the rnrtomy or the hehd-rnd mouth- parts 
 
 or 
 
 PL ECO PT ERR 
 
 
 
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THE ANATOMY OF THE HEADLAND MOUTH-PARTS OF PLECOPTERA 
 
 BY 
 
 GLADYS HOKE 
 
 B. S. Mississippi State College for Women, 1916 
 
 THESIS 
 
 Submitted in Partial Fulfillment of the Requirements for the 
 
 Degree of 
 
 MASTER OF SCIENCE 
 IN ENTOMOLOGY 
 
 IN 
 
 THE GRADUATE SCHOOL 
 
 OF THE 
 
 UNIVERSITY OF ILLINOIS 
 
 1921 
 

 
 
 Digitized by the Internet Archive 
 
 in 2016 
 
 https://archive.org/details/anatomyofheadmouOOhoke 
 
UNIVERSITY OF ILLINOIS 
 
 THE GRADUATE SCHOOL 
 
 J une 4, 1921 i 9 i 
 
 1 HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY 
 
 SUPERVISION BY Glady s He kg 
 
 ENTITLED The Ana tomy of t he H ead and Mou th-p a rts of 
 
 Plecoptera 
 
 BE ACCEPTED AS FULFILLING THIS PART OF THE REQUIREMENTS FOR 
 
 THE DEGREE OF 
 
 OH. *r. 
 
 In Charge of Thesis 
 
 
 
 Head of Department 
 
 Recommendation concurred in* 
 
 Committee 
 
 on 
 
 Final Examination* 
 
 *Required for doctor’s degree but not for master’; 
 

 
 
 
 ■ 
 
 
 
 
 , 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1 
 
 C 0 1'j T 3 II T S 
 
 Introduction 
 Fixed Parts of the Head 
 movable Parts of the Head 
 Summary 
 
 Explanation of Plates 
 
 5 
 
 23 
 
 34 
 
 37 
 
2 
 
 INTRODUCTION 
 
 The students of the various groups of insects are seriously 
 handicapped in their studies of the homology of structures on ac- 
 count of a lack of sufficient accurate morphological data on relat- 
 ed groups. This study of the Plecoptera is an attempt to increase 
 the present knowledge of the morphology of the head of generalized 
 insects as well as to amplify the studies of the order. 
 
 The stoneflies are usually thought of as a very generalized 
 group. This fact has "been emphasized by both Comstock and Crampton c 
 An examination of the heads of the order will reveal at once 
 
 the presence of many Orthopteran characteristics. Altho this is 
 true in many respects, in others they exhibit striking speciliza- 
 tions. Smith ( 1917 ) calls attention to the highly specialized con- 
 dition of the genitalia; Klapalek ( 1909 ), probably the most import- 
 ant of the European students of the group, has examined with con- 
 siderable detail the genitalia and the venation of the wings; 
 Koponen ( 1917 ) has studied in some detail the nymphs and adults 
 of the species of Finland; Tillyard ( 1921 ) points out specializa- 
 tions in the wing venation, the coriae and the mouth-parts of 
 Australian species. Studies of the morphology of the head have been 
 made by Schoenemund ( 1912 ) in connection with the morphology of 
 the thorax and abdomen, and by Comstock and Kochi ( 1902 ) who des- 
 cribed and figured the head of a nymph of Pteronarcys in their 
 study of the skeleton of the head of insects, oo far as i have been 
 able to ascertain, no work ha3 been done on the morphology of the 
 head and the tentorium of a representative series of the genera 
 of the order. An effort has been made to study as comprehensive a 
 series as possible, one which would show both the generalized and 
 

 
 
 
 
 
 
 
 
 
 
 
 
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the specialized conditions of the head- capsule, the tentorium and 
 the mouth-parts. 
 
 The specimens were prepared for study by placing in vials con- 
 taining a two per cent solution of potassium hydroxide and by heat- 
 ing the vials in a water-bath from fifteen to thirty minutes. The 
 potash was removed by washing the specimens in distilled water. 
 
 The specimens were studied under a binocular microscope. Occasion- 
 ally staining with Gage’s Saurfuchsin was necessary and in a few 
 instances the specimens were placed in lactic acid for fifteen to 
 thirty minutes in order to distend wrinkled or collapsed parts. 
 
 The use of an ocular micrometer ruled in squares facilitated the 
 measuring of the specimens. The following species have been des- 
 cribed and figured ; Acroneuria abnormis and a specimen labeled 
 as this species purchased from an American dealer but which has 
 mandibles of a different form (Figs. ), Alloperla minuta 
 
 banks, Capnia necydaloides Newn., Chloraperla grammatica Sep., 
 Isopteryx tripunctat Sp. Leuctra Klapalekihny. , wemura varigata 
 Oliv. , Perla immarginata nymph, Perlesta placida nag., Pteronarcys 
 regalis Hewn. , nymph and adult, and Taeniopteryx frigida Hag. 
 
 I wish here to express my hearty thanks to Professor Alex. 
 
 D. MacGillivray , under whose direction the work was done, for his 
 efforts in securing material for study, for his constant interest, 
 his ever ready assistance and valuable counsel which lightened the 
 routine of the work, and also for the permission to use his un- 
 published terminology much of which appears here for the first time, 
 and the manuscript of his forth coming book on the external mor- 
 phology of insects. This manuscript proved indispensable as a refer- 
 ence and of intrinsic value as a guide. I am exceedingly grateful 
 
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 to Dr. Hachiro Yuasa for his unstinted kindness, generosity and 
 skill in inking my drawings, one of the most tedious and irksome 
 tasks connected with the preparation of the paper. 1 am also in- 
 debted to Professor S. A. Forbes for the use of specimens from the 
 collections of the Illinois State Laboratory of Natural History, 
 to Dr. C. P. Alexander for many courtesies, and to Mr. Nathan 
 Hanks of the M useum of Comparative Zoology, for the identification 
 of specimens, i'he European species studied were purchased from Dr. 
 0. Standinger and A. Eang-Hass of Blasewitz bei Dresden, Germany, 
 ihese specimens were identified by Dr. Franz Klapalek. 
 
FIXED PARTS OF THE HEAD, 
 
 The epicranial suture is the inverted Y-shaped line on the 
 dorsal aspect of the head usually present in generalized insects. 
 
 It marks the line of separation of the sclerites developed from the 
 procephalon from the sclerites originating from the cephalic 
 lobes. In generalized Exoptera and in larvae and nymphs the suture 
 is usually more or less complete. The stem is the first portion of: 
 the suture to disappear. In a hypothetical type of a generalized 
 insect the part of the suture which is called the stem, begins 
 as a single line at the occipital foramen and extends cephalad 
 to a point on the meson between the compound eyes, where it branches 
 dichotomously ; each branch, which is known as an arm, extends 
 laterad and cephalad to a point on the f ronto- clypeal suture near 
 the precoila. 
 
 In the Plecoptera each arm of the epicranial stem always 
 ends at a point near a lateral ocellus or between a lateral ocellus 
 and the mesal or cephalic margin of a compound eye. The nymph of 
 Pteronarcys (Fig. 9 ) which represents the most generalized type 
 of the forms studied, has a long epicranial 3tem. The stem in 
 Acroneuria (Fig. 3 ) is shorter. The nymph of Perla (Fig. ) 
 shows a still shorter stem. The caudal portion of the stem has been 
 retained while the cephalic portion has been lost in Leuctra 
 (Fig. / ). The adult Bieronarcys (Fig. fr' ) shows the stem almost 
 
 obsolete. In Taeniopteryx (Fig. ^ ) the stem is wanting with the 
 exception of a short cephalic portion. There is only a slight 
 indication of the stem in Alloperla (Fig. y ). The cephalic portion 
 of the stem is obsolete in Chloraperla (Fig. // ) which shows a 
 

 
 
slight indication of the caudal part. The stem is entirely obsolete 
 
 in Capnia (Fig. /0 ) and in Nemjira (Fig. ) Leuctra (Fig.l) and Isop- 
 teryx. (Fig. 2). 
 
 The nymphs of Pteronarcys (Fig. ) and Perla show the long- 
 est epicranial arms found in the forms studied. Each arm extends 
 to the cephalic margin of a compound eye and terminates near the 
 antennaria. The arms are slightly shorter in Acroneuria (Fig. 3 ), 
 each ends near the mesal margin of a compound eye. Each arm of 
 Leuctra (Fig. / ) extends to the lateral margin of a lateral ocellus 
 
 where it bends abruptly cephalad. In the adult of Pteronarcys, 
 Taeniopteryx, Perlesta, and Capnia (Figs.y,/^/oJeach arm ends at a 
 point near a lateral ocellus. The arms of Capnia are not well 
 developed. In Chlora^perla (Fig. // ) the mesal portion of the arms 
 is obsolete, the lateral parts are distinct altho not well devel- 
 oped. There is only a slight indication of the arms in Alloperla 
 
 (Fig. 7 ), while in Isopteryx (Fig. ) and in Nemura (Fig. X ) 
 
 <TT 
 
 the arms are entirely obsolete. The vertex is one of the paired 
 sclerites formed from the cephalic lobes. In the generalized 
 forms of the Exoptera the epicranial suture is present and separ- 
 ates the vertex from the front and divides the sclerite into two 
 parts. The vertex extends from the epicranial arms to the occip- 
 ital suture. It bears the compound eyes and extends on to the 
 ventral aspect. The vertex on the ventral aspect includes the 
 portion of the head between the antennaria and the postgenae and 
 the occiput. The comparative size and shape of the vertex, when 
 clearly defined, varies only slightly in the Plecoptera studied. 
 
