MORPHOLOGICAL STUDIES. ON THE HEAD 
 AND MOUTH -PARTS OF THE 
 THYSANOPTERA 
 
 BY 
 
 AivVAH Peterson. 
 
 TKfc LIBRARY •* 
 
 Wk\' OF THE 
 liSWBSiTY OF SLUISiS 
 
 REPRINTED- EROM 
 
 ANNALS OF THE ENTOMOLOGICAL/ SOCIETY OF AMERICA 
 Volume VIII— No. i 
 
 Columbus, Ohio 
 March, 1915 
 
THE 'LIBRARY 
 OF THE 
 
 UHWEBS5TY Of BJJMB 
 
 MORPHOLOGICAL STUDIES ON THE HEAD 
 AND MOUTH -PARTS OF THE 
 THYSANOPTERA 
 
 BY 
 
 Alvah Peterson. 
 
 * 
 
MORPHOLOGICAL STUDIES ON THE HEAD AND 
 MOUTH-PARTS OF THE THYSANOPTERA.* 
 
 By Alvah Peterson. 
 
 TABLE OF CONTENTS. 
 
 PAGE 
 
 I. Introduction 20 
 
 II. Methods , 23 
 
 III. Acknowledgments 24 
 
 IV. Fixed Parts of the Head 24 
 
 Head-capsule 24 
 
 Clypeus and Labrum 27 
 
 Compound Eyes 28 
 
 Ocelli 29 
 
 Tentorium or Internal Head-Skeleton 29 
 
 V. Movable parts of the Head 34 
 
 Antennae 35 
 
 Labium 35 
 
 Maxillae . 36 
 
 Mandibles 43 
 
 VI. Pharynx 48 
 
 VII. Salivary Glands 51 
 
 VIII. Head-glands 54 
 
 IX. Summary 55 
 
 X. Bibliography 57 
 
 INTRODUCTION. 
 
 The small insects of the order Thysanoptera have four 
 long, narrow, membranous, flat, fringed wings. Only a few 
 veins are present in the wings and when at rest they are laid 
 horizontally along the back. The sucking mouth-parts form a 
 cone at the caudo-ventral margin of the head-capsule. The 
 maxillae, in part, and the left mandible are modified into 
 piercing organs and enclosed within the mouth-cone. The 
 mandibles, clypeus and maxillary sclerites are asymmetrical. 
 The tarsi are two-jointed, "bladder-like at the distal end and 
 without claws. The metamorphosis is incomplete. 
 
 The order is divided into two suborders, Terebrantia and 
 Tubulifera. The more important distinguishing characters 
 of these are as follows: The female of the Terebrantia has a 
 .saw-like ovipositor. This is wanting in the female of the 
 Tubulifera. In the Terebrantia the terminal segment of the 
 abdomen of the female is conical, while that of the male is 
 rounded. In the Tubulifera the distal segment of the abdomen 
 is tubular in both sexes. One or more longitudinal veins 
 
 * Contribution from the Entomological Laboratories of the University of 
 Illinois, No. 42. 
 
 20 
 
1915] The Head and Mouth-Parts of Thysanoptera 21 
 
 extend from the proximal to the distal end of the wings in the 
 Terebrantia, while only one, partially developed, median 
 vein occurs in the Tubulifera. 
 
 The Terebrantia are divided into two families, Aeolothripidae 
 and Thripidae, while only one family, Phloeothripidae, occurs in 
 the Tubulifera. It is generally stated by investigators in this 
 field that the Terebrantia are the more generalized and of the 
 two families in this suborder the Aeolothripidae are the more 
 primitive. The following brief summary of the evidence as 
 discussed by Uzel in his "Monographic der Ordnung Thy- 
 sanoptera," page 22, supports the above statement. 
 
 The wing characters of the Terebrantia are more generalized 
 than those of the Tubulifera. The wings of the Terebrantia 
 possess one or more complete longitudinal veins and in some 
 cases cross veins, while the Tubulifera have only one partially 
 developed longitudinal vein and no cross veins. Cross veins 
 among the Terebrantia are present only in the family Aeo- 
 lothripidae. The antennae of most Aeolothripidae are nine- 
 segmented while in all other thrips except the genus Heter- 
 othrips there are six to eight segments found. The maxillary 
 palpi of the Aeolothripidae are four-segmented, while those of 
 the Thripidae are usually three-segmented and those of the 
 Phloeothripidae are composed of two long segments. The 
 labial palpi of the Aeolothripidae are always four-segmented 
 while those of other thrips have but two segments. The 
 above data permit the assumption that the Terebrantia are 
 more primitive than the Tubulifera. This assumption is 
 substantiated by the comparative studies of the suborders 
 made in this paper. 
 
 The primary purpose of this research is to reach as definite 
 a conclusion as possible in respect to the interpretation of the 
 asymmetrical mouth-parts of the Thysanoptera. The litera- 
 ture of this subject shows a decided diversity of views as to 
 their homology and function. Some investigators consider 
 the mouth-parts as fitted for biting, and others as fitted for 
 sucking. The more recent workers, while agreeing that they 
 are of a sucking type, yet disagree as to the homology of the 
 mouth-parts. Besides the study of the mouth-parts, many 
 interesting head structures have been observed. Of these the 
 pharynx, salivary glands and head-glands are discussed and 
 figured. 
 
22 Annals Entomological Society of America [Vol. VIII, 
 
 In view of the limited nature of the morphological studies 
 of former workers, specimens of as many species of thrips as 
 possible were secured for this work in order that an extensive 
 view of the conditions in the order might be observed. Twelve 
 or more species were used and of these the following nine were 
 identified: (1) Heliothrips femoralis Reuter, (2) Frankliniella 
 tritici Fitch, (3) Thrips physapus Linne, (4) Cephalothrips 
 yuccae Hinds, (5) Haplothrips verbasci Osborn, (6) Thrips 
 tabaci Lindeman, (7) Chirothrips manicatus Haliday, (8) 
 Anaphothrips striatus Osborn, (9) Limothrips cerealium 
 Haliday. 
 
 The first five species in the above list are very abundant in 
 the vicinity of Urbana and as living material was necessary 
 for certain methods of preparation used, the most of my obser- 
 vations were made on these. Fortunately these have proven 
 to be typical and also comparatively easy to dissect and section. 
 Heliothrips femoralis was present thruout the year in its 
 nymphal and adult stages in the city and university green- 
 houses. Cephalothrips yuccae was found thruout the year 
 between the closely appressed leaves of the crown of the yucca 
 plant, Yucca filamentosa. Its nymphal stages are abundant 
 from April to December. Only the adult stages of the following 
 were found: Frankliniella tritici occurs in great numbers 
 in the flowers of peonies, roses, composites, etc. Thrips 
 physapus was very abundant in the flowers of dandelions and 
 Haplothrips verbasci can be secured the year around on mullein. 
 Of the above five genera, Heliothrips, Frankliniella and Thrips 
 belong to the suborder Terebrantia while Cephalothrips and 
 Haplothrips belong to the suborder Tubulifera. The nymphs 
 of Heliothrips and Cephalothrips and the adults of all five 
 species mentioned have received similar treatment and 
 observation. 
 
 Thruout the following discussions the structures as they 
 exist in the generalized Terebrantia are considered first and 
 the Tubulifera are compared with them. The generic names 
 have been used in the different discussions and on the figures 
 since only a single species of each genus has been considered. 
 The term nymph is used in the following pages to designate 
 the feeding, active, immature stages. In most cases only the 
 older nymphs were used, however, a few observations were 
 made on the early instars, but these did not differ from the 
 
 uiuc 
 
1915] The Head and Mouth-Parts of Thysanoptera 23 
 
 later instars. The term semi-pupa is used in this paper for 
 the semi-quiescent instar just before the adult stage and it is 
 only referred to in the suborder Tubulifera. 
 
 METHODS. 
 
 For general purposes thrips should be killed in boiling water 
 and preserved in 70 per cent, alcohol. When only the chitinized 
 parts are desired, living or preserved specimens should be boiled 
 for ten or fifteen minutes in a 10 per cent, solution of potassium 
 hydroxide, then reboiled in water to remove the alkali, and 
 finally preserved in 70 per cent, alcohol. Living material 
 treated in the above manner is better than preserved material, 
 for preservation in alcohol tends to make the specimens too 
 brittle for careful dissection. 
 
