less elon- gated, slightly club-shaped, hollow (in which they differ from the retinal rods), and terminate, in Graber's opinion,* in a separate end-piece (Fig. 66, ho). In ^rod ^of~a^grS different insects, besides being in some hopper, Gryllus ■, j. i j.i • >} viridissimus ^nfter casos more elongated than m others, /JC^Audito'iy ro°d; tliev prcseut various minor modifica- ;to, terminal piece. ^.^^^ -^ ^^^.^^^ ^^^^ ^^,^ ^^^^^^ Ulliform iu size— about '016 mm. ; being as large, for instance, in the young larva of a Tabanus (2 mm. long) as in much larger insects. They are, as we shall see, widely distributed in insects, but as yet unknown in other animals. At the upper part of the tibial organ of Ephippigera there is, as already mentioned, a group of cells, and below them a single row (Fig. 6d) of cells gradually diminishing in size from above downwards. One can- not but ask one's self whether the gradually diminish- ing size of the cells in the organ of Siebold (Fig. 66) may not have reference to the perception of different * Graber, " Die cTiordotonalen Sinucsorgane imd das Gehor der InsekteD," Arch, fiir Mic. Anat., 1882. POSITION OF AUDITORY RODS. 105 notes, as is the case with the series of diminishing arches in the organ of Corti {arde^ p. 80) of our own ears. I have already alluded to the supra-tympanal ganglion; this also terminates in a number of vesicles Fig, 67.— Diagram of a section tlirough the auditory organ of a Grasshopper (Meco- nema). c, cuticle; a.r, auditory rod; a.c, auditory cell; tr, tracheae. containing auditory rods, which are said to be somewhat more elongated than those in the organ of Siebold. The arrangement of the organ is very curious, and will best be understood by reference to Fig. Q^. The great auditory nerve, as already mentioned, bifurcates almost immediately after entering the tibia, and one of the branches swell into a ganglion : from this ganglion proceed fibres which enlarge into vesicles (Fig. QS), each containing an auditory rod; and then again contract, approximate into a close bundle, and coalesce with the hypoderm (inner skin) of the wall of the tibia. The supra-tympanal organ of the crickets closely resembles that of the grasshoppers, while, on the other hand, they appear entirely to want the organ of Siebold (Fig. 65). This is a very remark- able difference to exist in two organs otherwise so similar. There appear to be two ways in which the atmospheric 106 EAR OF LOCUSTS. vibrations may be communicated to the nerve: either the vibrations of the tympanum may act upon the air in the tracheae, and so upon the auditory rods, or the air in the tracheae may remain passive, and the vibrations may act upon the auditory rods through the fluid in the anterior chamber of the leg. The fact that the auditory rod is turned away from the trachea would seem to favour this hypothesis. Fig, 68.— Outer part of a section through the tibia of a Gryllus viridissimus (after Graber). /), Hind surface of leg; p, wall of trachea; F. fat bodies; Su, suspensor of the trachea; vW, tracheal wall; TX, nerve; gz, ganglionic cells; rB, tissue connecting the ganglionic cells; E.Sch., end tubes of the ganglionic cells, each containing an auditory rod ; fa, terminal threads of ditto. In the true Locustidse (Acridiodeae of Graber) the organ of hearing is situated, not in the anterior tibiae, but in the first segment of the abdomen ; externally it is marked by a glistening appearance, and it is oval, or in some cases nearly ear-shaped. It was first noticed by Degeer. Behind the tympanum is a large tracheal sac, as in the families already described, and the tension of the tympanum is regulated by one, or in some cases by two muscles. The tympanum also presents two chitin- EAR OF LOCUSTS. 107 ous or horny thickenings, a small triangular knob, and a larger, somewhat complicated piece, consisting of two processes — a shorter upper, and a longer lower one, making a broad angle with one another. As in the preceding families, so also in the Locust id^e, the acoustic nerve is in close connection with the tracheae ; it swells into a ganglion, which con- tains in some species as many as 150 auditory rods, and then, as in the supra-tympanal organ (see p. 105), con- tracts into a tapering end, which is attached to the small chitinous knob. The auditory rods differ in no respect, as yet ascertained, from those already described. For many years no structure corresponding to the tibial auditory organ of the Orthoptera w^as known in any other insect. In 1877, however, I discovered * in ants a structure which in some remarkable points resembles that of the Orthoptera, and which I described as follows : — " The large trachea of the leg (Fig. 69) is considerably Fig. 69.— Tibia of yellow ant (Lasius flavits), x 15. S, S, Swellings of large trachea; rt, small branch of trachea; x, chord otonal organ. swollen in the tibia, and sends off, shortly after entering the tibia, a branch which, after running for some time parallel to the principal trunk, joins it again. "Now, I observed that in many other insects the * Lubbock, "On the Anatomy of Ants," Microscopical Journal, 1877. 108 PECULIAR STRUCTURE IN LEG OF ANT. trachece of the tibia are dilated, sometimes with a recurrent branch. The same is the case even in some mites. I will, however, reserve w^hat I have to say on this subject, with reference to other insects, for another occasion, and will at present confine myself to the ants. If we examine the tibia, say of Lasius flavus, we shall see that the trachea presents a remarkable arrange- ment (Fig. 69), which at once reminds us of that which occurs in Gryllus and other Orthoptera. In the femur it has a diameter of about 30^0 ^^ ^^ ^^^^ > ^^ soon, however, as it enters the tibia, it swells to a diameter of about ^^ of an inch, then contracts again to gio* and then again, at the apical extremity of the tibia, once more expands to -^Iq-, Moreover, as in Gryllus, so also in Formica, a small branch rises from the upper sac, runs almost straight down the tibia, and falls again into the main trachea just above the lower sac. "The remarkable sacs (Fig. 69, S, S) at the two extremities of the trachea in the tibia may also be well seen in other transparent species, such, for instance, as Myrmica ruginodis and Pheidole 7negacephala. " At the place where the upper tracheal sac contracts (Fig. 69) there is, moreover, a conical striated organ (x), which is situated at the back of the leg, just at the apical end of the upper tracheal sac. The broad base lies against the external wall of the leg, and the fibres converge inwards. Indications of bright rods may also be perceived, but I was never able to make them out very clearly." This closely resembles both in structure and position the supra-tympanal auditory organ of the Orthoptera. Graber has entirely confirmed this account and dis- covered some insects in which the structure is more OEIGIN OF EAE. 109 clearly yisible than in any which I had examined. Fig. 70 represents part of the tibia of Isoi^terijx ajncaUs. These organs do not, however, appear to be univer- sally present. In some very transparent species no trace of them can be found. But though so similar in structure, and probably in Fig. 70,*— Part of the tibia of Isopteryx apicalis (after Graber). Sc, Auditory organ ; e/", terminal filament; Cm, cuticle ; (?, ganglion cells; e/, terminal filaments ; tr, trachea ; n, nerve. function, it may be doubted whether this tibial organ in the ants can be traced to a common origin with that of the Orthoptera. According to Graber, the direction of the rods is reversed in the two cases, which he regards as clear proofs that they have arisen independently. He is even of opinion that the tympana themselves have originated independently in the different groups of Orthoptera. Moreover, Graber has found this organ in certain insects not only in the anterior, but also in the two other pairs of legs. Indeed, rods of the same character have been found in other regions of the body. *■ In this, as iu one or two of the other figures, the explanation of some of the lettering appears to be omitted in the original. At least, I have been unable to find it. 110 EAR OF FLY. ^' As long ago as 1764 Keller * observed that tlie base of the curious club-like "halteres," or rudimentary hind-wings of flies, *'est garnie de polls tres courts, ou la tige a le plus d'epaisseur pres du corps ; elle est inflexible, et presque garrotte par en haut de plusieurs nerfs ; en un mot, elle est faite de maniere que Ton peut juger par sa force par les dehors." This observation remained unnoticed, and no further descrij^tion appears to have been given of the organ until it was redis- covered by Hicks in 1856, and more fully described in 1857.t He found that though in the Diptera (flies and gnats) the hind wings are reduced to two minute, club-shaped oigans, they still receive a nerve which is the largest in the insect, except that which goes to the eyes. This proves that they must serve some important function, and renders it almost certain that they are the seats of some sense. He also found at the base of the halteres a number of " vesicles," arranged in four groups, and to each of which the nerve sends a branch, though the mode of pre- paration w^hich he adopted did not see the finer structure of the nerves, which he figures as mere fine, hard lines. He describes the " vesicles " as " thin, transparent, hemispherical, or Fig. Tl.— One of the halteres of a fly (after Lowne). permit him to * "Geschichte der gemeiuen Stubcufliege," 17G4. I have not seen the original, and quote from Hicks's paper. t Transactions of the Linnean Society, vol. xvii. PECULIAR SENSE-ORGANS. Ill more nearly spherical projections from the cuticular surface," and as placed in rows. The number and arrangement differ in different species: the blowfly {Sarcopliaga carnaria) has ten rows, Sj/rphus luniger as many as twenty. These organs have recently been again examined by BoUes Lee.* The vesicles are, according to him, un- doubtedly perforated, contain a minute hair, and those of the upper groups are protected by hoods of chitine. He inclines to correlate them with the similar antennal organs, which he regards as olfactory. His view of the minute structure of these rods differs from that of previous authors, and the subject requires further study. He finds, moreover, that the sense-organ containing the rods has nothing to do with the vesicular plates, but that they are attached to the cuticle in a different place, and where it presents no special modification. The numerous small membranes in the halteres of insects seem to bear somewhat the same relation to the single tympanum of, say, the locust, as the many- faceted eyes do to those with a single cornea. The head of the halteres is divided into two separate spaces by a membrane composed of elongated hypo- dermal cells. The upper part contains a number of large vesicular cells, like those which are in connection with the ends of the tracheao. It does not appear to contain any special sense-organ, and, in fact, the large nerve is almost entirely devoted to the sense-organs at the base. M. Bolles Lee suggests that it perhaps serves principally to regulate the pressure on these delicate structures. * " Lcs Balaaciers des Dipteres," Eecueil Zool. Suisse, 18S5. 112 AUDITOKY RODS IN BEETLES. Special sense-organs occnr also on the wings of other insects. Hicks found them " most perfect in the Diptera, next so in the Coleoptera, rather less so in the Lepidop- tera, but slightly developed in the Neuroptera, scarcely at all in the Orthoptera (though this assertion may be hereafter modified), and that only a trace of them exists in the Hemiptera." They are similarly constituted and equally developed in both sexes. Hicks regarded them as organs of smell. Leydig,* on the contrary, considered them as auditory organs. His mode of preparation dis- played better the structure of the nerves, and he found that they end in peculiar, club-shaped rods {Stabchen oder Stafle), closely resembling those in the ears of Orthoptera. He observes that, as in the case of the tibial auditory rods of Orthoptera these rods are of two sorts, which are arranged separately, those in one part of the organs being shorter and blunter, those in another more pointed and elongated. Bolles Lee, on the contrary, considers that the supposed existence of two forms, pointed and rounded, is merely due to an optical deception, and that in reality they are all similar. Leydig also observed in some cases that the rods were thrown into fine ridges. He found also somewhat similar papillae on the front wings of certain insects, but could not detect in them the characteristic nerve-ends. It must be confessed that the base of the wing would not seem a convenient place for an organ of hearing. The movements of the wing, it might well be supposed, would interfere with any delicate sensations. Still, this objection would apply to almost any sense being thus placed. "Auditory rods" are now, moreover, known to occur * MUller's Archiv., 1860. POSITION OF AUDITORY RODS. 113 in other parts of the body ; for instance, they have been discoyered in the antennae of a water-beetle (Dytiscus) and of Telephorus by Hicks, Leydig, and Graber, and in the body segments of several larv?e by Leydig, Weiss- mann, Graber, Grobben, and BoUes Lee. In the larva of Dytiscus, indeed, they have been observed in the body, antennae, palpi, under lip, and legs. Moreover, while, as we have seen, in the tibiae of Orthoptera and the halteres of ilies they are numerous, in some of these cases they are few, sometimes, indeed, only a single rod being present, as discovered by Grobben in Ptychoptera.* Nevertheless the evidence that they are really acoustic organs is, in the case of the Orthoptera, so strong, their structure is so peculiar, and the gradation of these organs from the most com- plex to the most simple is so complete, that it seems reasonable to attribute to them the same function. Moreover, as regards the very simplest forms there is another consideration pointing to this conclusion. We have seen that in the Orthoptera the terminal filaments close up, and are attached to the skin. Now, it seems to be a very general rule, in reference to these organs, that they are attached to the skin at two points, between which is situated the attachment of the nerve. These points, moreover, are so selected as to be main- tained at the same distance from one another, thus pre- serving an equable tension in the connecting filament. Fig. 72, for instance, represents part of one segment of the body of the larva of a gnat (Corethra). This larva is as transparent as glass, and very common in ponds, a most beautiful and instructive microscopic object. EG is the ganglion ; a is the nerve in question, which * Sitz. der K Akad. der Wiss. Wien, 1876. 114 CHORDOTONAL ORGAN OF GNAT-LARVA. swells into a little triangular ganglion at g) from g the auditory organ runs straight to the skin at e, and contains two or three auditory rods (not, how- ever, shown in the figure) at the point Chs ; in the opposite direction, a fine ligament passes from g to the Fig. T2. — Right half of eighth segment of the b:)dy of the larva of a gnat (Corethra plumicornis) ; after Graber. EG. Ganglia; JV, nerve; g, auditory ganglion; pb, auditory ligament ; C7is, auditory rods; a, auditory nerve; e, attachment ot auditory organ to the skin ; b, attachment of auditory ligament to the skin ; hn, hn', termination of skin-nerve; tb, plumose tactile hair; h, simple hairj tg, ganglion of tactile hair; Im, longitudinal muscle. skin at h. Hence the organ ge is suspended in a certain state of tension, and is favourably situated to receive even very fine vibrations.* There are, as we have seen, a large number of observations which point to the antennsB as organs of hearing, and many more might have been given. When we come to consider, however, the anatomical provision which renders the perception of sound * Similar organs occur in other insects, as, for instance, in Ptychoptera. AUDITORY HAIRS ON ANTENNA OF GNAT. 115 possible, we are met by great difficulties. The evidence is, I think, conclusive that the antennae are olfactory as well as tactile organs, and I believe that they serve also as organs of hearing. There are, moreover, as shown in the last chapter, various remarkable structures in the antennae, and I have given reasons for thinking some of them to be the seat of the sense of smell. Which, if any, of the remainder convey the sense of sound, it is not easy to determine. I have suggested that Hicks's bottles (Fig. 43) may act as microscopic stethoscopes ; * but they occur, so far as we at present know, oDly in ants and certain bees. Fig. 73,— Head of gnat. That some of the antennal hairs are auditory can,' I think, no longer be doubted. Johnson, whose figure T give (Fig. 73), suggestedf in 1855 that the hairs on the antennae of gnats serve for hearing. Mayer also,{ * I am glad to see that Leydig, who, however, does not appear to have read either Hicks's paper or mine, also regards these as chordotonal organs (Zool. Anz., 1886). t Quarterly Journal of Microscojneal Science, 1855. i American Journal of Science ani Arts, 1874. il6 SYMPATHETIC VIBRATIONS. led by the observations of Hensen, has made similar experiments with the mosquito, the male of which has beautifully feathered antenna?. He fastened one down on a glass slide, and then sounded a series of tuning- forks. With an Ut^ fork of 512 vibrations per second he found that some of the hairs were thrown into vigorous movement, while others remained nearly stationary. The lower (Uts) and higher (Utg) harmo- nics of Ut4 also caused more vibration than any intermediate notes. These hairs, then, are specially tuned so as to respond to vibrations numbering 512 per second. Other hairs vibrated to other notes, extending through the middle and next higher octave of the piano. Mayer then made large wooden models of these hairs, and, on counting the number of vibra- tions they made when they were clamped at one end and then drawn on one side, he found that it " coincided with the ratio existing between the numbers of vibrations of the forks to which co-vibrated the fibrils." It is interesting that the hum of the female gnat corresponds nearly to this note, and would consequently set the hairs in vibration. Moreover, those auditory hairs are most affected which are at right angles to the direction from which the sound comes. Hence, from the position of the antennae and the hairs, a sound wdll act most intensely if it is directly in front of the head. Suppose, then, a male gnat hears the hum of a female at some little distance. Perhaps the sound affects one antenna more than the other. He turns his head until the two antennae are equally affected, and is thus able to direct his flight straight towards the female. The auditory organs of insects, then, are situated in ORGANS OF HEARING IN VARIOUS PARTS OF BODY. 117 different insects in different parts of the body, and there is strons^ reason to believe that even in the same animal the sensitiveness to sounds is not necessarily confined to one part. In the cricket, for instance, the sense of hearing appears to be seated partly in the antennae, and partly in the anterior legs. In other cases, as in Corethra, the division appears to be carried still further, and a " chordotonal " organ occurs in each of several segments. No doubt the multiplication of complex organs, like our ears, arranged as they are to appreciate a great variety of sounds, would be so great a waste that any theory implying such a state of things would be quite untenable ; but with simple organs, such, for instance, as that of Corethra * (gnat ; Fig. 72), the case is different, and there would seem to be an obvious advantage in such organs occurring in different parts of the body, ready to receive sound-waves coming from different directions. Moreover, the different organs exist ; they do not appear to be organs of touch, yet they are clearly organs of sense, and that sense, what- ever it be, whether hearing or any other, and though it may well be simple, and even perhaps confused, must be seated in various parts of the body. The fact of their being so distributed does not make it more improbable that they should be organs of hearing, than of any other sense. At the same time, it is an interesting result of recent investigations that the auditory organs of insects are not only situated in various parts of the body, but are constructed on such different principles. * Where, however, the number does not approach to that in certain Medusae (see ante, p. 84). ( 118 ) CHAPTER VI. THE SENSE OF SIGHT. It might at first siglit seem easy enough to answer the question whether an animal can see or not. In reality, however, the problem is by no means so simple. We find, in fact, every gradation from the mere power of distinguishing a difference between light and darkness up to the perception of form and colour which we ourselves enjoy. The undifferentiated tissues of the lower animals, and even of plants, are, as we all know, affected in a marked manner by the action of light. But to see, in the sense of perceiving the forms of objects, an animal must possess some apparatus by means of which — firstly, the light coming from different points, a, h, c, d, e, etc., is caused to act on separate parts of the retina in the same relative positions ; and secondly, by means of which these points of the retina can be protected from the light coming in other directions. There are three modes in which it is theoretically possible that this might be effected. Firstly, let 8 8' be an opaque screen, with a small orifice at o. Let ah c d e he a body in front of the THREE POSSIBLE MODES OF SIGHT. 119 screen. In this case the rays from the point c can pass straight through the orifice o, and fall on the retina ot an eye, or on a flat surface at c'. There is no other direction in which the rays from c could pass through 0. In the same way, the light from a would fall on the point a\ that from h on h\ from d on d\ and e on e'. The results which would be given in this way would be, however, very imperfect, and, as a matter of fact, no eye con- structed on this system is known to exist. Secondly, let a number of transparent tubes or cones with opaque walls be ranged side by side in front of the retina, and separated from one another by black pigment. In this case the only light which can reach the optic nerve will be that which falls on any given tube in the direction of its axis. Fig. 75. For instance, in Fig. 75 the light from a will pass to a\ that from h to h', that from c to c\ and so on. The light from c, which falls on the other tubes, will not 120 DIFFEEENT FOEMS OF EYE. reach the nerve, but will impinge on the sides and be absorbed by the pigment. Thus, though the light from c will illuminate the whole surface of the eye, it will only affect the nerve at c'. In this mode of vision, which was first clearly explained by Johannes Miiller, the distinctness of the image will be greater in proportion to the number of separate cones. "An image," he says,* "formed by several thousand separate points, of which each corre- sponds to a distinct field of vision in the external world, will resemble a piece of mosaic work, and a better idea cannot be conceived of the image of external objects which will be depicted on the retina of beings endowed with such organs of vision, than by comparing it with perfect work of that kind." There is, it will presently be seen, reason to suppose that the compound eyes of insects, Crustacea, and some molluscs, are constructed on this plan. Thirdly, let L (Fig. 76) be a lens of such a form Fig. 76. that all the light which falls upon its surface from the point a is re-collected at the point a', that from h at V , from c at c', and so on. If now other light be excluded, * " Phys. of the Senses," by Johannes Miiller, iranslated by Dr. Baly. THE VERTEBRATE EYE. 121 an image o^ a h c will be tlirown on a screen or on a retina at a' h' c'. The image, it will be observed, is necessarily reversed. This is the form of eye which we possess ourselves : it is, in fact, a camera obscura. It is that of all the higher animals, of most molluscs, the ocelli of insects, etc. Fig. 77, taken from Helmholtz, ^Yill give an idea cf the manner in which we see. Fig. 11. — (?, Vitreous liumor; L, lens; W, aqueous humor; c, ciliary process; d, optic nerve ; e e, suspensory ligament ; k k, hyaloid membrane ; //, h h, cornea ; g g, choroid; i, retina; 1 1, ciliary muscle; mf, nf, sclerotic coat; j^ P> iris; s, the yellow spot. The eyeball is surrounded by a dense fibrous mem- brane, the sclerotic coat, or icJiite of the eye, mf, nf, which 122 STKUCTURE OF THE EYE. passes in front into the glassy, transparent cornea, //, h h ; the greater part of the centre of the eye is occupied by a clear gelatinous mass, the vitreous humor, (r, in front of which is the lens, L ; while between the lens and the cornea is the aqueous humor, W. The sclerotic coat is lined at the back of the eye by a delicate, vascular, and pigmented membrane — the choroid, g g, so called from the great number of blood-vessels which it contains ; in front this membrane joins the iris, jo lo, which leaves a central opening, the pupil, so called from the little image of ourselves, which we see re- flected from an eye when we look into it. The iris gives its colour to the eye, its posterior membrane con- taining pigment-cells ; if these are few in number, it appears blue, from the layer behind shining through, and the greater the number of these cells the deeper the colour, e e, is a peculiar membrane, which serves to retain the lens in its place. The optic nerve, d, enters at the back of the eye, and, spreading out on all sides, forms the retina, i, of which one spot, s, the yellow spot, is pre-eminently sensitive. The action of the eye re- sembles that of a camera obscura, and, as shown in Fig. 76, the rays which fall upon it are refracted so as to form a reversed picture on the back of the eye. The retina (Fig. 78) is very complicated, and, though no thicker than a sheet of thin paper, consists of no less than nine separate layers, the innermost (Figs. 78, 79) being the rods and cones, which are the immediate recipients of the undulations of light. Fig. 79 represents the rods and cones isolated and somewhat more enlarged. The number of rods and cones in the human eye is enormous. At a moderate computation the cones may THE RETINA. 123 be estimated at over 3,000,000; and the rods at 30,000,000.* Fig. Y8.— Section through the retina (after Mas. Schultze). Beginning from the outside, 1, limitary membrane ; 2, layer of nerve-fibres ; 3. layer of nerve-cells; 4, nuclear layer; 5, inner nuclear layer; 6, intermediate nuclear layer; 7, outer nuclear layer ; 8, posterior membrane ; 9, layer of small rods and cones ; 10, choroid. * Sulzer estimates the cones at 3,360,000 ; Krause places the cones at 7,000,000, the rods at 130,000,000 ; but Professor M. Foster tells me that he thinks the latter figure is too higli. 124 THE RODS AND CONES. It will be observed that the nerve does not, as one might naturally have expected, enter the eye and then spread itself out at the back of the retiua ; but, on the contrary, pierces the retina and spreads itself out on the front, so that the cones and rods look inwards, and not outwards — towards the back of the eye, and not at the object itself. In fact, we do not look outwards at the actual object, but we see the object as reflected from the base of our own eye. From the arrangement of the rods in the eyes of verte- brata, then, the light has necessarily to pass through the retina, and is then re- flected back on it. This involves some loss of light ; on the other hand, it perhaps secures the advantage that the sensitive terminations of the rods and cones can be more readily supplied with blood. I do not propose to enter into the reason for this peculiar arrangement, which is connected with the development of the eye. But it is so different from what might have been expected, is in itself so interesting, and makes so important a Fig. 79. — A, Inner segments of rods (s, s, s) and cones {z, z') from man, the latter in connection with the cone-granules and fibres as far as the external molecular laj'er, 6. In the interior of the inner segment of both rod and cone fibrillar structure is visible, X 800. THE BLIND SPOT IN THE EYE. 125 contrast with the form which is general, though not universal among the lower animals, that I think it will not be out of place to mention a very simple and beautiful experiment by which every one can satisfy himself that it is so. One result is that we have in each eye a blind spot, that at which the nerve enters. Tarn the present page, so that the white circle is in front of the left eye and the small cross in front of the right. Til en close the right eye, look steadily across at the cross with the left, and move the book slowly backwards and forw^ards. At one particular distance, about ten inches, the white circle will come opposite the blind spot and will instantaneously disappear. Across an ordinary room, if a man stands in front of a screen, his head may in the same way be made entirely to vanish. The ordinary vertebrate eye consists of two main divisions : the refractive ^'^°- ^°- part, which is a modified portion of the skin ; and the 126 INVERSION OF THE RODS. receptive part, which arises from the central nervous system ; and the inverted arrangement of the rods is, "vve can hardly doubt, connected with the develop- ment of the eye, though it is not yet, I think, satis- factorily explained. There is, however, another eye in vertebrates, with reference to which I must say something, and which, though now rudimentary, is most interesting. Our brain contains a small organ, about as large as a hazel- nut, known, from its being shaped somewhat like a cone of a pine, as the pineal gland. Its function has long been a puzzle to physiologists. Descartes suggested that it was perhaps the seat of the soul ; and though this idea, of course, could not be entertained, no suggestion even plausible had been made. So matters stood until quite recently, when a most unexpected light has been thrown upon the question. As long ago as 1829, Brandt, describing the skull of a lizard (Lacerta agilis), pointed out that in the centre of the top of the head was a peculiar spot, one of the scales being quite unlike the rest. Leydig* subsequently observed that on the head of the slow- worm {Anguis fragilis) there is a dark spot surrounding a small unpigmented body immediately over the pineal gland. Eabl-Kuckhard,t in 1884, again called atten- tion to this structure, and suggested that it might serve for the perception of warmth. Ahlborn,| in the same year, was the first to suggest that it was a rudimentary eye. De Graaf § has the merit of dis- * " Die Arten der Saurier." t " Eutw. des Knochenfiscligebirn," Bericht der Sitz. nafurf, Freunde, Berliu: 1882. t "Ueber d. Bedeiitung der Zirbeldrlise," Zeit. fiir Wis?. Zool, 1884. § " Zur. Aiiat. uud Ent. der Epi. b. Arapbibieu uud Reptilien," Zool. Anz., 1886. THE PINEAL GLAND, 127 covering tbat iu the slow-worm the pineal gland is actually modified into a structure resembling an inver- tebrate eye. Tiiis remarkable structure has since been examined in various Keptilia by Mr. Spencer.* It appears to be more highly organized in Hatteria than in any other form yet studied ; but the retrogression of the different structures has not proceeded ixtri imssu, in some cases the lens, in some the retina, in others the nerve, having been most modified, or having dis- appeared. In Hatteria and Varanus the eye is very distinct ; the interior parts being more perfect in the former; while in the latter it is externally most con- S]3icuous, standing out prominently from its creamy whiteness. The lens is cellular in structure, and thins away rapidly at the sides. The "rods " are well developed, and embedded in pigment. Spencer describes the various modifications of the organ in the iguanas, chame- leons, flying lizards, geckos, etc. Fig. 81 represents the ex- ternal aspects of the eye-scale in a small lizard (Calotis), with the transparent cornea in the middle, through which the eye is seen ; and the diagram riof. 82 a section throuo^h the eye-scale of a small lizard (Lacerta). A very interesting point in connection with the pineal eye consists in the fact that the penetrate the retina, and then spread out on its outer * Quarterly Journal of ^licroscopical Science, October, 1886. ^^'T/A --id Fig. 81.— Pineal eye-scale on the head of a small lizard (Calotis) ; after Spencer. optic nerve does not 128 THE RUDIMENTARY MEDIAN EYE. surface, as in the lateral eyes of all vertebrates, but, on the contrary, is distributed over its exterior surface. It is, therefore, as De Graaf pointed out, formed in this respect on the type of the usual invertebrate eye ; so that we have the remarkable fact that in the same m, CM E.p OpL Fig. 82.