 The vertex in the nymph of Pteronarcys (Fig. ? ) is larger 
 than in most of the other forms and is completely divided by the 
 epicranial stem. It is smaller in Acroneuria and in the nymph 
 

 
 
 
 
 
 
 
 
 
 
 
 
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7 
 
 of Perla (FigS.3, 6) but is distinctly divided and separated from 
 the front by the epicranial suture. The two halves of the sclerite 
 are not completely divided, due to the partial or complete absence 
 of the epicranial stem in Leuctra, the adult of Pteronarcys, 
 
 Taeni opteryx, Capnia and Perlesta (Figs./, K /<£.) . 
 
 In Chloraperla (Fig. t! ) and Alloperla (Fig. 7 ) the halves 
 of the sclerite are not completely separated due to the partial 
 or total absence of the epicranial stem. The sclerite is fused to 
 the front in the absence of parts of the epicranial arms. There 
 is no epicranial suture in Isopteryx (Fig. S ) and Nenfura (Fig. <2. ) 
 so the vertex in these forms is not divided and is fused with the 
 front. In Isopteryx (Fig. d * ) the area which goes to form the 
 vertex is much narrower than in any of the other species. The 
 front is one of the unpaired sclerites formed from the procephalon. 
 In a hypothetical type of a generalized insect and in some of the 
 0 rthoptera the front is separated from the vertex by the epi- 
 cranial arms, from the genae by the front o-genal suture, and from 
 the clypeus by the f ront o- clypeal suture. Some of the Plecoptera 
 studied show the front distinctly separated from the vertex and 
 from the clypeus, in other more specialized forms it is fused 
 with the vertex and the clypeus. 
 
 The nymph of Pteronarcys, the adults of Leuctra, Taeni- 
 opteryx, Capnia, and Pteronarcys (Figs.? /,/y,/4#) show the front 
 distinctly separated from the vertex and from the clypeus. The 
 nymph of Perla, Acroneuria, and Perlesta (Figs. £.3, /JL) have the 
 front separated from the vertex but fused with the clypeus. In 
 Nemura (Fig. X. ) the front is separated from the clypeus only. 
 Chloraperla (Fig. II ) and Alloperla (Fig. 7 ) have the front al- 
 
8 
 
 most completely fused with the vertex and totally fused with the 
 clypeus. The front apparently occupies the greater portion of the 
 dorsal aspect of the head in Isopteryx (Fig. ) altho it is com- 
 pletely fused with the front, the genae, and the clypeus. 
 
 The gena is not a separate sclerite. The term is applied 
 to the lateral area of the head cephalad of a compound eye and 
 caudad of the mandihle. In orthopterous insects the genae are more 
 or less completely separated from the front by the fronto-genal 
 sutures. The antennal suture, when present, separates the vertex 
 from the antennaria. The Plecoptera studied show this suture but 
 the genae are continuous with the front and bear the antennariae. 
 
 In Acroneuria, Chloraperla, Perlesta, Alloperla, and Isopteryx 
 (Figs.3,//,/3, 6") the two areas are easily distinguished by a folding 
 of the lateral aspect of the head due to the inturning of the front 
 and the clypeus under the frontal shelf. 
 
 The fronto-genal sutures are the lines which separate the 
 front And the genae in the generalized Exoptera. This suture is the 
 cephalic portion of an epicranial arm and unites with the fronto- 
 clypeal suture at a precoila. Each of these sutures is interrupted 
 near the antennaria and the lateral ocellus in the Orthoptera, but 
 the cephalic portion of the suture is generally very distinct. 
 
 Such a suture is not present in any of the Plecoptera examined. The 
 folds between the genae and the front in Acroneuria, Perlesta, 
 
 All opejr.la, Choraperla and Isopteryx (Figs.3/4, >,//, should not be 
 considered as fronto-genal sutures for a smoothing or flattening 
 of the lateral aspect of the head removes the lines made by the 
 folding of this region and reveals the surface as continuous and 
 uninterrupted by sutures. 
 
9 
 
 The compound eye is a large globular projection on the lat- 
 
 V 
 
 eral aspect of the head. In the adults examined the compound eye 
 occupies a considerable portion of the lateral aspect of the dorsal 
 and the ventral surfaces. In the nymphs the extent of the compound 
 eye on the ventral aspect is considerably less than in the adult. 
 
 The oculata is usually present as a narrow band surrounding 
 the compound eye. In the nymph of Perla the oculata becomes very 
 much wider on the caudal margin of the eye and with the vertex forms 
 a sharp ridge which is fringed with numerous stout setae on the 
 caudal and the lateral margin. 
 
 The three ocelli are usually located on the front or the 
 vertex of the Exoptera. The median ocellus is usally situated on 
 the front, the two lateral, on the vertex. In the Plecoptera exam- 
 ined the ocelli are located on the front, the two lateral generally 
 in close proximity to the epicranial arms. The lateral ocelli in 
 Isopteryx (Fig. if ) are large and much nearer the occiforamen than 
 in any of the other specimens examined. As the epicranial arms are 
 obsolete it is impossible to determine the caudal extremity of the 
 front. The length of the vertex has been greatly reduced or the 
 lateral ocelli have migrated caudad on to the vertex. A study of 
 the immature forms is desirable as it would show just what has 
 occurred. In the nymphs (Figs. 6 , 9 ) studied the ocelli are very 
 
 much smaller than in the adult forms and do not protrude beyond 
 the surface of the adjacent areas. On the adult of ? teronarcys, 
 
 (Fig. S' ) as in other forms, the ocelli are large, globular struc- 
 tures which project as distinct convex protuberances beyond the 
 adjacent surface. The margins of the ocelli in the nymphs are not 
 as sharply defined as in the adults. The f&rm, the size, the 
 

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10 
 
 absence of a definite margin, and the external appearance show 
 clearly that the ocelli are not functional in the nympha. 
 
 The antennaria is a small sclerite to which the antacoria 
 of the antenna is attached in generalized insects. Some of the 
 largest antennariae are found in generalized Plecoptera hut in some 
 of the more specialized forms of this order the antennariae are 
 very rudimentary. The adult of Pteronarcys has the largest anten- 
 nariae of the forms studied. The nymph of P t eronarcy s (Fig. ? ) 
 
 and Isopteryx (Fig. & ) have well developed antennariae which are 
 broadest near the middle on the dorsal aspect of the head. Acron- 
 euria (Fig. 3 ) and the nymph of Perla (Fig. ) have well devel- 
 oped antennariae which are more nearly uniform in width on the 
 dorsal aspect. Chloroperla Taeniop teryx, and beuctra (Figs. //, 'V, /) 
 have the antennariae more reduced at the caudal extremity and lar- 
 gest at the cephalic end. The antennariae of Nemjira (Fig. 3J are 
 uniform in width on the dorsal aspect hut are distinctly curved. 
 
 The arc of the curve is directed mesad. Perlesta, Alloperla, and 
 Capnia ( Figs. /3, >, /o) have antennariae which are very much reduced. 
 In Capnia they are quite rudimentary on the dorsal and the ventral 
 aspect, while in Alloperla and Perlesta they are somewhat enlarged 
 on the ventral aspect (Figs./tf, /<2_ ). 
 
 The antacoria is the membrane between the antenna and the 
 antennaria. It is usually quite pronounced on the caudal and the 
 cephalic aspects and very narrow between the antacoila and the 
 antartis 
 
 The f ronto- clypeal suture, when present, separates the front 
 from the clypeus. The primitive PI ecoptera evidently possessed this 
 suture but early in the development of the order there seems to 
 
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11 
 
 have been a dichotomous branching, one group of genera retaining 
 the suture and developing from the pretentoria a lamella- like ex- 
 tension along the ental surface of this suture; the other group 
 of genera lost the suture and did not develop the lamella- like ex- 
 tension of the pretentoria. The genera belonging to the group which 
 lost the suture are described first. 
 
 ihe nymph of Pteronarcys (Fig. 9 ) has a distinct fronto- 
 clypeal suture. On the ental surface of the head-capsule there is 
 a slight lamella, a thickening due to the infolding of the coria 
 along its entire length. The lamella is not an extension of the 
 pretentoria. Ihe median portion of the suture is obsolete in the 
 adult (Fig. 9 ) ) and the lateral portions are only faintly indicated, 
 i'he lamella has entirely disappeared. The nymph of Pteronarcys 
 probably resembles the condition in the primitive ancestors before 
 any modification had taken place. The disappearance of the suture 
 and the lamella in the adult seems to indicate a beginning of the 
 modification which has been continued in the group in which there 
 is no suture. Acroneuria, the nymph of Perla, Chloraperla, Isop- 
 teryx, Perlesta, and Alloperla have no f r ont o- clypeal suture. The 
 suture and the tentorial lamella is very distinct in Leuctra. Suc- 
 cessive stages in the enlargement of the tentorial lamella along 
 the suture are seen in Capnia, Taeni opteryx, and iveufura. In each 
 of these forms it is well developed. The tentorial lamella is a 
 characteristic feature of the Orthoptera. 
 