 The use of a Leitz binocular microscope made possible the 
 dissection of the minute mouth-parts. A number of media 
 were tried in which to make dissections; carbol-aniline oil 
 proved to be the best. Its good qualities are that it evaporates 
 slowly and will clear specimens from any grade of alcohol 
 above 50 per cent. If it be desirable to stain with safranin 
 or orange G the stain can be dissolved in 95 per cent, alcohol 
 and will readily mix with the carbol-aniline oil. 
 
 The staining of the material for dissection with safranin 
 proved to be very useful in differentiating the almost colorless 
 mouth-parts of some of the species. In using aniline oil in any 
 form one precaution must be observed ; as much as possible of the 
 oil should be removed with a blotting paper or a dry rag or by 
 replacing the carbol-aniline oil with carbol-xylene or xylene 
 before mounting in balsam. If the oil be not removed the 
 media in which the parts are immersed will eventually darken. 
 
 Material for sections was fixed with hot (80° C.) corrosive 
 sublimate (saturated corrosive sublimate in 35% alcohol 
 plus 2% of glacial acetic acid) for 15 to 30 minutes, which was 
 replaced by 70% alcohol containing a few drops of iodine 
 for 24 or more hours. 
 
 Paraffin having a melting point of 52-54 C. gave a suf- 
 ficiently firm medium in which to cut sections as thin as five 
 microns. Erhlich's and Delafield's haematoxylins were used 
 for staining sections on slides and orange G. or safranin for 
 counter-staining. The best results were obtained by staining 
 punctured specimens in toto for 24 hours in Delafield's 
 haematoxylin or from 3 to 7 days in borax carmine. 
 
24 Annals Entomological Society of America [Vol. VIII, 
 
 ACKNOWLEDGMENTS. 
 
 This investigation was carried on under the supervision of 
 Dr. A. D. MacGillivray and Dr. J. W. Folsom, and to these 
 men I am greatly indebted for the sincere interest shown and 
 the many valuable suggestions received. For the use of the data 
 on the tentorial structures and their relations in insects in 
 general referred to in this paper, I wish to express my thanks 
 to Dr. A. D. MacGillivray, who allowed me the free use of his 
 unpublished manuscript ; and to Miss Margaret Washington for 
 the loan of drawings of the tentorium. I am indebted to Mr. 
 J. D. Hood, of the U. S. Biological Survey, for the identification 
 of the following five species: Heliothrips femoralis Reuter, 
 Frankliniella tritici Fitch, Thrips physapus Linn., Cepha- 
 lothrips yuccae Hinds, and Haplothrips verbasci Osborn; to 
 Dr. A. F. Shull, of the University of Michigan, who kindly 
 furnished several species of named thrips ; Euthrips ( = Franklin- 
 iella) tritici Fitch, Thrips tabaci Lindeman, Chirothrips man- 
 icatus Haliday and Anaphothrips striatus Osborn; to Professor 
 H. Garman, who supplied me with a few specimens of Limoth- 
 rips cerealium, the species he used in making his observations; 
 to Dr. W. E. Hinds for identifications and information, and 
 to Professor S. A. Forbes and Dr. J. S. Kingsley for corrections 
 and suggestions. 
 
 FIXED PARTS OF THE HEAD. 
 
 In the order Thysanoptera the gross arrangement of the 
 head and mouth-parts (fig. 1, 2 r 5, 8, and 11.) is homologous 
 with the corresponding arrangement of the head and mouth- 
 parts of a generalized homopteron. The cone-like mouth- 
 parts are attached to the caudo-ventral portion of the head- 
 capsule and project caudad between the prothoracic legs, and 
 the antennae are located at the extreme cephalic margin of 
 the head between the compound eyes (fig. 1 and 8). 
 
 Head-capsule (fig. 1, 2, 5, 7, 8, 9, and 11). — Head structures 
 in thrips differ in many respects from those of a generalized 
 insect. In the head-capsule nearly all of the sclerites are com- 
 pletely united and all traces of sutures have been lost. On 
 account of this union the following areas of the head, front 
 (fr.), vertex (vt.), genae (g.), and occiput (o.), are designated 
 only in a general way on the figures. The labrum and the 
 
1915] The Head and Mouth-Parts of Thysanoptera 25 
 
 asymmetrical clypeus are the mesal pieces on the ventral 
 aspect of the mouth-cone. The compound eyes are located 
 at the latero-cephalic corners of the head-capsule, and the 
 ocelli when present are between the compound eyes on the 
 dorsal aspect of the head. 
 
 Heliothrips femoralis (fig. 1, 2, 5, 7, 12, 13, and 19). — A line 
 extending between the vertex and the mouth-cone on the head 
 of a nymph of Heliothrips has a dorso- ventral position, while a 
 similar line on the head of an adult has a cephalo-caudal 
 direction. The position of this line or axis in the nymph 
 suggests a similarity to the position of the head of an orthopteron 
 (Acrididae) , while the position of the head of the adult is similar 
 to the position of the head of an homopteron (Cicada). 
 
 The head-capsule of the nymph is non-reticulated and 
 slightly chitinized, while that of the adult is highly reticulated 
 and heavily chitinized. The mouth-cone of the nymph occupies 
 more than one-half of the cephalic aspect of the head-capsule 
 and extends ventro-cephalad, while in the adult it is com- 
 paratively smaller and projects ventro-caudad. 
 
 Two similar caudal projections (c. a.) arise from the ventro- 
 caudal margin of the head-capsule of the nymph and extend 
 for a short distance as narrow pieces between the maxillary 
 sclerites (mx. s.) and the submentum (sm.). In the adult 
 of Heliothrips the caudal projections (c. a.) differ from those 
 of the nymph in that they are decidedly asymmetrical. The 
 left projection is broadly joined to the head-capsule while the 
 broad right projection has a narrow, neck-like attachment. 
 This striking asymmetry is possibly due to the excessive 
 lateral extension of the clypeus toward the right side. The 
 asymmetry of the caudal projections of other adult Terebrantia 
 such as Thrips physapus Linne (fig. 3 and 6) is not as prominent 
 as that found in Heliothrips. The caudal projections articulate 
 along their dorsal sides against certain sclerites of the prothorax. 
 
 The asymmetry of the head-capsule of the nymph and adult 
 is very evident in a frontal view (fig. 1 and 13). About half-way 
 between the meson and the left side of the head-capsule on the 
 ventral margin of the front there is a decided recurrent angle 
 which is very distinct in the adult. The thickenings, depres- 
 sions, and invaginations on the frontal area of the head will be 
 discussed under the internal head-skeleton. 
 
26 Annals Entomological Society of America [Vol. VIII, 
 
 Cephalothrips yuccce (fig. 8, 9, 11, 17, 18, and 20). — A line 
 extending between the vertex and the mouth-cone on the head 
 of a nymph or adult of Cephalothrips has a cephalo-caudal 
 position and thus in all its stages the position of the head- 
 capsule resembles the position of the head of an homopteron 
 (Cicada.). The nymph of Cephalothrips is very different from 
 the nymph of Heliothrips. 
 
 The head-capsule of the nymph and adult is smooth and 
 non-reticulated. The mouth-cone of the nymph occupies 
 about one-half the ventral aspect of the head (fig. 17), while in 
 the adult it is reduced to about one-third the ventral aspect 
 of the head (fig. 8). This reduction is clearly shown in the 
 semi-pupal stage (fig. 20). The dotted line across the head 
 indicates approximately the point of attachment of the mouth- 
 cone as it would be in an active nymph. This line is dis- 
 tinguishable in the early stages of the semi-pupa by a difference 
 in the staining quality of the two areas on the ventral aspect 
 of the head. With the reduction of the mouth-cone of the 
 Tubulifera, striking modifications have resulted in the form 
 of the clypeus, the left maxillary sclerite, and the piercing 
 organs. The modification and the reduction of these parts 
 during metamorphosis are of great value in determining their 
 homology. 
 
 Symmetrical, caudal projections (c. a.) are present at the 
 ventro-lateral margins of the head-capsule of the nymph and 
 adult. The caudal projections of the adult are prominent and 
 terminate in distinct acetabula which fit against certain sclerites 
 of the prothorax (p. s.). This arrangement permits of a dorso- 
 ventral movement of the head. 
 
 The asymmetry of the caudal margin of the front of the 
 adult is not prominent and in the nymph no asymmetry can 
 be seen. The recurrent angle in the adult is located in a 
 position similar to that in Heliothrips. 
 