— Diagram of a section through the skull and pineal eye of Lacerta viridis. C, Cuticle ; Pa, parietal bone ; Ep, epidermis ; L, lens ; Pig, Pigment ; E, rete muscosum ; Cff, cerebral hemisphere ; N, nerve ; E.p, epiphysis ; OpL, optic lobe of brain. vertebrate animal we find eyes formed on two different types. Not only so, but the development is dissimilar, the lens of the pineal eye being formed out of the walls of the neural canal. So that the lens of the pineal eye is a totally different structure from that of the lateral eyes. Spencer observed no effect whatever when he threw a strong light on the pineal eye. In fact, he does not believe that in any of the species examined by him the organ is at present in a functional condition. Indeed, in some cases the cornea is quite opaque, and in others the nerve to the brain is not continuous ; so that there can be no vision. At the same time, it seems to be established that this organ is the degraded relic of what was once a true eye. From the size of the pineal orifice in the skull of THE MEDIAN VERTEBRATE EYE. 129 the huge extinct reptiles, such as Ichthyosaurus and Plesiosaurus, it has been, I think, fairly inferred that the pineal eye was much more developed than in any known living form. In living fish and Amphibia, so far as they have been yet examined, the organ is even more rudimentary than in reptiles. But in the fossil Labyrinthodonts the skull possesses a large and well-marked orifice for the passage of the pineal nerve. This orifice is, in fact, so large that it can scarcely be doubted that the eye in these remarkable amphibia was also well developed, and served as a third organ of vision. In birds the organ is present, but retains no re- semblance to an eye. It is solid and highly vascular. In mammals it is still more degenerate, though a trace is still present even in man himself. The larval Ascidians, which present so many points of resemblance to the lowest vertebrates, and especially to the Lancelot (Amphioxus), have hitherto been re- garded as differing from them in the possession of a central eye. It now, however, appears that the verte- brate type did originally possess a central eye, of which the so-called pineal gland is the last trace. It seems, then, very tempting to regard the pineal eye as representing the central eye of Amphioxus; but Spencer points out that the two organs differ greatly in structure, and he himself doubts whether the pineal eye is really the direct representative of the central eye in the Tunicata. Beraneck* also regards the pineal as entirely different from the central eye of the Tunicata. Indeed, he considers its differentiation as an eye to be a * " Ueber d, Parietal Auge der Eeptilien," Jenaische Zeit., 1887. K 130 OEGANS OF VISION IN THE LOWER ANIMALS. secondary modification, and considers that it had previously served some other function. However this may be, it cannot be doubted that the pineal gland in Mammalia is the representative of the cerebral lobe which supplies the rudimentary pineal eye of Keptilia, and this itself is probably the degenerate descendant of an organ which in former ages performed the functions of a true organ of vision. The Organs of Vision in the Lower Animals. Mere sensibility to light is possible without any optical apparatus. Even plants, as we know, can well distinguish between light and darkness; and though it seems that in our own case the general surface of the skin has lost its sensitiveness to light, still, in many of the lower animals, light seems to act generally and directly on the tissues. Some microscopic vegetable forms even, as, for in- stance, Englena (Fig. 83), possess a red spot,* which appears to be specially sensi- tive to light. The loNver animals are, in a great many cases, very transparent. Light passes 'Fis.83.-Engienaviridis. ©asily through them, and, e. Eye-spot exccpt iu SO far as it is ab- sorbed, can hardly be supposed to produce any effect. The most rudimentary form of a light-organ, then, may be considered to be a coloured spot. In the first chapter I have endeavoured to show how * The moving zoospores of certain algse also possess a red spot, which may perhaps have special reference to light COLOR-SPOTS. 131 it may be possible to trace an almost complete series from such a mere spot of colour in the skin up to a complex organ of vision, such, for instance, as that of a snail ; indeed, in the development of the eye in the individual animal may be traced some of the same stages as have probably been passed through by the ancestral forms of the animal itself in long bygone ages. We must not, however, suppose that all eyes can be traced back to one and the same origin, or have been developed in the same manner. There are even cases in which an organ fulfilling a different function appears to have been modified into an eye. Look, for instance, at the organ of touch of Onchidium* (Fig. 16). The cuticle (see p. 14) is thickened into a biconvex, almost lens-like body; the epithelial cells are elongated, and below is a mass of cells, to which runs a nerve. A very little change would make this an organ capable of distinguishing light from darkness, and ^ 7 some of the eyes of On- chidium appear, indeed, to have thus originated. Compare with this, for instance, the ocellus of the young larva of a water-beetle (Fig. 84), as figured by Grenacher.. The eye-spots of Me- dusae were first noticed by Ehrenberg in 1836, and the lens was discovered many years afterwards by de Quatrefages. It is, in fact, by no means universally * A slug-like genus of molluscs. Fig. 84. — Section through the simple eye of a young Dytiscus larva (after Grenacher). ?, Corneal lens ; g, cells forming the vitreous humor ; r, retina ; o, optic nerve ; h, hypo- derm. 132 EOHINODERMS. present ; the eye, if so it can be called, in many species consisting merely of a coloured spot, while in others it is entirely absent.* Fig. 85.— Eye-spot of Lizzia (after Hertwig). oc, Ocellus ; I, leus. Fig. 86. — Eye-bulb of Astropecten (after Haeckel). In the Eehinoderms, the eyes, which were discovered by Ehrenberg, have been described by Haeckel,t Wilson, J Lange, and others. § They are in some cases situated, as in Astropecten, on a pear-shaped bulb (Fig. 86). They consist of a lens (Fig. 87), supplied with a nerve, and lying in a mass of pigment. In Solaster or * Allman, " Mon. of the Hydroids," Ray Society, 1871. t "Ueberdie Augeuund Nerven der Seestevne,"Zeit.furWiss.,\o\. x. X Transactions of the Linnean Society. § Lange, " Beit. z. Anat. iind Hist, der Asterien und Ophinren," Morpli: Jahrhuch, 1876. WORMS. 133 Asteracanthion the lenses look like brilliant eggs, " each in its own scarlet nest." In some species there are as many as two hundred eyes; but there appears to ^, be no retina, so that they can do little more than dis- /_— tino-uish between light and ^J Fig. 87.— Eye of Asteracanthion (after Haeckel). c. Cuticle ; e, epithelium ; I, lens ; p, pigment. darkness. It is quite possible that in some of the lower animals, where the eye-spot is sup- posed to consist merely of a layer of pigment at the end of a nerve, a lens may here- after be discovered. In the Turbellaria* the eyes, which were first noticed by de Quatrefages, are numerous, and lie immediately under the epithelium (skin). They consist of a certain number of crystalline rods and corresponding retinal cells, resting on a cup-shaped bed of pigment, and con- nected with a nerve. There is often a group on each side of the head, immediately over the brain. In species which possess tentacles the eyes are generally combined with them ; in others they are scattered over the whole periphery of the body, and look in all direc- tions. They differ greatly in size, and in the number of rods and retinal cells— the larger tentacular eyes having several; the small, scattered ones, which are generally more deeply situated, even as few as two or three. * "Die Polyckden," Fauna und Flora des Golfes von Neapel, 1884. Carriere, "Die Augen vou Planaria," Arch, fur Mic. Anat, 1882. 134 WORMS. In most of the Annulata (worms), the eyes, so far as they have yet been described, are very simple, and probably in most cases not capable of giving more than a mere impression of light. In some species the eye- spot is merely a group of pigmented epithelial cells. Iq many (Fig. 87) there is, besides the pigment, a well-marked Iods. At the same time, it is probable that in some cases this supposed sim|)licity is more apparent than real. The dioptric part is often cellular, consisting sometimes of one cell, sometimes of several. They are generally, but not always, situated on the head. The Polyophthalmians (Fig. 90), as already mentioned, have a series along the sides of the body, in pairs from the seventh to the eighteenth segments. I agree with Carriere that there is no sufficient reason for considering the supposed "eyes" of the leech as organs for the perception of light, but other species of the same group (Clepsine) possess well-marked, though rudiDientary eyes.* Certain leeches — for instance, Piscicola respirans — in addition to the pigmented spots on the head, have also some on the posterior sucking disc. These somewhat resemble the supposed organs of touch, but are larger, and surrounded by pigment. There is no lens, but the large cells are very transparent. It is not supposed that they give any distinct image, or can do more than distinguish light from darkness — as Leydig says, *' feel " the light. Still, I must confess that the deter- mination of these curious organs as eyes seems to me very doubtful. Fig. 88 represents the anterior extremity of a small freshwater worm (Bohemilla). * Grabcr, "Morpli. Unt. uber die Augen der frei-lebenden Borsten- Tfurmer," Arch, fur Mic. AnaL, 1880. WORMS. 135 Fig. 89 represents an eye-dot of Nereis. ' In this genus there are t^YO pairs of eyes, wliich differ some- rig, ss.— Anterior extremity of a freshwater worm {Boliemilla comata); after Vejdovsky).* a. Eye; 6, brain; c, cuticle; hp, hypoderm; lb, tactile hair; ne, nerve ; v, blood-vessel, what in structure, the lens in the anterior pair being flatter, that in the posterior more conical. In Hesione the difference is even more marked.f In Polyophthalmus, besides the eyes in the head, there is, as already mentioned, a series along the sides of the body, which differ some- what in structure from those in the head. As a general rule, in the Annelids each eye contains a single lens, but the cephalic eyes of Polyophthalmus, according to Mayer, contain three. * "Sys. und Morph. der Oligochseten." t Graber, "Morph. Unt. iiber die Augeu der frei-lebenden Borsten- wurmer," Arch. fUr Mic. Anal, 1880. Fig. 89.— Eye-dot of Nereis (after MuUer). In B the pigment is partly removed so as to show the lens. 136 WOEMS. F^//f septa J*^-^" -*"*"^ FdlS Fig. 90.— The first twelve segments of PolyopJdhalmus pictus, seen from below (after Mayer). The Roman numerals indicate the segments. St, Papilla; on the head ; KS, head ; au, head eye ; s.au, side eyes ; 01, upper lip ; Ul, under lip ; vph pharyngeal vein ; V.subinta, anterior ventral vein ; V.d.V-\ veins connecting the superior lateral and vessels ; sejyV-'^, intersegmentary membranes ; m.ocs.l, lateral muscle of the oesophagus ; V.ann, pulsating circular vessel; Md.dr, stomach- glands ; V.v-l, vein connecting the inferior and lateral blood-vessels ; Md, stomach ; Bm, muscles of the hairs ; G, brain ; jl.o, ciliated organ ; qm, transverse muscle. MOLLUSCS. 137 The most highly organized eyes in Annelids appear to be those of the Alciopidae, which have been described by Krohn,* de Quatrefages,t and especially by Greef J and Graber.§ The Alciopidse are small sea-worms ; they* live principally in 5 the open sea, and, like many other J^ pelagic animals, are extremely trans- ^ parent. It is, indeed, often difficult ^ to see more of them than the two ;3^ very large eyes, red or orange, and a ^ pair of dark violet dots (the seg- ^ mental organs) on each ring. :^ The principal parts of their eyes are % ^ % — (1) the outer integument, the whole ^ % ^J of which is so transparent that it needs '^ ^ ^ scarcely any modification ; (2) the so- '^/^^ ^ called "eye-skin," as to the true ^% nature of which there is still much J^ difference of opinion; (3) the lens; (4) ^ the "corpus ciliare ; " (5) the vitreous % humor; and (6) the retina, which i again is composed of four layers — (a) f the rods; (h) pigment layer; (c) \ granular layer ; (d) fibrous layer. ng. gi.-Aiciope (alter In Mollusca the eyes are variously de Quatrefages). situated ; being, for instance, either placed on the pos- terior tentacles ; or between the feelers, as in the fresh- water species ; or on a short stalk at the side of the * "Zool. imd Anat. Bemerk. iiber die Alciopeden," Wiegmann's Arch., 1845. t " Etudes s. 1. typ. Inf. de I'emb. des AnneUs;' Ann. Sci. Nat, 1850. J/'Unt. iiber die Alciopideu," Nova Ada Acad. Leop. Carol, vol. xxxix. 11, 1876. § Arch, fur Mic. Anat., 1880. 138 MOLLUSCS. feelers, as in the Prosobrancliiata ; or on the back. In some cases they are deeply sunk, even into the brain. Fig. 92.— Perpendicular section through the eye-pit of a limpet (Patella) ; after Carriers. 1, Epithelial cells ; 2, retina cells , 3, vitreous body. The mussels are generally deficient in eyes; and some which are, as larvEe, provided with an eye, lose their eyes when mature. Fig. 93.— Eye of Trochus magus (after Hilger).* Gl, Vitreous body ; No, nerve. In the limpet (Patella),* on the outer side of the tentacles, where the eyes are situated in more highly organized species, are certain spots, which may be * "Fraisse. Ueber Molluskenaiigen," Zeit. filr Wiss. Zool, 188L t *' Beit, zur Kennt. der Gastropodenaiigen," Gegenbaiir's Morph. Jahrhuch, 1885. MOLLUSCS. 139 regarded as a very rudimentary organ for tlie per- ception of light. The skin is thrown into a pit, within which the epithelial cells are elongated and pigmented. In the sea-ear (Haliotis), and in Trochus (Fig. 93), the arrangement is similar, but the depression is deeper, the mouth is very much restricted, and the interior is filled by a vitreous body. In Murex (Fig. 94) the eye is still further developed, and is entirely closed iu, a lens being present. ^. Fig. 94.— Eye of Murex Irandaris (after Hilger). L, Lens; Gl, vitreous body; Ko, nerve. In the snail (Helix) the eye is still more highly organized. It consists of a cornea, which lies imme- diately below the skin ; a lens, behind which is the retina, consisting of three layers, (1) the rods, (2) a cellular layer, (3) a fibrous layer. This, indeed, appears 140 CUTTLE-FISH. to be a very general arrangement in the Mollusca. The power of sight given by such an eye can be but small. Indeed, it is probable that it does little more Fig. 95. — Eye of nenxpomatia (after Simroth).* ct. Cuticle ; a, epithelium ; b, cornea ; c, envelope of the eye ; d, cellular laj^er ; e, fibrils of the optic nerve ; /, feeler cell ; na, nerve of the tentacle ; no, optic nerve. than distinguish degrees of light. According to Lespes, a Cyclostoma only perceives the shadow of a hand at a distance of five inches, and a Paludina of eight. It is interesting that, as Lankester first showed, f the eye of Mollusca, in its gradual development, passes through the stages which we find are the permanent conditions in Patella and Haliotis, commencing as a depression, which grows deeper and deeper, and gradually closes over. Even in the Nautilus the cornea leaves an opening, * Simroth, "Ueber die Sinneswerkzeuge uus. einh. Weiclitbiere," Zeit. fur Wiss. Zool, 1876. t " Obs. on the Dev. of Cepbalopoda," Quarterly Journal of Micro- scopical Science, 1875. COMPOUND EYES IN MOLLUSCS. 141 through which the water has free access to the interior of the eye. In the higher cuttle-fishes (Cephalopoda) the eye is very complex, and the optic ganglion is in some cases the largest part of the brain ; but, while we find the same parts, as, for instance, in Helix, though in a higher state of development, there does not seem sufficient reason to regard the two organs as homologous, but it appears possible that the eye of the cuttle-fish had an independent origin. Certain bivalves (Lamellibranchiata) possess bright spots round the edge of the mantle, or on the siphon, which some naturalists maintain to be eyes, while others deny them this character, leaving their true function, however, undecided. But though there is much doubt in some cases, there are other eye-spots which are certainly true eyes. Of these there are two distinct types — those of Spondylus, Pecten, etc., on the one hand ; of Area, Pectunculus, etc., on the other. The latter present several features of the compound insect's eye. This was first noticed by Will,* and they have since been more fully described by Carrierej and Patten.if They are composed (Fig. 96) of large conical cells wdth the points turned inwards. Pigment is deposited in the periphery of the cells. The outer surface is arched, and forms a biconvex lens. These cells pass gradually into those of the ordinary epithelium. It will be most convenient to consider the mode in which these compound eyes act when we come to * " Ueber die Augen der Bivalven," Frorieps Notizen, 1844. t "Die Sehorgane der Thiere," 1885. X " Eyes of Molluscs and Arthropods," Miit. Zool. Stat. Neapel, 1886. 142 ARC A— SPOND YLUS. consider those of insects, where they are more highly- developed. The eyes of Pecten and Spondjdus are, again, formed on a totally different plan. It has been already obseryed that there is an Fig. 96.— Perpendicular section through an eye of Area JVoce (after Carriere). 1, Epithelium of the edge of the mantle ; 2, cells of vision ; 3, lens ; 4, 5, connective tissue ; 6, section of one of the cells. essential difference between the typical vertebrate and the typical invertebrate eye; in that while in the former, the optic nerve (Fig. 77) penetrates the retina and then spreads out on the anterior surface, so that the " rods " point away from the light ; in the normal invertebrate eye, on the contrary, the nerve spreads out on the back of the retina, so that the rods point towards the light. Krohn,* however, made the remark- able discovery that in the genus Pecten the rods, like those of the vertebrates, are turned away from the light. In this case, however, the optic nerve does not enter the retina directly from behind, but runs round it and passes, so to say, over the lip of the cup. Here, then, we get a remarkable approach to the vertebrate eye ; but the similarity is still greater in * Miiller's Arch., 1840. See also Hansen, " Ueber das Auge einiger Lamellibranchiaten," Zeit.fur Wiss. Zool., 1865. PECTEN. 143 Oncliidium (a genus of slugs, widely spread over the Southern Hemisphere), in which Semper has shown * that the nerve actually pierces the retina as in verte- ■k Fig. 97.— Diagram of eye of Pecteu (after Hickson). a. Cornea ; 5, transparent base- ment membrane supporting the epithelial cells of cornea; c, the pigmented epithelium ; d, the lining epithelium of the mantle ; e, the lens ; /, the ligament supporting the lens ; g, the retina ; h, the tapetum ; Tc, the pigment ; w, the retinal nerve ; n, complementary nerve. brates. That this distinctive character should thus reappear in so distant a group is very interesting, and it is also remarkable that Onchidium possesses two kinds of eyes : some on the head, which are constructed on the same type as those of other molluscs ; while the peculiar eyes just mentioned are scattered over the back, and their nerves arise, not from the cephalic, but from the visceral ganglion. Moreover, they differ in number, not only in the different species, some having one hun- dred, some as few as twelve, and others none at all, but even in different individuals of the same species. Indeed, they are continually growing and being re- absorbed. But while thus resembling a simple verte- brate eye, the dorsal eyes of Onchidium have a totally * " Ueber Schnecken Augen am Wirbelthier typus," Arclu fur Mic. Anat, 1877. 144 ONCHIDIUM. different development, arising, except the nerve, entirely from the integument ; on the contrary, in the vertebrate eye, while the cornea and lens are formed from the shin, the retina is an outgrowth from the brain. Semper does not suppose that the Onchidia perceive any actual image with their dorsal eyes, and thinks that they are merely able to distinguish differences in the amount of light. They are shore-living molluscs, and are preyed on by small fishes belonging to the genus Perophthalmus, w^hich has the curious habit of leaving the water and ■svalking about on the sand in search of food. The back of the Onchidium contains a number of glands, each opening by a minute pore ; and Semper suggests that, when warned by the shadow of the fish, the little slugs eject a shower of spray, drive off their enemy, and save themselves. This is not quite so far-fetched as might at first sight appear, for we know that there are many other animals, the sepia, many ants, the bombardier and other beetles, etc., which defend them- selves in a similar manner. It seems difficult to understand why the Oncliidia should be endowed with so many eyes. The irrelative repetition of organs meets us, however, continually in the lower animals. Moreover, in the present case Semper has thrown out a plausible suggestion. The organs of touch (see ante, p. 14) curiously resemble eyes in structure, and a very slight change might make them capable of perceiving light. It is possible, then, that some of them may undergo a change of function, and that this may throw some light on the variability in number. In the Chitonidte, where dorsal eyes have recently SENSE-ORGANS OF CHITON. 145 been discovered by Moseley,* tliey are even more numerous. Chiton itself, indeed, Las none; but in Scliizocliiton there are 300, and in Corephium more Fig. 98. Schematic representation of the soft and some of the hard parts in a shell of a Chiton (Acanthopleura), as seen in a section vertical to the surface, and with the margin of the shell lying in the direction of the left side of the drawing, a. Conical termination of sense-organ ; h, l>', ends of nerve ; c, nerve ; /, calcareous cornea ; g, lens ; h, iris ; k, pigmented capsule of eye ; m, body of sense-organ cut across ; n, nerve of eye ; p, nerve of sense-organ ; r, rods of retina. than ten thousand. As in Onchidium, they probably arose as modifications of the organs of touch, and are supplied by the same nerves. They possess (1) a cornea, (2) a perfectly transparent and strongly biconvex lens, and (3) the retiDa, which presents a layer of short but well-defined rods. It is interesting that they point towards the light, and not, as in Onchidium, away from it. * " On the Presence of Eyes in Shells of certain Chitonida3," Quarterly Journal of Microscopal Science, 1885. 146 EYES OF CRUSTACEA AND INSECTS. CHAPTER VII. THE ORGANS OF VISION IN INSECTS AND CRUSTACEA. I NOW pass on to the eyes of insects. In most species of this group there are two distinct kinds: the large compound eyes, which are situated one on each side of the head; and the ocelli, or small eyes, of which there are generally three, arranged in a triangle, between the other two. Speaking roughly, the ocelli of insects may be said to see as our eyes do ; that is to say, the lens throws on the retina an image, which is perceived by the fine terminations of the optic nerve. One type of such an eye in a young water-beetle (Dytiscus) is shown in Fig. 84, p. 131. This illustrates the mode of develop- ment of an ocellus, which has been already referred to {ante, p. 131). The structure of fully formed ocelli is shown by Fig. 99. In details, indeed, they present many dif- ferences, and it is remarkable that in some species this is the case even with those of the same individual ; for instance, in those of one of our large spiders, Epeira diadema (Fig. 99). In this case the eye B would receive more light, and the image, therefore, would be brighter; but, on OCELLI. 147 the other hand, the image would be pictured in greater detail by the eye A. Fig. 99.— Long section through the front (^) and hinder (S) dorsal eyes of Epeira diadema (after Grenacher). A, Anterior eye; B, posterior eye; Hp, hypoderm ; Ct, cuticle ; ct, boundary membrane ; K, nuclei of the cells of the retina ; M, mus- cular fibres ; 31, M\ cross sections of ditto ; .S"^, rods ; Pg, P\ pigment cells ; L, lens ; Gk\ vitreous body ; Kt, crystalline cones ; Rt, retina ; Xojj, optic nerve. Speaking generally, an ocellus may be regarded as consisting of — 1. A lens, forming part of the general body covering. 2. A layer of transparent cells. 3. A retina, or second layer of deeper lying cells, each of which bears a rod in front, while their inner ends pass into the filaments of the optic nerve. 4. The pigment. From the convexity of the lens it would have a short focus, and the comparatively small number of rods would give but a very imperfect image, except of very near objects. But though these eyes agree so far with ours, there is an essential difference between them. It will be at 148 COMPOUND EYES. once seen that the pigment is differently placed, being in front of the rods, while in the vertebrate eye it is behind them. Again, the position of the rods them- selves is reversed in the two cases. Passing on to the compound eye. Fig. 100 gives a section of the eye of a cockchafer (Melolontha), after Strauss-Diirckheim. The separate facets of such an Fig. 100.— Section through the eye of a cockchafer (Melolontha) ; after Strauss- Diirckheim. eye act themselves as lenses, and give a very perfect As regards the numbsr of facets, Leeuwenhoek calcu- lated that there were 3180 facets in the compound eye of a beetle which, however, he does not name. In the house-fly (Musca) there are about 4,000 ; in the gadfly (CEstrus), 7,000; in the goat moth (Cossus), 11,000; in the death's-head moth {Sphinx atropos), 12,000 ; in a butterfly (Papilio), 17,000; in a dragon-fly (^schna), 20,000; in a small beetle (Mordella), as many as 25,000. STRUCTUEE OF THE COMPOUND EYE. 149 The size of the facets seems to bear some relation to the size of the insect, but even in the smallest species none have been observed less than tt^qq of an inch in diameter. Butterflies, which fly in the day, have the facets smaller than those of moths, which are generally evening insects. The facets are in most cases similar, six-sided, and cap. <¥M-''^F' Fig. 101. — Section through the eye of a fly (after Hickson). b.m, Basilar membratie ; c, cuticle; cop, epioptic ganglion; n.c, nuclei; n.c.s., nerve-cell sheath; X.f, decussating nerve-fibres ; op, optic ganglion ; pc, pseudocoue ; pg, pigment cells ; p.op, perioptic ganglion; ?•, retinula ; FJi., rhabdom ; T, trachea; t.a., terminal anastomosis ; Tt, trachea ; ti, tracheal vesicle. very regular. In locusts, however, they vary a good deal both in form and size. In some flies (Diptera) and dragon-flies (Libellulidae) those in the upper part of the eye are larger than the lower ones, and the junction of the two often forms a well-marked, curved line. 150 CORNEA— CRYSTALLINE CONES. The wonderful complexity is well shown in the pre- ceding figure, which rej^resents a section through the eye of a fly, after Hickson.* In illustration of the finer structure, I may take the eye of the bee (Apis) (Fig. 102), as described and fio;ured bv Grenacher in his beautiful w^ork.f Fig. 102, the general accuracy of which has been confirmed recently by Dr. Hickson, represents two of the elements of the faceted eye. The structure of the eyes varies considerably in different groups. They may be said to consist of the following principal parts : — 1. The cornea (L/, Fig. 102). 2. The crystalline cones {Kh), of which there is one immediately behind each facet. The development of the crystalline cone has been carefully studied by Claparede. It consists of from four to sixteen original, but com- pletely combined segments, secreted by cells which lie immediately behind each facet, but of which, when the eye is completely developed, only the nuclei, known as Semper's nuclei {n), finally remain. 3. Next comes the retinula {rl), which stands in more or less intimate connection with the pointed inner end of the crystalline cone. It is generally composed of seven, but sometimes of as few as four, or as many Urn, ig. 102.— Two sepa- rate elements of the faceted eye of a bee (after Grenacher), Lf, Cornea ; n, nu- cleus of Semper ; Kk, crystalline cone ; Pg, Pg\ pigment cells ; El, retinula ; Em, rbabdom. * " The Eye and Optic Tract of Insects," Quarterly Journal of Microscopical Science, 1885. t " Untersucliungen liber das Sehorgan der Arthropoden." 1879. KETINUL A— PIGMENT. 151 as eight, originally separate, but closely combined cells. They converge on the optic lobe, and form an outer nucleated sheath, enclosing a strongly refractive, generally quadrangular, rod (the rhabdom, Bm), the relation of which to the filaments of the optic nerve is not yet well understood. 4. The pigment (Pg). Between each separate eyelet (ommateum, or omma- tidium, as it is termed by Hickson), is — at least, in some insects — a long, tubular, thin-walled trachea. These are difficult to see in prepared specimens, but have been mentioned by several observers. They were first, I think, figured by Leydig,* and more recently by Hickson. Finally, the eye is bounded by a basilar membrane, which is perforated by two sets of apertures, a series of larger ones for the passage of the tracheal vessels, and of smaller ones for the nerve-fibrils. The crystalline cone is not, however, always present, and Grenacher divides the compound eyes of insects into [three types : acone eyes, in which the crystalline cone is not present, but is represented throughout life by distinct cells ; pseudocone eyes, in which there is a special conical and transparent medium ; and, lastly, eucone eyes, with true crystalline cones." t * " Zum feineren Bau der Insekten," Mailer's Arch, fur Anat. u. Phys., 1855. f Acone eyes occur in Nematocera (gnats), Hemiptera (bugs), For- ficula (earwigs), and those Coleoptera (beetles), which have less than five tarsal joints. Pseudocone eyes occur in the true flies (Muscidse). Eucone eyes prevail among other insects : Lepidoptera, Hymenoptera, Neuroptera, Orthoptera, Cicadidse, the Coleoptera with five tarsal segments, and among Diptera the single genus Corethra, which, more- over, is remarkable as possessing compound eyes, even in the larva, and pupa. 152 DIFFERENT FORMS OF EYES. The last form differs principally from the two first in that the elements which constitute the crystalline cone and the retinula have become completely coalesced and solidified. The differences are, no doubt, im- portant, but I need not enter into them at length here. Even the eucone eyes differ considerably, as may be seen from the following figures, representing (Fig. 103) an eyelet from the eye of a cockroach (Periplaneta), and (Fig. 104) one from that of a cockchafer (Melolontha), both taken from Grenacher. •t2 Fig. 103.— Eyelet of cockroach (after Grenacher). If, Cornea ; kk, crys- talline cone ; pg', pigment cell ; rl, retinula ; rm, rhabdom. Fig. 104.— Eyelet of cockchafer (after Grenacher). If, Cor- nea; kk, crystalline cone; PO^ Pg\ pigment cells ; rl, retinula ; rV, rhabdom. With some few exceptions (Corethra, Libellula, etc.), the larvae of insects do not possess faceted eyes; indeed, as a general rule their powers of vision are very limited, or they are altogether blind. Most caterpillars have STEUCTURE OF THE OPTIC LOBES. 153 on each side of the head five or six eye-spots^ contain- ing each a crystalline body, but, as we shall presently see, they can probably do little more than distinguish between light and darkness. I do not propose to attempt to give here any detailed account of the structure of the insect brain, but I must say a few words on the subject. Between the brain proper and the eye itself there are, in, for instance, the blow-fly {Musca vomitoria), three distinct ganglionic swellings, which Hickson, a copy of whose beautiful figure I have given (Fig. 101), terms the"opticon" (op), epiopticon (cop), and periopticon (p.op). It will be seeu that the nerve-fibrils do not pass in a direct course, but actually decussate, or cross from one side to the other, three times, once between each two ganglionic swellings. The optic lobes of the two sides are also con- nected by a fibrous bundle. The structure of the three nervous swellings is also very complex. It consists of a fine granular matrix, traversed by a meshwork of very minute fibrillae, and, at least in the periopticon, is col- lected into a series of cylindrical masses. It is entirely beyond our present range of knowledge to explain the origin or purpose of these complex arrangements, though we cannot doubt that they do serve important functions. It is remarkable that these arrangements, though apparently very constant in individual species and genera, differ greatly in different groups of insects ; for instance, Hickson asserts that in the water-scorpion (Nepa), there is no decussation, and Carriere makes the same statement as regards Libellula; but it seems very extraordinary that this arrangement should be present in some insect eyes, and absent in others formed apparently on so nearly the same plan. 154 KELATIONS OF OCELLUS AND EYE. On the Kelation of the Eye to the Ocellus. In considering the relation of the eye to the ocellus, it is obvious that we cannot regard either as derived from the other. They are, as Grenacher says, " sisters," and derived from a common origin. The ocellus consists of a single lens in front of a larger or smaller number of visual rods. The com- pound eye consists of a number of facets, each in front of a single rod ; which is produced by from four to sixteen cells : in some cases each cell at first produces a separate rod, and these then subsequently coalesce more or less completely. Starting, then, from a simple form of eye consisting of a lens and a nerve-fibre, which would be capable of perceiving light, but would give no picture of the external world, we should arrive at the compound eye by bringing together a number of such eye-spots, and increasing the number of lenses, while the separate cells beneath each com- bined to form a single cone and rod ; while, on the other hand, by increasing the size of the lens, and multiplying the nervous elements behind it, we should obtain the ocellus of an insect, or the typical eyes of a vertebrate animal. There is, indeed, no need to suppose that these two eyes are derived from a common origin. We know that, while very similar eyes occur in distant groups of animals, on the other hand nearly allied species often difi'er greatly in the structure of their eyes; that, indeed, eyes of very different types often occur even in the same animal, so that we have strong reasons for assuming that they had an independent and separate origin. OCELLI OF SPIDEKS— MYKIAPODS. 