 The clypeus is one of the unpaired sclerites developed from 
 the procephalon. It is located cephalad of the front and caudad of 
 the labrum. In the more generalized, insects it is separated from 
 the front and from the labrum by distinct sutures. In the P 1 ecoptera 
 there is no indication of a subdivision of the clypeus. The 
 

 
 
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12 
 
 clypeus is a large well defined sclerite in the nymph of Pteron- 
 arcys (Fig. 9 ). It is separated from the front by the fronto- 
 
 clypeal suture and from the labrum "by the clypeo-labral suture. 
 
 The caudal portion is covered with short setae, the cephalic por- 
 tion is glabrous. The sutures separating the clypeus from the front 
 and the labrum are distinct in .Leuctra and Taeniopteryx (Figs*/, 'V ). 
 In Nemura and Capnia (Figs. <2. , / 0 ) The clypeus is much broader on 
 
 the caudal margin than it is on the cephalic and is bounded by 
 clearly defined sutures. In the adult of Pteronarcys (Fig. Y ) the 
 f ronto- clypeal suture is almost obsolete and the clypeus is partly 
 fused with the front. It is separated from the labrum by a well 
 defined suture. The clypeus is fused with the front in Acroneuria, 
 the nymph of Perla, Chloraperla, Isopteryx, Perlesta, and Allo- 
 perla (Figs. 3, 6, "'Sax., 7). 
 
 The precoila is the point of articulation of the mandible 
 on the dorsal aspect with the clypeus, adjacent to the lateral 
 extremities of the f ronto- clypeal suture when this suture is present 
 In orthopterous insects the precoila is distinct and visible from 
 the dorsal or cephalic aspect. In the Plecoptera examined the pre- 
 coila is never visible from the dorsal aspect. The nymph and adult 
 of Pteronarcys, Leuctra, Capnia and i\emura ( Figs.?,?', /, (0 , ZL ) have 
 the precoila visible only from the lateral aspect, due to a bending 
 of each lateral portion of the clypeus and sometimes of the front 
 ventrad. It is at this ventral edge of the clypeus that the precoila 
 is found adjacent to the f ronto- clypeal suture, when it is present, 
 which also bends ventrad with the clypeus. In the nymph of Perla 
 the clypeus bends ventrad and mesad and it is under this ledge 
 formed by the clypeus that the precoila is located, ihe front and 
 the clypeus bend ventrad and mesad forming a similar ledge beneath 
 

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 which the precoila is located in Acroneuria, Isopteryx, Alloperla, 
 Chloraperla, and Perlesta. The f ront o- clypeal suture is lacking in 
 the forms having a frontal ledge as has "been stated previously. 
 
 The labrum is one of the unpaired sclerites formed from 
 the procephalon. It is movable and is the upper lip. In the Plec- 
 optera it is separated from the clypeus by a distinct suture, and 
 in the forms examined, is a well defined and usually a large area. 
 The labrum is a distinct quadrangular sclerite in the nymph of 
 Pteronarcys. Its caudal portion is densely setaceous, the cephalic 
 portion is bare. In Taeni apteryx, Isopteryx, Alloperla, and Chlora- 
 perla the labrum is of the same general shape as in the nymph of 
 Pteronarcys ( Pigs. 7 ^^ 7, // )• The posterior portion is more or 
 
 less membranous. The labrum of heuctra (Fig. ( ) and Capnia (Fig. JO) 
 has the same general shape as in the preceeding group, but, due 
 to a bending ventrad, appears short from the dorsal aspect. The 
 labrum in the adult of Pteronarcys (Fig. ) and Perlesta (Fig. /-2. ) 
 is crescentic and narrow, when measured on the meson: the labrum 
 bends ventrad and from the dorsal aspect appears narrower and more 
 crescentic than it really is. This is strikingly true of h'enmra in 
 which the labrum is almost as long as broad (Fig. ,2.60 but due to 
 a ventral incurving appears very narrow from the dorsal aspect 
 ' Fig. CL ) . In the nymph of Perla (Fig. i? ) the labrum is biemar- 
 ginate on the cephalic margin and the mesal lobe bears a small 
 
 membranous flap which is densely setaceous. The labrum in Acron- 
 euria is very much smaller than in any of the other species 
 examined. 
 
 ihe clyp eo- labral suture is the line or coria between the 
 clypeus and the labrum. It is present in all of the forms studied. 
 

 
 
 
 
 
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1A 
 
 In the nymph of Perla (Fig. ^ ) it is long and narrow; while in 
 the nymph of Pteronarcys (Fig. 2 ) it is wider and membranous. 
 
 It is membranous in Acroneuria (Fig. Q ) and contains a small 
 slightly crescent- shaped secondary sclerite or thickening near the 
 cephalic extremity in close proximity to the labrum. 
 
 The tormae are thickenings between the clypeus and the lab- 
 rum at the lateral extremities of the clypeo-labral suture. They 
 are well developed in all of the specimens examined with the single 
 exception of Acroneuria. Here they are rudimentary and consist of 
 
 an unbranched thickening which extends caudad from the labrum. In 
 
 and are 
 
 all of the other forms studied the tormae are conspicuous/usually 
 branched (Fig.^^A^ $ Zj. 
 
 The opening in the caudal aspect of the head thru which the 
 
 nerve 
 
 alimentary canal- cord, and other organs pass into the cervix is 
 the occipital foramen. In the cockroach the occipital foramen is 
 located on the mesal and caudal portion of the ventral aspect of 
 the head. The vertex and the occiput extend on to the ventral aspect 
 caudad of the occipital foramen and in no instance is the occipital 
 foramen visible from the dorsal aspect. In the Plecoptera studied 
 the occipital foramen is located on the caudal portion of the ven- 
 tral aspect and occupies the mesal portion of the caudal aspect of 
 the head extending caudad and dorsal, the caudal portion always 
 being visible from the dorsal aspect. In the more generalized Stone- 
 flies a large part of it is on the ventral aspect. In the special- 
 ized genera the portion on the ventral aspect is smaller and the 
 caudal portion extends further on to the dorsal aspect. The posi- 
 tion of the occipital foramen in the Plecoptera is intermediate 
 between the forms, in which the occipital foramen is located on the 
 

 
 
 
 
 
 
 
 
 
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 ventral aspect as in the Orthoptera and not opposite the mouth open- 
 ing, and those in which the occipital foramen is limited to the 
 caudal aspect and opposite the mouth opening. 
 
 The most generalized condition of the occipital foramen in the 
 Plecoptera is found in Leuctra (Fig. 3.0" ). The occipital foramen is 
 ss long as broad and roughly quadrangular in shape. The length of 
 the occipital foramen in Taeniopteryx (Fig. 3 3 . ) is greater than 
 the breadth. Specialization is shown in the migration of the occ- 
 ipital foramen caudad and dorsal. The nymph of Pteronarcys, Capnia, 
 the adult of Pteronarcys, Isopteryx, Alloperla, Nemura, chloraperla, 
 
 A 
 
 Acroneuria, Perla, and Perlesta ( Figs. 2?, 3d, Q.°l l 3^, 36,2.6) show increas- 
 ing specialization. Perlesta (Fig. ) has the most specialized 
 occipital foramen of the forms figured. Here the length is much less 
 than the breadth. On the dorsal aspect the vertex and the occiput 
 are distinctly emarginate. 
 
 The odontoidea is the point of articulation of a cerv e-pister- 
 num with the head-capsule. Each is a triangular projection located 
 on the lateral edge of the occipital foramen. In the Plecoptera the 
 odontoidea is usually well developed and distinct. 
 
 The occipital suture is not present in the nymphs of Pteronar- 
 cys and Perla, and in Leuctra and Nemura (Figs.aX, d f j . In 
 
 Capnia (Figf 36} the suture is obsolete on the dorsal aspect but 
 is clearly defined on the ventral aspect opposite the mesal portion 
 of a compound eye. In Pteronarcys, Taeniopteryx, Acroneuria, Isop- 
 teryx, Chloraperla, Alloperla, and Perlesta ( Figs.<*%33 ( i'>, 35 ) 
 
 the occipital suture is distinct on the dorsal aspect but is indis- 
 tinct opposite a metatent orina and is difficult to identify on 
 account of the folding of the cuticle on this area. In most of the 
 

 
 
 
 
 
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16 
 
 last mentioned forms the occipital suture can he identified thruout 
 its course. (i*'igs. 27,2. ^.3 CT , -3 &) • 
 
 The postgena is the area on the ventral aspect of the head 
 mesad of the occipital suture, when this suture is present, extend- 
 ing to the postcoila and fusing with the occiput near a metatento- 
 rina. In Capnia (Fig. SO ) there is an indication of a line which 
 extends from the cephalic margin of a met atent orina to the occipi- 
 tal suture. This line is undoubtedly secondary and is not found in 
 any of the other forms examined. Its position near a metatent orina 
 also suggests that it is secondary. In the nymphs of Pteronarcys 
 and Perla, and in peuctra and hemura (Figs.?S^3 1,2^/ ^6) the post- 
 genae are not separated from the vertex or the occiput. Each post- 
 gena is more or less completely separated from the vertex hy sin 
 occipital suture in Taeni opteryx, Acroneuria, Isopteryx, Chlora- 
 perla, the adult of Pteronarcys, Perlesta, and Alloperla (Figs. 0-2., 
 2 - 7 , 3 ^, 3 3 , Z.°I, id, 367 . 
 