 An unidentified suture (s.) occurs on the lateral aspect 
 of the head-capsule of the adult dorsad of the caudal projec- 
 tions. There is also an indication of a suture along the dorso- 
 caudal margin of the head. This suture along with the 
 thickenings and invaginations on the frontal and genal areas 
 of the head will be considered later with the discussion of the 
 internal head-skeleton. 
 
1915] The Head and Mouth-Parts of Thysanoptera 27 
 
 Clypeus and Labrum. — The clypeus and labrum are the 
 mesal pieces on the ventral aspect of the mouth-cone. 
 
 Heliothrips femoralis (fig. 1 and 13). — The adult and nymph 
 show no marked differences in the form of the clypeus and 
 labrum. The clypeus (cl.) is the large asymmetrical piece, 
 and the convex piece at its distal end is the labrum (Ir.) . Garman 
 and other workers have considered the area cephalad of the 
 line f as the clypeus and the area ventrad of this line as the 
 labrum. With this interpretation one meets with considerable 
 difficulty in homologizing the parts in the two suborders. 
 If the line f is a suture, it can not be the clypeo-labral suture, 
 but must be the proximal margin of the clypeus. The area 
 between this so-called suture and the front is a broad hyaline 
 membrane (me.). In safranin-stained material, it is slightly 
 colored while in sagittal sections stained in haematoxylin it 
 shows a decided blue tinge. The above interpretation of the 
 line f may be correct, but a better interpretation seems to be 
 that it marks the point where the heavily chitinized, proximal 
 end of the clypeus becomes more or less membranous, and thus 
 permits the dorsal and lateral movements of the mouth-cone. 
 The line f is located in a deep fold from which point the mouth- 
 cone extends in a caudo-ventral direction in the adult and 
 cephalo- ventrad in the nymph. A distinct line or suture 
 (s.) can be seen extending from the recurrent angle on the front 
 to the left lateral end of- the line f. This suture is the left 
 lateral margin of the clypeus and shows distinctly in the nymph 
 of Heliothrips (fig. 13) and in the adult of Thrips physapus 
 (fig. 3). This evidence supports the interpretation that the' 
 entire area from the front to the small convex labrum is the 
 clypeus. Garman's drawing of Limothrips cerealium shows 
 clearly the above clypeal and labral areas even tho he interprets 
 the parts differently. 
 
 The clypeus (cl.) is an asymmetrical triangle with its right 
 latero-cephalic angle decidedly more cephalad than its left 
 latero-cephalic angle. The left margin of the clypeus is a 
 comparatively straight line from the front to the labrum and 
 nearly parallel with the meson, while the right margin in the 
 adult is a curved line and extends at a decided angle to the 
 meson from the labrum to the point of attachment of the right 
 caudal projection with the head-capsule. This unique asym- 
 metry is characteristic of all the Terebrantia examined. 
 
28 Annals Entomological Society of America [Vol. VIII, 
 
 Cephalothrips yuccce (fig. 8 and 17). — In the nymph of 
 Cephalothrips the clypeus (cl.) is a long, V-shaped, nearly 
 symmetrical piece attached to the caudal margin of the front 
 (fr.). The fronto-clypeal suture between the clypeus and the 
 head-capsule is wanting. The small, distinct, convex piece 
 at the distal end of the clypeus is the labrum (lr.). The clypeus 
 of the semi-pupal stage is smaller than that of the nymph 
 on account of the reduction of the mouth-cone. It is sym- 
 metrical and shows a distinct fronto-clypeal suture, but the 
 clypeo-labral suture is not present, however, it is indicated by 
 indentations on the lateral margins of the clypeus near the 
 distal end of the mouth-cone. The ventral margin of the 
 labrum is bilobed indicating its future adult structure. Within 
 the semi-pupal parts the developing adult structures can 
 be seen. 
 
 In the adult (fig. 8) the clypeus and labrum have been 
 modified thru the reduction of the mouth-cone, but they are 
 distinctly differentiated by sutures and color differences. 
 The above interpretation of the clypeus and labrum, which is 
 the one given by Muir and Kershaw, agrees with the nymph 
 and adult condition found in all Tubulifera observed and it 
 also explains the condition found in the Terebrantia. Garman's 
 interpretation of the clypeus and labrum in the Terebrantia 
 presents difficulties when an attempt is made to apply it to 
 the conditions found in the Tubulifera. 
 
 Compound Eyes. — The compound eyes of Heliothrips (fig. 
 12, 13 and 19) are present in the nymph as small, oval areas 
 on the lateral aspects of the head-capsule in the dorso-caudal 
 region. There are five, round, indistinct bodies in each elevated 
 area. In the nymph of Cephalothrips (fig. 17 and 18) two 
 similar, elevated, small, oval areas are present on the cephalo- 
 lateral aspects of the head, but these possess no facets or round 
 bodies. In the adult stage of both species (fig. 1, 2, 5, 7, 8, 9, 
 and 11) there is a large, reniform area filled with facets, cover- 
 ing the latero-cephalic areas of the head. In the adult of 
 Heliothrips the eyes protrude beyond the general curvature of 
 the head, the facets are scattered, few in number and variable 
 in size. From a ventral view six large, opaque facets can be 
 seen. These larger facets retain their muddy brown color, 
 and do not become clear and transparent as do the remainder 
 
1915] The Head and Mouth-Parts of Thysanoptera 29 
 
 of the facets when they have been treated with potassium 
 hydroxide. In the adult of Cephalothrips the eyes do not 
 protrude from the head-capsule, the facets are numerous, 
 clear, and all approximately of the same size. 
 
 Ocelli. — The ocelli as far as observed are three in number. 
 They are situated on the dorsal aspect of the head between the 
 compound eyes in the form of a triangle with the median ocellus 
 on the cephalic side. In the two species figured the ocelli of 
 Heliothrips (fig. 7) are clear oval bodies located on a small, 
 elevated, triangular area, while the ocelli of Cephalothrips 
 (fig. 9) are clear, circular bodies, larger than the facets of the 
 compound eyes, more distant from each other than those of 
 Heliothrips and not elevated above the general plane of the 
 head-capsule. No traces of ocelli were found in the nymphs. 
 
 Tentorium or Internal Head-skeleton. — Under this 
 heading only those internal structures found in the head that 
 pertain to the tentorium will be considered. For other internal 
 parts the discussions on the maxillae, pharynx and salivary 
 glands should be consulted. 
 
 One finds within the head of a generalized insect a definite 
 arrangement of rod- and plate-like structures which go to 
 support the internal organs and furnish points of attachment 
 for muscles. These rods or plates extend from three pairs 
 of openings on the head-capsule. These openings are known 
 as the invaginations of the anterior arms, dorsal arms and 
 posterior arms of the tentorium. The invaginations of the 
 anterior arms are usually associated with the lateral margins 
 of the clypeus and with one of the points of articulation of the 
 mandibles. The invaginations of the dorsal arms are associated 
 with the points of attachment of the antennae. The invagi- 
 nations of the posterior arms are associated with the occipital 
 foramen, and the point of attachment of the maxillae. These 
 generalized relations become modified in the various orders 
 and families of insects. The invaginations may disappear and the 
 rods and plates undergo striking changes or become atrophied. 
 The important point, however, is that in all orders of insects, 
 so far as observed, the invaginations and the arms of the tent- 
 orium are always associated with the appendages named. 
 These associations exist in thrips and an attempt will be made 
 to show that they have an important bearing on the homology 
 of their mouth-parts. 
 