155 The spiders have simple ocelli only, the higher Crustacea compound eyes only, while many of the lower Crustacea and of the great class of the insects possess both eyes and ocelli. It would seem probable, therefore, that the ancestral stock must have possessed both, though not perhaps in so perfect a form as that which has now been attained, and that the spiders have lost the compound eyes, while, on the contrary, in the higher Crustacea the ocelli have disappeared. Moreover, though the ocellus of a spider at first sight closely resembles the eye of a Scolopendra, the internal structure is, according to Grenacher, altogether different. In the ocellus of a spider or an insect we find, at a greater or less distance behind the lens, a retina consisting of a receptive surface, extended con- centrically with that of the lens, and consisting of a number of more or less rod-like perceptive elements so arranged that the light falls on their ends. On the contrary, in the eyes of Myriapods there is, he says, either a single element behind the cornea, or where there are many such elements, they are arranged with their longer axes perpendicular to the direction of light ; so that any separate perception of the rays of liglit coming from different points seems to be an impossibility. In the eye of Lithobius, behind the biconvex lens, he states that the cells lining what I may call the tube of each separate eye, terminate in filaments, between the free ends of which is left a narrow passage, down which the light must pass to reach the end of the optic nerve. Such a structure is certainly very remarkable, and seems entirely to preclude the possibility of the formation of a true image. Altogether the account given by Grenacher, both as to the mode 156 EYES OF CRUSTACEA. of action of the eyes of the Myriapods and as to their internal structure, differs entirely from that of Graber, Fig. 105.— Leptodora bjalina. The Eyes of Crustacea. The eyes of many Crustacea are highly developed. In the higher families (thence named Podophthalmata, STRUCTURE OF EYE. 157 or stalk-eyed) they are situated on more or less elongated pedestals. In some of the lower forms, though less complex, they are very large, occupying, as in the curious Leptodora (Fig. 105) of our deep lakes, the whole front of the head ; while in Corycaeus -a' / Nr Ifly Fig. 106.— Eye of Mysis (after Grenacher). n, Nuclei; Lf, facets; Kk, crystalline cones; n\ cells of the retinula; Rl, retinula ; Em, rhabdom ; Cp, blood-vessels; iV. fibres of the optic nerve; iV', iV", iV^", j\n'", decussations of the fibres of the optic nerve; G, G\ G", (?'", ganglia; M, muscles for the movement of the eye-stalk Km\ Km}\ mxc\Q\. (Fig. 107) they extend to more than one -half of the whole length of the body. The higher Crustacea possess no ocelli. In the lower species, on the contrary, a central ocellus is often present, especially in the young state. 1 58 MYSIS— CORYCiEUS— COPILTA. In illustration of the compound eyes of GrLi&tacea, I give a figure of an eye of My sis (Fig. 106). In the higher Crustacea the nervous elements of the eye are, moreover, very complex. There are no less than four optic ganglia (Fig. 106), and there is a chiasma, or decussation of fibres {N^, N^^, N^^^, N^^^^), between each. The eyes of lobsters and of crabs offer a curious difference. In the former, the crystalline cones are very long, and the retinulse comparatively short ; while in the crabs, on the contrary, the crystalline cone is short, and the retinulae long. The eye of Corycseus (Fig. 107) is very interesting. It is extremely large in proportion to the size of the Fig. 107.— Cory cfeus (after Leuckart), a. b, The eye. animal, extending from the front of the head to the beginning of the abdomen. The perceptive part of the eye (h) is, therefore, far removed from the lens (a). The eye of Corycaeus appears to represent, in fact, a single element of a compound eye. The eye of Copilia is also very remarkable, the retinula being, at about the end of the first third of its length, bent at a right angle. Here also the eye is about one-third as long as the body. The ocelli of Crustacea have not been much studied with reference to their microscopic structure. Those CALANELLA— LIMULUS. 159 of Calanella are very remarkable, and, indeed, but for their position and the presence of pigment, would hardly be recognized as eyes. They are three in number, and together form an X-shaped body (Fig. 108), supplied by a large nerve (iVIop.), and consisting of three groups of large nerve-cells, embedded in pig- ment. There are eight in each of the two side groups, and ten in the central. In form they are pear-shaped, with the narrow end turned towards the nerve. The organ contains no lens nor rods. Fig. 108.— Eyes of Calanella Mediterranea (after Gerstarker). Pg., pigment cells; N.fr., frontal nerves; N.op., nervus opticus. The numbers show the numbers of the cells. The eyes of the king crab (Limulus) have been described by Grenacher and by Lankester and Bourne.* The two lateral eyes form a polished, kidney-shaped protuberance on each side of the great shield. The outer side is smooth, but on the inner surface it is produced into a number of conical processes (Fig. 109), * " On the Eyes of Scorpio and Limulus," Quarterly Journal of Microscopical Science, 1883. 160 LIMULUS. each of which forms a special lens. Underneath each of these secondary lenses is a group of large, elongated pigmented cells, arranged round a central space, and the lens with their outer ends, while the touching Fig. 109. — Diagram of a vertical section through a portion of the lateral eye of Limulus polyphemus, showing some of the conical lenses, and corresponding retinulse (after Lankester and Bourne), a, Cuticle ; hb, cuticular lens ; cc, hypoderm ; Rn, retinula ; m, nerves. inner ones are continued into the optic nerve. These nerve-end cells form the " retinula," while their sides, which face one another, are thickened, and coalesce into a rod, the rhabdom, which is hollow at the end nearest the lens, but solid towards the nerve. The central eye is very different. It possesses a single lens, like that of an ordinary ocellus, underneath which is a layer of cells not differiDg much in appear- ance from those of the hypoderm, and below which again is another layer of large nerve-cells, which, how- ever, are so irregular as to suggest the idea that the central eye of the king crab may have partially lost its function. The king crab, then, so remarkable in other ways, is also very interesting in reference to the peculiar SCOEPIONS— LIGHT-ORGANS OF EUPHAUSIA. 161 structure of its eyes. These can hardly be regarded as homologous with the compound eyes of insects and Crustacea, but appear to have originated independently. They have, indeed, hardly anything in common, except that of being compound eyes. Lastly, I may allude to the eyes of scorpions, which, though very different from those of Limulus in appearaace, in Lankester's opinion approach them more nearly in essential constitution than any other known eyes. Before quitting this part of my subject, I must mention the curious eye-like organs of Euphausia. Euphausia (Fig. 110) — a shrimp-like crustacean, be- Fig. IIO.— Euphausia pellucida (after Sars). l.o., Luminous organ. longing to the same group as Mysis — and some of its allies, are remarkable for possessing at the base of some of the thoracic legs, and on the four anterior abdominal segments, luminous eye-like organs. They form small bulbs, each containing a vitreous body, some pigment, a lens, and a fan-shaped bundle of delicate fibres, and are very conspicuous from their beautiful red color and glistening lustre. M 162 LIGHT-ORGANS OF EUPHAUSIA. Clans * regards them as true accessory eyes. Sars,t on the contrary, considers that they have no power of sight, but are highly differentiated luminous organs. He admits that they present a deceptive resemblance to true eyes, but has convinced himself by observations- of the living animal that they have no power of vision. The fibrous fascicle (Fig. Ill,/) he finds to be the chief light-producing part.t and the lens-like body in front serves, as he supposes, for a condenser, producing a bright flash of light, the direction of which the animal, by means of its muscles, is able to control. The anterior pair (Fig. 112, h), which differ some- what in structure from the rest, are situated on the Y\g. ni.— Luminous organ of Eiiphausia (alter Sars). /, Fibres ; e, lens. Fig. 112.— Eye-stalk of Euphausia (after Sars). lo. Luminous organ; a, lower eye. eye-stalks, and appear to serve as " bull's-eyes " to the true organs of vision. Sars considers that the luminous organs do not serve as eyes, on the grounds * " Ueber einige Schizopoden und niedere Malacostraceen," Zeit. fur Wiss. ZooU 1863. t "On the Scbizopoda," "Challenger Reports," vol. xiii. X Valentine and Cimningharo, in a memoir just published (^Quarterly Journal of 3Iicroscopical Science, vol. xxviii.) deny this, and attribute it to the inner surface of the reflector. MODE OF VISION BY COMPOUND EYES. 163 that the nerve which supplies them is but small ; that the structure is not really analogous to that of a true eye, and that the position would be very unsuitable, one of them being actually situated on the stalk of the compound eye. The question does not, however, seem to be by any means clearly solved, and it must, I think, be admitted that, with the exception of the anterior pair, if the position does not seem suitable for true eyes, neither is it that which one would expect in light-organs. On the Mode op Vision by Means op Compound Eyes. Johannes Miiller, in his great work on the Physiology of Vision,* was the first to give an intelligible explana- tion of the manner in which insects see with their com- pound eyes. According to his view (see Fig. 75), those rays of light only which pass directly through the crystalline cones, or are reflected from their sides, can reach the corresponding nerve-fibre. The others fall on and are absorbed by the pigment which separates the different facets. Hence each cone receives light only from a very small portion of the field of vision, and the rays so received are collected into one spot of light. The larger and more convex, therefore, is the eye, the wider will be its field of vision ; while the smaller and more numerous are the facets, the more distinct will the vision be. In fact, the picture per- ceived by the insect will be a mosaic, in which the number of points will correspond with the number of facets. * "Zur vergleichenden Physiologie des Gesichtsinnes.'* 164 MULLER'S THEORY OF MOSAIC VISION. This theory was at first received with much favour. In 1852, however, Gottsche * attacked Miiller's view, pointing out that each separate cornea of a compound eye can, and in fact does, give a separate and distinct image. This had, indeed, long previously been ob- served by Leeuwenhoek, who said, " When I removed the tunica cornea a little from the focus of the micro- scope, and placed a lighted candle at a short distance, so that the light of it must pass through the tunica cornea, I then saw through it the flame of the candle inverted, and not a single one, but some hundreds of flames appeared to me, and these so distinctly (though wonderfully minute) that I could discern the motion of trembling in each of them." t Of this, indeed, it is easy to satisfy one's self. It is only necessary to look at a candle through the cornea of an insect, and then slightly draw back the micro- scope, when a thousand small images of the candle, each formed by one of the lenses, will be plainly seen. If, then, in such cases there was a retina placed at the proper distance, a true image w^ould be formed, as on the retina in our own eyes. This paper of Gottsche's threw great doubt on Miiller's explanation, which, indeed, was, in Dors's words, " abandonnee par tout le monde." { It is one thing, however, to see that the lenses throw distinct pictures, but quite another to understand how such pictures could be received on the retina, or com- bined into one distinct image. * " Beit, zur Anat. und Phys. der Fliegen und Krebse," 3Iuller's Arch., 1852. t A. Van Leeuwenhoek, " Select Works," translated by S. Hoole. X " De la vision chez les Artbropodes," Ar. des Sci. Phys. et Nat. Geneva: 1861. IMAGES THROWN BY THE CORNEA. 165 It must, moreover, be remembered that in our eyes the whole field of vision is reversed, so that different objects remain in the same relative position. In the case of insects, however, it would be the image thrown by each facet which would be reversed, and hence the general effect would be altogether false. We must not attach too much importance to the mere presence of an image. Any lens-like object, even a globule of fat, will give one. Moreover, as Miiller and Helmholtz have shown, the lenses of the cornea would be an advantage on the theory of mosaic vision, by assisting to condense the rays of light on the termination of the nerve. Gottsche's observation was made on the eye of the blow-fiy (Miisca vomitoria), and, as a matter of fact, the fly is one of those insects which do not possess a true crystalline cone. It is, therefore, probable that the image which he saw was that of the cornea. Moreover, as is shown by his figure, which I give below (Fig. 113), he states* that the image was formed at x, while the retina is far away at y. He suggested, indeed, that the so-called optic ganglion really corresponds with the retina of our own eye ; but this would not remove * His words are — " An der hintern Flache der Crystallkorper im Fliegenauge kehrt sich sicker das Bild urn, weil das Bild dem object in der Lage gleich ist, und da das Mikroskoj) das Object einmal umkehrt, so muss hier eine doppelte Umkehiung stattfinden, einmal durch das Mikroskop und vorlier durch den paraboliscben Crystall- korper. Entstebt nun bei x (Fig. 113) ein umgekebrtes Bild, so ist die Frage, wird das gauze Bild von x durch den Stiel zur Retina und zur Perception bei y hingeleitet oder wirkt dieser diinne Stiel gleicbsam wie ein Diapbragma und giebt er nur eiuen Tbeil des Bildes bei x nach y " (Gottscbe, " Beit, zur Anat. und Pbys. des Auges der Krebse und Flicgen," Arch, fiir Anat. Phys. und Wiss. Bledicin., 1852> 166 MOSAIC VISION. tlie difficulty, because, if any definite picture is to be formed, the seusitive rods, cones, or other structures must lie in the plane of tlie image, and this is not, in fact, the case. Dor suggested that the crystalline cones are nervous structures, and cor- respond to the rods of the vertebrate eye (Fig. 79). He admits, however, that, as a matter of fact, the image is not formed at the anterior surface of the crystalline cones.* And yet in his final summary, having shown that the image is formed, not at the anterior surface, but deep down in the crystalline cones, he expresses quite a different view, compares the crystalline cone to the vitreous body, and considers that the true retina is to be found in an envelope which surrounds the cone. Plateau j regards the mosaic theory of Miiller as definitively abandoned, but rather seems to have had in his mind that of Gottsche. At least, he states that, accord- ing to Miiller, the mosaic is formed by a number of partial images, each occupying the base of one of the elements composing the compound eye. This, how- ever, is not Miiller's theory. * "La cornee avec sa convexite posterieure correspond a la coruee et au cristallin des vertel^res, le corps cristallin (avec le soi-disant corps vitre) et la fibre nerveuse qui s'y attache a la couclie des batonnets, enfia le ganglion optique a celles des conches de la re'tine, qui sent compose'es des granulations, des cellules, et des fibres nerveuses." t " Rech. Exp. sur la Vision des Arthropodcs." Bruxelles : 1887. Fig. 113.— One of the elements of the eye of a fly (afier Gottsche). JcTc, Crys- talline cone ; x, posi- tion of the image; s, rod ; sc, sheath ; scm, outer sheath ; r, retina ; y, seat of vision. OBJECTIONS TO OTHER THEORIES. 167 On the other hand, Boll,* Exner,t and Grenacher se'^m to me to have proved that the compound eyes of insects cannot act as ours do ; that the theory which assumes that each facet acts as a separate eye and projects an image on a retina, is physically untenable. In the first place, there are cases — for instance, Forficula, Dytiscus, and Stratiomys among insects ; Ligia and many others among Crustacea — where the cornese are not sufficiently arched to give any distinct image. But even where an image is thrown by the cornea, it would be destroyed by the crystalline cone. In certain Crustacea the crystalline cones are elongated and curved ; this, which Oscar Schmidt f regarded as fatal to Miiller's theory, is, on the con- trary, as Exner has pointed out, quite compatible with it, but, on the contrary, cannot be reconciled with the theory of an image. There are few beetles in which the cornea give better images than in the firefly (Lampijris splendidula). On the other hand, the crystalline cones entirely destroy these images. If the eye is looked at through a microscope, and the crystalline cones are left in situ, the field of view appears perfei-tly black, with a bright spot of light at the end of each cone. No trace of an image can be any longer perceived. In fact, the images seen by Leeuwenhoek and Grottsche are thrown by the cornea only. In most cases, then, it would appear that the image formed by the cornea is destroyed by the crystalline * "Beit, zur Phys. Optik," Arch.fiir Anat. PJiys. und Wiss. Medicin., 1871. t *' Ueber das Sehen von Bewegungen und der Theorie des zusammengesetzten Auges," Sitz. K. AJcad. d. Wiss. Wien., 1875. % Ibid., 1876. 168 POSITION OF THE IMAGE. cone. This does not, indeed, always occur ; but even in such cases the image does not coincide with the posterior end of the cone. Grenacher repeated the experiment of Grottsche with moths. Here the crystalline cones are firm, and are attached to the cornea. Thus he was able to remove the soft parts, and to look through the cones and the cornea. When the microscope was focussed at the inner end of the cone, a spot of light was visible, but no image. As the object-glass was moved forward, the image gradually came into view, and then disappeared again. Here, then, the image is formed in the interior of the cone itself. Exner had endeavoured to make this experiment with the eye of Hydrophilus (the great black water-beetle), but the crystalline cones always came away from the cornea. He, how^ever, calculated the focal length, refraction, etc., of the cornea, and concluded that, even if, in spite of the crystalline cone, an image could be formed, it would fall much behind the retinula. In these cases, then, an image is out of the question. Moreover, as the cone tapers to a point, there would, in fact, be no room for an image, which must be received on an appropriate surface. In many insect eyes, indeed, as' in those of the cockchafer (Fig. 100), the crystalline cone is drawn out into a thread, which expands again before reaching the retinula. Such an arrangement seems fatal to any idea of an image. Moreover, for definite vision by the formation of an image, it is necessary that the eye should possess some powder of accommodation for different distances. It is obvious, from Fig. 76, that no distinct vision would be given unless the receptive surface follows the line a' V c\ But the position of this surface will ABSENCE OF POWER OF ACCOMMODATION. 169 depend upon the distance o{ ah c from the lens. As a matter of fact, Leydig * and Leuckart t thought they had discovered, between the cornea and the crystalline cones, certain muscular fibres which might regulate the distance between the two, and thus effect this object. Subsequent observers, however, have failed to detect these fibres. Again, it will be seen, from a glance at Fig. 76, that in an eye constituted like ours, on the principle of a camera obscura, the retina must follow a regular curve. If it is brought at all too far forward, or forced the least too far back, the image is at once blurred. Hence, in our own case the frequent need for spectacles, and hence it would seem that a conical retina is a physical impossibility. Plateau, indeed, adopts t a suggestion made by Grenacher that the absence of any means of adaptation may be rendered unnecessary by the length of the cones, the rays coming from distant objects acting on the anterior end, those from nearer ones at a greater or less depth. This, I confess, seems to me inadmissible. In the first place, the light must surely act immedi- ately it impinges on the organ of perception ; and, in the second, the cones are, as a general rule, abso- lutely transparent — the light passes unimpeded through them. Again, if insects see with their compound eyes as we do with ours, they must, of course, possess a retina. No such structure, however, has been as yet show^n to * " Zum feineren Bau der Arthropoden," BluUer^s Arch, fur Anat. unci rjiys., 1855. t " Carcinologisches," Wiegmann's Arch., 1858. X "Rech. Exp. feur la virion chez les Arthropodes," 1887. 170 ABSENCE OE EETINA. exist. Wagner,* indeed, observed that in some cases the optic nerve embraces the end of the cone, and he supposed that it thus forms a sort of retina, for which, however, its form is little suited. I ought also to mention that Max Schultzef con- sidered that he had, in some few cases — for instance, in Syrphus — been able to observe that the termina- tion of the nerve does divide into a number of fibres. Patten,t more recently, has also maintained the existence of numerous nerve-fibrils, which, however, subsequent observers — for instance, Kingsley § and Beddard || — have been unable to discover. Even, how- ever, if we admit the perfect correctness of Schultze's observation, these cases are exceptional, and the fibres so few that they can hardly, I think, affect the general conclusion. To give anything like a distinct vision, a very large number would be required. A last objection is the extreme difficulty which would exist of combining so many different images into one idea, though it must be admitted that at first sight this difficulty (though to a minor degree) exists even in the case of simple eyes, the number of which varies considerably. Spiders have six to eight ; some aquatic larvse twelve ; while the Oniscoidse (wood-lice), assuming that their eyes are aggregates of simple eyes, as Miiller supposed, have as many as twenty to forty. * Einige Bemerk. iiber den Bau der zus. Aiigen," Arch, fiir Nat., 1835. t " Uut. iiber die zus. Augen der Krebse und Insecten," 1868. X "Eyes of Molluscs and Arthropods," Mdth. Zool. St. Neapel, 1886. § *' On the Divisions of the Compound 'Eye," Journal of Morphology, 1887. II " On the Structure of the Eye in Cymothoidsc," Trans. Boy. Sac. Edtn., 1887. SUMMARY. 171 These, however, take in different parts of the field of vision. The principal reasons, then, which seem to favour Miiller's theory of mosaic vision are as follows : — (1) in certain cases — for iDstance, in Hyperia — there are no lenses, and consequently there can be no image ; (2) the image would generally be destroyed by the crystalline cone ; (3) in some cases it would seem that the image would be formed completely behind the eye, while in others, again, it '^vould be too near the cornea ; (4) a pointed retina seems incompatible with a clear image; (5) any true projection of an image would in certain species be precluded by the presence of im- penetrable pigment, which only leaves a minute central passage for the light-rays ; (6) even the clearest image would be useless, from the absence of a suit- able receptive surface, since both the small number and mode of combination of the elements composing that surface seem to preclude it from receiving more than a single impression; (7) no system of accommoda- tion has yet been discovered. Finally, (8) a combina- tion of many thousand relatively complete eyes seems quite useless and incomprehensible. On the Power of Vision in Insects, etc. As regards the practical vision of insects, oiir know- ledge is still very imperfect. No one, indeed, who has observed them can doubt that in some the sight is highly developed. It is impossible, for instance, to watch a dragon-fly hawking over a pond, — to see the rapidity and accuracy of its movements, and doubt that it can see well. 172 ON THE POWER OF VISION IN INSECTS. On the other hand, CJaparede asserts that at a distance of twenty feet a hive bee would be unable to see any object which was less than eight or nine inches in diameter, and even at a distance of a foot he says that each facet w^ould correspond to an inch and a third. To determine how far a faceted eye could see, he takes the breadth of a facet, the radius of the eye- sphere, and the smallest angle of vision, and the dis- tance in centimetres at which the facet would cover a centimetre, and finds for the bee, for instance, 6*7 centimetres. He then proceeds to inquire at what distance from the faceted eye the image is as clear as in the human eye, and he thinks this would be about a millimetre, from which it would rapidly diminish, being only ^ at a centimetre, and at a metre no distant vision being possible; so that at a very little distance such eyes would be as good as useless. "In the human eye, for example, the distance between the centres of two adjacent cones is only •^-^QQ mm., but in Musca the distance between adjacent ommatidia is you ^^* ^^ ^^^^ *^® picture, as received by the nerve-end cells of the Vertebrate eye, is much more complete in itself than it can possibly be in any Arthropod eye, and consequently the latter possesses a much more elaborate and complete translating appa- ratus in its retina than the former possesses." * Claparede arrives at this conclusion by taking the average curvature of the whole eye, as being true for each part. This, however, is not the case, and in the central region of the eye the adjacent facets * S. J. Hickson, " The Eye and Optic Tract of Insects," Quarterly Journal of 31icroscopical Science, vol. xxv., new series, 1885, p. 242. EXPERIMENTS ON VISION OF INSECTS. 173 make but a small angle with one another. Lowne has calculated that wasps, humble bees, dragon-flies, etc , would, at a distance of twenty feet, be able to distinguish objects from half an inch to an inch in diameter. Thus a dragon-fly would see an object twenty feet from its eye in the same detail tliat a man would perceive it at a distance of a hundred and sixty feet. Moreover, when Claparede * observes that bees will return from a considerable distance straight to the door of their nest, and that, under Muller's theory, the door would at such a distance be absolutely invisible, he forgets that the bee first probably guides itself by the known position of the door in relation to some tree or other large object, then with reference to the hive itself, and that it is quite unnecessary to assume that the door is actually seen from a distance. With reference to the power which insects possess of determining form. Plateau f has recently made some ingenious experiments. Suppose a room into which the light enters by two equal and similar orifices, and suppose an insect set free at the back of the room, it will at once fly to the light, but the two openings being alike it will go indifferently to either one or the other. That such is the case Plateau's experiments clearly show, and, moreover, prove that a comparatively small increase in the amount of light will attract the insect to one orifice in preference to the other. It occurred then to Plateau to utilize this by varying the form of the opening, so that the light admitted being * "Zur Morph. der zus. Augen bei den Arthropodeu," Zeit fur Wiss. Zool, 18G0. t Bull de VAcad. Boy. de Belgique, t. x., 1885 ; Comptes Rendus de la Soe. Ent. de Belg., 1887; " Rech. Exp. sur la Vision clicz lea Arthropodes," 1887. 174 EXPEKIMENTS ON VISION OF INSECTS. equal, the opening on the one side should leave a clear passage, while that on the other should be divided by- bars large enough to be easily visible, and sufficiently close to prevent the insect from passing. His experiments were conducted in a room five metres square, lighted by two similar windows looking to the west. It was on the first floor, and looked out on to fields. Moreover, he had the glass of the windows slightly ground, so that, while the light penetrated, nothing outside could be seen. He then covered up the windows, leaving only two orifices, one of which was simple and square, while the other was divided by cross-bars. To secure equality of light, the latter was left somewhat larger than the other, and the equivalence of the two was determined by a Kumford's photometer. The insects were set free on a table at the back of the room, exactly between the two open- ings, and at a distance of four metres. He states that a very slight diiference in the intensity of the light determined the flight of the insect to either one or the other opening; while, if the amount of light was as nearly as possible equal, they flew as often to the one as to the other. Omitting the cases when the light was not equal, the numbers were as follows : — Clear Trellised opening, opening. Musca vomitoria (the hlnehotile) ... ... ... 8 ... 7 On the other hand, they were — for EHstcdis tenax (the hee Q.y) 4 ... 8 Vanessa urticse (tovtoisebhell hnttev&.y) 1 ... 5 13 20 In fact, then, the insects seem to have gone more EXPERIMENTS ON VISION OF INSECTS. 175 often to the trellised opening. M. Plateau concludes that iusects do not distinguish differences of form, or can only do so very badly (" lis ne distinguent pas la forme des objects ou la distinguent fort mal "). I confess, however, that these experiments, ingenious as they are, do not seem to me to justify the conclu- sions which M. Plateau has deduced from them. Unless the insects had some means of measuring distance (of which we have no clear evidence), they could not tell that even the smaller orifice might not be quite large enough to afford them a free passage. The bars, moreover, Avould probably appear to them somewhat blurred. Again, they could not possibly tell that the bars really crossed the orifice, and if they were situated an inch or two further off they would constitute no barrier. I have tried some experiments, not yet enough to be conclusive, but which lead me to a different conclusion from that of M. Plateau. I trained wasps to come to a drop of honey placed on paper, and, when the insects had learned their lesson, changed the form of the paper, as I had previously changed the color. It certainly seemed to me that the insect recognized the change. M. Forel has also tried similar experiments, and with the same result. We know, however, as yet very little with reference to the actual power of vision possessed by insects. On the Function of Ocelli. Another interesting question remains. What is the function of the ocelli ? Why do insects have two sorts of eyes ? 