 The occiput is one of the paired sclerites formed from the 
 cephalic lobes. It is the area between the vertex and the occipital 
 foramen. The suture separating the vertex from the occiput is often 
 wanting. In the nymphs of Pteronarcys and Perla, and in Leuctra and 
 iiemura (Figs. If, 3 the occiput is indist inguishably fused with 
 
 the vertex. In Capnia the occiput is also fused with the vertex. 
 
 The occiput and postgenae are more or less completely separated 
 from the vertex by the occipital suture in Taeniopteryx, Acroneuria, 
 Isopteryx, chloraperla, Alloperla, Perlesta, and Pteronarcys 
 ( jfigs.93, A}, ‘34^33 36) • On the dorsal aspect the occiput is a very 
 narrow band which becomes enlarged on the lateral aspects adjacent 
 to the lateral portions of the occipital foramen. 
 

 
 
 
 
 
 
 
 
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17 
 
 The crassa is present in all of the forms studied. It usually 
 extends from near the central portion of a postcoila to a point 
 opposite the cephalic margin of a compound eye. 
 
 The paracoila is the point of articulation of a maxilla on 
 the ventral aspect. In the Plecoptera it is always at the cephalic 
 margin of a metatent orina. The maxillaria is not well developed in 
 the Plecoptera. 
 
 The postcoila is the point of articulation of a mandible on 
 the ventral aspect at the cephalic margin of a postgena. as a rule 
 the relative distance between the postcoila and the paracoila is 
 much less in generalized than in specialized forms. In the Plecop- 
 tera this is illustrated by referring to figures«itf* and3o .Leuctra 
 (Fig. ) is a generalized form in which the distance between the 
 postcoila and the paracoila is just one-half that between these 
 points in Perlesta (Fig. 3 0 ) which represents a specialized con- 
 dition. A comparison of figures shows that the other species studied 
 are intermediate between these forms. In a linear series the speci- 
 mens would be arranged: Leuctra, Pteronarcys (nymph), jNem^ura, 
 isopteryx, Acroneuria, Pteronarcys (adult), Perla, Taeniopteryx, 
 Capnia, Alloperla, Chloraperla, Perlesta. 
 
 The tentorium is the end oskel eton of the head and is formed 
 from paired invaginations of the head-capsule. These invaginations 
 of the wall of the head extend entad, meet, and fuse, forming a 
 strong framework for the attachment of muscles and tendons and for 
 the support of the internal organs of the head. The fused portion 
 of the tentorium is usually called the body and the three pairs of 
 projections extending from the head-capsule to the fused portion are 
 called the arms. 
 
f 
 
 
 
 
 
 
 
 
 
 
 - 
 
 
 - 
 
 
 
 
 
 
 
 . 
 
 
 . 
 
 
 
 
 
 , 
 
 ■ 
 
 . 
 
 
 
 
 - 
 
 
 . 
 
 . 
 
 
 , 
 
 . 
 
 
 
 
 
 
 
 
 
 
One pair of invaginations, the metaten torina* , occurs on the 
 ventral aspect where they are considered to have originated as apo- 
 demes of the maxillary segment. The cephalic pair of invaginations, 
 the pretentorinae occurs on the dorsal or later o-dor sal aspect 
 near the cephalic extremities of the epicranial arms and the lateral 
 limits of the f ronto-clypeal suture, when this is present. Each 
 pretentorina is usually also closely associated with a precoila. 
 Carriere and burger concluded from their embryologi cal studies that 
 each pretentorium originates from a spiracle of the mandibular seg- 
 ment while Riley is of the opinion that the originate as apodemes. 
 The third pair of arms, the supratentoria, are attached to the head- 
 capsule on the dorsal aspect cephalad and laterad of a lateral 
 ocellus and fuse with the pretentoria cephalad of the metatentoria 
 in the Plecoptera. The supratentoria are said by some students to 
 be projections of the pretentoria, others suggest that they origin- 
 ate as invaginations similar to those of the posterior and anterior 
 arms. 
 
 Erom embry ological studies of Blatta it has been pointed out 
 that the invaginations for the posterior pair of arms are much less 
 prominent than those of the anterior arms, and, that, due to an 
 apparent retardation in their development in embryos which clearly 
 showed the invaginations for the anterior arms in the mandibular 
 segment, the invaginations for the posterior arms could not be iden- 
 tified. The pretentorinae are small in the embryo of Blatta altho 
 the arms are well developed in the adult. The metatentoria and the 
 metatentorinae are also quite distinct and large in the adult, tho 
 as has been pointed out by Riley, it is sometimes impossible to 
 identify them on embryos that show the pretentorinae. As the 
 
19 
 
 supratent oria are very much reduced in the adult of filatta the 
 aupratentor ia and the supratent orinae must he much smaller in the 
 embryo and probably somewhat more retarded in their development 
 than the posterior arms, and therefore, easily overlooked. The 
 supratentoria in Blatta are very much smaller than in Melanoplus, 
 Gryllus, or Diapheromera which have the supratentoria firmly att- 
 ached to the head-capBule and larger at the point of attachment 
 than near the pretentoria. In the Plecoptera the adult of Pteron- 
 arcys (Pig. y. ^ ) and Isopteryx (Pig. 3V ) are the only forms exam- 
 
 ined in which the supratentoria are smaller at the head- capsule 
 than toward the body of the tentorium as described by Comstock and 
 Kochi, while in Leuctra (Pig. 2. C ), which appears to be one of the 
 most generalized forms examined, the supratentoria are greatly 
 enlarged at the head-capsule. 
 
 As all studies of the morphology of the cockroach have been 
 based on three or four closely related domestic species, is it not 
 possible that more generalized species have large supratentoria like 
 Melanoplus, or Gryllus or even like some of the generalized Plecop- 
 tera and may they not be found on further study of other forms to 
 be invaginated? The subject offers, at least, a broad and rich 
 field for further investigation. 
 
 In many insects the metatent orinae appear as open pockets be- 
 tween the postgenae and the maxillariae. In the Plecoptera the 
 metatent orinae are very prominent and distinct. They are largest in 
 Acroneuria (Pig. S'? ) and Chloraperla, (Pig. 3 3 ) and most reduced 
 
 in Taeniopteryx (Pig. 3 2.) and Nemura (Pig. *16 ). The unfused por- 
 
 A. 
 
 tion of this invagination within the cavity of the head, on each 
 side, is a metatentorium; the fused portion is the corpotent orium. 
 
20 
 
 In the Plecoptera examined the metatentoria are seldom very con- 
 spicuous. In the nymphs of Perla and Pteronarcys, and in Taeniop- 
 teryx, Nem^.ra, Leuctra, Isopteryx, and Capnia (Figs.3 i j sl '3 2 / :L 6f L6 
 the metatentoria extend around the occipital foramen forming a 
 slightly thickened ridge on the ental surface. 
 
 The cephalic arms of the tentorium are the preten toriae, and 
 originate as invaginations which, in generalized insects are usually 
 located on the dorsal aspect of the head. In the Plecoptera exam- 
 ined each pretent orina iB located at the lateral extremity of the 
 f ronto- clypeal suture, when it is present, hut on the lateral as- 
 pect or beneath the frontal ledge which is formed by a bending of 
 the front and clypeus ventrad and mesad. In none of the specimens 
 examined are the pretent orinae visible from the dorsal aspect. In 
 Leuctra, hemura, Capnia, the adult of Pteronarcys, Isopteryx, and 
 Taeniopteryx (Figs.*^^, 36 /- 2 -? 3^32.) each pretentorium is short, very 
 much flattened, and broadly triangular in outline at the peripheral 
 end. One angle of the plate is adjacent to the precoila, one ex- 
 tends laterad and dorsal toward an antafossa, and the third to the 
 laminatent orium. Each pretentcrium is longer and slenderer in the 
 nymph of Pteronarcys, Chloraperla, and Alloperla (Figs. AS, 33, 36 ) 
 than in the preceeding group but they have the same general form as 
 found in that group. Acroneuria, the nymph of Perla, and Perlesta 
 (Figs. ^ \ b) have long, slender, rod-like pretentoria. 
 
 Each supratent orium is attached to the head- capsule on the dor- 
 sal aspect cephalad and laterad of a lateral ocellus and fuses with 
 a pretentorium cephalad of the metatentorium in the Plecoptera. 
 Leuctra (Fig. ) which appears to be one of the most generalized 
 forms studied, and Capnia (Fig. 3 o ) show the supratent oria, 
 

 
 , 
 
 
 
 
 
 
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 ■ 
 
 
 
 - 
 
 , 
 
 
 
 
 
 
 
 
 
 * 
 
 V 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 r 
 
 
 
 • 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 * 
 
 * 
 
 . 
 