30 Annals Entomological Society of America [Vol. VIII, 
 
 On account of the close relation existing between the 
 Homoptera and the Thysanoptera, a comparison will be made 
 between the internal head-structures of Cicada and those of 
 thrips. Figure 29 is a lateral view of the entire tentorium 
 of Cicada showing the invaginations and the arms. The 
 invaginations of the anterior arms of the tentorium (i. a.) 
 occur at the ventro-lateral ends of the clypeus while the dorsal 
 arms are invaginated (i. d.) just ventrad of the antennae. 
 Between these two pairs of invaginations distinct, thin, broad, 
 chitinized plates extend which are the anterior arms of the 
 tentorium (a. a.). The invaginations of the posterior arms 
 (i. p.) are found on the lateral margins of the occipital foramen 
 adjacent to the maxillary plates described by Muir and Ker- 
 shaw. These invaginations give rise to the broad, plate-like 
 posterior arms (p. a.) which nearly surround the occipital 
 foramen. A small chitinized rod extends across the occipital 
 foramen between the dorsal ends of the posterior arms. This 
 rod is the body of the tentorium (b. t.) and from its mesal 
 portion two long, slender rods arise which extend cephalad 
 (d. a.) and unite with the invaginations of the dorsal arms 
 (i. d.). These two rods are the dors'al arms of the tentorium. 
 In Cicada the ventral ends of the posterior and anterior arms 
 unite on each side (xc.) and between these united arms there 
 extends across the meson a distinct plate or bridge (zc). The 
 usual association between the appendages of the head and the 
 tentorium among insects may be found in Cicada; that is, the 
 invaginations of the posterior arms (i. p.) are near the point 
 of attachment of the maxillary, piercing organs with the head- 
 capsule; the mandibles are associated with the anterior arms 
 but on account of their great length and the excessive develop- 
 ment of the clypeus, they are not connected near the invagina- 
 tions of the anterior arms (i. e.) but connect with the head- 
 capsule slightly laterad of the anterior arms near the invagina- 
 tions of the dorsal arms ; and the invaginations of the dorsal 
 arms (i. d.) are adjacent to the antennae. 
 
 As mentioned before, the internal head-skeleton of thrips 
 undergoes considerable modification and this is well illustrated 
 in the various heads figured. In the nymph of Heliothrips 
 (fig 12, 13, 19 and 28) the tentorial structures are nearly all 
 present, while in the adult of Cephalothrips (fig. 8 and 33) 
 
1915] The Head and Mouth-Parts of Thysanoptera 31 
 
 extensive atrophy has taken place and those tentorial parts 
 which still remain can only be interpreted by comparison 
 with the more generalized conditions in the nymphs. 
 
 On the cephalic aspect of the head of a nymph of Heliothrips 
 (fig. 13) two sets of invaginations can be seen. One pair 
 exists on the ventral margin of the front (i. a.) and the second 
 pair (i. d.) a short distance dorsad of these. Since the invagina- 
 tions on the ventral margin of the front are close to the lateral 
 edges of the wide clypeus, they actually occur on the lateral 
 aspects of the head-capsule. This is especially true of the 
 invaginations on the right side. Distinct chitinous arms extend 
 between the invaginations on each side (a. a.). The invagi- 
 nations, as described, are homologous with the two pairs of 
 invaginations on the cephalic aspect of the head of Cicada. 
 The two invaginations on the ventral margin of the front 
 are believed to be the invaginations of the anterior arms of 
 the tentorium (i. a.) while the invaginations dorsad of these 
 are the invaginations of the dorsal arms of the tentorium 
 (i. d.). The rods between the invaginations on each side are 
 the anterior arms (a. a.). Chitinized thickenings (d. a.) 
 extend dorso-caudad from each of the invaginations of the 
 dorsal arms (i. d.) and these thickenings on reaching the caudal 
 margin of the head-capsule turn caudad and run parallel 
 with the caudal margin of the head (p. a.) to the distal ends 
 of the caudal projections (c. a.). On comparison with Cicada 
 the dorso-caudal extensions that project from the invaginations 
 of the dorsal arms to the caudal margin of the head are homol- 
 ogous with the dorsal arms of the tentorium (d. a.) while the 
 chitinized thickenings along the lateral margins of the occipital 
 forearm (p. a.) are considered as homologous with the posterior 
 arms of the tentorium. In thrips the body of the tentorium 
 is wanting, consequently the dorsal arms (d. a.) connect directly 
 with the posterior arms (p. a.). The invaginations for the 
 posterior arms (i. p.) were not identified in any of the thrips 
 examined. In a few cases it seemed as though the invaginations 
 were present near the ventral end of the posterior arms (p. a.) 
 where the thickenings (x.) on the ventral margin of the head- 
 capsule came in contact with the posterior arms. Thus far 
 the tentorium of a nymph of the Terebrantia can be homologized 
 with the tentorium of Cicada. 
 
32 Annals Entomological Society of America [Vol. VIII, 
 
 The differences which occur between the tentorium of a 
 Cicada and a thrips can be largely accounted for in the dif- 
 ferences between the head-structures. The head-capsule of a 
 thrips is elongated cephalo-caudad while this extension of the 
 head-capsule of a Cicada is short. Furthermore in a Cicada 
 the clypeus is large and occupies the greater part of the cephalic 
 aspect of the head but this is not so in thrips. The invaginations 
 of the dorsal arms (i. d.) in thrips are apparently not associated 
 with the antennae. This seemingly lost association is undoubt- 
 edly due to the excessive cephalic or dorsal growth of the head- 
 capsule. The location of the invaginations of the anterior 
 arms of the tentorium (i. a.) in thrips resembles the location 
 in a generalized insect since they are near the cephalo-lateral 
 corners of the clypeus and not near the distal ends of the 
 clypeus as in Cicada. 
 
 The following tentorial structures are present in thrips 
 which are difficult to account for on the basis of the homology 
 with the structures in Cicada. A distinct thickening extends 
 along the caudal or ventral margin of the head-capsule between 
 the caudal projections (x. and z.). This thickening is present 
 in all thrips. In the nymph of Heliothrips the portion of the 
 thickening (x.) between the ventral end of the right anterior 
 arm (a. a.) and the right posterior arm (p. a.) is wanting. The 
 above thickening may be secondary or possibly may be homol- 
 ogized with the union of the ventral ends of the anterior and 
 posterior arms of Cicada (xc. and zc). In all the thrips 
 examined, distinct, chitinous, ental projections arise from the 
 invaginations of the dorsal arms. These projections are not 
 present in Cicada. 
 
 In considering the tentorium of thrips figured in this paper, 
 a distinct atrophy of certain portions and parts can be seen. In 
 the adult of Heliothrips (fig. 1 and 27) a thickening occurs 
 (x. and z.) along the caudo-ventral margin of the head-capsule 
 and from this thickening other thickenings (p. a.) arise which 
 extend to the caudal projections (c. a.) of the head-capsule. 
 Of the anterior arms (a. a.) only the left one is present. The 
 left anterior arm is located in a depressed line extending between 
 the invagination of the left dorsal arm and the recurrent 
 angle on the caudal margin of the front. The dorsal arms 
 (d. a.) are wanting and also the posterior arms (p. a.) except 
 for a small portion on the caudal projections (c. a.) of the 
 
1915] The Head and Mouth-Parts of Thysanoptera 33 
 
 head-capsule. The invaginations of the anterior arms are very- 
 indistinct in this species due to the thick, highly chitinized 
 and reticulated nature of the head-capsule. This characteristic 
 also makes it difficult to differentiate the tentorial thickenings. 
 However in Thrips physapus (fig. 3 and 6) the invaginations 
 of the anterior (i. a.) as well as the tentorial thickenings are 
 readily made out in dissected heads that have been stained. 
 The invaginations of the dorsal arms (i. d.) with their ental 
 projections show very clearly in the adult of Heliothrips and 
 likewise in all thrips. 
 
 In the nymph of Cephalothrips (fig. 22) a still greater 
 reduction of the tentorium apparently exists. This apparent 
 loss, however, may be due to the fact that it is impossible to 
 distinguish all the parts on account of the extremely thin and 
 membranous nature of the head-capsule. Only the invagin- 
 ations of the dorsal arms (i. d.) with their ental projections 
 show very clearly. By careful staining one can distinguish 
 a thickening (a. a.) occupying the position of the left anterior 
 arm of the tentorium. This thickening extends from the 
 invagination of the left dorsal arm (i. d.) to the point where the 
 asymmetrical piercing organ (1. md.) unites with the head- 
 capsule. In this feature there is a remarkable similarity 
 between the nymphal head of the Tubulifera and the adult 
 head of the generalized Terebrantia. 
 
 The tentorium of the adult of Cephalothrips resembles 
 somewhat that of Heliothrips. In Cephalothrips the anterior 
 arms of the tentorium are atrophied but the invaginations 
 of the dorsal arms with their projections are very distinct 
 and the invaginations of the anterior arms can be identified 
 in dissected material that has been stained, if a careful examina- 
 tion is made of the caudal boundary of the head-capsule (z.) 
 at the point where the lateral margins of the clypeus are in 
 contact with the front. Besides the thickenings that are similar 
 to those of Heliothrips two thickenings extend cephalad (mx. p.) 
 on the lateral areas of the head-capsule. These cephalic- 
 extending thickenings clearly arise from the thickenings (z.) 
 about the caudo-ventral margin of the head-capsule and 
 terminate in enlarged, elevated ends which possess acetabula 
 in which the proximal pieces of the paired, piercing organs 
 (mx.) articulate. One can readily see that these mandibular 
 pillars, as Muir and Kershaw call them, are of a tentorial origin. 
 