176 ON THE FUNCTION OF OCELLI. Johannes Miiller considered that the power of vision of ocelli "is probably confined to the perception of very near objects. This may be inferred partly from their existing principally in larvae and apterous insects, and partly from several observations which I have made relative to the position of these simple eyes. In the genus Empusa the head is so prolonged over the middle inferior eye that, in the locomotion of the animal, the nearest objects can only come within tlie range. In the Locusfa cornuta, also, the same eye lies beneath the prolongation of the head. ... In the Orthoptera generally, also, the simple eyes are, in consequence of the depressed position of the head, directed downwards towards the surface upon which the insects are moving." From these facts, he considers himself justified in concluding that the simple eyes of insects are intended principally for myopic vision. The simple eyes bear a similar relation to the compound eyes, as the palpi to the antennae. Both the antennae and compound eyes are absent in the larvae of insects." * Lowne observes t that " the great convexity of the lens in the ocellus of Eristalis must give it a very short focus, and it is manifestly but ill adapted for the formation of a picture. The comparatively small number of rods must further render the production of anything like a perfect picture, even of very near objects, useless for purposes of vision. I strongly suspect that the function of the ocelli is the perception of the intensity and the direction of light rather than of vision in the ordinary acceptation of the term." * " Physiology of the Senses," translated by Baly. t "On the Modification of the Eyes of Insects," Phil. Trans., 1878. DIFFICULTY OF SUBJECT. 177 Eeaumnr, Marcel de Serres, Duges, and Forel also have shown that in insects which possess both ocelli and compound eyes, the ocelli may be covered over without materially affecting the movements of the animal ; while, on the contrary, if the compound eyes are so treated, they behave just as if in the dark. For instance, Forel varnished over the compound eyes of some flies (Musea vomitoria and Lucilia ciesar), and found that, if placed on the ground, they made no attempt to rise; while, if thrown in the air, they flew first in one direction and then in another, striking against any object that came in their way, and being apparently quite unable to guide themselves. They flew repeatedly against a wall, falling to the ground, and unable to alight against it, as they do so cleverly when they have their eyes to guide them. Finally, they ended by flying straight up into the air, and quite out of sight. It seems, indeed, to be a very general rule that insects of which the eyes are covered, whether they are totally blinded, or whether the ocelli are left uncovered, fly straight up into the air — a very curious and significant fact of which I think no satisfactory explanation has yet been given. Plateau * regards the simple eyes, or ocelli, as rudi- mentary organs of scarcely any use to the insect. Forel also states, as the result of his observations, that wasps, humble bees, ants, etc., find their way both in the air and on the ground, almost equally well \^ithout as with the aid of their ocelli. I confess that I am not satisfied on this point. In such experiments great care is necessary. M. Forel's interesting experiments with ants, whose compound eyes ♦ Bull, de VAcad. Boy. de Belgique, t. x., 1885. N 178 EXPERIMENTS. he had covered with opaque varnish, might almost, for instance, be quoted to prove the same with reference to the compound eyes. "Mes Camponotus aux yeux vernis," he says, " attaquaient et tuaient aussitot une Formica fusea mise au milieu d'eux, la saisissaient presque aussi adroitement que ceux qui avaient leurs yeux. lis demenageaient un tas de larves d'un coin de leur recipient a I'autre avec autant de precision qu' avec leurs yeux." * On the other hand, Forel goes so far as to say that if the compound eyes are covered with black varnish, insects cannot even perceive light (" Cela prouve qu'elles ne voyaient plus memo la lueur"). In fact, the use of the ocelli seems a great enigma, at least when the compound eyes are present. We must remember that some other xirticulata — spiders, for instance — possess ocelli only, and they certainly see, though not probably very well. Plateau has made some ingenious observations, from which it appears that spiders are very short-sighted, and have little power of appreciating form. He found they were easily deceived by artificial flies of most inartistic construction; and he concludes that even hunting spiders do not perceive their prey at a greater distance than ten centimetres (about four inches), and in most cases even less. Scorpions appeared scarcely to see beyond their own pincers. I have also made some experiments on this point with spiders (Lycosa saccata). In this species, which is very common, the female, after laying her eggs, collects them into a ball, which she surrounds with a silken envelope and carries about with her. I captured a * Eecueil Zool. Suisse, 18S7. SHORT SIGHT OF OCELLI. 17y female, and, after taking the bag of eggs from her, put her on a table. She ran about awhile, looking for her eggs. When she became still, I placed the ball of eggs gently about two inches in front of her. She evidently did not see it. I pushed it gradually towards her, but she took no notice till it nearly touched her, when she eagerly seized it. I then took it away a second time, and put it in the middle of the table, which was two feet four inches by one foot four, and had nothing else on it. The spider wandered about, and sometimes passed close to the bag of eggs, but took no notice of it. She wandered about for an hour and fifty minutes before she found it — apparently by accident. I then took it away again, and put it down as before, when she wandered about for an hour without finding it. The same experiment was tried with other individuals, and with the same results. It certainly appeared as if they could not see more than half an inch before them — in fact, scarcely further than the tips of their feet. I may also mention that they did not appear to recognize their own bags of eggs, but were equally happy if they were interchanged. On the other hand, it must be remembered that the sac is spun from the spinnerets, and the Lycosa had perhaps actually never seen the bag of eggs. Hunting spiders certainly appear to perceive their prey at a distance of at least several inches. Plateau has shown, in a recent memoir, that cater- pillars, which possess ocelli, but no compound eyes, are very short-sighted, not seeing above one to two centimetres.* * "Rech. Esp. sur la Vision chez les Artliropocles." Bull, de 'Acad. Roy. de Belgique, 1888. 180 OCELLI OF CAVE-DWELLING SPIDEKS. Lebert has expressed the opinion * " that in spiders some of their eight eyes — those which are most convex and brightly coloured — serve to see during daylight ; the others, flatter and colorless, during the dusk." Pavesi has observed f that in a cave-dwelling species {Nestims sjpeluncarum), which belongs to a genus in which the other species have eight eyes, the four middle eyes are atrophied. This suggests that they serve specially in daylight. Eeturning for a moment to the ocelli of true insects, it seems almost incredible that such complex organs should be rudimentary or useless. Moreover, the evidence afforded by the genus Eciton seems difficult to reconcile with this theory. The species of this genus are hunting ants, which move about in large armies and attack almost all sorts of insects, whence they are known as driver ants, or army ants. They have no compound eyes, but in the place of them most species have a single large ocellus on each side of the head, while others, on the contrary, are blind. Now, while the former hunt in the open, and have all the appearance of seeing fairly well, the latter con- struct covered galleries, and seek their prey in hollow trees and other dark localities. Insects with good sight generally have the crystalline lenses narrow and long, which involves a great loss of light. The ocelli are specially developed in insects, such as ants, bees, and wasps, which live partly in the open light and partly in the dark recesses of nests. Again, the night-flying moths all possess ocelli ; while they are entirely absent in butterflies, with, accord- * " Die Spinnen der Schweiz." t " Sopra una nuova Specie di Ragui." PROBABLE FUNCTION OF OCELLI. 181 ing to Scudder, one exception, namely, the genus Pamphila. On the whole, then, perhaps the most probable view is that, as regards insects, the ocelli are useful in dark places and for near vision.* Whatever the special function of ocelli may be, it seems clear that they must see in the same manner as our eyes do — that is to say, the image must be reversed. On the other hand, in the case of compound eyes, it seems probable that the vision is direct, and the diffi- culty of accounting for the existence in the same animal of two such different kinds of eyes is certainly enhanced by the fact that, as it would seem, the image given by the medial eyes is reversed, while that of the lateral ones is direct. * Forel, in his last memoir, inclines to this opinion. ( i82 ) CHAPTER yill. ON PROBLEMATICAL ORGANS OF SENSE. In addition to the organs of which I have attempted in the preceding chapters to give some idea, and to those which from their structure we may suppose to perform analogous functions, there are others of con- siderable importance and complexity, which are evi- dently organs of some sense, but the use and purpose of which are still unknown . " It is almost impossible," says Gegenbaur,* " to say what is the physiological duty of a number of organs, which are clearly sensory, and are connected with the integument. These enlargements are generally formed by ciliated regions to which a nerve passes, and at which it often forms enlargements. It is doubtful what part of the surrounding medium acts on these organs, and we have to make a somewhat far- fetched analogy to be able to regard them as olfactory organs." Among the structures of which the use is still quite uncertain are the muciferous canals of fishes. The skin of fishes, indeed, contains a whole series of organs of whose functions we know little. As regards the * " Elements of Comparative Anatomy." MUCIFEROUS CANALS OF FISH. 183 muciferous canal, Schultze has suggested * that it is a sense-organ adapted to receive vibrations of the water with wave-lengths too great to be perceived as ordinary sounds. Beard also leans to this same view. However this may be, it is remarkably developed in many deep- sea fish. In some cases peculiar eye-like bodies are developed in connection (though not exclusively so) with the muciferous canal. Leuckart,t by whom they were discovered, at first considered them to be accessory eyes, but subsequent researches led him to modify this opinion, and to regard them as luminous organs. Ussowt has more recently maintained that they are eyes, and Leydig considers them as organs which approach very nearly to true eyes (" welche wirblichen sehorganen sehr nahe stehen "). Whatever doubt there may be whether they have any power of sight, there is no longer any question but that they are luminous, and they are especially developed in the fishes of the deep sea. These are very peculiar. The abysses of the ocean are quite still, and black darkness reigns. The pressure of the water is also very great. Hence the deep seas have a peculiar fauna of their own. Surface species could not generally bear the enormous pressure, and do not descend to any great depth. The true deep-sea forms are, however, as yet little known. They are but seldom seen, and when * "Ueber die Sinnesorgane der Seitenlinie bei Fischen und Amphibien," Arch.fiir Mic. Anat., 1870. t " Ueber muthmassliche Nebenaugen bei einem Fische." Bericht iiber die 39 Vers., Deutscher Naturforscher, Giessen, 1864. X " Ueber den Bau der sog. angenahnlichen Flecken einiger Knochenfische," Bull. Soc. Imp. Moscow, 1879. 184 DEEP-SEA FISH. obtained are generally in a bad state of preservation. Their tissues seem to be unusually lax, and liable to destruction. Moreover, in every living organism, besides those usually present in the digestive organs, the blood and other fluids contain gases in solution. These, of course, expand when the pressure is diminished, and tend to rupture the tissues. The circumstances under which some deep-sea fish have occasionally been met with on the surface bears this out. They are generally found to have perished while endeavouring to swallow some prey not much smaller, or even in some cases larger, than themselves. What, then, has happened ? During the struggle they were carried into an upper layer of water. Immediately the gases within them began to expand, and raised them higher; the process continued, and they were carried up more and more rapidly, until they reached the surface in a dying condition.* It is, however, but rarely that deep-sea fish are found thus floating on the surface, and our knowledge of them is mainly derived from the dredge, and especially from the specimens thus obtained during the voyage of the Challenger, In other respects, moreover, their conditions of life in the ocean depths are very peculiar. The light of the sun cannot penetrate beyond about two hundred fathoms ; deeper than this, complete darkness prevails. Hence in many species the eyes have more or less completely disappeared. In others, on the contrary, they are well developed, and these may be said to be a light to themselves. In some species there are a number of luminous organs arranged within the area ♦ Guather, " Introduction to the Study of Fishes." LIGHT-OKGANS. 185 of, and in relation to, the muciferous systeai ; while in others they are variously situated. These luminous organs were first roentioned by Cocco.* They have since been studied by Giinther, Leuckart, Ussow, Leydig, and Emery. Lastly, they have been carefully described by Giintber, Moseley, and von Lendenfeld in the work on "Deep-Sea Fishes," in vol. xxvii. of the "Challenger Eeports." The deep-sea fish are either silvery, pink, or in many cases black, sometimes relieved with scarlet, and, when the luminous organs flash out, must present a very remarkable appearance. We have still much to learn as to the structure and functions of these organs, but there are cases in which their use can be surmised with some probability. The light is evidently under the will of the fish. It is easy to imagine a Photichthys (Fig. 114), swimming Fig. Hi.— Photichthys argenteus (" Challeuger Eeports," vol. xxvii.). in the black depths of the ocean, suddenly flashing out light from its luminous organs, and thus bringing into view any prey which may be near; while, if danger is disclosed, the light is again at once extiDguished. It may be observed that the largest of these organs is situated just under the eye, so that the fish is actually provided with a bull's eye lantern. In other cases * Kuovi Ann. dti Sci. Nat., 1838. 186 LIVING LAMPS. the light may rather serve as a defence, some having — as, for instance, in the genus Scopelus — a pair of large ones in the tail, so that " a strong ray of light shot forth from the stern-chaser may dazzle and frighten an enemy." * In other cases they probably serve as lures. The " sea-devil," or " angler," of our coasts has on its head three long, very flexible, reddish filaments, while all round its head are fringed appendages, closely resembling fronds of seaweed. The fish conceals itself at the bottom , in the sand or among seaweed, and dangles the long filaments in front of its mouth. Other little fishes, taking them for worms, unsuspect- ingly approach, and themselves fall victims. Several species of the same family live at great Fig. 115.— Ceratius bisjpinosus (" Challenger Reports," vol. xxvii.). depths, and have very similar habits. A mere red filament would, however, be invisible in the dark, and therefore useless. They have, however, developed (Fig. 115) a luminous organ, a living " glow-lamp," at * Gunther, " Challenger Eepoits," vol. xxvii. PKOBLEMATICAL ORGANS IN LOWER ANIMALS. 187 the end of the filament, which doubtless proves a very- effective lure.* These cases, however, though very interesting, throw little light on the use of the muciferous system in ordinary fish, which, I think, still remains an enigma. In some of the lower animals, the nerves terminate on reaching the skin at the base of rod-like structures similar, in many respects, to the rods of the retina, or the auditory rods of the ear, and of which it is very difficult to say whether they are organs of touch or of some higher sense. Round the margin of the common sea-anemone is a circle of bright blue spots, or small bladders. If a section be made, there will be found a number of cylindrical organs, each containing a fine thread, and terminating in a " cnidocil (Fig. 14) ; " and, secondly, fibres very like nerve-threads, swelling from time to time with ganglionic expansions, and also terminating in a cnidocil. These structures, in all probability, serve as an organ of sense, but what impressions they convey it is impossible to say. Some jelly-fishes (Trachynemadse) have groups of long hairs arranged in pairs at the base of the tentacles (Fig. 116), which have been regarded as organs of touch, and it is certainly difficult to suggest any other function for them. They are obviously sense-hairs, but I see no reason for attributing to them the sense of touch. The so-called eyes of the leech, in Leydig's f opinion, * Gunther, " Study of Fishes." t "Die Augen imd neue Sinnesorgane der Egel.," ReicherVs Arch., 18G1. 188 MEDUSiE— INSECTS— CRUSTACEA. which is confirmed by Ranke,* are also developed from the supposed special organs of touch. The latter are much more numerous, as many as sixty being developed % ^"^^ .._„-IS^ Fig. 116.— Ed»e of a portion of the mantle of Aglaura hemistoma, with a pair of sense- organs (after Hertwig). v, Velum; k,- sense-organ; ro, layer of nettle cells; t, tentacle. on the head alone. They are cylindrical organs, lined with large nucleated refractive cells, which occupy nearly all the interior. A special nerve penetrates each, and, after passing some way up, appears to terminate in a free end. I may also allude to the very varied bristles and cirrhi of worms, with their great diversity of forms. Among Insects and Crustacea, there are a great number of peculiarly formed skin appendages, for which it is very difficult to suggest any probable function. The lower antennsB of the male in Gammarus, for instance, bear a very peculiar slipper-shaped organ, situated on a short stalk : this was first mentioned by * " Beit, zu der Lehre. von den Uebergangs Sinnesorganen," Zeit fur Wiss. Zool., 1875. DIFFICULTY OF PROBLEM. 189 Milne Edwards, and subsequently by other authors, especially by Leydig.* The short stalk contains a canal, which appears to divide into radiating branches on reaching the " slipper," which itself is marked by a series of rings. Among other problematical organs, I might refer to the remarkable pyriform sensory organs on the antennae of Pleuromma,t the appendages on the second thoracic leg of Serolis, those on the maxilli- peds of Eurycopa, on the me- tatarsus of spiders, the finger- shaped organ on the antennae of Polydesmus, the singular pleural eye (?) of Pleuromma, and many others. There is every reason to hope that future studies will throw much light on these in- teresting structures. We may, no doubt, expect much from the improvement in our microscopes, the use of new reagents, and of mechanical appliances, such as the microtome ; but the ultimate atoms of which matter is composed are so infinitesiraally minute, that it is difficult to foresee any manner in which we may hope for a final solution of these problems. Loschmidt, who has since been confirmed by Stoney and Sir W. Thomson, calculates that each of the Fi,?. 117. — Sense-organ of leech (from Carriere, after Ranke). 1, Epithelium ; 2, pigment ; 3, cells; 4, nerve. The longer axis equals ••! mm. * Zeit. fur TFtss. Zool.,'' 1878. t Brady, " On the Copepo^la of the Challenger Expedition," vol. viii. 190 SIZE OF ULTIMATE ATOMS. ultimate atoms of matter is at most su.oo'o^.oim ^^ ^^ inch in diameter. Under these circumstances, we cannot, it would seem, hope at present for any great increase of our knowledge of atoms by improvements in the microscope. With our present instruments we can perceive lines ruled on glass which are -g-Q.oiro ^^ ^^^ inch apart. But, owing to the properties of light itself, the fringes due to interference begin to produce con- fusion at distances of 7 4,|)-oo> and in the brightest part of the spectrum, at little more than 9-0,0 o'o* ^^^7 would make the obscurity more or less complete. If, indeed, we could use the blue rays by themselves, their waves being much shorter, the limit of possible visibility might be extended to i2o!ooo » ^^^» ^^ Helmholtz has suggested, this perhaps accounts for Stinde having actually been able to obtain a photographic image of lines only T^-oaiWo^ ^^ ^^ ^^^^ apart. This, however, would appear to be the limit, and it would seem, then, that, owing to the physical characters of light, we can scarcely hope for any great improvement so far as the mere visibility of structure is concerned, though in other respects, no doubt, much may be hoped for. At the same time, Dallinger and Koyston Pigott have shown that, as far as the mere presence of simple objects is concerned, bodies of even smaller dimensions can be perceived. According to the views of Helmholtz, the smallest particle that could be distinctly defined, when associated with others, is about -go^.Voo ^^ ^^ ^^^^ ^^ diameter. Now, it has been estimated that a particle of albumen of this size contains 125,000,000 of molecules. In the case of such a simple compound as water, the number would be no less than 8,000,000,000. Even then, if we could THE RANGE OF VISION AND OF HEARING. 191 construct microscopes far more powerful than any we now possess, they could not enable us to obtain by direct vision any idea of the ultimate molecules of matter. The smallest sphere of organic matter which could be clearly defined with our most powerful micro- scopes may be, in reality, very complex ; may be built up of many millions of molecules, and it follows that there may be an almost infinite number of structural characters in organic tissues which w^e can at present foresee no mode of examining. Again, it has been shown that animals hear sounds ■which are beyond the range of our hearing, and that they can perceive the ultra-violet rays, which are invisible to our eyes.* Now, as every ray of homogeneous light which we can perceive at all, apj)ears to us as a distinct color, it becomes probable that these ultra-violet rays must make themselves apparent to the ants as a distinct and separate color (of which we can form no idea), but as different from the rest as red is from yellow, or green from violet. The question also arises whether Avhite light to these insects would differ from our white light in containing this additional color. At any rate, as few of the colors in nature are pure, but almost all arise from the combination of rays of different wave- lengths, and as in such cases the visible resultant would be composed not only of the rays we see, but of these and the ultra-violet, it would appear that the colors of objects and the general aspect of nature must present to animals a very different appearance from what it does to us. These considerations cannot but raise the reflection * " Ants, Lees, aud Wasps." 192 UNKNOWN SENSES. how different the world may — I was going to say must — appear to other animals from what it does to us. Sound is the sensation produced on us when the vibra- tions of the air strike on the drum of our ear. When they are few, the sound is deep ; as they increase in number, it becomes shriller and shriller ; but when they reach 40,000 in a second, they cease to be audible. Light is the effect produced on us when waves of light strike on the eye. When 400 millions of millions of vibrations of ether strike the retina in a second, they produce red, and as the number increases the color passes into orange, then yellow, green, blue, and violet. But between 40,000 vibrations in a second and 400 millions of millions we have no organ of sense capable of receiving the impression. Yet between these limits any number of sensations may exist. We have five senses, and sometimes fancy that no others are possible. But it is obvious that we cannot measure the infinite by our own narrow limitations. Moreover, looking at the question from the other side, we find in animals complex organs of sense, richly supplied with nerves, but the function of which we are as yet powerless to explain. There may be fifty otlier senses as different from ours as sound is from sight ; and even within the boundaries of our own senses there may be endless sounds which we cannot hear, and colors, as different as red from green, of which we have no conception. These and a thousand other questions remain for solution. The familiar world which sur- rounds us may be a totally different place to other animals. To them it may be full of music which we cannot hear, of color which we cannot see, of sensations which we cannot conceive. To place stuffed birds and THE UNKNOWN WORLD. 193 beasts in glass cases, to arrange insects in cabinets, and dried plants in drawers, is merely the drudgery and preliminary of study ; to watch their habits, to understand their relations to one another, to study their instincts and intelligence, to ascertain their adaptations and their relations to the forces of nature, to realize ^Yhat the world appears to them ; these constitute, as it seems to me at least, the true interest of natural history, and may even give us the clue to senses and perceptions of which at present we have no conception. ( 194 ) CHAPTER IX. ON BEES AND COLOES. In my book on " Ants, Bees, and Wasps," * I have recorded a number of observations which seemed to me to prove that bees possess the power of distinguish- ing colors — a power implied, of course, in the now generally accepted views as to the origin of the colors of flowers, but which had not up to that time been proved by direct experiment. Amongst other experiments, I brought a bee to some honey which I placed on a slip of glass laid on blue paper, and about three feet off I placed a similar drop of honey on orange paper. With a drop of honey before her a bee takes two or three minutes to fill herself, then flies away, stores up the honey, and returns for more. My hives were about two hundred yards from the window, and the bees were absent about three minutes, or even less; when working quietly they fly very quickl}^, and the actual journeys to and fro did not take more than a few seconds. After the bee had returned twice, I transposed the papers ; but she returned to the honey on the blue paper. I allowed her to continue this for some time, and then again transposed the papers. Slie * " Ants, Bees, and Wasps," International Scientific Series. Kegan Paul, Trench & Co. EXPERIMENTS WITH COLORED PAPERS. 195 returned to the old spot, and was just going to alight, when she observed the change of color, pulled herself up, and without a moment's hesitation darted off to the blue. jSTo one who saw her at that moment could have the slightest doubt about her perceiving the difference between the two colors. I also made a number of similar observations with red, yellow, green, and white. But I was anxious to carry the matter further, and ascertain, if possible whether they have any preference for one color over another, which had been denied by M. Bonnier. To test this I took slips of glass of the size used for slides for the microscope, viz. three inches by one, and pasted on them slips of paper of the same size, coloured re- spectively blue, green, orange, red, white, and yellow. I then put them on a lawn, in a row, about a foot apart, and on each put a second slip of glass with a drop of honey. I also put with them a slip of plain glass with a similar drop of honey. I had previously trained a marked bee to come to the place for honey. My plan then was, when the bee returned and had sipped for about a quarter of a minute, to remove the honey, when she flev/ to another slip. This I then took away, when she went to a third, and so on. In this way, as bees generally suck for three or four minutes, I induced her to visit all the drops successively before returning to the nest. When she had gone to the nest, I trans- posed all the upper glasses with the honey, and also moved the colored glasses. Thus, as the drop of honey was changed each time, and also the position of the colored glasses, neither of these could influence the selection by the bee. In recording the results, I marked down successively 196 EXPERIMENTS WITH COLORED PAPERS. the order in which the bee went to the different coloured glasses. For instance, in the first journey from the nest, as recorded below, the bee lit first on the blue, which accordingly I marked 1 ; when the blue was removed, she flew about a little, and then lit on the white; when the white was removed, she settled on the green, and so on successively on the orange, yellow, plain, and red. I repeated the experiment a hundred times, using two different hives — one in Kent and one in Middlesex — and spreading the observations over some time, so as to experiment with different bees, and under varied circnmstances. I believe that the precautions taken placed the colors on an equal footing, and that the number of ex- periments is sufficient to give a fair average. More- over, they were spread over several days, and the daily totals did not differ much from one another. The result shows a marked preference for blue, then white, then successively yellow, red, green, and orange. The red I used was a scarlet ; pink would, I believe from subsequent observations, have been more popular. I may also observe that the honey on plain glass was less visited than that on any of the colors, which was the more significant because when I was not actually observing, the colors were removed, and some drops of honey left on plain glass, which naturally gave the plain glass an advantage. Another mode of testing the result is to take the number of times in which the bee went first to each color, for instance, in a hundred visits she came to the blue first thirty-one times, and last only four; while to tbe plain glass she came first only five times, and last twenty-four times. It may be worth while to add that I by no means expected such a result. DR. MtJLLER'S OBJECTIONS. 