 
 
 . 
 
 . 
 
 
 
 
greatly enlarged at the head- capsule. Acroneuria (Fig. a ) and the 
 nymph of Perla (Fig. ^ / ) have cluh-shaped suprat ent oria with the 
 large part of the club toward the head- capsule. The suprat entori a 
 of Taeni opteryx, Nem^ira, Chloraperla, Alloperla, and Perlesta, 
 
 (Figs . 3 ^6.53^4,**) are approximately uniform in size thruout their 
 
 length, while Isopteryx, (Fig.^^ ) and the nymph of Pteronarcys 
 (Figs, j ^ ) have the supratent oria much smaller at the head- 
 
 capsule than at the body of the tentorium. 
 
 The laminatent orium is formed by a fusion of the pretentoria 
 cephalad of and adjacent to the corpotentorium. This fusion is 
 usually indicated by a line or ridge on the meson. The laminatent or- 
 ium is long and the line of fusion is distinctly indicated on the 
 meson in Taeni opteryx, Leuctra, Capnia, Chloraperla, and Alloperla 
 (Figs.3^,*F . In Isopteryx (Fig.^ ) it is narrow. The lam- 
 
 inat ent orium is longer than the corpotentorium in the nymph of 
 Perla (Fig. 3/ ). The laminatent or ium and the corpotentorium are 
 indistinguisedably fused in the nymph of the adult of Pteronarcys, 
 hemjira, Acroneuria, and Perlesta ( Fig/^^ 6 , * >, 53”) , 
 
 The corpotentorium, which is formed by the fusion of the mesal 
 portions of the metatentoria and which fuse with the laminatent orium 
 never has any indication of a line of fusion on the meson. In the 
 Plecoptera it can usually be distinguished from the laminatent orium, 
 measured on the meson, is short in Leuctra, the nymph of Perla, 
 
 Taeni op teryx, Capnia, Alloperla, and Chloraperla, (Figs.*- 36} 3 j) 
 
 is larger in Isopteryx (Fig. ZH ). There is no indication of a 
 line of fusion of the corpotentorium and the laminatent orium in the 
 nymph and adult of Pteronarcys, .wemjira, Acroneuria and Perlesta 
 (Figs.ijja^u^. 3 &). 
 
. 
 
 - 
 
 
 
 
 
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 4 * 
 
 
 
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22 
 
 The laminatent orium hears a single distinct cesctendcn on the 
 cephalic margin in the nymph of Perla, Perlesta, Chloraperla,^ t y. 
 
 x 'j) 
 
 and Acroneuria; in Isopteryx and Alloperla (Figs. 3^ 36) it is rudi- 
 mentary. In the nymph of Perla and Acroneuria (Figs. 3 } A > ) 
 
 there are two flexotendons on the ventral surface of the corpoten- 
 torium near the laminatent crium which extend ventrad. 
 

 
 - 
 
 . 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
MOVABLE PARTS OP HEAD 
 
 23 
 
 The mandibles are appendages of the fourth segment of the head, 
 xhe form of the mandibles varies in the different orders and in 
 the species of the same order according to the food habits of the 
 insects. A reduction in the size, cutting edge, and strength indic- 
 ates specialization in food habits. 
 
 In the mandibles the nymphs figured are typically orthopteran 
 in form, Those of the nymph of Pteronarcys (FigA are slightly 
 
 broader than long with the lateral edge of the proximal end, the 
 distance between the preartis and the postartis, equal to at least 
 two- thirds of the entire length of the mandible while the width 
 of the distal end is slightly less than one-half that of the prox- 
 imal end. The dorsal aspect is convex, the ventral is slightly 
 concave. The dentes or teeth are well developed and heavily chit- 
 inized. The smooth rounded protuberance, the mola, is present at 
 the proximal end of the dentes on the ventral aspect. The setae 
 near the mola, the inf ra^brust iae , are prominent, rhe rectotendon 
 is large and is attached to the mesal edge of the proximal end. 
 
 The extensotendon is slender and is attached on the dorsal aspect iro 
 the latero-ventral extremity. The point of articulation of a man- 
 dible on the dorsal aspect with a precoila is a preartis. It 
 is adjacent to the extensotendon and is convex, in striking contrast 
 to the globular, protruding postartis, the point of articulation 
 with a postcoila on the ventral aspect. The mandibles of the nymph 
 of Perla (Fi gS.6Ln<*) show the same general condition as described 
 above. The length exceeds the breadth and the mandible is thinner 
 and more convexo-concave than in the nymph of Pteronarcys. The mola 
 is not present. 
 
 rhe form of the mandibles of the adults varies greatly.. This 
 

 
 
 
 
 
 
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variation is strikingly shown in the features of the lateral as- 
 pects. Kem^ira, Capnia, Leuctra, and Isopteryx (Figs. 6<>) 
 
 have mandibles very similar to those of the nymph of Pt eronarcys. 
 in Nemura (Fig. ^2-) the distance between the preartis and the 
 postartis is as shown on the lateral aspect, equal to the width of 
 the mandible, while in Alloperla (Fig. (s>°l ) the distance between 
 
 the preartis and the postartis exceeds the width of the proximal 
 end. fhe distal end is greatly reduced but the dentes are sharp and 
 prominent. The mandible of the adult of Pteronarcys (Fig. ) has 
 no prominent angularities, is broader than long and rudimentary in 
 appearance. A very different type is found in Acroneuria, Perlesta, 
 and Chloraperla (Figs. 6%, ) • In these species the mandible is 
 
 not heavily chitinized and are longer than wide. The distance 
 between the preartis and the postartis varies but the proximal end 
 is always much wider than the distal which is always thin and bears 
 from one to several dentes. 
 
 The most generalized condition of the mandibles of any of the 
 species figured is found in the nymph of Pteronarcys (Fig. \y ). 
 They are typically orthopteran in form. The lateral aspect is very 
 broad at the proximal end and is one-half as wide at the distal 
 end. The same general condition is found in Nemura, Taeniopteryx 
 and Capnia, (Figs.4:^, V ) . Leuctra and Isopteryx (Figs.^?. && ) 
 
 have mandibles of the same general type as those described above 
 but the distal end is narrower, tho not printed, and the mandibles 
 are more curved. The mandibles of the adult of Pteronarcys and the 
 nymph of Perla (Figs. ; c'Jr' ) taper to a distinct point as seen 
 from the lateral aspect. Perla is distinctly convexo-concave. The 
 mandibles described above are well chitinized. Two specimens 
 

 
 
 
 . 
 
 
 
 
 
 
 
 
 
 ■ 
 
 - 
 
 
 
 
 
 
 
 
 
 
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 . 
 
 
 
 
 
 
 
 
 
 
 
 
 
 > I 
 
 
 
 
 
 
 
 
 
 
 
 
 
 , 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 • 
 
 - 
 
 , 
 
 
 
 
 
 
 
 
 
 
 
 
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*-!? 
 
 labeled Acroneuria abnormis ( Figs. ) are shown as the 
 
 mandibles are very different in shape. They are only slightly chit- 
 inized, are broad at the proximal end and from the lateral or mesal 
 aspect taper to a sharp point at the distal end. The mandibles of 
 Alloperla (Fig. b°t ) are distinctive in having the proximal end 
 wider on the lateral than on the dorsal ventral aspect. They do not 
 taper to a distinct point as in Acroneuria, Chloraperla and Per- 
 lesta but agree with these forms in being only slightly chitinized. 
 chloraperla and Perlesta (Figs. 72. , ) are very broad at the 
 
 proximal end but are much narrower at the distal end of the prox- 
 imal half. The distal half is dagger-like in form. These two species 
 have the most specialized mandibles figured. 
 
 The maxillae are appendages of the fifth segment of the 
 head. In the Plecoptera the maxillae are typically orthopteran 
 in form. The generalized species have all the parts of a normal 
 generalized type, but the specialized species show a considerable 
 degree of fusion of the different parts, and many sutures which in 
 generalized species are prominent are entirely wanting in the more 
 specialized forms. A subcardo, alacardo , stipes, subgalea, pal- 
 pifer, labial palpus, lacinia, proxagalea, and distagalea are 
 present in most generalised forms. 
 
 Leuctra (Figs. 76 , 77) has a generalized maxillae, ana 
 shows all the different parts. The subcardo, which bears the 
 parartis, is large and distinctly separated from the alacardo which 
 is also large and prominent. The parartis is the chitinized area 
 on the dor s al a sp e c t of the subcardo which articulates with a 
 paracoila. The exparartis (Fig. 86) is at the proximal end, and 
 is enlarge^, slightly concave, and articulates with the outside of 
 
26 
 
 the head-capsule at the paracoila. The entopararti a is tne end 
 which bears the premaxs©:endon and articulates on the ental sur- 
 face. The premaxa tendon is long and slender. It is attached to 
 the entoparartis in close proximity to the alacardo and the rnaxa- 
 coria. The parts of the parartis are not well developed. 
 