34 Annals Entomological Society oj America [Vol. VIII, 
 
 A distinct strip (st.) is present on the caudal margin of the 
 head-capsule of Cephalothrips and extends between the short 
 sutures (s.) which arise from the latero-caudal margin. Stained 
 material shows clearly that this area (st.) is structurally dif- 
 ferent from the head-capsule cephalad of it. On the ends 
 of this area (st.) two depressions are present which are similar 
 in many respects to the invaginations of the tentorium. If 
 these are invaginations of the posterior arms then this is a 
 thickening composed of the union of the posterior arms about 
 the dorsal margin of the occipital foramen. This interpretation 
 is rather questionable since no invaginations of the posterior 
 arms have been seen in the more generalized thrips. 
 
 As a conclusion to the above discussion on the internal 
 head-skeleton, the following statements are of importance. 
 The tentorium and the associated mouth-parts of an homopteron 
 such as Cicada can be interpreted on comparison with the same 
 parts in a generalized insect. The arms of the tentorium and 
 their respective invaginations in the nymph of one of the 
 Terebrantia can be homologized with the tentorial parts of a 
 Cicada. On the basis of this homology the specialized and 
 atrophied conditions existing in the adults of the Terebrantia 
 and in the nymphs and adults of the Tubulifera can be inter- 
 preted. 
 
 Furthermore, on the basis of this homology between the 
 tentorium of thrips and other insects an attempt will be made 
 to demonstrate the association of the mandibles and the 
 maxillae with their respective tentorial parts. This should 
 give conclusive evidence as to the correct interpretation of 
 the two sets of piercing organs. See discussion on mandibles 
 and maxillae for this demonstration. 
 
 MOVABLE PARTS OF THE HEAD. 
 
 The movable parts of the head will be considered in the 
 following order: antennae, labium, maxillae and mandibles. 
 In the discussion of the movable appendages those of the 
 generalized Terebrantia will be considered first and then the 
 homologous structures of the Tubulifera will be compared 
 with them. 
 
1915] The Head and Mouth-Parts of Thysanoptera 35 
 
 Antennae (fig. 4) . — The antennae of the adult of Helio- 
 thrips are about twice the length of the head-capsule, eight- 
 segmented and reticulated with chitinous elevations cor- 
 responding to the reticulations (rt.) on the head-capsule. 
 The shape, size, reticulated character and the setal arrangement 
 are shown in the figure. Segments one, two, six, seven and 
 eight are of a brownish color similar to the head-capsule while 
 the remaining segments are but slightly pigmented. The 
 distal end of segment four gives rise to a thin, hyaline, two- 
 branched sense cone (s. c). The sense cones are not present 
 in the antennae of the nymph. Except for this the antennae 
 of the nymph are structurally similar to those of the adult. 
 
 The antennae of the nymph and adult of Cephalothrips 
 resemble each other and are of the same number of segments, 
 eight, providing the very small pieces at the distal ends of the 
 nymphal antennae are considered as segments. In the first 
 stages of the semi-pupa (fig. 20) the antennae appear as 
 mere buds at the cephalic margin of the head-capsule. As the 
 insect becomes older, the antennal cases increase in size and 
 length until they extend around the lateral margins of the head- 
 capsule. A late stage of the semi-pupal instar shows seg- 
 mentation on the distal portions of the antennal cases. The 
 eight-segmented antennae of the adult are about one and one- 
 third times longer than the dorsal aspect of the head-capsule 
 and have a yellow color thruout, except the two basal segments, 
 which are of a brown tinge. The general shape, size and setal 
 arrangement of the segments is shown in figure 10. Segments 
 three, four, five and six each possess at their distal end a pair 
 of simple, hyaline, spine-like sense cones (s. a). The sense 
 cones are wanting in the nymph. 
 
 Labium. — The mouth-parts of Hemiptera are fitted ' for 
 sucking. In this adaptation the labium is modified into a long, 
 trough-like beak, enclosing the bristle-like mandibles and 
 maxillae. The mouth-parts of Thysanoptera are also fitted 
 for sucking. The adaptation in thrips, however, is not confined 
 to the specialization of one part of the mouth, but the clypeus, 
 labrum, maxillary sclerites and labium together form a broad 
 and blunt mouth-cone, enclosing the needle-like mandibles 
 and maxillae. Of these two types of sucking mouth-parts, 
 those of the Thysanoptera more closely resemble a generalized 
 
36 Annals Entomological Society of America [Vol. VIII, 
 
 biting type of mouth. The arrangement of the parts of the 
 mouth-cone, the presence of maxillary and labial palpi and 
 other characteristics in Thysanoptera show this close similarity. 
 
 The labium of the nymph and adult of thrips is similar 
 (fig. 60 and 61). The entire convex, caudal area of the mouth- 
 cone is the labium. From a caudal view it is distinctly tri- 
 angular in outline, with the apex at its distal end and from a 
 ventral or lateral view it is convex. The labium is attached 
 along its lateral margins to the triangular, palpus-bearing 
 sclerites (mx. s.) while its dorsal margin is united with the ventral 
 membranous portion of the prothorax. It is composed of 
 two distinct sclerites separated by a transverse suture (s.). 
 The proximal piece is the submentum (sm.) and the distal 
 piece is the mentum (m.). The mentum has at its distal 
 end a membranous area which gives rise to a pair of small, 
 two-segmented palpi (lb. pi.). The distal margin of the 
 mentum possesses two small, chitinous projections considered 
 by Hinds as paraglossae (pr.). 
 
 Heliothrips femoralis. — The labium of Heliothrips (fig. 60) 
 corresponds to the general description. The dorso- ventral 
 length of the labium is about the width of the submentum. 
 The ectal surface of the labium is slightly reticulated. 
 
 Cephalothrips yuccce. — The labium of Cephalothrips (fig. 61) 
 corresponds to the general description. The mouth-cone of 
 Cephalothrips is short, consequently the dorso-ventral length 
 of the labium is less than one-half the width of its submentum 
 (s. m.). The submentum is heavily chitinized along its proxi- 
 mal margin as indicated in the figure. The mesal portion of 
 the suture (s.) between the submentum and the mentum is 
 obsolete, but its lateral ends show distinctly. The two pro- 
 jections or paraglossae at the distal end of the labium are united 
 and form a lip-like structure over which the movable, paired, 
 piercing organs pass. This lip is indicated by shading in 
 figure 8. - 
 
 Maxillae. — A pair of asymmetrical, triangular, palpus- 
 bearing sclerites (mx. s.) are situated on the lateral sides of the 
 clypeus and labrum in the mouth-cone of the Thysanoptera 
 (fig. 1 and 8). The palpi (mx. pi.) of these pieces are usually 
 two- or three-segmented. A comparison of these palpus- 
 bearing sclerites with similar sclerites in Cicada shows that 
 
1915] The Head and Mouth-Parts of Thysanoptera 37 
 
 they are homologous with the so-called maxillary plates 
 described by Muir and Kershaw. The segmented palpi 
 however are wanting on the maxillary plates of the Hemiptera. 
 Their presence in Thysanoptera indicates a more primitive 
 condition and also is conclusive evidence that these sclerites 
 are maxillary in origin. 
 
 The lateral margins of the maxillary sclerites (mx. s.) are 
 turned into the mouth-cone, thus giving rise to ento-mesal 
 extensions (et.) or plates. These extensions, toward the distal 
 end of the mouth-cone, unite with the lateral edges of the 
 pharynx and form sheaths or troughs over which the needle- 
 like portions of the paired piercing organs (mx.) pass. This 
 is clearly shown in cross-sections of the pharynx (fig. 38-44 and 
 51-57). 
 