197 A recent number of Kosmos contains a very courte- ous and complimentary notice of these observations by Dr. H. Miiller, which, coming from so high an authority, is especially gratifying. Dr. Miiller, however, criticizes some of the above-mentioned experiments, and remarks that, in order to make the test absolutely correct, the seven glasses should have been arranged in every possible order, and that this would give no less than 5040 combinations. I did not, however, suppose that I had attained to mathematical accuracy, or shown the exact degree of preference ; all I claimed to show was the existence, and order, of preference, and I think that, as in my experiments the position of the colors was continually being changed, the result in this respect would have been substantially the same. Dr. Miiller also observes that when a bee has been accustomed to come to one jDlace for honey, she returns to it, and will tend to alight there whatever the color may be ; and he shows, by the record of his own experiences, that this has a considerable influence. This is so. Of course, however, it applies mainly to bees which had been used for some time, and were accustomed to a particular spot. I was fully alive to this tendency of the bees, and neutralized it to a considerable extent, partly by frequently changing the bee, and partly by moving the glasses. While, how- ever, I admit that it is a factor which has to be taken into consideration, I do not see that it afi'ords any argument against my conclusions. The tendency would be to weaken the effect of preference for any particular color, and to equalize the visits to all the glasses. This tendency on the part of the bees was, as my experiments show, overborne by the effect produced upon them 198 REPLY TO OBJECTIONS. by the color. So far, then, from weakening my con- clusions, the fact, so far as it goes, tends to strengthen them, because it shows that notwithstanding this tendency the blue was preferred, and the honey on colorless glass neglected. The legitimate conclusion to be drawn seems, I confess, to me, not that my mode of observation was faulty, but rather that the pre- ference of the bees for particular colors is even some- what greater than the numbers would indicate. Next, Dr. Miiller objects that when disturbed from one drop of honey, the bees naturally would, and that in his experiments they actually did, fly to the next. As a matter of fact, however, this did not happen in mine, because, to avoid this source of error, when I removed the color I gave the bee a good shake, au'l so made her take a flight befoi-e settling down again. According to my experience, bees differ considerably in character, or, I should rather perhaps say, in humour. Some are much shyer and more restless than others. When disturbed from the first drop of honey, some are much longer before they settle on the next than others. Much also, of course, depends on how long the bee has been experimented on. Bees, like men, settle down to their work. Moreover, it is no doubt true that, cseteris 'paribus^ a bee in search of honey will go to the nearest source. But, as a matter of fact, in my hundred experiments I had but very few cases like those quoted above from Dr. Miiller. This arose partly from the fact that my bees were frequently changed, and partly because, as. already mentioned, I took care, in removing the color, to startle the bee enough to make her take a little flight before alighting again. Dr. Miiller says that in PREFERENCES OF BEES. iD9 his experiments, when the bee did not go to the next honey, it was when he shook her off too vigorously. I should rather say that in his observations he did not shake the bee off vigorously enough. The whole objection, however, is open to the same remark as the last. The bee would have a tendency, of course, like any one else, to go to its goal by the nearest route. Hence I never supposed that the figures exactly indi- cate the degree of preference. The very fact, however, that there would naturally be a tendency on the part of the bees to save themselves labour by going to the nearest honey, makes the contrast shown by my observations all the more striking. I have never alleged that it was possible, in the case of bees (or, for that matter, of men either), to get any absolute and exact measure of preference for one color over another. It would be easy to suggest many con- siderations which would prevent this. For instance, something would probably depend on the kind of flower the bee had been in the habit of visiting. A bee which had been sucking daisies miglit probably behave very differently from one which had beea frequenting a blue flower. So far, however, as the conclusions which I ventured to draw are concerned, I cannot see that they are in any way invalidated by the objections which Dr. Miiller has urged, which, on the other hand, as it seems to me, rather tend to strengthen my argument. I may perhaps be asked, If blue is the favourite color of bees, and then pink, and if bees have had so much to do with the origin of flowers, how is it there are so few blue and pink ones ? The explanation I believe to be that all blue flowers 200 THE COLORS OF FLOWERS. have descended from ancestors in which the flowers were red, these from others in which they were yellow, while originally they were all green — or, to speak more precisely, in which the leaves immediately surrounding the stamens and pistil were green ; that they have passed through stages of yellow, and generally if not always red, before becoming blue. It is, of course, easy to see that the possession of color is an advantage to flowers in rendering them more conspicuous, more easily seen, and less readily over- looked, by the insects which fertilize them ; but it is not quite so clear why, apart from brilliancy and visibility at a distance, one color should be more advantageous than another. These experiments how- ever, which show that insects have their preference, throw some light on the subject. Where insects are beguiled into visits, as is the case especially with flies, they are obviously more likely to be deceived if the flowers not only, as is often the case, smell like decaying animal substance, but almost re- semble them in appearance. Hence many fly flowers not oidy emit a most ofiensive smell, but also are dingy yellow or red, often mottled, and very closely resemble in color decaying meat. There remains another case in which allied flowers, and species, moreover, which are fertilized by very much the same insects, are yet characterized by distinct colors. We have, for instance, three nearly allied species of dead nettle — one white (Lamium album), one red {Lamium maculatiim), and one yellow {Lamium galeobdolon or luteum). Now, if we imagine the existence in a single genus of three separate species, similar in general habit and THE COLORS OF FLOWEHS. 201 appearance, and yet mutually infertile, it is easy to see that it would be an advantage to them to have their flowers diflerently colored. The three species of Lamium above mentioned may be growing together, and yet the bees, without difficulty or loss of time, can distinguish the species from one another, and collect pollen and honey without confusing them together. On the other hand, if they were similarly colored, the bees could only distinguish them with comparative difficulty, involving some loss of time and probably many mistakes. I have not yet alluded especially to white flowers. They seem to stand in a somewhat special position. The general sequence, as I have suggested, is from green, through yellow and red, to blue. Flowers normally yellow seldom sport into red or blue ; those normally red often sport into yellow, but seldom into blue. On the other hand, flowers of almost any color may sport into white. White is produced by the absence of color, may therefore appear at any stage, and will be stereotyped if for any reason it should prove to be an advantage.* * The genesis of the color is a large and interesting question. It may be due to various causes, and is by no means always owing to the presence of a diflerent coloring matter. For instance, as Professor Foster has observed to me, many species of Iris occur in blue and yellow forms. The yellow is largely, or wholly, produced by chroma- toplacts, the purple or blue to cell-sap, and if the latter is absent the yellow becomes apparent. ( 202 ) CHAPTEK X. on the limits of vision of animals. Ants and Colors. I HAVE elsewhere * recorded a series of experiments on ants with light of different wave-lengths, in order, if possible, to determine whether ants have the power of distinguishing colors. For this purpose I utilized the dislike which ants, when in their nest, have for light. Not unnaturally, if a nest is uncovered, they think they are being attacked, and hasten to carry their young away to a darker and, as they suppose, a safer place. I satisfied myself, by hundreds of experiments, that if I exposed to light the greater part of a nest, but left any of it covered over, the young would certainly be conveyed to the dark part. In this manner I satisfied myself that the various rays of the spectrum act on them in a different manner from that in which they affect us; for instance, that ants are specially sensitive to the violet rays. But I was anxious to go beyond this, and to attempt to determine whether, as M. Paul Bert supposed, their limits of vision are the same as ours. We all know that * •' Auts, Bees, and "Wasps." THE ULTRA-VIOLET RAYS. 203 if a ray of white light is passed through a prism, it is broken up into a beautiful band of colors, known as the spectrum. To our eyes this spectrum, like the rainbow, which is, in fact, a spectrum, is bounded by red at the one end and violet at the other, the edge being sharply marked at the red end, but less abruptly at the violet But a ray of light contains, besides the rays visible to our eyes, others which are called, though not with absolute correctness, heat-rays and chemical rays. These, so far from falling within the limits of our vision, extend far beyond it, the heat-rays at the red end, the chemical or ultra-violet rays at the violet end. I made a number of experiments which satisfied me that ants are sensitive to the ultra-violet rays, which lie beyond the range of our vision. I was also anxious to see how two colors identical to our eyes, but one of which transmitted and the other intercepted the ultra-violet rays, would affect the ants. Mr. Wigner was good enough to prepare for me a solution of iodine in bisulphide of carbon, and a second of indigo, carmine, and roseine mixed so as to produce the same tint. To our eyes the two were identical both in color and capacity ; but of course the ultra-violet rays were cut off by the bisulphide-of-carbon solution, while they were, at least for the most part, transmitted by the other. I placed equal amounts in flat-sided glass bottles, so as to have the same depth of each liquid. I then laid them, as in previous experiments, over a nest of Formica fusca. In twenty observations the 'ants went seventeen times in all under the iodine and bisulphide, twice under the solution of indigo and carmine, while once there were some under each. These observations, therefore, show that the solutions. 204 PERCEPTION OF LIGHT though apparently identical to us, appeared to the ants very different, and that, as before, they preferred to rest under the liquid which intercepted the ultra-violet rays. In two or three cases only they went under the other bottle ; but I ought to add that my observations were made in winter, when the ants were rather sluggish. I am disposed to think that in summer perhaps these exceptional cases would not have occurred. Professor Graber, however, while admitting the accuracy of my observations, has attempted to prove that the perception of the ultra-violet rays is not a case of sight in the ordinary acceptation of the words, but is due to the general sensitiveness of the skin. It has long been known that some of the lower animals which do not possess eyes are, nevertheless, sensitive to light. Hoffmeister,* in his work on earth- w^orms, states that, with some exceptions, they are very sensitive to light. Darwin, perhaps, experimented with a different species (for there are many different kinds) ; at any rate, his specimens seemed to be less keenly affected, though if one was suddenly illumi- nated it dashed "like a rabbit into its burrow." He observed, how^ever, that some individuals were more sensitive to light than others, and that the same indi- viduals by no means always acted in the same way. Moreover, if they " were employed in dragging leaves into their burrows or in eating them, and CA^en during the short intervals when they rested from their work, they either did not perceive the light or were regard- less of it."t He observes, however, that it is only the * " Familie der Eegenwiirmcr," IS45. t Dar.viu's ''Earthworms." BY THE GENERAL SURFACE OF THE SKIN. 205 anterior extremity of the body, where the cerebral ganglia lie, which is affected by light, and he suggests that the light may pass tlirough the skin and acts directly on the nervous centres. Lacaze-Duthiers, Haeckel, Engelmann, Graber, Plateau, and other naturalists have abundantly proved the sensitiveness to light of other eyeless animals. There has, indeed, long been a vague idea that blind people have some faint perception of light through the general surfoce of the shin. So far as I am aware there is not the slightest evidence or foundation for this belief; nor, indeed, has it been advocated by any com- petent authority. It seems a 'priori improbable that an animal with complex eyes should still retain a power which would be almost entirely useless. On the other hand, it is unquestionable that light can, and often does, act directly on the nerve termi- nations without the intermediate operation of any optical-apparatus. Some of them might, perhaps, be open to criticism. The effect of heat may not have been always sufficiently guarded against. Again, it is quite true that, as Plateau observes " Lorsque les Myriapodes chilopodes aveugles ou munis d'yeux, deposes sur le sol, s'introduisent avec empressement dans la premiere fente qu'ils rencon- trent, cet acte n'est pas determine par le seul besoin de fuir la lumiere, ces aniraaux cherchent en meme temps un milieu humide et avec lequel la plus grande partie de la surface de leur corps soit en contact direct." * But though this is no doubt true, and though, perhaps, the moisture may be some help, still, whatever be their * Plateau, " Rech. sur la perception de la lumiere par les Myriapodes aveugles," Jour, de I'Anatomie, etc., T. xxii. 1886. 206 PERCEPTION OF LIGHT object, we can hardly doubt that the absence of light is the principal guide. Professor Graber,* in his interesting memoir on this subject confirms the observations on ants and Daphnias, in which I showed that they are sensitive to the ultra-violet rays, by similar observations on earth- worms, newts, etc. It is interesting, moreover, that the species examined by him showed themselves, like the ants, specially sensitive to the blue, violet, and ultra- violet rays. Graber, however, states^that he differs from me inasmuch as I attribute the sensitiveness to the ultra-violet rays exclusively to vision ; — that it is " ausschliesslich durch die Augen vermittelt." I am not, however, of that opinion as a general expression, though I believe it to be true of ants, where the opacity of the chitine renders it unlikely that the light could be perceived except by the medium of the eyes or ocelli. Graber has shown in earthworms and newts, and Plateau t in certain Myriapods, that tliese animals perceive the difference between light and darkness by the general surface of the skin. But more than this, Graber seems to have demonstrated that earthworms and newts distinguish not only between light of differ- ent intensity, but also between rays of different wave- lengths, preferring red to blue or green, and green to blue. He found, moreover, as I did, that they are sensitive to the ultra-violet rays. Earthworms, of course, have no eyes; but, thinking that the light might * "Fundamental Versuche iiber die Helligkeits und Farben Em- pfindlichkeit augcnloser uud geblendeter Thiere," Sitz. Kais. AJcad. d. Wiss. Wien: 1883. t Journ. de VAnatomie et de la PhysioJogie, 1886. BY THE GENERAL SURFACE OF THE SKIN. 207 act directly on the cephalic gauglia, Graber decapi- tated a certain number, and found that the light still acted on them in the same manner, though the differ- ences were not so marked. He also covered over the eyes of newts, and foand that the same held good vdtli them. Heuce he concludes that the general surface of the skin is sensitive to light. These results are certainly curious and interesting, but even if we admit the absolute correctness of his deductions, I do not see that they are in opposition to those at which I had arrived. My main conclusions were that auts, Daphnias, etc., "were able to perceive light of different wave-lengths, and that their eyes were sensitive to the ultra-violet rays much beyond our limits of vision. His observa- tions do not in any way controvert these deductions; indeed, the argument by which he endeavours to prove that the effect is due to true light, and not to warmth, presupposes that sensations which can be felt by the general surface of the skin, would be still more vividly perceived by the special organs of vision. In connection with this subject, I may add that I do not at all doubt the sensitiveness to light of eyeless animals. In experimenting on this subject, I have always found that though the blind woodlice (Platy- arthrus), which live with the ants, have no eyes, yet if part of the nest be uncovered and part kept dark, they soon find their way into the shaded part. It is, however, easy to imagine that in unpigmented animaJs, whose skins are more or less semi-transparent, the light might act directly on the nervous system, even though it could not produce anything which could be called vision. 208 EXPERIMENTS WITH HOODWINKED ANTS. Forel, in some recent experiments, varnished over the eyes of fifteen ants (Camjoonotus Ugniperdus) and put them with fifteen others, wliich were left in their normal condition, in a flat box with a glass top and divided in the middle into two halves by a cardboard division, which, however, left room enough underneath for the ants to pass freely from one half to the other. After some other experiments, in the course of which one of the varnished ants was accidentally killed, at 1 p.m. all the varnished ants and thirteen of the un- varnished were in the right half of the box, and two unvarnished in the left. He then placed over the whole box two flat bottles containing water to inter- cept heat-rays — over the right half a piece of cobalt (violet) glass ; and over the left, a flat bottle containing a solution of esculine, which is quite transparent, but cuts off the ultra-violet rays. At 1.55 the result was as follows : — Under the esculine. Under the cobalt. 5 varnislied. 9 varnished. 13 normal. 2 normal. The esculine and cobalt were then transposed. At 2.3 the position was — Under the cobalt. Under the esculine. 4 varnished. 13 varnished. 3 normal. 12 normal. The esculine and cobalt were again transposed, and one normal ant was accidentally wounded and removed. At 3.8— Under the esculine Under the cobalt. 3 varnished. 12 varnished. 11 normal. 3 normal. EXPERIMENTS WITH HOODWINKED ANTS. 209 The escnline and cobalt were once more transposed, and at 3.13 there were — Under the cobalt. Under the esculiue. 3 varnished. 11 varnished. 1 normal. 13 normal. Thus the number of ants which followed the esculine and moved from one half of the box to the other at each transposition of the esculine and cobalt, was as follows : — Varnished. Normal. First change ... Second „ Third „ Fourth „ 5 ... 1 ... ... ... ... 11 ... 10 9 ... 10 6 40 And the number remaining under the cobalt and esculine respectively was — Under the Varnished. cobalt. Normal. Under the esculine. Varnished. Normal First experiment ... Second „ Third Fourth „ ... 9 ... 4 ... 12 ... 3 2 3 ... 3 ... 1 ... ... 5 13 ... 10 12 ... 3 11 ... 12 13 28 9 30 49 These experiments clearly showed that, while the normal ants moved from side to side so as to be under the esculine and consequently protected from the ultra- violet rays, those in which the eyes had been varnished remained unaffected by the transposition of the esculine and the cobalt, showing that the difference was per- ceived, not by the general surface of the skin, but by the eyes, and that when these were covered the ants were unaffected by the change. P 210 CONFIRMATION OF MY EXPERIMENTS ON ANTS. It might be suggested that possibly the ants had been injured or stupefied by the varnishiug. M. Forel accordingl^v, on the following day at 8 a.m., placed over one half of the box a layer of water six centimetres dee^D, and on the other a piece of red glass, which, while intercepting some of the lic^ht, allows almost all the heat to pass through. At 9.25 there were — Under the red glass. Under the layer of water. 3 varnished. 11 varnislied. 12 normal. 2 normal. Here, it seems that the ants which could see pre- ferred the shade, even though they were rather too warm; while the hoodwinked ants went under the cool water. This indicated that the varnished ants remained sensitive to heat, though not to light. Indeed, Forel states that they were just as lively, just as sensitive to currents of air, as the normal ants.* These experiments, then, entirely confirm those I had made. "C'est une confirmation entiere," says Forel, "des resultats de Lubbock t " and he sums up as follows : — The ants " paraissent percevoir I'ultra-violet priucipalement avec leurs yeux, c'est-a-dire qu'elles le voient, car lorsque leurs yeux sent vernis elles s'y montrent presque indifferentes ; elles ne reagissent alors nettement qu'a une lumiere solaire directe ou moins forte. Les experiences ci-dessus semblent in- diquer que les sensations dermatoptiques sont plus faibles chez les fourmis que chez les animaux etudies par Graber." From these and other experiments M. Forel comes *Loc. cit, p. 1G7. t Ibid., p. 174. EXPERIMENTS WITH DAPHNIAS. 211 to tlie same conclusion as I did, that the ants perceive the ultra-violet rays with their eyes, and not as suggested by Graber, by the skin generally. It is very gratifying that my experiments and conclusions should thus be entirely confirmed by an observer so careful oncl so experienced as M. Forel. Fig. lli.—Daphnia pulex. a, Antenna?; b, brain; e, eye; 7i, heart; m, muscle of eye ; n, nerve of eye ; o, ovary ; ol, olfactory organ ; .s stomach ; y, three eggs deposited in the space between the back and the shell. Experiments with Daphnias. The late M. Paul Bert made some very interesting experiments on a small fresh-water crustacean belong- 212 DAPHNIAS AND COLORS. ing to the genus Daphnia (Fig. 118), from which he concludes that they perceive all the colors known to us, being, however, especially sensitive to the yellow and green, and that their limits of vision are the same as ours. Nay, he even goes further than this, and feels justi- fied in concluding, from the experience of two species — Man and Daphnia — that the limits of vision would be the same in all cases. His words are — 1. " Tons les animaux voient les rayons spectraux que nous voyoiis." 2. '• lis ne voient aucun de ceux que nous ne voyons pas. 3. " Dans I'etendue de la region visible, les differences entre les pouvoirs eclairants des diflferents rayons colores sont les memos pour eux et pour nous." He also adds, "Puisque les li mites de visibilite semblent etre les memos pour les animaux et pour nous, ne trouvons-nous pas la une raison de plus pour supposer que le role des milieux de I'oeil est tout a fait secondaire, et que la visibilite tient a I'impression- nabilite de I'appareil nerveux lui-meme ? " These generalizations would seem to rest on a very narrow foundation. I have already attempted to show that the conclusion does not appear to hold good in the case of ants ; and I determinedj therefore, to make some experiments myself on Daphnias, the results of which are here embodied.* Professor Dewar was kind enough to arrange for me, at the Royal Institution, a spectrum, which, by means of a mirror, was thrown on to the floor. I then placed some * These observations were published in the Journal of the Linnean boviety lor 1881. PREFERENCE FOR YELLOWISH GREEN. 213 Daphnias in a shallow wooden trough fourteen inches by four inches, and divided by cross partitions of glass into divisions, so that I could isolate the parts illumi- nated by the different coloured rays. The two ends of the trough extended somewhat beyond the visible spectrum. I then placed fifty specimens of Baplmia piilex in the trough, removing the glass partitions so that they could circulate freely from one end of the trough to the other. Theu, after scattering them equally through the water, I exposed them to the light for ten minutes, after which I inserted the glass partitions, and then counted the Daphnias in each division. The results were as follows: — Number OF Daphnias . Beyond the red. In the red and yellow. In the greenish yellow and green. In the blue. In the violet. Beyond tbe violet. Obs.l ... 20 28 2 „ 2 ... 1 21 25 3 „ 3 ... 2 21 2i 8 „ 4 ... 1 19 29 1 „ 5 ... 20 27 3 4 101 183 12 I may add that the blue and violet divisions were naturally longer than the red and green. May 25. — Tried again the same arrangement, but separating the yellow, and giving the Daphnias the choice between red, vellow, green, blue, violet, and dark :— Dark. Violet. Blue. Green. Yellow. Red Exp. 1 ... .. 3 39 5 3 „ 2 ... .. 1 2 37 7 3 „ 3 ... .. 4 31 10 5 „ 4 ... .. 1 5 30 8 6 „ 5 ... .. 1 4 33 6 6 18 170 36 23 214 EXPERIMENTS. Of course, it must be remembered that the j^ellow band is much narrower tlian the green. I reckoned as yellow a width of three-quarters of an inch, and the width of the green two inches. Again — Dark. Violet. Blue. Groen. Yellow. Red. Exp. 1 ... ... 4 30 6 10 „ 2 ... ... 1 3 25 8 13 „ 3 ... 2 24 9 15 „ 4 ... 1 3 25 8 13 „ 5 ... 1 2 24 7 IG 1 2 14 128 38 67 Adding there I to- — — — — — gether, we get 1 5 32 298 74 90 M. Paul Bert observes (he. cit.) that in his experiments the Daphnias followed exactly the brilliance of the light. It will be observed, however, that in my expe- riments this was not the case, as there were more Daphnias in proportion, as well as absolutely, in the green, although the yellow is the brightest portion of the spectrum. In fact, they follow the light up to a certain brightness; but, as will be seen presently, they do not like direct sunshine. I then arranged the trough so that the yellow fell in the middle of one of the divisions. The result was — NUMBEK OF DArHNIAS. Exp. 1 .. 2 25 113 12 May 18. — In order to test the limits of vision at the Ultra- red and lower red. Upper edge. of red, yellow, and lower green. Greenish blue and blue. Violet. Ultra- violet. .. 8 38 4 .. 9 36 5 ,.. 8 39 3 LIMITS OF VISIOX OF DAPHNIAS. 215 red end of the spectrum, I used the same arrangement as before, phiciiig the trough so that the extreme division was in the ultra-red, and the second in the red. I then placed sixty Daphnias in the ultra-red. After five minutes' exposure, I counted them. There were in the— Red. Ultra- red, Exp. 1 54 ... 5 „ 2 56 ... 4 I now gave them four divisions to select from — dark, red, ultra-red, and dark again. The numbers were — Dark. Red. Ultra-red Dark. Exp. 1 5 47 6 2 „ 2 9 41 7 3 I then shut them off from all the colors excepting red, giving them only the option between red and ultra-red : — Exp. 1 « 2 „ 3 I then left them access to a division on the other side of the red, which, however, I darkened by interposing a piece of wood. This enabled me better to compare the ultra-red rays wath a really dark space : — Red. Ultra-red, 46 4 47 3 44 6 Dark. Red. Ultra-red. 4 43 3 3 45 2 Exp. 1 „ 2 These observations appear to indicate that their limits of vision at the red end of the spectrum coincide approximately with ours. I then proceeded to examine their behaviour with reference to the other end of the spectrum. In the first place, I shut them off from all the rays 216 PERCEPTION OF ULTRA-VIOLET RAYS. except the blue, violet, and ultra-violet. The result was as follows : — Number of Daphntas. Ultra-violet. Violet. Blue. Dark. Exp. 1 1 9 38 2 „ 2 4 G 38 2 „ 3 2 46 2 5 17 122 6 This shows that they greatly prefer blue and violet to darkness or ultra-violet. I afterwards gave them only the option of ultra-violet, violet, and darkness : — Exp. 1 „ 2 « S « 4 „ 5 45 238 17 They preferred the violet; but there were many more in the ultra-violet than in the dark. I then tried ultra-violet and dark. The width of the violet was two inches; and I divided the ultra-violet portion again into divisions each of two inches, which we may call ultra-violet, further ultra-violet, and still further ultra-violet. The results were — Ultra-violet. Violet. Dark. 8 48 4 6 48 6 ... 12 47 1 ... 15 42 3 4 53 3 Number of DArnNiAS. Exp. Still further ultra-violet. Further ultra-violet. Ultra-violet. Dark. 1 ... ... 6 52 2 2 ... ... 5 52 3 3 ... ... 6 50 4 4 ... ... 4 53 3 5 ... ... 4 54 2 28G 14 PERCEPTION OF ULTRA-VIOLET RAYS. 217 In this case the preference for ultra-violet over dark was very marked. May 18. — I again tried them with the ultra-violet rays, using three divisions — namely, further ultra-violet, ultra-violet, and dark. The numbers were as follows, viz. under the — Exp. 1 „ 2 ultra-violet. Ultra-violet. Dark. ... 6 50 4 ,.. 3 55 2 105 To my eye there was no perceptible diflerence be- tween the further ultra-violet and the ultra-violet portion ; but slightly undiffused light reached the two extreme divisions. It may be asked why the still further ultra-violet division should have been entirely deserted, while in each case two or three Daphnias were in the darkened one. This, I doubt not, was due to the fact that, the darkened division being next to the ultra- violet, one or two in each case straggled into it. Not satisfied with this, I tried another test. There are some liquids which, though transparent to the rays we see, are quite opaque to the ultra-violet rays. Bisulphide of carbon, for instance, is quite colourless and transparent : it looks just like water, but it entirely cuts off the ultra-violet rays. If, then, we place the trough containing Daphnias, as I had previously done my nest of ants, in the ultra-violet part of the spectrum, and then place over one half of it a flat bottle contain- ing water, and over the other half a similar bottle con- taining bisulphide of carbon, both halves will seem equally dark to us, but the ultra-violet rays reach one half of the vessel, while they are cut off from the other. 218 .PEKCEPTION OP ULTRA-VIOLET RAYS. To our eyes both, as I say, are equally dark, and so they would be to the Daphnias if their limits of vision were the same as ours. As a matter of fact, however, the Daphnias all collected in the part of the trough under the water, and avoided that under the bisulphide of car- bon, showing that this, therefore, was to them darker tlian the other. I varied the experiments in several ways, but always with similar results. Bichromate of potash is also impervious to the ultra-violet rays, and had the same effect. Not satisfied with this, I tried to test it in another way. I took a cell, in which I placed a layer of five-per- cent, solution of chromate of potash less than an eighth of an inch in depth, and which, though almost colourless to our eyes, completely cut off the ultra-violet rays. I then turned my trough at right angles, so that I could cover one side of the ultra-violet portion of the spectrum with the chromate and leave the other exposed. The numbers were as follows : — Side of the ultra- violet covered with Side chromate of potash, uncovered. Dark. Exp. 1 5 ... 55 ... I now covered up the other side. Esp. 2 ... 3 ... 57 ... Again covered up the same side as at first. Exp. 3 4 ... 56 ... Again covered up the other side. Exp. 4 ... ... ... 3 ... 57 ... May 19. — I again tried the same arrangement, re- ducing the chromate of potash to a mere film, which, Under the film of chromate of potash. Under the water. ... 8 52 ... 4 56 ... 10 50 7 53 OBJECTIONS OF M. MEHEJKOWSKY. 219 however, still cut off the ultra-violet rays. I then placed it, as before, over one half of the ultra-violet portion of the spectrum ; and over the other half I placed a similar cell containing water. Between each experiment I reversed the position of the two cells. The numbers were — Exp. 1 ... „ 2 ... „ 3 ... „ 4 .. Evidently, then, even a film of chromate of potash exercises a very considerable influence ; and, indeed, I doubt not that, if a longer time had been allowed, the difference would have been even greater. It seems clear, therefore, that a five-per cent, solution of chromate of potash only one-eighth of an inch in thickness, which cuts off the ultra-violet rays, though absolutely transparent to our eyes, is by no means so to the Daphnias. These observations seem to prove, though I differ with great reluctance from so eminent an authority as M. Paul Bert, that the limits of vision of Daphnias do not, at the violet end of the spectrum, coincide with ours, but that the Daphnia, like the ant, is affected by the ultra-violet rays. Since these observations were published, M. Merej- kowsky has experimented on the subject, and come to the conclusion that the Daphnias are attracted wherever there is most light, that they are conscious only of the intensity of the light, and that they have no power of distinguishing colors. It is no doubt true tbatin ordinary diffused daylight the Daphnias generally 220 DAPHNIAS SUPPOSED TO PERCEIVE congregate wherever the light is strongest. Their eyes are, however, so delicate that one would naturally expect, a priori, that there would be a limit to this ; and, in fact, direct sunshine is somewhat too strong for their comfort. For instance, I took a porcelain trough, seven and a half inches long, two and a half broad, and one deep, and put in it some water containing fifty Daphnias. One half I exposed to direct sunlight, and the other I shaded, counting the Daphnias from time to time, and trans- posing the exposed and shaded halves. The numbers were as follows: — At 10.40 a.m, « 12.50 „ »3 1.10 „ >> 1.35 „ 55 1.50 „ « 2.5 „ >5 2.40 „ » 3.0 „ 5> 4.0 „ )) 4.30 „ In the sun. In the shade. ,. 4 46 . 8 42 . 7 43 ,. 7 43 . 4 4G ,. 3 47 ,. 4 46 . 5 45 ,. 7 43 .. 4 46 53 447 This seems clearly to show that they avoid the full sunlight. I believe, then, that in some of my previous experi- ments the yellow light was too brilliant for them ; and the following experiments seem to show that, when sufficiently diffused, they prefer yellow to white light. M. Merejkowsky, however, denies to the Crustacea any sense of color whatever. His experiments were made with larvse of Balanus and with a marine cope- pod. Bias longiremis. These, if I understand him correctly, have given identical results. He considers BRIGHTNESS, BUT NOT COLOR. 