 The stipes extends onto the dorsal aspect but is much 
 narrower than on the ventral aspect. It bears long setae on the 
 lateral aspect. The subgalea is the long triangular sclerite on tae 
 ventral aspect between the stipes and the labicoria. The palpifer 
 is not prominent. It bears the palpus which is twice as long as 
 the maxilla. The proxagalea is distinctly separated from tne 
 distagalea and is not heavily chitinized on the ventral aspect. 
 
 The distagalea is sickle-shaped and is setiferous on the dorsal and 
 lateral aspects. It bears a row of seta on the ventral surface of 
 the mesal edge which is grooved. The lacinia is thin and shorter 
 than the galea, the mesal edge is sharp, and a lacinara.stra is 
 present. The lacinia, fits into the groove of the galea. The maxa- 
 coris. and the labiacoria are distinct. 
 
 are distinct 
 
 The subcardo and the alacardo. /in the nymph of Perla (Pig. 
 79), Isopteryx (Pig. 8l), Taeniopteryx (Pig. 83 ), kemura (Fig. 85 ), 
 Capnia(Fig. 88), and Pteronarcus, nymph (Fig. 91), and adult (Fig. 
 90 ). In Chloraperla (Fig. 95) the subcarda is present but not as 
 distinctly separated from the alaca.rdo as in the preceding species. 
 In Alloperla (Fig. 97), Perlesta (Fig. 100 ) , and Acroneuria (Figs. 
 93, 9 4), the subfardo is completely fused with the alacardo, and in 
 the latter there is only a slight indication of a separation of the 
 cardo as a whole from the stipes. 
 
 The stipes is large and densely setiferous in most of the 
 
 n. 
 
27 
 
 species. In the nymph of Perla (Pig. 78 ) its mesal extremity on the 
 dorsal aspect bears a conspicuous row of stout setae which extends 
 from the alacardo to the palpifer. The stipes is partly fused 
 with the subgalea on the ventral aspect in the nymph cf Perla (Pig. 
 79 ) » Acroneuria (Pig. 94 ), and Chloraperla (Pig. 95 ).' In Isop- 
 teryx (Pig. 8l ) , Nemura (Pig. 85 ), and Perlesta (Pig. 100), it is 
 completely fused with the subgalea. The stipes articulates with 
 the alacardo on the dorsal aspect and with the subcardo on the ven- 
 tral aspect. The palpifer is distinct in most of the species fig- 
 ured. It is trie rounded protuberance which bears the palpus. 
 
 The galea is divided into two segments in the generalized 
 forms. In Leuctra (Pigs. 76 , 77), Isopteryx (Pigs. 80 , 8l), Taeniop- 
 teryx (Pigs. 82, 83 ), Nemura (Fjfcgs. 84, 85 ), Capnia (Pigs. 87 , 88), 
 and Pteronarcys nymph (Pig. 91) , and adult (Pig. 90) , the proxagalea 
 'is shorter than the distagalea. In the nymph of Perla (Pig. 78 ) 
 the distagalea is slender and shorter than the proxagalea and re- 
 sembles in general form the galea of Harpalus, a carabid. The 
 distagalea is enlarged at the distal end in Isopteryx (Fig. 8l). 
 There is a dense cluster of setae near the end of the distagalea in 
 Pteronarcys numph (Pig. 91) 2 -nd adult (Fig, 90). The proxagalea 
 and the distagalea are indistinguishably fused in Acroneuria (Pig. 
 93), Chloraperla (Pig. 96 ), Alloperla (Pig. 98), and Perlesta (Pig. 
 99). 
 
 The lacinia is thin and the mesal edge is sharp. The 
 setae of the lacinarastra are stout in Isopteryx (Pig. 80 ) and 
 ITeirftira (Pig. 84), long in the numpt cf Perla (Pig. 78 ), slightly 
 shorter in the nymph of Pteronarcys (Fig. 92) but promiscuously ar- 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ' 
 
 ' 
 
 
 
 
 . 
 
 . 
 
 ‘ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
28 
 
 ranged. The lacinarastra is present in Taeniopteryx (Fig. 82) , 
 Capnia (Fig. 87 ), the adult of Pteronarcys' (Fig. 90 ) , Chloraperla 
 (Fig. 96 ) and Acroneuria (Fig. 94-) » in/the last two it is greatly re- 
 duced. Alloperla (Fig. 98 ) and Perlesta (Fig. 99) have ncjlacinar- 
 astra. Capnia (Fig. 88) and thd nymph of Pteronarcys (Fig. 92) 
 have,, distinct maxadentes. There is a distinct movable hook between 
 the lacinaristra and the distal end of the lacinia which extends as 
 a sharp curved projection beyond the end of the hook. The lacinia 
 is longer than the galea in the nymph of Perla (Fig. 7 f , ) and in 
 
 Perlesta, (Fig. 100) , approximately the same length in Capnia (Fig. 
 
 \ 
 
 88 ), and shorter than the galea in Isopteryx (Fig. 80 ) , Taeniop- 
 teryx (Fig. 82) Xempra (Fig. 84), Pteronarcys adult (Fig. yo) , 
 nymph (Fig. 91), Chloraperla (Fig. 95), and Alloperla (Fig. 97). 
 
 The palpi of the nymphs of Perla (Fig. 78 ) and Pteronar- 
 cys(Fig. 9l), and of Taeniopteryx (Fig. 82 ) are shorter than the 
 maxillae. Each palpus consists of five segments. The proximal seg- 
 ment is always short. The relative length of the different seg- 
 ments varies according to the species. 
 
 The parartis is well developed in Nemura (Fig. 86) , the 
 nymph of Pteronarcys (Fig. 92), and Capnia (Fig. 87 ). It is large 
 and chitinized. Tne exparartis is enlarged and articulates with a 
 par, coila (Fig. 86), The premaxatendon is attached to the entcparar 
 tis adjacent to the alacardo snd the maxacoria. In generalized 
 stone-flies the premaxatendon is well developed. It is rudimentary 
 in the specialized species. In Taeniopteryx (Fig. 82 ) it is 
 short but dilated while in most of the other species it is longer 
 and slenderer. 
 

29 
 
 The maxacoria (Fig. 82) is the membrane on the dorsal as- 
 pect attached to the cardo, the stipes, and the lacinia. The 
 labicoria (Fig. 83) is the membrane on the ventral aspect which 
 fuses with the cardo, the subgalea, and the lacinia. 
 
 The labium is the appendage of th§ sixth segment of the 
 head, a.nd is formed by the fusion of a pair of appendages on the 
 meson. It is attached to the cervicoria and to the ventral as- 
 pect of each maxilla by labicoria. The labium is typically or- 
 thopteran in form in the Plecoptera. The submentum is the large 
 subquadrangular sclerite attached to the cervicoria. The lateral 
 edges are folded under and are continuous with the labicoria. The 
 proximal and the distal ends of the submentum are often emarginate 
 and when not emarginate are usually convex. The mentum is always 
 small. The stipulae are the sclerites distad of the mentum. They 
 bear the glossae and the paraglossae at the distal end, and a pal- 
 piger on either edge. Each palpiger is a rounded protuberance at 
 the disto-lateral or lateral aspect of a stipula and bears a pal- 
 pus. Each palpiger is usually separated from the stipula by a 
 suture or a furrow. The glcssae are the two mesal projections from, 
 the stipul e. The fissure separating the glossae is always pres- 
 ent in the Plecoptera. It is the mesarima. Laterad of each 
 glcssa is a paraglossa which is usually distinctly separated from 
 the glossa by a furrow, the latarima. The labia of the Plecoptera 
 a.re of two types: those which have the paraglossae separated from 
 the stipulae by a suture, as in the cockroach, and those on which 
 the paraglossae are indistinguishably fused with the stipulae. The 
 first group represents the most generalized condition, the latter 
 the specialized. 
 

 
 
 
 N • I 
 
 'N 
 
 
 
 
 
 
 
 
 
 
The subme ntum in Acroneuria (Pig, //o) is broader than 
 long. It is convex on the proximal end and concave on the distal. 
 The same condition 'is found in the nymph of Perla (Fig. /d'*), Per- 
 lesta (Fig.// 3 ) # Alloperla (Fig. //&) has a submentum which is slight 
 
 V* . 
 
 ly concave at both ends and convex on the sides. It is slightly 
 
 brrader than long, Chloraperla (Fig. W) hnd Pteronarcys adult 
 
 (Fig, /<$<?) and nymph (Pig. /dfif show the, same general form. In Isop- 
 
 teryx (Pig, /t*>) and TaeniOpteryx, the submentum is longer than 
 
 broad. It is emarginate at both ends. In Idopteryx (Fig. n*>) the 
 
 sides at the proximal end are roughly parallel" on the distal half, 
 toward which 
 
 they converge mm ti e distc 1 end. /is much narrower than the proximal. 
 The submentum of Leuctrgi (Pig, to*) is roughly hexagonal in outline, 
 in ITemura (Fi g./aC*) and Capnia (Fig./ 7 ^.), rectangular, broader than 
 
 A 
 
 long. In Capnia (Fig. //a.) the distal margin is slightly convex, 
 the proximal concave. 
 