 Two sets of piercing organs are found in the mouth-cone of 
 all Thysanoptera (fig. 22, 27, 28 and 33). The paired, sym- 
 metrical set will be considered first because of their relation 
 to the maxillary sclerites. Each one of the paired piercing 
 organs is composed of three parts (mx.). Its distal portion 
 is long, grooved, needle-like, and swollen at the proximal end. 
 The middle portion is short and thick and separated from the 
 distal portion by a distinct suture while its proximal end is 
 connected to the proximal portion by a movable joint. The 
 proximal portion is a heavy piece and in some cases three or 
 four times as long as the middle portion. Also its proximal 
 end articulates against the cephalic edge of the maxillary 
 sclerite (mx. s.) or in an acetabulum on an elevated pillar 
 (mx. p.) arising from the head-capsule. This articulation in 
 all cases is either directly or indirectly associated "with the 
 maxillary sclerite. The presence of two movable joints in 
 this piercing organ makes its extension into a straight line pos- 
 sible. This extension occurs when the organ functions. When 
 the paired piercing organs are extended beyond the mouth-cone 
 the grooved, needle-like piercing portions interlock and such a 
 union naturally makes an effective tool. This interlocking is 
 readily seen in sectioned material (fig. 25). 
 
 The muscles which bring about the extension of the paired 
 piercing organs (mx.) are attached to the proximal pieces. 
 Each extensor muscle is connected with the ental surface of the 
 maxillary sclerite and the mesal edge of the proximal piece 
 when the piercing organs (mx.) are within the head-capsule. 
 
38 
 
 Annals Entomological Society of America [Vol. VIII, 
 
 The above characteristics concerning the maxillary sclerites 
 (mx. s.) and the paired piercing organs (mx.) may be found in 
 all thrips as far as observed and one can safely infer that the 
 similar parts in all thrips are homologous and should have the 
 same interpretation thruout the order. In a like manner it 
 will be found that a similar homology exists between the 
 asymmetrical piercing organs of all thrips. This homology 
 eliminates the suggestion offered by Muir and Kershaw that 
 in the two suborders of the Thysanoptera a distinct difference 
 of interpretation of the piercing organs might be possible. 
 
 Heliothrips femoralis (fig. 1, 2, 5, 12, 13, 19, 27, and 28).— 
 The maxillae of Heliothrips are in general typical of the sub- 
 order Terebrantia and accord with the general description 
 of the maxillae just given. The maxillary sclerites (mx. s.) 
 of the nymph and adult resemble each other except that the 
 palpi of the nymph are four-segmented while those of the 
 adult consist of only two long segments. The palpi of some of 
 the more generalized adult Aeolothripidae are four-segmented 
 and thus the four-segmented palpi of the nymph indicate a 
 generalized condition. The maxillary sclerites (mx. s.) are 
 decidedly asymmetrical. This asymmetry is more pronounced 
 in the adult (fig. 1). The proximal ends of the maxillary sclerites 
 of the adult are not in direct contact with the head-capsule. 
 There are distinct membranous areas (me.) between them. 
 In this particular species the membranous areas (me.) continue 
 from the cephalic margin of the maxillary sclerites to the bases 
 of the palpi where the membranes broaden and surround the 
 proximal segments. The membranous strip on the left maxil- 
 lary sclerite is more extensive than the one on the right. The 
 ento-mesal extensions (et.) arising from the lateral margins 
 of thejmaxillary sclerites resemble closely the general descrip- 
 tion of these parts and they can be readily made out in figures 
 28 and 39 to 44. 
 
 The paired piercing organs of Heliothrips (fig. 27 and 28, 
 mx.) are but slightly longer than the maxillary sclerites and 
 this is true of the Terebrantia in general. The structure of 
 their paired piercing organs resembles the general description 
 of these parts. The proximal piece in the nymph is L-shaped 
 and the base of the L articulates along the dorsal margin of the 
 maxillary sclerite (mx. s.). The proximal piece in the adult, 
 
1915] The Head and Mouth-Parts of Thysanoptera 39 
 
 likewise articulates against the cephalic margin of the maxillary 
 sclerite and has an L-shape, but the base of the L is greatly 
 reduced and the erect portion is longer, heavier and of the 
 same thickness thruout. The above relation between the 
 paired piercing organs and the maxillary sclerites is true of all 
 Terebrantia as far as observed (fig. 14, 21 and 32). Since 
 the ' paired piercing organs in the Terebrantia are directly 
 connected with the cephalic margin or the latero-cephalic 
 corners of the maxillary sclerite, one may conclude, as Garman 
 has already done in his work on Limothrips cerealium, that these 
 piercing organs are parts of the maxillae. If such is the case 
 the paired piercing organs may be modified lacinia as Garman 
 has suggested. According to Garman and other workers 
 these organs consist of only two pieces, but in all species observed 
 three distinct pieces can be found. The suture between the 
 needle-like distal portion and the middle piece is wanting in 
 the figures of other workers. 
 
 Karl Jordan and other workers have interpreted the paired 
 piercing organs (mx.) as mandibles. Jordan worked with 
 Heliothrips haemorrhoidalis, a form closely related to Helio- 
 thrips femoralis. His proof is largely founded on the state- 
 ment that the embryonic development shows that these pieces 
 are mandibles. The evidence in support of this statement 
 is wanting in both figures and discussion. He has but one 
 figure of the embryonic condition and in this the relation of the 
 so-called mandibles to the head-capsule and other parts is 
 not clear. The real difficulty with Jordan's interpretation 
 rests in the fact that he has wrongly identified the asym- 
 metrical piercing organ (1. md.) and consequently in his 
 search for mandibles he has from necessity tried to show that 
 the paired piercing organs are mandibles. 
 
 The conclusion in regard to the paired piercing organs of 
 the Terebrantia is that they are portions of the maxillae, and 
 the following additional evidence will be presented to sub- 
 stantiate the same. 
 
 In the first place, if these paired piercing organs are mandi- 
 bles, how can one explain their three piece structure? Mandibles 
 usually have only one solid piece, but maxillae on the other hand 
 are composed of several sclerites and upon modification they 
 might assume the conditions found in thrips. As to the exact 
 
40 Annals Entomological Society of America [Vol. VIII, 
 
 homology of these pieces it is difficult to determine. However, 
 on the basis of the maxillae of other insects, one finds that the 
 galea is usually segmented while the lacinia is not. In most 
 sucking insects the galea is present and the lacinia may be 
 wanting. It would seem that the paired piercing organs were 
 galea rather than lacinia as Garman has suggested. For 
 convenience of description the paired piercing organs will be 
 designated as the maxillary setae (mx.). 
 
 Strong evidence is likewise .found in the relation existing 
 between the paired piercing organs and the tentorium of the 
 head. The maxillae in generalized insects are associated with 
 the invaginations of the posterior arms of the tentorium and 
 this relation was shown to be true with Cicada. On the basis 
 of the homology between the tentorium of Cicada and thrips 
 it was shown that the thickening about the occipital foramen in 
 the nymph of Heliothrips (p. a.) are homologous with the 
 posterior arms of Cicada. In the nymph and adult of 
 Heliothrips the paired piercing organs are closely associ- 
 ated with the ventral or caudal ends of these thickenings or 
 so-called posterior arms. As mentioned before, no invagina- 
 tion of the posterior arms of the tentorium could be found in 
 this genus of thrips; however, from the close association of 
 the paired piercing organs with the posterior arms of the 
 tentorium we may conclude that the generalized relation 
 between the posterior arms of the tentorium and the maxillae 
 holds for thrips as well as for more primitive insects. 
 
 Further evidence is found in the fact that in generalized 
 insects the maxillae are always attached to the head-capsule 
 caudad of the mandibles. As was pointed out- in the dis- 
 cussion of the head-capsule of thrips, the mouth-cone has been 
 carried around the head until it is now found at the caudo- 
 ventral portion of the head. As a result the mouth-parts 
 have a cephalo-caudal extension rather than the generalized 
 dorso-ventral direction. This rotation of the mouth-parts 
 means that the maxillae should connect with the head-capsule 
 dorsad of the mandibles. The paired piercing organs of the 
 Terebrantia (mx.) are decidedly dorsad to the asymmetrical 
 piercing organ (1. md.), and it will be shown later that the 
 asymmetrical piercing organ is a mandible. 
 