221 that they perceive all the luminous rays, and can dis- tiuguish very slight differences of intensity ; but that they do not distinguish between different colors. He sums up his observations as follows : — " II resulte de ces experiences que ce qui agit sur les Crustaces, ce n'est point la qualite de la lumiere, c'est exclusivement sa quantite. Autrement dit, les Crus- taces inferieurs ont la perception de toute onde himi- neuse et de toutes les differences, meme trc? legeres, dans son intensite ; mais ils ne sont point capables de dis- tinguer la nature des ondes, de differentes couleurs. lis distinguent tres bien I'intensite des vibrations etherees, leur amplitude, mais point leur nombre. H y a done, dans le mode de perception de la lumiere, une grande difference entre les Crustaces inferieurs et I'Homme, et meme entre eux et les Fourmis ; tandis que nous voyons les differentes couleurs et leurs differentes intensites, les Crustaces inferieurs ne voient qu'une seule couleur dans ses differentes variations d'intensite. Nous percevons des couleurs comme couleurs ; ils ne les percoient que comme lumiere." * It is by no means easy to decide such a question absolutely ; but the subject is of much interest, and accordingly I made some further experiments, as it did riot seem to me that those of M. Merejkowsky bore out the conclusion he has deduced from them. Professor Dewar most kindly arranged the apparatus for me again. He prepared a normal diffraction-spec- trum, produced by a Kutherfurd grating with 17,000 lines to the inch ; the spectrum of the first order was thrown on the trough. In this case the distribution of * M. C. Merejkowsky, " Les Crustaces inferieurs distinguent-ils les couleurs ? " 222 FURTHER EXPERIMENTS. lumiDous intensity las been shown to be uniform on each side of the line having the mean wave-length, i.e. a little above the line D in the yellowish green of the spectrum. I then took a long shallow trough in which were a number of Daphnias, and placed it so that the centre of the trough was at the brightest part of the spectrum, a little, however, if anything, towards the green end. After scattering the Daphnias equably I left them for five minutes, and. then put a piece of blackened cardboard over the brightest part. After five minutes more, there were at the green end, 410; in the dark, 14 ; at the red end, 76. Here the two ends of the trough were equally illuminated ; but the preference for the green over the red side was very marked. I then took five porcelain vessels, seven and a half inches long, two and a half broad, and one deep, and in each I put water containing fifty Daphnias. One half of the water I left uncovered ; the other half I covered respectively with an opaque porcelain plate, a solution of aurine (bright yellow), of chlorate of copper (bright green), a piece of red glass, and a piece of blue glass. Every half-hour I counted the Daphnias in each half of every vessel, and then transposed the coverings, so that the half which had been covered was left exposed, and vice versa. I also changed the Daph- nias from time to time. Here, then, in each case the Daphnias had a choice betw^een two kinds of light. It seemed to me that this would be a crucial test, because in every case the colored media act by cutting o^T certain rays. Thus the aurine owes its yellow color to the fact that it cuts FURTHER EXPERIMENTS. 223 off the violet and blue rays. The light beneath it con- tains no more yellow rays than elsewhere; but those rays produce the impression of yellow, because the yellow is not neutralized by the violet and blue. In each case, therefore, there was less light in the covered than in the uncovered part. After every five experiments I added up the number of the Daphnias; and the following table gives twenty such totals, each containing the result of five observa- tions, making in all one hundred. My reason for adding one vessel in which one half had an opaque cover was to meet the objection that possibly the light might have been too strong for the Daphnias ; so that when they went under the sheltered part they did so, not for color, but for shade. I was not very sanguine as to the result of this arrangement, because I had expected that the preference of the Daphnias for light would overcome their attachment to yellow. The numbers were as in the following table (p. 224). The result was very marked. The first two columns show the usual preference for light. If the covered half had been quite dark, no doubt the difference in numbers would have been greater ; but a good deal of light found its way into the covered half. Still the result clearly shows that the Daphnias preferred the lighter half. The numbers were 204S in the dark to 2952 in the light; and it will be seen that the preference for the light was shown, though in different degrees, in almost every series. The result in the blue gives, I think, no evidence as to color-sense. The numbers were respectively 2046 against 2954, and were therefore practically the same 224 EVIDENCE THAT DAPHNIAS. 1 OOiOOOCO^CXMlOCO'*! a5(MrHOco-*x»ococo CO ■* 1- CO CC lO T^l CO lO -*! rH Oi Ttl c» lOI>-OOC a rHOOaiO(MCO(MiOl-t^ t^ CO 1 r-ir-iOr-IOOOiOCOiOO Ot-COCOOOOOSrHCO C-('tlrH^HCOC^lCNOOCOO ^ C5 > I— i I— It-ti— (i-Ht— 1 I— (I— I CO Oi lO .5i I— 1 ""• (M *=» d CO-^iCt^-HOCOtCh-l^ o t^COC^Ixrq'-OrHOTHCO o CO 2 cccor-iooo^i-iocccq TtH coococOi-HCMcqi>]^Tt( CO Q rt pH r-i rH 1-, r-H rH (M T^ ;5 '"' (M "i -+iC0eC'*lTtTH CO COOCO(M'*lr-lUO'*COlO a (M OOlOGOOOt^t-tMO-SCOO:! o ■C0»OC0C0C0'*Ht^C0iO»O a t^ > lO 1— (rHi— IrnrHrHrHi— IrlrH ■* O « 8 rH CO . -o a co-t. •d coi>r-'o:ooooco«o t-- THO'+ICOCOOilOCO'^lO rH 00 oaiOOI>lX00t^lO00'rt^cq ^ l>CO-H-Ht>-HCO-HCrHiOt>t>CDrH;0(Mi-H CO aicocorHO C<1 o > I— t I— 1 I— ( rH (X) rH rH r-( i-l rH rl o 05 o 1— ( rH o i-'5Oi-HTt^(Mf0l0-*(a>t:-t^COCOCX)CNCO OCOtMiOCOTtit-TtlCOTtl CO Ol "3 CD ■* o >« CO 1 -tiCiOCOOWCOOrH-H CO C500-*COrHCO-HOrHCO 05 ffq > (MOCC rH(MCO-(Ha5'^OiOC5t> l—t 00 (MTtHCOOrHr-ICTiCMGCtM uo COrHOrHlOOCOOOlOi S5 :S a, rH rH r-t rH CO o O (M :::::::::: : :::::::: i . '^^^••••Ci : :::::::: * lOuOCOOOOOJCSCiOrH ^ CO t> CO GO GO 00 05 Ci s rQ ^^^^„^^^- ^ £ (X) RRrSK^»-.-r. 5 12.30. Two were >» 1.30. Two were None were JJ 2.30. One was » »> » I do not give these results as by any means proving that ants do not recognize their friends by means of smell. They do seem, however, at any rate, to show that not even six months of close companionship under pre- cisely similar conditions will so far assimilate the odour as to lead to confusion. If the recognition is due in any degree to this cause, the odour is therefore probably an hereditary characteristic. In the interesting memoir already cited, Forel says,* "Lubbock {loG. cit.) a cru demontrer que les fourmis enlevees de leur nid a I'etat de nymphe et ecloses hors de chez elles etaient neanmoins reconnues par leurs * EecueilZool Suisse, 1887. 236 EXPERIMENTS WITH ANTS REMOVED FROM THE compagnes lorsqii'on les leur rendait. Dans mes Fourmis de la Suisse, j'avais cm demontrer le contraire. Voici ime experience que j'ai faite ces jours-ci : Le 7 aoiit, je donne des nymplies de Formica i^fcf-tensis pres d'eclore a quelques Formica sanguinea dans une boite. Le 9 aout quelques-unes eclosent. Le 11 aout, au matin, je prends Tune de jeunes pratensis agee de denx ou trois jours seulement et je la porte a sa fourmiliere natale dont elle etait sortie com me nymphe seulement 4 jours auparavant. Elle y est fort mal repue. Ses nourrices d'il y a 4 jours I'empoignent qui par la tete, qui par le thorax, qui par les pattes en recourbant leiir abdomen d'un air mena^ant. Deux d'entre elles la tinrent longtemps en sens inverse cliacune par une patte en i'ecartelant. Enfin cependant on finit par la tolerer, comme on le fait aussi pour de si jeunes fourmis (encore blanc jaimatre) provenant de fourmilieres dif- ierentes. J 'attends encore deux jours pour laisser durcir mi peu mes nouvelles ecloses. Puis j'en reporte deux sur leur nid. Elles sont violemment attaquees. L'une d'elles est inondee de venin, tiraillee et tuee. L'autre est longtemps tiraillee et mordue, mais finalement laissee tranquille (toleree?). On m'objectera I'odeur des sanguinea qui avait vecu 4 jours avec la premiere et 6 jours avec les deux dernieres. A cela je repondrai simplement par Texperience de la page 278 a 282 de mes Fourmis de la Suisse, ou des F. pratensis adultes separees depuis deux mois de leurs compagnes par une alliance forcee avec des F. sanguinea, alliance que j'avais provoquee, reconnurent immediatement leurs anciennes compagnes et s'allierent presque sans dispute avec elles. Je maintiens done mon opinion : les fourmis apprennent a se connaitre petit a petit a partir de leur eclosion. NEST AS VUFJE AND SUBSEQUENTLY RESTORED. 237 Je crois du reste que c'est au moyen de perceptions olfactives de contact." * I have, however, repeated my previous observations, with the same results. At the beginning of August I brought in a nest of Lasiits niger containing a large number of pupae. Some of these I placed by themselves, in charge of three ants belonging to the same species, but taken from a nest I have had under observation for rather more than ten years. On August 28 I took twelve of the young ants, wliich in the mean time had emerged from the sepa- rated pupse, selecting some which had almost acquired their full colour. Four of them I placed in their old nest, and four in that from which their nurses were taken. At 4.30 in their own nest none were attacked. „ „ „ nurses' nest one was attacked. „ 5.0 „ own nest none were attacked. „ „ „ nurses' nest all four were attacked. „ 8.0 „ own nest none were attacked. „ „ „ nurses' nest three were attacked. The next day I took six more and marked them with a spot of paint as usual, and at 7.30 replaced them in their own nest. At 8.0 I found 5 quite at home ; tlie others I could not see, but nono were attacked. js n ti " »» « »> ?> 5> 3> >J it i> J» JJ J> „ 8.30 »j 5 „ 9.0 » 3 „ 10.0 ?5 4 „ 11.0 ?5 5 „ 12.0 »5 3 „ 1.0 5> 3 „ 4.0 ?' 4 „ 7.0 1 „ 9.0 ., 2 * "Forel. Exp. et Rem. crit. sur les Sensations des Insectcs, Jiecueil Zool. Suisse., 1887. 238 EXPERIMENTS WITH DROWNED ANTS. Tlie next morning I could only see two, but none were being attacked, and there were no dead ones. It is probable that the paint had been cleaned off the others, but it was not easy to find them all among so many. At any rate, none were being attacked, nor had any been killed. These observations, therefore, quite confirm those previously made, and seem to show that if pupae are taken from a nest, kept till they become perfect insects, and then replaced in the nest, they are recognized as friends. As regards the mode of recognition, Mr. McCook considers that it is by scent, and states that if ants are more or less soaked in water, they are no longer recog- nized by their friends, but are attacked. He mentions a case in wliich an ant fell accidentally into some water: *• She remained in the liquid several moments, and crept out of it. Immediately she was seized in a hostile manner, first by one, and then another, then by a third, the two antenupe and one leg were thus held. A fourth one assaulted the middle thorax and petiole. The poor little bather was thus dragged helplessly to and fro for a long time, and was evidently ordained to death. Presently I took up the struggling heap. Two of the assailants kept their hold, one finally dropped ; the other I could not tear loose, and so put the pair back upon the tree, leaving the doomed immersionist to her hard fate." His attention having been called to this, he noticed several other cases, always with tlie same result. I have not myself been able to repeat the observation with the same species, but with two at least of our native ants the results were exactly reversed. In one EECOGNITION AFTER A YEAR AND NINE ]\IONTHS. 230 case five specimens of Lasius niger fell into water and remained immersed for three hours. I then took them out and put them into a bottle to recover themselves. The following morning I allowed them to return. They were received as friends, and, though we watched them from 7.30 till 1.30 every hour, there was not the slightest sign of hostility. The nest was, moreover, placed in a closed box, so that if any ant were killed we could inevitably find the body, and no ant died. In this case, therefore, it is clear that the immersion did not prevent them from being recognized. Again, three specimens of Formica fusca dropped into water. After three hours I took them out, and, after keeping them by themselves for the night to recover, I put them back into the nest. They were unquestionably received as friends, without the slightest sign of hostility or even of doubt. I do not, however, by any means intend to express the opinion that smell is not the mode by which recognition is effected. It will be remembered, perhaps, that my ants {For- mica fusca) recognized one another after a separation of a year and nine months, though '^ after some months' separation they were occasionally attacked, as some of the ants, perhaps the young ones, did not recognize them. Still, they were never killed or driven out of the nest, so that evidently when a mistake was made it v>^as soon discovered." Hence it would appear that there are differences in the memory of different species. In one case Forel liad taken some ants from a large nest of Comjoonotus, for the experiments on their sensibility to the ultra-violet rays, to which I have alreadv referred. After his observations were 240 SUPPOSED RECOGNITION BY SCENT. concluded, he returned them to the nest, some after eight, some after forty-one days. Those which were returned after eight days were at once recognized, while as regards those which had been forty-one days away from home. " On reculait de part et d'autre, se menapait des mandibules, s'examinait a fond avec les antennes, se mordait memo. Plusieurs meme allerent dans leur irritation jusqu' a essayer de decapiter et meme a decapiter quelques-nnes de leurs anciennes compagnes et soeurs avec leurs mandibules (c'est le mode de combat des Camj)onotus) ! Les fourmis vernies prirent part a ces rixes aussi bien que les non vernies ; je les vis meme attaquer, et elles etaient a peine moins adroites. Les combats ne cesserent entierement qu'au bout d'un on deux jours, et, a part les quelques victimes du premier jour, I'incident se tormina par une alliance." Forel seems to entertain no doubt that the recog- nition is effected by a form of smell, which he terms " odorat au contact." He says, " Beaucoup d'insectes ont en outre une sorte d'odorat au contact que nous ne possedons pas et qui permet entre autres aux fourmis de distinguer leurs compagnes de leurs ennemies." His observations, however, do not favour the hy- pothesis that the recognition may be by smell. If the ants recognized their companions by any odour characteristic of the community, the lapse of thirty days could not have made any difference. Here the question of memory would not enter, because the per- ception of the odour would in both cases be continually before them. M. Forel is so excellent an observer, and has so great a knowledge of the ways of ants, that his opinion is entitled to great weight. It EECOGNITION BY MEANS OF THE ANTENNiE. 241 would be very interesting to repeat similar observations, for if it turn out to be the case that separations of comparatively few days lead, in some species, to a want of recognition, it would be a strong argument against the hypothesis that this recognition is due to smell. It certainly seems as if the recognition was effected to a great extent by the antennae. Not only do the ants cross and recross them, almost, so to say, as two deaf mutes conversing by their fingers ; but, as M. Forel has shown, if ants of different species are brought together after the removal of their antennae they show no signs of hostility. That this latter statement is correct I am quite content to take on M. ForeFs authority; but it is not so conclusive as might seem at first sight, because in ants, as in men, " a fellow- feeling makes us wondrous kind," and ants when isolated, and especially when suffering, are much less pugnacious than they arc under normal conditions. ( 242 ) CHAPTER XII. ON THE INSTINCTS OF SOLITARY VfASPS AND BEES. The hive bee and the common wasps are so familiar and so interesting that they have to a great extent diverted attention from the so-called solitary species of the same groups. Few, for instance, are aware that about 4500 species of wild bees are known, and of wasps 1100, of which some 170 and 16 respectively live in Britain. These insects often live in association, but do not form true communities. Speaking generally, we may say that each female constructs a cell, every species having its own favourite site, sometimes underground, sometimes in a hollow stick, in an empty snail-shell, or built against a wall, a stone, or the branch of a tree. Ilaving completed her cell, the female stores up in it a sufficient supply of food, which in the case of bees consists of pollen and honey ; while the wasps select small animals, such as beetles, caterpillars, spiders, etc., each species generally having one kind of prey. The mother then lays an egg, after which she closes up the cell, and commences another. Having thus pro- vided sufficiently for her offspring, she generally takes no further heed of it. This is not, however, an invari- able rule : in the genus Bembex, for instance, the mother, instead of provisioning her cell once for all, brings food to the young grub from day to day. This, however, is an exceptional case, and the mode of life of the solitary wasps raises one of the most interesting questions in connection with instinct. The Ammophila, for instance, having built her cell, places in it, as food for her young, the full-grown caterpillar of a moth, Nocfiia segetum. Now, if the caterpillar were un- injured, it would strnggle to escape and almost inevit- ably destroy the egg ; nor would it permit itself to be eaten. On the other hand, if it were killed, it would decay and soon become unfit for food. The wasp, however,, avoids both horns of this dilemma. Having found her prey, she pierces with her sting the membrane between the head and the first segment of the body, thus nearly disabling the caterpillar, and then proceeds to inflict eight more wounds between the following segments; lastly crushing the head, and thus completely paralyzing her victim, but not actually killing it; so that it lies helpless and motionless, but, though living, let us hope insensible. M. Fabre, to whom we are indebted for a most interesting and entertaining series of essays on this group of insects, argues that this remarkable instinct cannot have been gradually acquired. The spots selected are, he says, exactly those occupied by the ganglia. ISTo others among the in- numerable points which might have been chosen would have answered the purpose; not one wound is mis- placed or without efiect. M. Fabre truly ob-rerves that chance offers no explanation.* Moreover, he unhesi- * In the case of other insects, such as Mutilla, Chrysis, Leucospi?, Anthrax, etc., which do not possess the instinct of paralyzing their victims, the young feed on tlie chrysalis, Aihich is normally without po^Yer of movement. 244 ORIGIN OF INSTINCTS. tatingly asserts that ''Si de son cote rhymenoptere excelle dans son art, c'est qu'il est fait pour I'exercer ; c'est qu'il est done, non senlement d'outils, mais encore de la maniere de s'en servir. Et ce don est originel, parfait des le debut ; le passe n'y a rien ajoute, I'avenir n'y ajoutera rien." * But how was it acquired ? M. Fabre cuts the Gordian knot. *' Et tout naivement je me dis : Puisqu'il faut des Araignees aux Pompiles, de tout temps ceux-ci ont possede leur patiente astuce et les autres leur sotte audace. C'est pueril, si Ton veut, peu conforme aux visees transcendantes des theories a la mode ; il n'y a la ni objectif ni subjectif, ni adapta- tion ni differentiation, ni attavisme ni transformisme ; soit, mais du moins je comprends." "Je comprends!" M. Fabre says he understands, and no doubt he thinks so ; but I confess that his explanation seems to me lo leave us just where we were. To my mind, I confess, it seems to me to throw no light whatever on the matter. M. Fabre asserts that the habits of these insects have been *''de tout temps" exactly what they are now. I pass by the fact that the Hymenoptera are, geologically speaking, of comparatively recent appearance. But is it the case that habits are so invariable ? Quite the reverse. The cases of variation are innumerable. Eomanesf refers to a criticism of the same nature by Eirby and Spence. "Why," they ask, "if instincts are open to modification by experience and inteUigeuce, are not bees sometimes found to use mud or mortar instead of wax or propolis ? Show us," they say, " but one instance of their having substituted mud for * J. H. Fabre, " Nouveaux Souvenirs Entomologiqiics." t " jNrent:il Evolution in Animals." HABITS NOT INVARIABLE. 245 propolir, . . . and there could be no doubt of their having been guided by reason." Such cases have, how- ever, been observed. Andrew Knight found that his bees collected some wax and turpentine with which he had covered some decorticated trees, and used it instead of propolis, the manufacture of which they discontinued. Nay, M. Fabre has himself placed on record some cases of the same kind, and shown that the instincts of these animals are not absolutely unalterable. Thus one solitary wasp, Si:)liex flaviioennis, which provisions its nest with small grasshoppers, when it returns to the cell, leaves the victim outside, and goes down for a moment to see that all is right. During her absence M. Fabre moved the grasshopper a little. Out came the Sphex, soon found her victim, dragged it to the mouth of the cell, and left it as before. Again and again M. Fabre moved the grasshopper, but every time the Sphex did exactly the same thing, until M. Fabre was tired out. All the insects of this colony had the same curious habit; but on trying the same experiment with a Sphex of the following year, after two or three dis- appointments she learned wisdom by experience, and carried the grasshopper directly down into the cell. Eumenes fomiformis builds, as already mentioned, a cell in the open air. If attached to a broad base, *' C'est un dome avec goulot central, evase en embou- chure d'urne. Mais quand I'appui se reduit a un point, sur un rameau d'arbuste par exemple, le nid devient une capsule spherique, surmontee tonjours d'un goulot, bien entendu." * Again, he has shown good reason for believing that, although the Tachjtes nigra generally makes its * Loc cit, p. 66. 246 CHANGE OF INSTINCTS— BEMBEX. own burrow and stores it with paralyzed prey for its own larvse to feed on, yet that, when this insect finds a burrow already made and stored by another Sphex, it takes advantage of the prize, and becomes for the occasion parasitic. On which Mr. Darwin has justly observed that he could see no difficulty in natural selection making an occasional habit permanent, if of advantage to the species, and if the insect whose nest and stored food are thus feloniously appropriated be not thus exterminated. The problem is certainly one of great difSculty, and it is with diffidence that I would suggest to M. Fabre certain considerations which may perhaps throw some light on it. Let us examine some of the other solitary wasps, and see whether their habits aftbrd us any clue. That an animal of prey knows where its victim is most vulnerable, has not in itself anything unusual or unaccountable. The genus Bembex kills the insects on which its young are fed, and supplies the cell with a fresh victim from time to time. Eumenes, like Ammophila and Sphex, stores up the victims once for all. They are grievously wounded, but not altogether paralyzed. Here, then, we have the very condition which M. Fabre considers would be fatal to the tender egg of the wasp. But not necessarily so. The wretched caterpillars lie in a wriggling mass at the bottom of the cell; a clear space is teft above them, and from the summit of the cell the delicate egg is suspended by a fine thread, so that, even if touched by a caterpillar in one of its con- vulsive struggles, it would simj^ly swing away in safety. When the young grub is hatched, it suspends itself to this tliread'by a silken sheath, in which it hangs liead ODYNERUS — AMMOPHILA. 247 downwards over its victims. Does one of them struggle ? quick as lightning it retreats up the sheath out of harm's way. In Odynerus the arrangement is very similar, but the grub simply attaches itself to the support, and does not construct a tube. Moreover, while in the solitary bees and w^asps the laying of the egg is generally the final operation before the closing of the cell, in Odynerus, on the contrary, or at least in Ochjnerus reniformis, the egg is laid before the food is provided. This, perhaps, may have reference to the different con- dition of the victims. According to Marchal,* Cerceris ornata practically kills her victim ; moreover, she stings it not iw, but between, the ganglia, and though the first sting is planted between the head and thorax, the following ones do not always follow the same order. At present the Ammopliila supplies each cell with one large caterpillar ; but w\as this always so ? One species of Odynerus deposits in each cell no less than twenty-four victims, another only eight. Eumenes Amedei regulates the number according to the sex : ten for the female grub, five only for the smaller male. Moreover, while phytophagous larvae will not gene- rally eat any plants but those to which they are accustomed, it has been proved that, as a matter of fact, these larvce will feed and thrive on other insects almost, if not quite, as well as on their natural food. Is it, then, impossible that in far bygone ages the larvae may have grown more rapidly, so that the victims had not time to decay; or that the ancestors * IMarchal, " Sur I'liistinct du Cerceris ornata" Arch. d. Zool. Exper., 1887. 248 MODIFIABILITY OF INSTINCTS. of our present Ammopliilas may have fed tlieir young from day to day with fresh food, as Bembex does even now ; that they may then have gradually brought the provisions at longer intervals, choosing small and weak victims, and layiug the egg in a special part of the cell, as Eumenes does? that during these long ages they may have gradually learnt the spots where tlieir sting would be most effective, and, thus saving themselves the trouble of capturing a number of victims, have found that it involved less labour to select a fine fat common caterpillar, such as that of Noctua segetum, and so have gradually acquired their present habits'? Wonderful doubtless they are; but, though I hint the suggestion with all deference, such a sequence does not seem to me to present any in- superable difficulty. This suggestion was made in the Contemporary Bevieiv for 1885, and I was much interested to fiud in Mr. Darwin's life that he had made a similar suggestion in a letter to M. Fabre. He refers to the great skill of the Gauchos in killing cattle, and suggests that each young Gaucho sees how the others do it, and with a very little practice learns the art. "I suppose that the sand-wasps originally merely killed their prey by stinging them in many places (see p. 129 of Fabre's * Souvenirs,' and p. 24.1), and that to sting a certain segment was found by far the most successful method, and was inherited like the tendency of a bulldog to pin the nose of a bull, or of a ferret to bite the cerebellum. It would not be a very great step in advance to prick the ganglion of its prey only slightly, and thus to give its larvae fresh meat instead of only dried meat." * * "Life and Letters of Charles Darwiu." DIFFERENCES UNDER DIFFERENT CIRCUMSTANCES. 249 Perhaps, however, it may be asked, Why should the insect change its habits ? Several reasons might be sug- gested. The prey first selected might be exterminated, or at any rate diminish in numbers, and, though each species as a general 'rule confines itself to one special victim, some exceptions have already been noticed. For instance, Spliex flavi^ennis habitually preys on a species of grasshopper, but on the banks of the Khone M. Fabre found it, on the contrary, attacking a field cricket, whether from the absence of the grasshopper or not he was unable to determine. ; Take another case. M. Fabre denies* that the different species of Sphex can ever have been derived from one source. Every species now, he observes, has some one victim, some one insect on which it preys, to which it restricts itself, and which tlie other species do not attack. But " Que chassait, je vous prie, ce proto- type des Sphegiens ? Avait il regime varie ou regime uniforme ? Ne pouvant decider, examinons les deux cas." He begins by supposing tliat with the ancestor of the Sphex, *' Le regime etait varie. J 'en felicite hautement ce premier ne des Sphex. II etait dans les meilleures conditions pour laisser descendance prospere." Is it likely then, he says, that they would have limited themselves to one prey, and thus have foolishly diminished their chances in life? "Mais non," he adds, in his lively style, " mes beaux Sphex, vous n'avez pas ete aussi idiots que cela. Si vous etes de nos jours can- tonnes chacun dans un mets de famille, c'est que votre ancetre ne vous a pas enseignd la variete." He then discusses the alternative whether the * " Souv. Entom., tioisieme serie." 250 ORIGIN OF THE HABITS OF SPHEX. ancestral Sphex restricted itself to one victim, and that its descendants ^^subdivises en groupes et con- stitues enfin en aiitant d'especes distinctes par le lent travail des siecles, se sont avises qu'en dehors dii comestible des ancetrcs il y avait ime foule d'autres aliments." This, he says, supposes that they experimented on various victims, found several of them to their liking, and then, after a period of varied and plentiful diet, voluntarily abandoned so great an advantage. "Avoir decouvert, par vos essais d'age en age, la variete de I'alimentation ; i'avoir pratiquee, au grand avantage de votre race, ct finir par I'uniformite, cause de decadence ; avoir connu I'exceilent et le repudier pour le mediocre, * Oh ! mes Sphex, ce serait stupid e si le transformisme avait raison. ' " " J'estime," then he concludes, " que votre ancetre commun, votre precurseur, a gouts simples ou bien a gouts multiples, est une pure chime re." Ko doubt the habits of Hymenoptera present many difficulties, and have undoubtedly many surprises in store for us, and I cannot think the matter is so clear as M. Fabre imagines, or that he has exhausted the possible cases. It is possible, though it is, I admit, only a supposition, that the ancestral Sphex hunted some species which does not now exist — at least not in the south of France — and which might have disappeared gradually. As it became rarer, they might be driven to attack other prey, and M. Fabre has himself shown by a variety of most ingenious experiments that the larvae are by no means fastidious as to their food. The Hymenoptera vary considerably in size, and the larger individuals might be able to overmaster some large KACE DIFFERENCES. 251 insect, while tlio feebler specimens were compelled to content themselves wuth humbler fare. This is no purely imaginary case. M. Fabre himself distinguishes three races — or are they species ? — of Leu- cospis which live on the three species of Chalicodomas. ** Venn du Chalicodome des galets ou des murailles, dont I'opulente larve le sature de nourriture, il merite par sa grosseur le nom le Leucospis gigas, que lui donne Fabricius ; venu du Chalicodome des hangars, il ne merite plus que le nom de Leucosins grandis, que lui octroie Klug. Avec uue ratiou moindre, le geant baisse d'un degre et n'est plus que le grand. Yenu du Chalicodome des avbustes, il baisse encore, et si quelque nomenclateur s'avisait de le qualifier, il n'aurait plus droit qu'au titre de mediocre. The Anthrax, again, differs considerably according to the species on which it has fed, those coming from the cocoons of Osmia tricornis being much larger from those from 0. cyanea. Or it might well happen that while the victim was from some cause or other, say for instance the absence of food elsewhere, limited to a particular district, the region beyond was suited to the ancestress Sphex. In that case, would she not naturally try whether she could not find some other suitable food ? Tliis again, is not a purely imaginary case. M. Fabre himself tells us that while '•' la Scolie interrompue avait pour gibier aux environs d'Avignon, la larve de I'Anoxie velue {Anoxia villosa). Aux environs de Serignan, dans un sol sablon- neux semblable, sans autre vegetation que quelques maigres gramens, je lui trouve pour vivres I'Anoxie matutinale (Anoxia matutinalls), qui remplace ici la velue," 252 POWER OF DETERMINING SEX. That bees soon take to newly introduced flowers is a familiar case wliicli every one must have noticed, and which it is surely not logical to dismiss by the conve- nient process of referring it to " instinct." It is indeed difficult for any one who watches these insects to deny to bees the possession of a higher and conscious faculty. In considering the question whether these remarkable instincts were originall}^, so to say, engrafted in the insect, or whether they were the result of innumerable repetitions of similar actions carried on by a long series of ancestors, we may perhaps be aided by the consideration that, though the results would in either case be in many respects the same, there are some in which they would altogether differ. In the former, for instance, we might expect that the insect would be so gifted that no slight obstacle should interfere with the great end in view : in the latter, on the contrary, the very repetition which gave such remarkable results would tend to incapacitate the insect from dealing with any unusual conditions. Limitation of Instinct. We should, in fact, find side by side with these won- derful instincts almost equally surprising evidence of stupidity. Now, one species of Sphex preys on a large grasshopper (Ephippigera). Having disabled her vic- tim, she drags it along by one of the antennae, and M. Fabre found that if the antennae be cut off close to the head, the Sphex, after trying in vain to get a grip, gives the matter up as a bad job, and leaves her victim in despair, without ever thinking of dragging it by one of its legs. Again, when a Sphex had provisioned her cell, laid her egg, and was about to close it up, M. LIMITATION OF INSTINCT. 