 In the Plecoptera the mentum is usually a narrow sclerite 
 closely associated with the submentum. When the mentum is wanting 
 it is eigher fused with the submentum or is obsolete. The mentum is 
 a narrow band in, Acroneuria (Pig Mp ), nymph of Perla (Fig. /a ' ) ) , 
 Alloperla (Fig. // (?) , Capnia (Fig. //a) , Ueirjira (Fig. /Ob) , and 
 Chloraperla (Fig. y/V). In Chloraperla it is concave, in Capnia, 
 convex, and in the other forms, straight. The mentum is slightly 
 triangular in shape in Pteronarcys, adult and nymph . (Figs. /O e 7,/0£r') t 
 being widest on the meson. It is not well developed in Taeniopteryx 
 (Fig. ///) , Perlesta (Pig. //3), and Isopteryx (Fig.//<o). The stipu- 
 lae is a narrow area in the nymph of Perla (Fig. /o ')) . It is similar 
 though larger in Acroneuria (Fig. I/O), It is longest in the adult 
 
31 
 
 of Pteronarcys (Fig./<J^). 
 
 Each palpiger is distinct in Pteronarcys nymph (Fig./0ff), 
 and adult (Fij./^f), Nemura (Fi ./^6), Chlorape^a (Fig. //^) , Isop- 
 
 A 
 
 teryx (Fig. //<?“) , Alloperla (Fi ^,//6?) , Taeniopteryx (Fig. f// ), Cap- 
 
 nia (Fig.//^.) , Leuctra (Fig./Od*), Peilesta (Fig.//3 ) , and Acrone *i 
 
 (Fig. //0). In the nymph of Pe,rla (Fig. /07) it is not separated "by 
 
 a distinct suture. Tne glossae in trie most generalized forms are 
 
 divergent. The distal end is bluntly rounded. In the nymph of 
 
 Perla (Fig. 107) they are short and broad, triangular in shape. 
 
 The same type is found in Acroneuria (Figi//0 ), Alloperla (Fig. 
 
 //£? ) , Chloraperla (Fig .//£/ ), Isopteryx >(Fig J f& and Perle t 
 
 (Fig. 1 1?>). The adult of Pteronarcys (Fig, /0*7 ), Hemoura (Fig.’ 
 
 > 
 
 Jd&), and Taeniopteryx (Fig. ill ) have glossae intermediate in 
 form between the two types. They are distinctly separated from 
 each other by the mesarima and from the paraglossae by the latarima. 
 The lateral edge is as long as the mesal. They are slightly di- 
 vergent. They are shorter in -Taeniopteryx than in Pteronarcys. In 
 the nymph the glossae are of the same type as in the adult but are 
 convergent at the distal end and bear a thick cluster of long 
 setae. The glossae in Leuctra (Fig. (0$) are almost adjacent for 
 their entire length on the meson and in Capnia they are adjacent 
 througnout their mesal length. 
 
 The paraglossae of Acroneuria (Fig. //0) , Perlesta (Fig. 
 /h3 ), Allopdrla (Fig. //k) , Chloraperla (Fig. / r*y ) , and Isopteryx 
 (Fi g.//p~ ) are very similar in shape, they converge toward the 
 distal end and are roughly triangular in outline. The nymph of 
 Perla (Fig. 107) has paraglossae more nearly globular in shape than 
 in the preceding forms. The ventro.-lateral edge bears a fringe of 
 
32 
 
 long setae. 
 
 The paraglossae of Pteronarcys adult and nymph. (Figs. 
 
 }.08, 109) are longer and narrower than in the preceding forms. In 
 the nymph there is a row of distinct 3etae on the lateral aspect. 
 
 e paraglOBsae of Leuctra (Fig./tfCT) are not separated from the 
 stipulae but they are longer and the latarima is distinct. Their 
 distal eau is larger than the proximal. Lemur a (Fig. 106) and 
 Taeniopteryx (Fig. .Ill) have short paraglossae which are not 
 separated from the stipulae. Capnia (Fig. 112) has the most 
 specialized paraglossae figured. They are in close proximity to the 
 glossae and are almost uniform in width. The glossae and para- 
 glossae are the same length. The mesarima is the fissure which 
 spparates the glossae. In the nymph of Perla (Fig. 107) it is V- 
 shaped, broadly rounded at the base, and extends almost to the men- 
 turo. The base of the mesarima is wider in Acroneuria (Fig. 110) 
 than in the nymph of Perla but it is comparatively shorter. In 
 Perlesta the mesarima is narrow at the base. The same type mesa- 
 rima is found in Alloperla (Fig. Il6) , Isopteryx (Fig, 115) » 
 Chloraperla (Fig. 114), the adult of Pteronarcys (Fig, 109) , 
 
 Fermi r a (Fig. lOo) , and Taeniopteryx (Fig. Ill), but it is much 
 smaller. In the nymph of Pteronarcys (Fig. 108) the mesarima is 
 broader at the base than at the distal end. 
 
 The fundarima, is the proximal end of the mesarima when 
 this furrow extends between the stipulae, when the glossae are 
 fused it is the line which marks the fusion. It is present as such 
 in Leuctra (Fig. 105) , and in Capnia (Fig. 112) , where the glossae 
 are fused. In lie mu r a (Fig. 106) and Taeniopteryx (Fig. Ill ) it 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 * 
 
 
 
 
 . 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 . 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
33 
 
 marks the line of fusion of the stipulae. In Capnia (Fir. 112 ) it 
 is obsolete for a short distance proximad of tne glossae but appears 
 between the stipulae opposite the palpigers. The distal end of tne 
 fundarima is present in the adult of Pteronarcys (Fig. 109 ) as a 
 large dark area. 
 
 The latarimae are distinct in the nymphs of Perla (Fig, 
 
 1 °7), Acroneuria (Fig. 110), Pterinarcys nymph (Fig. 108) and adult 
 (Fig. 109), Perlesta (Fig. 113 ) , Chloraperla (Fig. 114), Isopteryx 
 (Fig. 115), Alloperla (Fig. llo) , Leuctra (Fig. 105), Nemoura (Fig. 
 106), and Taeniopteryx (Fig. 111). The laterimae appear as lines 
 in Capnia (Fig. 112). In Taeniopteryx (Fig. Ill), Leuctra (Fig. 
 105), a^d Uern^ira (Fig, 106) the glossae and the paraglossae are 
 adj acent . 
 
SULIMARY 
 
 34 
 
 The detailed anatomy of the head of several species of 
 Plecoptera which had not been previously studied were examined. 
 
 The parts have been homologized and their taxonomic relations 
 determined. A comprehensive and representative series of genera 
 was studied to determine the lines of development and specializa- 
 tion within the order. 
 
 Specialization is indicated by the absence of sutures, a 
 reduction in size, a change in form or position, and the loss of 
 chitinization of sclerites and also of the appendages. There is a 
 striking difference in structure between the most generalized and 
 the most specialized. The former are distinctively of the orthop- 
 teran type but a: e alv/a.. s, in practically every particular, more 
 specialized than the average of this type, as: 
 
 (1) The epicraneal suture is not as well developed in the 
 Plecoptera as it is in most of tne Orthoptera. The cephalic por- 
 tion of the epicraneal arms is obsolete in all of the species, whi^e 
 in Capnia and Leuctra the stern is obsolete. Both the stem and the 
 arms are wanting in Isopteryx and Xernoura. The epicraneal suture 
 
 is always present in tne Ortnop tera. The fronto-genal sutures 
 which represent the cephalic portions of the epicraneal amis are 
 never present in the Plecoptera. 
 
 (2) The precoil?„e which are always exposed on the dorsal 
 or cephalic aspect of the head of orthopterans are not visible from 
 the dorsal aspect. They are located on the lateral aspect and are 
 always concealed by the dorsal ledge. 
 
 (3) The f rente- clypeal suture is entirely wanting in 
 

 
 
 
 
 
35 
 
 many of the species and when present is usually supported by a lamel- 
 la-like extension of the pre tentonae „ . 
 
 (4) The occipital foramen, by its migration dorsal and 
 caudad, opening on the caudal aspect, indicates a greater degree of 
 specialisation than is found in any orthopte ans. 
 
 ( 5 ) The occipital foramen, by its migration caudad. and 
 dor. ad opening on the caudal aspect, indicates a greater degree 
 
 of specialization than is found in any orthopteran . The contour of 
 that portion of the iiead bearing the paracoila awi the postcoila 
 and bounding the maxillae and labium, is characteristic of those 
 specialized insects that have the mouth at one end of the head and 
 the occipital foramen at the other. 
 
 (6) The an tennaria is as small and of the same degree of 
 development in Capnia a s in the Orthoptera. In Pteronarcys it is 
 large but the increase in size seems to indicate specialization 
 rather than generalization as it is smaller in the nymph than in the 
 adult, Tnis is paralleled in the adult by the much greater size and 
 use of t:.e antennae. 
 