1915] The Head and Mouth-Parts of Thysanoptera 41 
 
 Cephalothrips yuccce. — The maxillae of Cephalothrips (fig. 
 8, 11, 17, 18, 20, 22, and 33) are typical of the suborder Tubu- 
 lifera. The maxillary sclerites (mx. s.) of the nymph (fig. 17) 
 and the adult (fig. 8) are dissimilar in comparative size and 
 symmetry, but both possess a two-segmented palpus (mx. pi.). 
 The long maxillary sclerites of the nymph are nearly symmetrical 
 while those in the reduced mouth-cone of the adult are asym- 
 metrical. This asymmetry in the adult is characterized by a 
 distinct knob (n.) at the cephalic end of the mesal margin 
 of the maxillary sclerite. This knob fits into a notch on the 
 left margin of the clypeus and appears to be a portion of the 
 maxillary sclerite, but a close examination shows that it is 
 only connected with the maxillary sclerite by a narrow strip 
 at its latero-cephalic corner; and a distinct fissure, which extends 
 cephalad and laterad from the point where the clypeo-maxil- 
 lary suture turns mesad, separates it from the maxillary 
 sclerite. The asymmetrical piercing organ of the adult is 
 associated with the above asymmetry. On comparison with 
 the asymmetry found in the Terebrantia the asymmetry of the 
 Tubulifera is of a decidedly different nature. 
 
 The mesal extensions (et.) of the lateral edges of the maxil- 
 lary sclerites resemble the general description of these parts 
 (fig. 22, 33 and 35). However, a unique modification of these 
 extensions is found in the development of two long, trough- 
 like, cephalic extending arms (mx. g.). These parts serve to 
 guide the long maxillary setae and have been designated as 
 the maxillary guides. These guides are only present in the 
 Tubulifera, and undoubtedly they have been developed in this 
 suborder on account of the necessity of some kind of a guiding 
 structure for the exceedingly long maxillary setae.. The max- 
 illary guides arise from the mesal part of the sheath (et.) that 
 extends between the lateral edges of the maxillary sclerites at 
 the distal end of the mouth-cone, and project forward into the 
 dorsal portion of the head-cavity to the region of the compound 
 eyes. Cross-sections of the pharynx (fig. 53 and 54) show 
 that in this forward projection they unite with the pharynx. 
 In the nymph they are narrow at their proximal ends but 
 broadly rounded and separate at their distal ends, while in the 
 adult they are trough-like and their distal ends are united in a 
 transverse plate. The long maxillary setae pass around the 
 
42 Annals Entomological Society of America [Vol. VIII, 
 
 cephalic ends of the maxillary guides and at first are dorsad of 
 the guides. In their caudo-ventral extension they pass along 
 the lateral sides of the trough-like guides and laterad of the 
 pharynx toward the tip of the mouth-cone beyond which the 
 two distal, grooved, needle-like portions interlock and pro- 
 ject as a single piercing organ (fig. 25). The maxillary guides 
 show no relation to the tentorial structures of the head. 
 
 The paired piercing organs (mx.) of Cephalothrips, except 
 for size, and point of attachment in the adult, resemble the 
 maxillary setae of Heliothrips. The linear extension of the 
 maxillary setae in Cephalothrips is due largely to the increased 
 length of the distal and proximal pieces. The large proximal 
 piece in the adult possesses a distinct knob on its mesal margin 
 which serves as a point of attachment for muscles. The 
 resemblance between the maxillary setae in the nymph or adult 
 of Heliothrips (fig. 27 or 28) and the paired piercing organs in 
 the nymph of Cephalothrips is very striking and important. 
 The feature of significance in the nymph of Cephalothrips is the 
 point of attachment of the paired piercing organs with the 
 cephalic margin of the maxillary sclerites. This shows clearly 
 that the paired piercing organs of Cephalothrips are not only 
 homologous in structure but in point of articulation with the 
 maxillary setae of the Terebrantia. The paired piercing organs 
 of the Tubulifera are therefore maxillary setae. The evidence 
 used to prove that the paired piercing organs of the Tere- 
 brantia are maxillary in origin will likewise hold for the Tu- 
 bulifera with the exception of the evidence based on the ten- 
 torium, since some of the tentorial structures are wanting 
 in the nymph of Cephalothrips. 
 
 The relation of the maxillary setae to the mouth-parts in 
 the adults of the Tubulifera (fig. 33) is apparently very different 
 from the conditions found in the Terebrantia. The reduction 
 of the mouth-cone explains to a large extent the characteristic 
 modifications of this suborder. In the reduction of the mouth- 
 cone the proximal pieces of the long maxillary setae (mx.) do 
 not retain their point of articulation with the maxillary scler- 
 ites but articulate in acetabula located in special elevated 
 maxillary pillars (mx. p.) on the ental surface of the head- 
 capsule cephalad of the maxillary sclerites. This modification 
 is a result of a mechanical necessity. It would be impossible 
 
1915] The Head and Mouth-Parts of Thysanoptera 43 
 
 for the long maxillary setae to function if they retained their 
 connection with the reduced maxillary sclerites. However, 
 their connection with the maxillary sclerites is not entirely 
 lost since the thickenings from which the maxillary pillars 
 (mx. p.) arise, extend to the tentorium (x. and z.), which is 
 adjacent to the maxillary sclerites. These thickenings are 
 undoubtedly derived from the tentorium, and since the maxillae 
 are associated with them they are possibly modifications of 
 the posterior arms (p. a.). 
 
 Muir and Kershaw in their work on a species of Tubulifera 
 interpret the paired, piercing organs as mandibles. They 
 give two lines of evidence. In the first place, they show that 
 the asymmetrical piercing organ (1. md.) is a part of the 
 maxillae, consequently the paired piercing organs must be man- 
 dibles. Also they have proved to their own satisfaction by 
 comparison with Rhynchota that the paired piercing organs 
 are mandibles. "The paired setae we consider as mandibles, 
 homologous to those of Rhynchota." The first part of their 
 evidence will be considered later. In regard to their own 
 statement here quoted there is considerable doubt. They 
 present no satisfactory evidence on this point, and as far as 
 observed, the relations existing in Cicada permit of no such 
 interpretation, as has already been shown. Jordan also studied 
 individuals of the Tubulifera and came to the same conclusions 
 as in his studies on the Terebrantia. The different inter- 
 pretations of the paired piercing organs in the Thysanoptera 
 of the authors mentioned above has been largely due to an 
 incomplete comparative study of the nymphal and adult stages 
 of both the suborders. 
 
 Mandibles. — A large, heavy, unpaired piercing organ 
 (1. md.) is present on the left side of the mouth-cone of all 
 thrips (fig. 2, 3, 11, 12, 18, 22, 27, 28, and 33) while on the right 
 side no such organ is found. Berlese figures the asymmetrical 
 piercing organ as occurring on the right side. Undoubtedly 
 this is a mistake for in no case has such an occurrence been 
 recorded by other workers. This asymmetry of mouth-parts 
 is unique among insects. The asymmetrical, piercing organ 
 connects with the head-capsule at the point where the left 
 margin of the clypeus or the right margin of the maxillary 
 sclerite comes in contact with the front. It is composed of 
 
44 Annals Entomological Society of America [Vol. VIII, 
 
 two parts, which are of about the same length. The 
 distal half is a long, heavily chitinized, hollow spine, while the 
 proximal half is broader, hollow and not as heavily chitinized. 
 Cross and longitudinal sections show a distinct lumen extending 
 thruout its length (fig. 26). At the very tip of the spine it 
 is impossible however to be sure that an opening is present. 
 If such an opening is present then it may serve as an exit for the 
 secretions from the glandular tissue within the basal half of the 
 piercing organ. If the lumen is not a duct for the secretions 
 but a blind tube, then one can explain its presence on the basis 
 that it is the place formerly occupied by the hypodermal cells 
 which formed the cuticle of the spine. 
 
 Cross-sections of the pharynx show how the asymetrical 
 piercing organ passes along the cephalic side of a left lateral 
 extension of the pharynx (fig. 40 and 41). This left lateral 
 extension is cephalad of the paired extensions over which the 
 maxillary setae pass, and only occurs on the left side. In the 
 Tubulifera the left maxillary guide helps to form this extension 
 (%• 53). 
 
 There are large muscles attached to the proximal end of the 
 asymmetrical piercing organ. These muscles connect with 
 the dorsal aspect of the head-capsule and on contraction 
 pull the asymmetrical piercing organ (1. md.) into the 
 mouth-cone. The majority of the specimens examined showed 
 the piercing organ protruding from the concave side of 
 the labrum (lr.). Apparently this is its normal position and 
 it is evidently used in making the first puncture thru the 
 outer cells of the plant tissue. After this puncture is made 
 it is probably withdrawn into the mouth-cone, and the 
 long maxillary setae are then used to puncture the inner and 
 deeper cell-layers. The withdrawal of the asymmetrical 
 piercing organ ' from the tip of the mouthrcone also permits 
 the plant juices to pass into the pharynx. 
 