253 Fabre drove her away, and took out both the Ephippi- gera and the egg. He then allowed the Sphex to return. She went down into the empty cell, and though she must have known that the grasshopper and the egg were no longer there, yet she proceeded calmly to stop up the orifice just as if nothing had happened. The genus Sphex paralyzes its victims and provisions its cell once for all. Bembex, on the contrary, as already mentioned, kills the insects on which its young are to feed, and, perhaps on this account, brings its young fresh food (mainly flies) from time to time. But while the Bembex thus preys on some flies, there are others which avenge their order. The genus Miltogramma lays its eggs in the cell of the Bembex ; and, though there seems no reason why the Bembex, which is by far the stronger insect, should tolerate this intrusion, which, moreover, she shows unmistakably to be most unpalatable, she never makes any attack on her enemy. Nay, when the young of the Miltogramma are hatched, so far from being killed or removed, these entomological cuckoos are actually fed until they reach maturity. ISTevertheless, it seems contrary to etiquette for the fly to enter the cell of the Bembex ; she watches the opportunity when the latter is in the cell and is dragging down the victim. Then is the Miltogramma's oj)portunity ; she pounces on the victim, and almost instantaneously lays on it two or three eggs, which are then transferred, with the insect on which they are to feed, to the cell. It is remarkable how the Bembex remembers (if one may use such a word) the entrance to her cell, covered as it is with sand, exactly to our eyes like that all round. On the other hand, M. Fabre found that if he 254 TOLERATION OF PARASITES. removed the surface of the eavtii and the passage, exposing the cell and the larva, the Bembex was quite at a loss, and did not even recognize her own offspring. It seems as if she knew tlie door, the nursery, and the passage, but not her child. Another ingenious experiment of M. Fabre's was made with a mason bee (Chalicodoma). This genus constructs an earthen cell, through which at maturity the young insect eats its way. M. Fabre found that if he pasted a piece of paper ronnd the cell, the insect had no difficulty in eating through it ; but if he enclosed the cell in a paj^er case, so that there was a space even of cnly a few lines betvveen the cell and the paper, in that case the paper formed an effectual prison. The instinct of the insect taught it to bite through one enclosure, but it had not wit enough to do so a second time. One of the most striking instances of stujDidity (may I say) is mentioned by M. Fabre, in the case of one of his favourite bees, the Chalicodoma pjrenaica. This species builds cells of masonry, which she fills with honey as she goes on, raising the rim a little, then making a few journeys for honey, then raising the rim again, and so on until the cell is completed. She then prepares a last load of mortar, brings it in her mandibles, lays her egg, and immediately closes up the cell ; having doubtless provided the mortar beforehand, lest during her absence an enemy should destroy the egg or any parasitic insect should gain admittance. This being so, M. Fabre chose a cell which was all but finished, and during the absence of the bee he broke away part of the cell-covering. Again, in some half- finished cells he broke away a little of the wall. In all these cases the bee, as might be expected, repaired CASES OF APPARENT STUPIDITY. 255 the mischief, the operation being in the natural order of her work. But now comes the curious fact. In another series of cells M. Fabre pierced a hole in the cell below the part where the bee was working, and through which the honey at once began to exude. The poor stupid little bee, howeyer, never thought of repairing the breach. She worked on as if nothing had happened. In her alternate journeys she brought first mortar and then honey, which, however, ran out again as fast as it was poured in. This experiment he repeated over and over again with various modifications in detail, but always with the same result. It may be suggested that possibly the bee was unable to stop up a hole once formed. But that could not have been the case. M. Fabre took one of the pellets of mortar brought by tlio bee, and successfully stopped the hole himself. The omission, therefore, was due, not to a want of power, but of intellect. But M. Fabre carried his experiment still further. Perhaps the bee had not noticed the inj ury. He chose, therefore, a cell which was only just begun and contained very little honey. In this he made a comparatively large hole. The bee returned with a supply of honey, and, seeming much surprised to find the hole in the bottom of the cell, examined it carefully, felt it with her antennae, and even pushed them through it. Did she then, as might uaturally liave been expected, stop it up ? iNot a bit. The unexpected catastrophe transcended the range of her intellect, and she calmly proceeded to pour into this vessel of the Danaides load after load of honey, which of course ran out of the bottom as fast as she poured it in at the top. All the afternoon she laboured at this fruitless task, and began again undiscouraged the next morning. At length, when she 256 M. FABRE'S EXPERIMENTS. had brought the usual complement of honey, she laid her egg, and gravely sealed up the empty cell. In another case, he made a large hole in the cell just above the level of the honey — a hole so large that through it he was able to see the bee lay her egg. Having done so, she carefully closed the top of the cell, but though she closely examined the hole in the side, it did not enter into the range of her ideas that such an accident could take place, and it never occurred to her to cover it up. Another curious point raised by these ingenious experiments has reference to the quantity of hone}^ The cell is by no means filled ; a space is always left between the honey and the roof of the cell. The usual depth of the honey in a completed cell is ten milli- metres. But the bee is not guided by this measure- ment, for in the preceding cases she sometimes closed the cell when the honey had a depth of only five milli- metres, of three, or even when the cell was almost empty. No ; in some mysterious manner the bee feels when she has provided as much honey as her ancestress had done before her, and regards her work as accomplished. What a wonderful, but what a narrow, nature ! She has built the cell and provided the honey, bat there her instinct stops : if the cell is pierced, if the honey is removed, it does not occur to her to repair the one or fill up the other. M. Fabre not unnaturally asks, " Avec la moindre lueur rationnelle, I'insecte deposerait- il son oeuf sur le tiers, sur le dixieme des vivres neces- saires ; le deposerait-il dans une cellule vide ; laisserait- il le nourrisson sans nourriture, incroyable aberration de la maternite? J'ai raconte, que le lecteur decide." The family of bees is generally reckoned to be one of great intelligence, but these and many other similar LIMITATION OF INSTINCT. 257 instances which might be recorded seem to show great limitation of intelligence. Let me give one other, which any person may easily test for himself. I took a glass shade or jar eighteen inches long, and with a mouth six and a half inches wide, turning the closed end to the window, and put in a common hive bee. She buzzed about for an hour, when, as there seemed no chance of her getting out, I put her back into the hive. Two flies, on the contrary, which I put in with h'er, got out at once. Again I put another bee and a fly into the same glass ; the latter flew out at once. For half an hour the bee tried to get out at the closed end ; I then turned the glass wdth its open end to the light when she flew out at once. To make sure, I rejoeated the experiment once more, wdth the same result. And yet there is, no doubt, ample foundation for the ordinary view which attributes considerable intelligence to the bse, within the sphere of her own operations. Several other poiLts of resemblance between instincts and habits could be pointed out. As in repeating a well-known song, so in instincts, one action follows another by a sort of rhythm. If a person be interrupted in a song, or in repeating anything by rote, he is often forced to go back to recover the habitual train of thought; so P. Huber found it was with a caterpillar, which makes a very complicated hammock; for if he took a caterpillar which had completed its hammock up to, say, the sixth stage of construction, and put it into a hammock completed up only to the third stage, the caterpillar simply re-performed the fourth, fifth, and sixth stages of construction. "If, how- ever, a caterpillar were taken out of a hammock made 258 INSTINCTS AND HABITS. up, for instance, to the third stage, and were put into one finished up to the sixth stage, so that much of its work was already done for it, far from feeling the benefit of this, it was much embarrassed, and, in order to complete its hammock, seemed forced to start from the third stage, where it had left off, and thus tried to complete the already finished work."* Another very interesting series of observations which ViO owe to M. Fabre has reference to the question of sex, and it would really seem that the mother can regulate the sex of the egg at will. In many of our wild bees, the females are much larger than the males. The male lives a life of pleasure, idle but short. " Quinze jours de bombance dans un magasin a miel, un an de sommeil sous terre, une minute d'amour au soleil, puis la mort." But the female " C'est la mere, la mere seule qui, peniblement, creuse sous terre des galeries et des cellules, petrit le stuc pour enduire les logos, ma9onne la demeure de ciment et de graviers, taraude le bois et subdivise le canal en etages, deeoupe des rondelles de feuilles qui seronf assemblees en pots a miel, malaxe la resine cueillie en larmes sur les blessures des pins pour edifier des voutes dans la rampe vide d'un es- cargot, chasse la proie, la paralyse et la traine au logis, cueille la poussiere pollinique, elabore le miel dans son jabot, emmagasine et mixtionne la patee. Ce rude labour, si imperieux, si actif, dans lequel se depense toute la vie de I'insecte, exige, c'est evident, une puissance corporelle bien inutile au male, I'amou- leux desoeuvre." In the hive bee the drone cells differ materially in shape from those of the queens and workers. * Darwin, " Origin of Species." INFLEXIBILITY OF INSTINCT. 259 In the solitary wasps, where the females are much larger than the males, the mother builds a larger cell and provides more food for the former than for the latter. The Chalicodoma (one of the mason bees) often lays her eggs in old cells of the previous year. These are of two sizes — large ones, originally built for the females, and small ones for the males. Now, in utilizing old cells, the bee always places male eggs in male cells and female eggs in female cells. If, how- ever, a female cell be cut down so as to reduce the size, then indeed the bee deposits in it a male egg. The bees belonging to the genus Osmia* arrange their cells in a row in a hollow stick, or some other similar situation, and it has long been known that in these and similar cases the cells first provisioned, and which are therefore furthest from the entrance, always contain females, while the outer cells always contain males. There is an obvious advantage in this, because the males come out a fortnight or more before the females, and it is, of course, convenient that those which have to come out first should be in the cells nearest the door. The bee does not, however, lay all the female eggs first, and then all the male eggs. By no means. She produces altogether from fifteen to thirty eggs, but seldom arranges them in one row ; generally they are in several series, and in every one the same sequence occurs — females further from, and males nearest to, the door. For instance, one of M. Fabre's marked bees — one, moreover, of exceptional fertility — occupied some glass * Osmia tridentata constitutes an exception to the general rule in this respect, as in some others. 260 DIFFERENT HABITS OF MALES AND FEMALES. tubes, which he arranged conveniently for her. From the 1st to the 10th of May she constructed, in one tube, eight cells — first seven female, and then one male. From the 10th to the 17th, in a second tube, she built first three female and then three male cells ; from the 17th to the 25th, in a third, three female and thea two male; on the 26th, in a fourth, one female; and, finally, from the 26th to the 30th, in a fifth, two female and three male : altogether twenty-five, seventeen female and eight male cells. The advantage of this is clear, but the manner in which it is secured is not so obvious. It might be suggested that the quantity of food was not regulated by the sex of the young one, but that the sex depended on the quantity of food. This would be very improb- able, and M. Fabre attempted to disprove it by some very ingenious experiments. He found that if he took some of the food from a female cell, the bee or wasp produced was still a female, though a starveling; while if he added food to a male cell, the larva still pro- duced a male, though a very large and fine one. M. Fabre then made some of his most ingenious experiments. He brought into his room a large number of cocoons of Osmia. When the perfect insects were about to emerge, he arranged for them a number of glass tubes, of which the Osmias gladly availed them- selves, and in which they proceeded to construct their cells. The usual arrangement, as already mentioned, is that the males are placed nearest to, and the female furthest from, the door. But M. Fabre so arranged the tubes that each was in two parts, an outer wider portion having a diameter of eight to twelve milli- metre's, which is sufficient fur a female cell; and an ARRANGEMENT OF MALE AND FEMALE CELLS. 231 inner narrower portion with a diameter of five to five and a half millimetres, which is too small for a female, but just large enougli for a male. This arrangement placed the Osmias in a difficulty. They could not follow their natural instinct and construct at the end of the tube cells large enough for females. What happened ? Some of the Osmias shut off the narrow ends, and used only the outer wider portion. Others, reluctant, as it were, to throw away a chance, built also in the narrow part of the tube, and under these circumstances, contrary to the otherwise invari- able rule, the inner and first constructed cells contained males. M. Fabre concludes then, and it seems to me has given very strong reasons for thinking so, that these privileged insects not only know the sex of the insect which will emerge from the egg they are about to lay, but that at their own will they can actually control it ! Certainly a most curious and interesting result ! He concludes'^ his charming work as follows : — " Mes chers insectes, dont I'etude m'a soutenu et continue a me soutenir au milieu de mes plus rudes epreuves, il faut ici, pour aujourd'hui, se dire adieu. Autour de nioi les rangs s'eclaircissent et les longs espoirs ont fui. Pourrai-je encore parler de vons ? " and every lover of nature will, I am sure, echo the wish. ( 262 ) CHAPTER XII r. ON THE SUPPOSED SENSE OF DIRECTION. One of the most interesting questions connected with the instincts and powers of animals has reference to the manner in which they fiad their way back, after having been carried to a distance from, home. This has by some been attributed to the possession of a special " sense of direction." Mr. Darwin suggested that it would be interesting to try the effect of putting animals " in a circular box ^^■ith an axle, which could be made to revolve very rapidly, first in one direction and then in another, so as to destroy for a time all sense of direction in the insects. I have sometimes," he said, "imagined that animals may feel in which direction they were at the first start carried." In fact, in parts of France it is considered that if a cat is carried from one house to another in a bag, and the bag is whirled round and round, the cat loses her direction and cannot return to her old home. On this subject M. Fabre has made some interesting and amusing experiments. He took ten bees belonging to the genus Chalicodoma, marked them on the back with a spot of white, and put them in a bag. He then carried them half a kilometre in one direction, stopping at a point where an old cross stands by the EXPERIMENTS WITH BEES. 263 wayside, and whirled the bag rapidly round his head. AVhile he was doing so a good woman came by, who was not a little surprised to find the professor stand- ing in front of the old cross, solemnly whirling a bag- round his head, and, M. Fabre fears, strongly suspected liim of some sfttanic practice. However this may be, M. Fabre, having sufficiently whirled his bees, started off back in the opposite direction, and carried his prisoners to a distance from their home of three kilometres. Here he again whirled them round, and then let them go one by one. They made one or two turns round him, and then flew off in the direction of home. In the meanwhile his daughter Antonia was on the watch. The first bee did the mile and three-quarters in a quarter of an hour. Some hours after two more re- turned ; the other seven did not reappear. The next day he repeated this experiment with ten other bees. The first returned in five minutes, and two more in about an hour. In this case, again, seven out of ten failed to find their way home. In another experiment he took forty-nine bees. When let out, a few started wrong, but he says that *'lorsque la rapidite du vol me laisse reconnaitre la direction suivie; " the great majority flew homewards. The first arrived in fifteen minutes. In an hour and a half eleven had returned, in five hours six more, making seventeen out of forty-nine. Again he experi- mented with twenty, of which seven found their way home. In the next experiment he took the bees rather further — to a distance of about two and a quarter miles. In an hour and a half two had returned, in three hours and a half seven more ; total, nine out of forty. Lastly, he took thirty bees: fifteen marked rose he took by 264 WHIRLING BEES. a roundabout route of over five miles ; the other fifteen marked blue ho sent straight to the rendezvous, about one and a half miles from home. All the thirty were let out at noon ; by five in the evening seven '* rose " bees and six "blue" bees had returned, so that the long detour had made no appreciable difference. These experiments seem to M. Fabre conclusive. " La demonstration," he says, " est suffisante. Ni les mouve- ments enchevetres d'une rotation comme je I'ai decrite; ni I'obstacle de collines a franchir et de bois a traverser; ni les embuches d'une voie qui s'avance, retrograde et revient par un ample circuit, ne peuvent troubler les Chali- codomes depayses et les empecher de revenir au nid." * I am not ashamed to confess that, charmed by M. Fabre's enthusiasm, dazzled by his eloquence and ingenuity, I was at first disposed to adopt this view. Calmer consideration, however, led me to doubt, and though M. Fabre's observations are most ingenious, and are very amusingly described, they do not carry conviction to my mind. There are two points specially to be considered — 1. The direction taken by the bees when released. 2. The success of the bees in making good their return home. As regards the first point, it will be observed that the successful bees were in the following proportion, viz. : — 3 out of 10 4 „ 10 17 „ 40 7 ,, 20 9 „ 40 7 „ 15 Or altogctlicr 47 „ 14 1 * J. II. Fabre, " Nouvcanx Souvenirs Eutomologiques." BEHAVIOUR OF BEES IF TAKEN FEOM HOME. 265 This is not a very large proportion. Oat of tlie M'hole number no less than ninety-seven appear to have lost their way. May not the forty-seven have found theirs by sight or by accident ? Instinct, how- ever inferior to reason, has the advantage of being generally unerring. AVhen two out of three bees went wrong, we may, I think, safely dismiss the idea of instinct. Moreover, the distance from home was only one and a half to two miles. Now, bees certainly know the country for some distance round their home ; how far they generally forage I believe we have no certain information, but it seems not unreasonable to suppose that if they once came withiQ a mile of their nest they would find themselves within ken of some familiar landmark. Now, if we suppose that 150 bees are let out two miles from home, and that they flew away at random, distributing themselves equally in all directions, a little consideration will show that some iwenty-five of them would find themselves within a mile of home, and consequently would know where they were. I have never myself experimented with Chalicodomas, but I have observed that if a hive bee is taken to a distance, she behaves as a pigeon does under similar circumstances ; that is to say, she flies round and round, gradually rising higher and higher and enlarging her circle, until I suppose her strength fails or she comes within sight of some known object. Again, if the bees had returned by a sense of direction, they would have been back in a few minutes. To fly one and a half or two miles would not take five minutes. One bee out of the 147 did it in that time ; but the others tcok one, two, three, or even five hours. Surely, then, it is reasonable to suppose that these lost some time before 266 BIODE OF FINDING THEIR WAY. they came in sight of any object lvno^vu to them. The second result of M. Fabre's observations is not open to these remarks. He observes that the great majority of his Chalicodomas at once took the direction home. He confesses, however, in the sentence I have already quoted, that it is not always easy to follow bees with the eye. Admitting the fact, however, it seems to me far from impossible that the bees knew where they were ; and, at any rate, this does not seem so improbable that we should be driven to admit the existence of a new sense, which we ought only to assume as a last resource. Moreover, M. Fabre himself says, " Lorsque la rapidite du vol me laisse reconnaitre la direction suiyie," which seems to imply a doubt. Indeed, some years previously he had made a similar experiment with the same species, but taking them direct to a point rather over two miles (four kilometres) from the nest, and not whirling them round his head. I looked back, there- fore, to his previous work to see how these behaved, and I found that he says — "Aussitot libres, les Chalicodomes fuient, comma effares, qui dans une direction, qui dans la direction tout opposee. Autant que le permet leur vol fougueux, je crois neanmoins reconnaitre un prompt retour des abeilles lancees a I'oppose de leur demeure, et la majorite me semble se diriger du cote de I'horizon oil se trouve le nid. Je laisse ce point avec des doutes, que rendent inevitables des insectes perdus de vue a une vingtaine de metres de distance." In this case, then, some went in one direction, some in another. It certainly would be remarkable if bees which were taken direct missed their way, while those EXPERIMEXTS WITH ANTS. 267 which were \^•lli^led round and round went straiglit home. Moreover, it appears that after all, as a matter of fact, they did not fly straight home. If they liad doue so they would have been back in three or four minutes, whereas they took far longer. Even then, if they started in the right direction, it is clear that they did not adhere to it. 1 have myself tried experiments of the same kind with hive bees and ants. For instance, I put down some honey en a piece of glass close to a nest of Lashis niger, and wlien the ants were feeding I placed it quietly on the middle of a board one foot square, and eighteen inches from the nest. I did this with thirteen ants, and marked the points at which they left the board. Five of them did so on the half of the board nearest the nest, and eight on that turned away from it. I then timed three of them. They all found the nest eventually, but it took them ten, twelve, and twenty minutes respectively. Again, I took forty ants which were feeding on some honey, and put them down on a gravel-path about fifty yards from the nest, and in tlie middle of a square eighteen inches in diameter, vrhich I marked out on the path by strav/s. I prepared a corresponding square on paper, and, having indicated by the arrow the direction of the nest, I marked down the spot where each ant passed the boundary. They crossed it in all directions; and dividing the square into two halves, one towards the nest and one away from it, the number in each were almost exactly the same. After leaving the square, they wandered about with every appearance of having lost themselves, and crossed 268 MR. ROMANES' EXPERIMENTS. the boundary backwards and forwards in all directions. Two of tliem, however, we watched for an hour each. They meandered about, and at the end of the time one was about two feet from where she started^ but scarcely any nearer liome; the other about six feet away, and nearly as much further from home. I then took them up and replaced them near the nest, which tljey at once joyfully entered. I mentioned some of the foregoing facts in a paper which I read at the meeting of the British Association at Aberdeen, and they have since been confirmed by Mr. Eomane?.* " In connection," he says, " with Sir John Lubbock's paper at the British Association, in which this subject is treated, it is perhaps worth while to describe some experiments which I made last year. The question to be answered is whether bees find their way home merely by their knowledge of landmarks, or by means of some mysterious faculty usually termed a sense of direction. The ordinary impression appears to have been that they do so in virtue of some such sense, and are therefore independent of any special knowledge of the district in which they may be suddenly liberated ; and, as Sir John Lubbock observes, this impression was corroborated by the experiments of M. Fabre. The conclusions drawn from these experiments, however, appeared to me, as they appeared to Sir John, un- warranted by the facts ; and therefore, like him, I re- peated them with certain variations. In the result I satisfied myself that the bees depend entirely upon their special knowdedge of district or landmarks, and it is because my experiments thus fully corroborate those * Nature, October 29, 1886. I MR. ROMANES' EXPERIMENTS. 269 which were made by Sir John that it novv' occurs to me to publish them. "The house where 1 conducted the observations is situated several hundred yards from the coast, with flower-gardens on each side, and lawns between the house and the sea. Therefore bees starting from the house would find their honey on either side of it, while the lawns in front would be rarely or never visited — being themselves barren of honey, and leading only to the sea. Such being the geographical conditions, I placed a hive of bees in one of the front rooms on the basement of the house. When the bees became thoroughly well acquainted with their new quarters by flying in and out of the open window for a fortnight, I began the experiments. The modus oioevancli consisted in closing the window after dark when all the bees were in their hive, and also slipping a glass sliutter in front of the hive door, so that all the bees were doubly im- prisoned. Next morning I slightly raised the glass shutter, thus enabling any desired number of bees to escape. When the desired number had escaped, the glass shutter was again closed, and all the liberated bees were caught as they buzzed about the inside of the shut window. These bees were then counted into a box, the window of the room opened, and a card well smeared over with birdlime placed ujDon the threshold of the beehive, or just in front of the closed glass shutter. The object of all these arrangements was to obviate the necessity of marking the bees, and so to enable me not merely to experiment with ease upon any number of individuals that I might desire, but also to feel confident that no one individual could return to the hive un- noticed. For whenever a bee returned it was certain 270 MR. EOMANES' EXPERIMENTS. to become entangled iu the bird-lime, and whenever I found a bee so entangled, I was certain that it was one which I had taken from the hive, as there were no other liives in the neighbourhood. " Such being the method, I began by taking a score of bees in the box out to sea, v/here tliere could be no land- marks to guide the insects home. Had any of these insects returned, I should next have taken another score out to sea (after an interval of several days, so as to be sure that tlie first lot had become permanently lost), and then, before liberating them, have rotated the box in a sling for a considerable time, in order to see whether this would have confused their sense of direction. But, as none of the bees returned after the fi;-st experiment, it was clearly needless to proceed to the second. Ac- cordingly, I liberated the next lot of bees on the sea- shore, and, as none of these returned, I liberated another lot on the lawn between the shore and the house. I was somewhat surprised to find that neither did any of these return, although the distance from the lawn to the hive was not above two hundred yards. Lastly, I liberated bees in different parts of the flower-garden, and these I always found stuck upon the bird-lime within a few minutes of their liberation. Indeed, they often arrived before I had had time to run from the place where I had liberated them to the hive. Now, as the garden was a large one, many of these bees had to fly a greater distance, in order to reach the hive, than was the case with their lost sisters upon the lawn, and therefore I could have no doubt that their imiform success in finding their way home so immediately was due to their special knowledge of the flower-garden, and not to any general sense of direction. NO EVIDENCE OF SEPARATE SENSE OF DIRECTION. 271 " I may add that, while in Germany a few weeks ago, I tried ou several species of ant the same experimoDts as Sir John Lubbock describes in his paper as having been tried by him upon English species, and here also I obtained identical results; in all cases the ants were hopelessly lost if liberated more than a moderate dis- tance from their nest. M. Komanes' experiments, therefore, as he himself says, entirely confirm tlie opinion I have ventured to express — that there is no sufficient evidence among insects of anything which can justly be callel a '• sense of directi n." ( 272 ) CHAPTER XIV. ON THE INTELLIGENCE OF THE DOG. Considering the long ages during which man and the other animals have shared this beautiful world, it is surely remarkable how little we know about them. We have recently had various interesting works on the intelligence and senses of animals, and yet I think the principal impression ^\hich they leave on the mind is that we know very little indeed on the subject. The Dog. As to the intelligence of the dog, a great many people, indeed, seem to me to entertain two entirely opposite and contradictory opinions. I often hear it said that the dog, for instance, is very wise and clever. But when I ask whether a dog can realize that two and two make four, which is a very simple arithmetical calculation, I generally find much doubt expressed. That the dog is a loyal, true, and affectionate friend must be gratefully admitted, but when we come to con- sider the psychical nature of the animal, the limits of our knowledge are almost immediately reached. I have else- where suggested that this arises in great measure from the fact that hitherto we have tried to teach animals, rather than to learn from them— --to convey our ideas to EDUCATION OF THE DEAF AND DUMB. 273 them, rather than to devise any language or code of signals by means of which they might communicate theirs to us. The former may be more important from a utilitarian point of view, though even this is questionable, but psychologically it is far less interest- ing. Under these circumstances, it occurred to me whether some such system as that followed with deaf mutes, and especially by Dr. Howe w^ith Laura Bridg- man, might not prove very instructive if adapted to the case of dogs. A very interesting account of Laura Bridgman has been published by Wright, compiled almost entirely from rej)orts of the Perkins Institution, and the Massachusetts Asylum for the Blind, in which Dr. Howe, the director of the establishment, details the history of Laura Bridg- man, who was deaf, dumb, and blind, almost without the power of smell and taste, but who, nearly alone among those thus grievously afflicted, possessed an average, if not more than an average, amount of intelligence, although, until brought under Dr. Howe's skilful treat- ment and care, her physical defects excluded her from all social intercourse. Laura Bridgman was born of intelligent ana respect- able parents, in Hanover, New Hampshire, U.S., in December, 1829. She is said to have been a sprightly, pretty infant, but subject to fits, and altogether very fragile. At two years old she was fairly forward, had mastered the difference between A and B, and, indeed, is said to have displayed a considerable degree of intelligence. She then became suddenly ill, and had to be kept in a darkened room for five months. When she recovered she was blind, deaf, and had nearly lost the power both of smell and taste. 