 (7 ) The mandibles of the generalized soecies and of the 
 nymphs are typically orthopteran in form. They are large, have well 
 develop d dentes, and are heavily chitinized. In the specialized 
 forms the mandibles are strikingly different. They are smaller, 
 
 the dentes are usually reduced or absent , as in the adult of 
 Pteronarcys, and are poorly chitinized. 
 
 (8) The orthopteran type of maxillae is characteristic 
 of the generalized Plecoptera. Great reduction in size, form, fu- 
 sion of parts and degee of chitinization has taken place in the 
 

 
 
 
 s 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 . 
 
 
 
 ■' 
 
 
 
 
 
 
 - 
 
 
 ' 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 . 
 
 
 
36 
 
 specialized species. The proxagalea and the distagalea are inc 
 tinguishably fused, and the suture "between the subgalea and the f. 
 stipes is obsolete. The dardo is not separated from the stipes 
 a distinct suture, and tne line separating its two parts can not be 
 distinguished. The parartis in Acroneuria is rudimentary or 
 wanting, and the cuticle of the proximal portion of the maxillae is 
 continuous with that of the head-capsule. 
 
 (9) The labia is also typically orthopteran in form. 
 
 whi c h 
 
 This is true of the speciejs/in other respects are distinctly 
 specialized, while in those that ordinarily are most specialized 
 have the most generalized labia. This is paralleled in the Hepiali- 
 dae, which xe the most generalized Lepidoptera in practically all 
 of their structures, yet have been specialized by the loss of 
 mouth-parts . 
 
 The Plecoptera has been regarded by many as a more 
 generalized group of insects than the Orthoptera. This may be 
 true in some respects but as an order the structure of the head 
 and mouth-parts shows a greater degree of specialization than do 
 tne orthopterans and many of tne most specialized species of 
 stone-flies show specializations that are more striking than those 
 found in any of the Orthoptera., 
 
PLATS I 
 
 Explanation of Figures 
 
 Dorsal Aspect of the Head 
 
 1. Leuctra klapaleki 
 
 2. Hemoura varigata 
 
 3. Acroneuria abnormis 
 
 4 . Taeniopteryx frigida 
 
 5. Isopteryx tripunctata 
 
 6. Perla immarginata, nymph 
 
 7. Alloperla minuta 
 
 8. Pteronarcys rdgalis, adult 
 
 9. Pteronarcys regalis, nymph 
 
 10. Capnia necydaldides 
 
 11 . Chloraperla grammatica 
 
 12 . Perlesta placida 
 
PLATE I 
 
PLATE II 
 
 Explanation of Figures 
 
 Ventral Aspect with. Mouth-parts in place 
 
 13. Leuctra klapaleki, 
 
 14 . ITemouro. vari gata . 
 
 15. Taeniopteryx frigida. 
 l6 Acroneuria atnomis, 
 
 17 . Pteronarcys regalis, adult. 
 
 18. Pteronarcys regalis, nymph. 
 
 19. Perla irnmarginata, nymph. 
 
 20. Ohio rape rla grammati ca, 
 
 21. Capnia necydaloides, 
 
 22. Isopteryx tripunctata. 
 
 23 . Perlesta placida. 
 
 24. All ope rla. rninuta. 
 
PLATE II 
 
39 
 
 PLATE III 
 
 Explanation of Figures 
 
 Ventral Aspect with Mouth-parts Removed 
 25. Leuctra klapaleki. 
 
 2o. Ner.iura varigata. 
 
 27. Acroneuria ahnormis. 
 
 28. Pteronarcys regalis, nymph. 
 
 29. Pteronarcys regalis, adult. 
 
 30. Capnia necydaloides. 
 
 31. Perla immarginata, nymph. 
 
 32. Taeniopteryx frigida. 
 
 33. Chloroperla grammatica. 
 
 34. Isopteryx tripunctata, 
 
 35. Perlesta placida. 
 
 36 . Alloperla minuta. 
 
PLATE IV 
 
 Explanation of Figures 
 Hand i Lies 
 
 37. Pteronarcys regalis nymph, dorsal. 
 3c. Pteronarcys regalis nymph, ventral. 
 39. Pteronarcys regalis nymph, lateral. 
 4-0. Nerijura varigata, ventral. 
 
 4-1. Neri^ira varigata, dorsal. 
 
 42. ITenfiira varigata, lateral. 
 
 43. Pteronarcys regalis, adult, ventral. 
 
 44. Pteronarcys regalis, adult, dorsal. 
 
 45. Pteronarcys regalis, adult, rnesal. 
 
 46. Taeniopteryx frigida, dorsal. 
 
 47. Taeniopteryx frigida, ventral. 
 
 48. Taeniopteryx frigida, lateral. 
 
 49. Leuctra klapalekt, dorsal. 
 
 50. Leuctra klapalekt, ventral. 
 
 7l. Leuctra klapalekt, lateral. 
 
 Oapmia necydaloides , ventral. 
 
 53. Vapnia necydaloides, dorsal. 
 
 54. Capnia necydaloides , me sal. 
 
 55. Capnia necydaloides, lateral. 
 
 56. Perla immarginata nymph, ventral. 
 
 . Perla immarginata nymph, dorsal. 
 
 5c. Perla immarginata nymph, mesal. 
 
41 
 
 PLATE IV (Continued) 
 
 59. Isopteryx tripunctata, ventral. 
 
 60. Isopteryx tripunctata, dorsal. 
 
 61. Isopteryx tripunctata, ental. 
 
 62. Acroneuria abnormis , 'dorsal. 
 
 63 . Acroneuria abnormis , Vventral . 
 
 64. Acroneuria abnormi s , V ental . 
 
 65 . Acroneuria. abnormis, ventral. 
 
 66. Acroneuria abnormis, meso-dorsal . 
 
 67 . Alloperla minuta, dorsal. 
 
 6.8. Alloperla minuta, ventral. 
 
 69 . Alloperla minuta, ental. 
 
 70. Chloraperla grammatica, dorsal. 
 
 71. Chloraperla grammatica, ventral. 
 78. Chloraperla. grammatica, lateral. 
 
 73. Perlesta placida, dorsal. 
 
 74. Perlesta placida, ventral. 
 
 75. Perlesta placida, lateral. 
 
PLATE IV 
 
PLATE V 
 
 Explanation of Figures 
 Maxillae 
 
 76 . Leuctra klapaleld, dorsal. 
 
 77. Leuctra klapaleki, ventre 1. 
 
 78 . Perla iramarginata, nymph, dorsal. 
 
 79. Perla immarginata, nymph, ventral. 
 
 80 . Isopteryx tripunctata, dorsal. 
 
 8 1. Isopteryx tripunctata, ventral. 
 
 82 . Taeniopteryx frigida, dorsal. 
 
 83 . Taeniopteryx frigida, ventral. 
 
 84. Nenura varigata, dorsal. 
 
 85 . ITemura varigata, ventral. 
 
 86. Lemur a varigata, 
 
 87 . Capnia necydaloides , dorsal. 
 
 88. Capnia necydaloides, ventral. 
 
 89 . Pteronarcys regalis adult, dorsal. 
 
 90. Pteronarcys regalis adult, ventral 
 
 91. Pteronarcys regalis nymph, ventral 
 
 92. Pteronarcys regalis nymph , dorsal. 
 
 93. Acroneuria abnormis, dorsal. 
 
 94. Acroneuria abnormis, ventral. 
 
 95. Chloraperla grammatica, ventral, 
 
 96 . Chloraperla grammatica, dorsal. 
 
 97. Alloperla minuta, ventral. 
 
 98 . Alloperla minuta, dorsal. 
 
PLATE V (Continued) 
 
 99. Perlesta placida, dorsal. 
 
 100. 
 
 Perlesta placida, ventral. 
 
 
 
 
 
 Antennae 
 
 
 
 101. 
 
 Pteronarcys 
 
 recalls , 
 
 nymph , 
 
 ventral 
 
 aspect. 
 
 102. 
 
 Pteronarcys 
 
 regalis, 
 
 adul t , 
 
 dorsal aspect. 
 
 103. 
 
 Pteronarcys 
 
 regal is , 
 
 nymph, 
 
 dorsal 
 
 aspect. 
 
 104. 
 
 Pteronarcys 
 
 rogalis, 
 
 adult , 
 
 ventral 
 
 aspect . 
 
PLATE V 
 
44 
 
 PLATE VI 
 
 Explanation of Figures 
 
 Labia 
 
 105. 
 
 Leuctra klapalekc. 
 
 106. 
 
 Nemoura varigata. 
 
 107. 
 
 Perla immarginata , nymph. 
 
 • 
 
 00 
 
 0 
 1 — 1 
 
 Pteronarcys regalis, nymph 
 
 109. 
 
 Pteronarcys regalis, adult 
 
 no. 
 
 Acroneuria abnormis. 
 
 ■111. 
 
 Taeniopteryx frigida. 
 
 112. 
 
 Gapnia necydaloides . 
 
 113. 
 
 Perlesta plucida. * 
 
 114 . 
 
 Chi 0 rap e rl a gramma t i ca . 
 
 115 . 
 
 Isopteryx tripunctata. 
 
 116. 
 
 Alloperla minuta.