274 LAURA BEIDGMAN. *'What a situation was hers! The darkness and silence of the tomb were around her ; no mother's smile gladdened her heart, or ' called forth an answering smile;' no father's voice taught her to imitate his sounds. To her, brothers and sisters were but forms of matter, which resisted her touch, but which differed not from the furniture of the house, save in warmth and in the power of locomotion, and in these respects not even from the dog or cat." Her mind, however, was unaffected, and the sense of touch remained. *' As soon as she was able to walk, Laura began to explore the room, and then the house ; she became familiar with the form, density, weight, and heat of every article she could lay her hands on. "She followed her mother, felt her hands and arms, as she was occupied about the house, and her disposi- tion to imitate led her to repeat everything herself. She even learnt to sew a little, and to knit. Her affectioiis, too, began to expand, and seemed to be lavished upon the members of her family with peculiar force. '•'The means of communication with her, however^ were very limited. She could only be told to go to a place by being pushed, or to come to one by a sign of drawing her. Patting*^ her gently on the head signified aj)probation ; on the back, the contrary." The power of communication was thus most limited, and her character began to suffer, when fortunately Dr. Howe heard of her, and in October, 1837, received her into the institution. "Eor a while she was much bewildered, till she became acquainted with her new locality, and somewhat familiar with the inmates ; the attempt was made to give her LAURA BRIDGMAN. 275 knowledge of arbitrary signs, by which she could interchange thoughts with others. ''The first experiments were made by taking the articles in common use, such as knives, forks, spoons, keys, etc., and pasting upon them labels, with their names embossed in raised letters. These she felt carefully, and soon, of course, distinguished that the crooked lines s-p-o-o-n differed as much from the crooked lines k-e-y, as the spoon differed from the key in form. Then small detached labels with the same words printed upon them were put into her hands; she soon observed that they were the same as those pasted upon the articles. She showed her perception of this similarity by laying the label k-e-y upon the key, and the label s-p-o-o-n upon the spoon. " Hitherto, the process had been mechanical, and the success about as great as that of teaching a very know- ing dog a variety of tricks. "The poor child sat in mute amazement, and patiently imitated everything Ler teacher did. But now her intellect began to work, the truth flashed upon her, and she perceived that there was a way by which she could herself make a sign of anything that was in her own mind, and show it to another mind. At once her countenance lighted up with a human expression. It was no longer as a mere instinctive animal ; it was an immortal spirit, eagerly seizing upon a new link of union with other spirits. I could almost fix upon the moment when this truth dawned upon her mind, and spread its beams upon her countenance ; I saw that the great obstacle was overcome, and that henceforth nothing but patient and persevering, but plain and straightforward, efforts were necessary. 276 ArPLICATiON OF THE METHOD FOLLOWED ''The result, tliiis far, is quickly related and easily conceived ; but not so was the process, for many weeks of apparently unprofitable labour were spent before it was effected. " The next step was to procure a set of metal typss, with the different letters of the alphabet cast separately on their ends; also a board, in which were square holes, into which slie could set the types, so that the letters could alone be felt above the surface. " Thus, on any article being handed to her, as a pencil or watch, she would select the component letters and arrange them on the board, and read them with apparent pleasure, assuring her teacher that she understood by taking all the letters of the word and putting them to her ear, or on the pencil." It is unnecessary, from my present point of view, to carry the narrative further, interesting as it is. I will only observe that even in the case of Laura Bridgman the process was one of much difficulty and requiring great patience. For a long while it was found im- possible to make her realize the use of adjectives ; she could not "understand any general expression of quality." Again, we are told that " Some idea of the difficulty of teaching her common expressions may be derived from the fact that a lesson of two hours upon the words 'right' and 'left' was deemed very profitable if she had in that time really mastered the idea." Now, it seemed to me that the ingenious method devised by Dr. Howe, and so successfully carried out in the case of Laura Bridgman, might be adapted to the case of dogs, and I have tried this in a smalj way with a black poodle named Van. WITH THE DEAF AND DUMB TO ANIMALS. 277 Van and his Caeds. I took two pieces of carlboarJ about ten inches by three, aud on one of them printed in large letters tlie word FOOD leaving the other blank. I then placed the two cards over two saucers, and in the one under the "food" card put a little bread and milk, which Van, after having his attention called to the card, was allowed to eat. This was repeated over and over again till he had had enough. In about ten days he began to distinguish between the two cards. I then put them on the floor and made him bring them to me, which he did readily enough. When he brought the plain card I simply threw it back, while when he brought the " food " card I gave him a piece of bread, and in about a month he had pretty well learned to realize the difference. I then had some other cards printed with the words "out," "tea," "bone," "water," and a certain number also with words to which 1 did not intend him to attach any significance, such as "nought," "plain," "ball," etc. Van soon learned that bringing a card was a request, and soon learned to distinguish between the plain and printed cards; it took him longer to realize the difference between words, but he gradually got to recognize several, such as " food," " out," " bone," " tea," etc. If he was asked whether he would like to go out for a walk, he would joyfully fish up the "out" card, choosing it from several others, and bring it to me, or run with it in evident triumph to the door. 278 MY DOG VAN. I need hardly say that the cards were not always put in the same places. They were varied quite indiscrimi- nately and in a great variety of positions. ^Ror could the dog recognize them by sccDt. They were all alike, and all continually handled by us. Still, I did not trust to that alone, but had a number printed for each word. When, for instance, he brought a card with " food " on it, we did not put down the same identical card, but another bearing the same word ; when he had brought that, a third, then a fourth, and so on. For a single meal, therefore, eighteen or twenty cards would be used, so that he evidently is not guided by scent. No one who has seen him look down a row of cards and pick up the one he wanted could, I think, doubt that in bringing a card he felt that lie is making a request, and that he could not only distinguish one card from another but also associate the word and object. I used to leave a card marked " water" in my dress- ing-room, the door of which we used to pass in going to or from my sitting-room. Van was my constant companion, and passed the door when I was at home several times in the day. Generally he took no heed of the card. Hundreds, or I may say thousands, of times he passed it unnoticed. Sometimes, however, he would run in, pick it up, and briug it to me, when of course I gave him some water, and on such occasions I invariably found that he wanted to drink. I might also mention, in corroboration, that one morning he seemed unwell. A friend, being at break- fast with us, was anxious to see him bring his cards, and I therefore pressed him to do so. To my surprise he brought three dummy cards successively, one marked COMMUNICATION BY MEANS OF CARDS. 270 " ham," one " bag," and one " brush." I said re- .proacbfally, *' Oh, Van ! bring " food," or " tea ; " on which he looked at me, went very slowly, and brought the ''tea" card. But w^hen I put some tea down as usual, he would not touch it. Generally he greatly enjoyed a cup of tea, and, indeed, this was the only time 1 ever knew him refuse it. A definite numerical statement always seems to me clearer and more satisfactory than a mere general assertion. I will, therefore, give the actual particulars of certain days. Twelve cards were put on the floor, one marked *'food" and one "tea." The others had more or less similar words. I may again add that every time a card was brought, another similarly marked was put in its place. Yan was not pressed to bring cards, but simply left to do as he pleased. 1 Y au br on Sht " food ' 4 times. "Tea' ' 2 times. 2 55 55 6 '5 3 55 55 8 55 55 2 5) 4 )' 5> 7 55 ,, 3 55 5 55 55 6 55 55 4 55 6 55 5' G 55 55 I) >5 " Noiiglit " one 7 ,j 55 8 ,, » 2 55 8 55 55 5) 55 3 55 9 55 5> 4 55 >» 2 „ 10 55 55 10 „ 5) 4 55 " Door " once. 11 5J 55 10 „ „ 3 >J 12 5> 5> G » 55 3 55 80 31 Thus out of 113 times he brought food 80 times, tea 81 times, and the other 10 cards only twice. Moreover, the last time he was wrong he brought a card — namely, "door" — in which three letters out of four were the same as in " food." 280 ATTEMPTS TO CONVEY IDEAS. This is, of course, only a beginrjing, but it is, I venture to think, suggestive, and might be carried further, though the limited wants and aspirations of the animal constitute a great difficulty. My wife has a beautiful and charming collie. Patience, to whom we are much attached. This dog was often in the room when Yan brought the " food " card and was rewarded with a piece of bread. She must have seen this thou- sands of times, and she begged in the usual manner, but never once did it occur to her to bring a card. She did not touch, or, indeed, even take the slightest notice of them. I then tried the following experiment : — I prepared six cards about ten inches by three, and coloured in pairs — two yellow, two blue, and two orange. I put one card of each colour on the floor, and then, holding up one of the others, endeavoured to teach Yan to bring me the duplicate. That is to say, that if the blue was held up, lie should fetch the corresponding colour from the floor ; if yellow, he should fetch the yellow^, and so on. When he brought the wrong card he w^as made to drop it and return for another, until he brought the right one, when he was rewarded with a little food. AYe continued the lessons for nearly three months, but as a few days were missed, we may say for ten weeks, and yet at the end of the time I cannot say that Yan appeared to have the least idea w^iat was expected of him. It seemed a matter of pure accident which card he brought. There is, I believe, no reason to doubt that dogs can distinguish colours ; but as it was just possible that Yan might be colour-blind, we then repeated the same experiment, only substitating for the coloured cards others marked respectively with one, ARITHMETICAL POWEES OF ANIMALS. 281 two, and three dark bands. This we continued for another three months, or, say, allowing for intermissions, ten weeks ; but, to my surprise, entirely without success, for we altogether failed to make Van understand what we wanted. I was rather disappointed at this, as, if it had succeeded, the plan would have opened out many interesting lines of inquiry. Still, in such a case one ought not to wish for one result more than another, as, of course, the object of all such experiments is merely to elicit the truth, and our result in the present case, though negative, is very interesting. I do not, however, regard it as by any means conclusive, and should be glad to see it repeated. If the result proved to be the same, it would certainly imply very little power of combining even extremely simple ideas. Can Animals count ? I then endeavoured to get some insight into the arithmetical condition of the dog's mind. On this subject I have been able to find but liltle in any of the standard works on the intelligence of animals. Considering, however, the very limited powers of savage men in this respect — that no Australian language, for instance, contains numerals even up to four, no Australian being able to count his own fingers even on one hand — we cannot be surprised if other animals have made but little progress. Still, it is curious that so little attention should have been directed to this subject. Leroy, who, though he ex- presses the opinion that " the nature of the soul of animals is unimportant," was an excellent observer, mentions a case in \Ahich a man was anxious to shoot 282 PREVIOUS OBSERVATIONS. a crow. ** To deceive this suspicious bird, the plan was hit upon of sencliug two men to the watch-house, one of^whom passed on, while the other remained ; but the crow counted, and kept her distance. The next day three went, and again she perceived that only two retired. In fine, it was found necessary to send five or six men to the watch-house to put her out in her calculation. The crow, thinking that this number of men had passed by, lost no time in returning." From this he inferred tbat crows could count up to four. Lichtenberg mentions a nightingale which was said to count up to three. Every day he gave it three meal- worms, one at a time; when it had finished one it returned for another, but after the third it knew that the feast was over. I do not find that any of the recent works on the intelligence of animals, either Buchner, or Peitz, or Eomanes in either of his books, give any additional evidence on this part of the subject. There are, however, various scattered notices. According to my bird-nesting recollections, which I have refreshed by more recent experience, if a nest contains four eggs, one may safely be taken ; but if two are removed, the bird generally deserts. Here, then, it would seem as if we had some reason for supposing that there is sufficient intelligence to distinguish three from four. An interesting consideration rises also vvith refer- ence to the number of the victims allotted to each cell by the solitary wasps. Ammophila considers one large caterpillar of Noctua segetitm enough ; one species of Eumenes .supplies its young with five victims; one ten, another fifteen, and one even as many as twenty-four. Tiie number is said to be constant in SUPPOSED POWEES OF COUNTING. 283 eacli species. How, then, does the insect know when her task is fulfilled ? Not by the cell being filled, for if some be removed she does not replace them. When she has brought her complement she considers her task accomplished, whether the victims are still there or not. How, then, does she know when she has made up the number twenty-four ? Perhaps it will be said that each species feels some mysterious and innate tendency to provide a certain number of victims. This would not under any circumstances be an ex- planation, nor is it in accordance with the facts. In the genus Eumenes the males are much smaller than the females. Now, in the hive bees, humble bees, wasps, and other insects Avhere such a differ- ence occurs, but where the young are directly fed, it is, of course, obvious that the quantity can be pro- portioned to the appetite of the grub. But in insects with the habits of Eumenes and Ammophila the case is different, because the food is stored up once for all. Now, it is evident that if a female grub was supplied with only food enough for a male, she would starve to death ; while if a male grub were given enough for a female it would have too much. No such waste, how- ever, occurs. In some mysterious manner the mother knows whether the egg will produce a male or female grub, and apportions the quantity of food accordingly. She does not change the species or size of her prey ; but if the egg is male she supplies five, if female ten, victims. Does she count ? Certainly this seems very like a commencement of arithmetic. At the same time, it would be very desirable to have additional evidence before we can arrive at any certaia conclusion. Considering how much has been written on instinct, 284 im. HUGGiNS's experiment. it seems surprising tliat so little attention has been directed to this part of the subject. One would fancy that there ought to be no great difficulty in determining how far an animal can count; and whether, for in- stance, it could realize some very simple sum, such as that two and two make four. But when we come to consider how this is to be done, the problem ceases to appear so simple. We tried our dogs by putting a piece of bread before them, and preventing them from touching it until we had counted seven. To prevent ourselves from unintentionally giving any indication, we used a metronome (the instrument used for marking time when practising the pianoforte), and to make the beats more evident we attached a slender rod to the pendulum. It certainly seemed as if our dogs knew wlien the moment of permission had arrived ; but their movement of taking the bread was scarcely so definite as to place the matter beyond a doubt. Moreover, dogs are so very quick in seizing any indication given them, even unintentionally, that, on the whole, the attempt was not satisfactory to my mind. I was the more discouraged from continuing the experiment in this maimer by an account Mr. Huggins gave me of a very intelligent dog belonging to him. A number of cards were placed on the ground, numbered respectively 1, 2, 3, and so on up to 10. A question was then asked : tlie square root of 9 or 16, or such a sum as 64-55 — 3. Mr. Huggins pointed consecutively to the cards, and the dog always barked when he came to the right one. Now, Mr. Huggins did not consciously give the dog any sign, yet so quick was the dog in seizing the slightest indication, that he was able to give the correct answer. " The mode of procedure is this. His master tells CONCLUSION. 285 him to sit down, and shows him a piece of cake. He is then questioned, and barks his answers. 8ay he is asked what is the square root of 16, or of 9; he will bark four or three times, as the case may be. Or such a sum as ^"^V^ he will always answer correctly. The piece of cake is, of course, the meed of such cleverness. It must not be supposed that in these performances any sign is consciously made by his questioner. None whatever. We explain the per- formance by supposing that he reads in his master's expression when he has barked rightly; certainly he never takes his eyes from his master's face." * This observation seems to me of great interest in connection with the so-called " thought-reading." No one, I suppose, will imagine that there was in this case any *Uhought-reading " in the sense in which this word is generally used. Evidently " Kepler " seized upon some slight indication unintentionally given by Mr. Huggins. The observation, however, shows the great difficulty of the subject. The experiments I have made are, I feel, very incomplete, but I have ventured to place them on record, partly in hope of receiving some suggestiouR, and partly in hope of inducing others with more leisure and opportunity to carry on similar observa- tions, which I cannot but think must lead to interestino- results. ♦ M. L. Iluggins, "Kepler: a Biograp]!}'." INDEX. Acalles, 6Q Acanthopleura, 15, 145 Acheta, 61, 63, 97, UV Acridiidje, 100, 106 Actinia, 13 Ageronia, 73 Aglaura, 188 Alciopidse, 14, 22, 137 Ammophila, 243, 282 Amphibia, 32, 129 Amphicora, 87 Amphioxus, 129 Angler, 186 Anguis, 126 Annelides, touch, 13 ; taste, 22 ; smell, 34 ; hearing, 87 ; sight, 134; problematical organs, 189 Anobium, 67 Anoxia, 251 Anthidium, 71 Anthrax, 251 Ants, 24, 31, 43, 56, 69, 107, 115, 178, 202 Apion, 94 Apis, 26, 29, 58, 69, 70, 115, 150, 172, 194, 258, 283 Area, 141 Arenicola, 87 Arithmetic of animals, 281 Arthropods, touch, 16 ; taste, 23 ; smell, 35 ; hearing, 88 ; sight, 146 ; problematical organs, 188 Articulata. See Annelides, Insects Ascidians, 129 Asellus, 48 Astacus, 23, 51, 88 Asteracanthion, 133 Asterope, 22 Astropecten, 132 Ateuches, 66 Auditory hairs, 16, 79, 85, 88, 116 organs, 77 rods, 18, 104, 111 B Balanus, 220 Bee, hive. See Apis Bee, solitary, 242" Beetles. See Coleoptera Bembex, 242, 246 Birds, 129, 282 Blatta, 46, 152 Blethisa, 68 Blind spot in eye, 125 Bohemilla, 13, 134 Bombardier beetle, 65 Bombus, 28, 70, 73, 178, 283 Bostrychida, 67 Brachinus, 64, 68 Brachyura, 90 Butterfly. See Lepidoptera a Calanella, 159 Calotis, 127 Callianassa, 50 288 INDEX. Cair.ponotus, 208 Capitellida?, 34 Capricorn beetle, 96 Carcinus, 92 Cards, Van and his, 277 Carinaria, 87 Caterpillars, 23, 2-1:3, 259 Cats, 262 Centipedes, 49, 74 Cephalopoda, 34, 141 Cerambyx, 67, 95, 96 Ccratius, 186 Ceratophyus, 63 Chalcidida?, 27 Chalicodoma, 251, 262 Chiasognathus, 68 Chitons, 15, 144 Cicadas, 61, 64, 151 Cioadida?, 151 Clepsine, 134 Cockchafer. See Mclolontha Cockroach, 46, 152 Ccelenterata, touch, 11; taste, 22; smell, 33; hearing, 82; sight, 131 Coleoptera, 58, 67, 111, 151 Collie, 280 Color of deep-sea fish, 185 . of flowers, 199 , sense of, 190, 194, 202, 280 Componotus, 239 Compound eyes, 163 Copepoda, 48 Copilia, 158 Copris, 68, 95 Corephium, 145 Corethra, 18, 113, 117, 151 Corixa, 75 Corti, the organ of, 80, 105 Corycseus, 157 Cossus, 148 Count? can animals, 281 Crabs, 90, 92 Crayfish. See Astacus Cricket. See Acheta Crioceris, 68 Crow, 282 Crustacea, touch, 16 ; taste, 23 ; smell, 46 ; hearing, 88 ; sight, 156 ; sense of color, 211 ; problematical organs, 188 Crystalline cone, 166 Culex, 68, 115 Curculionidae, 68 Cychrus, 68 Cyclostoma, 140 Cymbulia, 88 Daphnia, 48, 206, 212 Dead-nettle, 200 Death-watch, 66 Dias, 220 Dinetus, 39 Diptera, 52, 69, 110, 149, 151 Direction, sense of, 262 Dog, intelligence of the, 272 Dragon-fly. See Libel lula Dytiscus, 5, 6, 113, 131, 140, 137 E Ear. See Auditory organs in tail of Mysis, 92 , structure of the human, 78, 101 Earthworms, 206 Elaphrus, 68 Elaterida, 67 Empusa, 176 Endosmosis, 25 Englena, 130 Epeira, 146 Ephippigera, 103 Epithelial cells, 14, 20 Epithelium, 11, 19 Eristalis, 69, 174, 176 Eucopida?, 85 Eucorybar, 74 Eumenes, 245, 282 Euphausia, 161 Eurycopa, 189 Eutima, 83 Evaneadaj, 27 Eye, compound, 163 of man, 121 , pineal, 126 , simple, 170 F Fish, 182 Flowers, 200 INDEX. 289 Fly. See ^Musca Forficula, 151, 167 Formica. See Ants Gammarus, 49, 188 Gasteropods, 86 Geotrupes, 68 Geryonia, 86 Glomeris, 50 Glossopharyngeal nerves, 19 Gnat, Q^, 115 Gryllotalpa, 102 Gryllus, 63, 98, 106, 108 H Hairs, auditory, 16, 79, 85, 88, 116 , depressed, 17 , flattened, 56 , glandular, 29 , hollow, 17 in insects, 16 of touch. See Tactile , olfactory, 16, 25 , ordinary surface, 16, 56 , plumose natatory, 16, 94 , simple, 18 , solid, 17, 82 , tactile, 16, 18, 28, 29, 56 , taste, 16, 28 Haliotis, 5, 139 Hattaria, 127 Hearing, organs of, in Vertebrata, 77 ; Cceleuterata, 82 ; Mollusca, 86 ; Annelida, 87 ; Arthropods, 88 , sense of, 60, 97 Helix, 14, 139 Hemiptera, 112, 151 Hesioue, 135 Humble-bee. See Bombus Hydaticus, 40 Hydrachna, 28 Hydromedusse, 86 Hydrophilus, 168 Hydrozoa, 13 Hylceus, 58 Hymenoptera, 23, 25, 56, 57, 58, 69, 70, 96, 151, 181, 250 Hyperia, 171 Hypoderm, 5, 16 Ichneumon, 54, 58 Ichthyosaurus, 129 Infusoria, 11 Insects, touch, 16 ; taste, 23 ; smell, 35, 52; hearing, 61, 94; sight, 146; problematical organs, 138 Instinct — Ant, 202, 232, 267 Bee, hive, 194, 253 , solitarv, 255, 260, 262 Birds, 282 Bombardier beetle, 64 Change in, 244 Crustacea, 90 Daphnia, 229 Dog, 272 Fish, 186 Fly, 174, 177 Limitation of, 253 Of direction, 262 Onchidium, 144 Paussus, 65 Wasp, solitary, 243, 282 Isopteryx, 109 Jelly-fish. See INIedusse Julus, 49 Labyrinthodons, 129 Lacerta, 126, 128 Lamellibranchiata, 14, 141 Lamellicornia, 37, 52 Lamium, 200 Lampyris, 167 Lancelet, 129 Lasius. See Ants Laura Bridgmau, 273 Leech, 189 Lema, 68 Lepidoptera, 37, 71, 94, 111, 148, 151, 168, 181 Leptodora, 156 290 INDEX. Leucospis, 251 Libellula, 69, 70, 149, 152, 171 Light-organs, 161, 185 Ligia, 167 Limitation of instinct, 253 Limpet, 4, 138 Limulus, 159 Lithobius, 155 Lizzia, 132 Lobster, 90, 91 Locusts, 62, 99, 106, 111, 149, 176 Longicorn beetles, 66, 95 Lucamis, 43, 52 Lucilia, 177 Lycosa, 179 Lyda, 58 M Mammals, 129 Maxilla?, 25 Meconema, 102, 105 MedusEG, 6, 22, 82, 83, 84, 85, 86, 117 Meissner's corpuscles, 7 Melolontha, 52, 58, 67, 68, 148, 152, 168 Mesonotum, 67 Metronome, 284 Miltogramma, 254 Mollusca, 14, 22, 34, 61, 86, 120, 137, 140 Mordella, 148 Mosaic vision, 163 Mosquito. See Culex Moths. See Lepidoptera Murex, 139 Musca, 17, 29, 30, 45, 53, 58, 68, 71, 110, 113, 148, 153, 165, 172, 174, 177, 254 Mutilla, 69, 70 Myriapods, 155, 205 Myrmica. See Ants Mysis, 92, 98, 157, 161 N Nautilus, 140 Kecrophorus, QQ, 68 Needle cells, 21 Nematocera, 151 Nematocysts, 12 Nereis, 12, 135 Nesticus, 180 Neuroptera, 111, 151 Newts, 207 Noctua, 73, 243, 282 Oceanidse, 86 Ocypoda, 61 Odynerus, 247 CEstrus, 148 Olfactory organs. See Organs of smell Omaloplia, 68 Onchidiura, 14, 131, 143 Oniscoida?, 170 Ontorchis, 6, 84 Organs of hearing, 17, 19, 77, 81, 93, 109, 114 of sight, 19, 130, 146 of smell, 17, 88 of taste, 17, 19, 21 of temperature, 6, 10 of touch, 11, 14, 17, 19, 131 , problematical, 182 Origin of organs of sense, 3 Orthoptera, 37, 99, 107, 112, 131, 176 Oryctes, 68 Osmia, 251 Otolithes, 52, 82, 84, 85, 89, 90, 91, 92 , possible origin of, 3 Pacinian corpuscle, 8 Pagurus, 51 Palasmon, 51 Palinurus, 61 Palpi, 30, 37, 38, 39, 41, 73 Paludina, 140 Pamphila, 184 Paniscus, 58 Patella, 138, 140 Paussus, 65 Pectens, 61, 141 Pectunculus, 141 Pelagia, 86 INDEX. 291 PeloLius, 68 Periplaneta, 152 Perophthalnius, 144 Pheidole, 108 Phialidium, 85 Photichthys, 185 Pineal eye, 127 Pinnotheres, 51 Piscicola, 134 Platyarthrus, 207 Plesiosaurus, 129 Pleuroraona, 189 Podophthalmatn, 50, 156 Polydesmus, 189 Polyophthalmus, 33, 98, 134 Pompilus, 58 Ponera, 69 Pontella, 47, 48 Pontinia, 51 Poodle dog, 276 Pressure-point, 10 Prionus, 67 Proctotrupida?, 27 Pronotum, 67 Prosobranchiata, 138 Protoplasm, 21 Protozoa, 32, 61 Pteropods, 87 Ptychoptera, 113 R Recognition among ants, 234 Eeptilia, 127, 130 Respiration in insects, 35 Retina, 123 Rhopalonema, 85 Rods, auditory, 18, 104, 111, 187 , olfactory, 55 , retinal, 124 S Salivary gland, 30 Sarcophaga, 111 Schizochiton, 145 Scolopendra, 155 Scopelus, 186 Scorpions, 179 Sea-anemone, 12, 187 Sense-hairs. See Hairs Sense of direction, 262 Sense-organs, origin of, 3, 80, 111 Senses, unknown, 192 Serolis, 189 Setaj. See Hairs Sex, power of regulating, 262 Sigh-fc^rgans of, in Vertebrata, 121 ; Cceknterata, 131; Annelida, 133; Mollusca, 137 ; Arthropods, 146 , sense of, 118 , three possible modes of, 118 Silpha, 38, 41 Sirex, 58 Skin, termination of nerves in, 18 Smell, organs of, in Vertebrata, 32 ; Protozoa, 33; Coelenterata, 33; Annelida, 33; Mollusca,34; Arthro- pods, 35 Smeriuthus, 73 Solaster, 133 Sound, organs of, not known in Pro- tozoa or Coelenterata, 61; i\Iol]usca, 61 ; Crustacea, 61 ; Insects, 62 Sphex, 245 Sphinx, 73, 148 Sphcerotherium, 74 Spiders, 74, 146, 155, 170, 178 Spondylis, 67, 141 Squilla, 51 Stag-beetle, 43, 52 Staphylinus, 50 Stenobothrus, 62, 63 Stratiomys, 167 Syrphus, 69, 170 Tachytes, 246 Taste, organs of, in Vertebrata, 19 ; Annelida, 22; Mollusca, 22; Ar- thropods, 23 Telephorus, 112 Temperature, organs of, 10 Tenebrionida, 68 Tenthredo, 27, 58 Theridium, 75 Touch, organs of, in Vertebrata, 7 ; Protozoa, 11; Coelenterata, 11; Medusae, 12 ; Annelida, 13 ; Mol- lusca, 14; Arthropods, 16 292 INDEX. 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KEGAN PAUL, TREN'CH & CO.'S EDITIONS OF SHAKSPERE'S WORKS THE PARCHMENT LIBRARY EDITION. THE A VON EDITION The Text of these Editions is mainly that of Delius. Wher- ever a variant reading is adopted, some good and recognized Shaksperian Critic Jias been followed. In no case is a new 7'endering of the text proposed ; nor has it been thought ne- cessary to distract the readers attention by notes or comments. I, PATERNOSTER SQUARE. [p. T. O. SHAKSPERE'S WORKS. THE A VON EDITION. Printed on thin opaque paper, and forming 12 handy volumes, cloth, i8j-., or bound in 6 volumes^ 15-$-. The set of 1 2 volumes may also be had in a cloth box, price 2ii'., or bound in Roan, Persian, Crushed Persian Levant, Calf, or Morocco, and enclosed in an attractive leather box at prices from 31^'. dd. upwards. Sjp^ SOME PRESS NOTICES. " This edition will be useful to those who want a good text, well and clearly printed, in convenient little volumes that will slip easily into an overcoat pocket or a travelling-bag." — St. James's Gazelle. " We know no prettier edition of Shakspere for the price. " — Academy. " It is refreshing to meet with an edition of Shakspere of convenient size and low price, without either notes or introductions of any sort to distract the attention of the reader." — Saturday Review. "It is exquisite. Each volume is handy, is beautifully printed, and in every way lends itself to the taste of the cultivated student of Shak- spere. " — Scotsman. LoNDox : Kegan Paul, Trench & Co., i, Paternoster Square. SHAKSPERE'S WORKS. THE PARCHMENT LIBRARY EDETION. In 12 volumes Elzevir 8vo., choicely printed on hand-made paper, and bound in parchment or cloth, price J^i 12^., or in vellum, price £^^ \os. The set of 1 2 volumes may also be had in a strong cloth box, price ^3 i7->\, or with an oak hanging shelf, ^3 i8j-. SOME PRESS NOTICES. "... There is, perhaps, no edition in which the works of Shakspere can be read in such luxury of type and quiet distinction of form as this, and we warmly recommend it." — Pall A/all Gazette. " For elegance of form and beauty of typography, no edition of Shakspere hitherto published has excelled the * Parchment Library Edition.' . . . They are in the strictest sense pocket volumes, yet the type is bold, and, being on fine white hand-made paper, can hardly tax the weakest of sight. The print is judiciously confined to the text, notes being more appropriate to library editions. The whole will be comprised in the cream-coloured parchment which gives the name to the series." — Daily Keios. "The Parchment Library Edition of Shakspere needs no further praise. " — Saturday Reviczv. Just published. Price 5^". AN INDEX TO THE WORKS OF SHAKSPERE. Applicable to all editions of Shakspere, and giving reference, by topics, to notable passages and significant expressions ; brief histories of the plays ; geographical names and historic incidents ; mention of_ all characters and sketches of important ones ; together with explanations of allusions and obscure and obsolete words and phrases. By EVANGELINE M. O'CONNOR. London : Kegan Paul, Tjiench & Co., i, Paternoster Square. SHAKSPERE'S WORKS. SPECIMEN OF TYPE. 4 THE MERCHANT OF VENICE Act i Salar. My wind, cooling my broth, Would blow me to an ague, when I thought What harm a wind too great might do at sea. I should not see the sandy hour-glass run But I should think of shallows and of flats, And see my wealthy Andrew, dock'd in sand. Vailing her high-top lower than her ribs To kiss her burial. Should I go to church And see the holy edifice of stone. And not bethink me straight of dangerous rocks, Which touching but my gentle vessel's side, Would scatter all her spices on the stream. Enrobe the roaring waters with my silks, And, in a word, but even now worth this, And now worth nothing ? Shall I have the thought To think on this, and shall I lack the thought That such a thing bechanc'd would make me sad ? But tell not me : I know Antonio Is sad to think upon his merchandise. Ant. Believe me, no : I thank my fortune for it, My ventures are not in one bottom trusted. Nor to one place ; nor is my whole estate Upon the fortune of this present year : Therefore my merchandise makes me not sad. Salar. Why, then you are in love. Aiit. Fie, fie ! Salar. Not in love neither ? Then let us say you are sad. Because you are not merry ; and 'twere as easy For you to laugh, and leap, and say you are merry. Because you are not sad. Now, by two-headed Janus, Nature hath fram'd strange fellows in her time : Some that will evermore peep through their eyes And laugh like parrots at a bag- piper ; And other of such vinegar aspect London : )Kegax Paul, Trzxch & Co., i. Paternoster Square,' ^