 On the right side of the mouth-cone a piece (r. md.) is found 
 which is homologous in position to that of the left, asymmetrical 
 piercing organ (rig. 27). In all cases this piece arises adjacent 
 to the front, between the clypeus and the right maxillary 
 sclerite, and extends to the right cephalo-lateral margin of the 
 pharynx and unites with the same (fig. 40). This union is 
 indicated by a distinct suture in cross-sections of the pharynx. 
 
1915] The Head and Month-Parts of Thysanoptera 45 
 
 The invaginations of the anterior arms of the tentorium (i. a.) 
 in thrips are always located on the caudal margin of the front 
 and adjacent to the asymmetrical piercing organ and the above- 
 described piece. This evidence, undoubtedly shows that this 
 rudimentary piece on the right side is homologous with the 
 asymmetrical piercing organ on the left. 
 
 Heliothrips femoralis (fig. 2, 5, 12, 19, 27 and 28.).— The 
 left asymmetrical piercing organ and its right homologous 
 rudiment are typical of the suborder Terebrantia and correspond 
 to the general description given above. These parts in the 
 nymph and adult closely resemble each other. In the nymph 
 (fig. 28) the left, piercing organ is smaller, straighter, and its 
 two halves less differentiated than in the adult (fig. 27). The 
 basal half of the left, piercing organ is connected with the front 
 by a narrow, chitinized neck. This connection is readily seen 
 in frontal sections of the adult and nymph. 
 
 The right, rudimentary piece in the nymph and adult 
 extends as a distinct piece between the right cephalo-lateral 
 margin of the pharynx and the front. This piece is homologous 
 with the asymmetrical piercing organ (1. md.) and resembles 
 somewhat its basal half. The distinguishing differences in these 
 two are their size and the union of the right piece with the 
 pharynx. Jordan represents a prominent projection on the 
 right margin of the pharynx of Heliothrips haemorrhoidalis. 
 This probably represents the asymmetrical piece above de- 
 scribed. Garman found in his work on Limothrips a small, 
 right, asymmetrical structure which he considered to be a 
 rudimentary piece homologous with the left, piercing organ. 
 These two pieces Garman designated as mandibles. This 
 interpretation agrees with my own observations. Garman's 
 own words are, "The organ has every appearance of being a 
 mandible. Its form and its relation to the other mouth- 
 parts and to the epicranium all indicate this. Nothing cor- 
 responding to this conspicuous organ is apparent on the right 
 side of the head unless a very small, chitinous structure under 
 the edge of the clypeus is a rudiment of the organ for this side." 
 Garman records the structure of the left mandible as con- 
 sisting of one solid piece. The writer however, considers the 
 left mandible as composed of two parts. At least in all thrips 
 examined a distinct constriction occurs between the distal 
 spine and the large proximal portion, and to all appearances a 
 
46 Annals Entomological Society of America [Vol. VIII r 
 
 suture is located here. Without any question the mandible 
 is hollow, as heretofore explained. The following evidence goes 
 to show that the asymmetrical piercing organs of the Tere- 
 brantia are mandibular in origin. 
 
 In the first place, as Garman has already mentioned, the 
 structure, position, and point of connection of the asym- 
 metrical piercing organ with the head-capsule are strong 
 mandibular characteristics. It has already been pointed out 
 that the invaginations for the tentorial arms on the front of 
 thrips are homologous with the invaginations of the tentorial 
 arms on the cephalic aspect of the head of Cicada. The in- 
 vaginations of the anterior arms of the tentorium on the caudal 
 margin of the front (i. a.) are adjacent to the point of connection 
 of the left and right asymmetrical organs (1. md. and r. md.). 
 This relation shows that the asymmetrical piercing organs of 
 the Terebrantia are undoubtedly mandibles since in generalized 
 insects mandibles are usually associated with the invaginations 
 of the anterior arms of the tentorium. 
 
 Furthermore, mandibles in generalized insects are always 
 connected with the head-capsule cephalad of the maxillae. In 
 the nymph of Heliothrips the asymmetrical piercing organs 
 are connected cephalad of the maxillary setae but in the adult 
 on account of the change of position of the mouth-cone these 
 connections are ventrad of the maxillary setae. This relation 
 in the Terebrantia shows clearly that the asymmetrical pierc- 
 ing organs are mandibular in origin. 
 
 Karl Jordan interprets the left, asymmetrical piercing organ 
 (1. md.) as a modified epipharynx. He bases his conclusion 
 upon embryological studies. The one embryo figured shows 
 the asymmetrical organ as the upper portion of the anterior 
 end of the alimentary canal. The base of the so-called epi- 
 pharynx is also connected with the head-capsule. He con- 
 cludes from this figure that when the adult stage is reached the 
 epipharynx loses all connection with the alimentary canal and 
 becomes more firmly attached to the head-capsule. The 
 above hypothesis in regard to the behavior of the epipharynx 
 is contrary to the relations of these parts in other insects. 
 Jordan likewise failed to account for the right, rudimentary 
 piece (r. md.). On the whole Jordan's interpretation is very 
 unsatisfactory, for it does not explain numerous relations here 
 pointed out. 
 
1915] 
 
 The Head and Mouth-Parts of Thysanoptera 
 
 47 
 
 Cephalothrips yuccce (fig. 11, 18, 22, 26, 33, 34, 35 and 36).— A 
 striking similarity exists between the ental view of the mouth- 
 parts of a nymph of Cephalothrips (fig. 22) and the adult stage 
 of Heliothrips (fig. 27). This similarity is particularly notice- 
 able in respect to the structure and position of the asymmetrical 
 piercing parts. On the basis of this similarity it is possible 
 to derive the correcf interpretation of the asymmetrical, 
 piercing organs. The left mandible (1. md.) in the nymph is 
 a long, nearly straight, stout, two-segmented structure and con- 
 nects with the caudal margin of the front (f.) at the point where 
 the maxillary sclerite and clypeus unite with the head-capsule. 
 Between this point of connection and the invagination of the 
 left dorsal arm of the tentorium a thickening occurs (a. a.). 
 This thickening is the anterior arm of the tentorium and can be 
 seen in carefully stained and dissected material. 
 
 The right, rudimentary mandible in the nymph of Cephal- 
 othrips is represented by a distinct chitinized thickening be- 
 tween the clypeus and the right maxillary sclerite. The 
 distal end of this thickening (r. md.) unites with the right 
 lateral margin of the pharynx as seen in cross-sections of the 
 adult pharynx (fig. 55 and 56). Figures 33 and 35 do not 
 show this connection because of the spreading of the heads. 
 
 The above nymphal evidence clearly demonstrates the 
 homology between the asymmetrical mandibular parts of a 
 species of Tubulifera and those of a species of Terebrantia. 
 The evidence used in. proving that the asymmetrical piercing 
 organs of the Terebrantia are mandibles is equally applicable 
 in demonstrating that the asymmetrical parts in the Tubulifera 
 are also mandibles. One exception however occurs in the fact 
 that in the nymph of Cephalothrips the invaginations for the 
 anterior arms have not been identified along with other ten- 
 torial structures. 
 
 The mandibular parts of the adult (fig. 33) undergo a 
 distinct modification due to the reduction in the size of the 
 mouth-cone. In this change the left mandible (1. md.) retains 
 its position between the clypeus and the left maxillary sclerite, 
 but in so doing the heavy, proximal portion turns and forms 
 a half circle. This modification has resulted in the formation 
 of a distinct notch (n.) in the clypeus and the left maxillary 
 sclerite. It has also resulted in the uniting of the meso-cephalic 
 
48 Annals Entomological Society of America [Vol. VIII, 
 
 corner of the maxillary sclerite and the base of the left man- 
 dible. The right mandible in the reduction of the mouth-cone 
 has given rise to a distinct, crook-like, ental projection. The 
 above modifications are typical for the suborder Tubulifera as 
 far as observed. 
 
 Muir and Kershaw interpreted the left, asymmetrical 
 piercing organ as a part of the maxillae in their work on one 
 species of Tubulifera. "The unpaired setae arises from the left 
 maxillae and is a part thereof." It is readily understood how 
 such an error came about, since only, the adult stage of a single 
 species of Tubulifera was studied. The connection of the left 
 asymmetrical piercing organ in the adult with the left maxil- 
 lary sclerite is secondary. This connection does not show in 
 the nymph. 
 
 PHARYNX. 
 
 If one examines the mouth-cone of various species of thrips, a 
 short, heavily chitinized, ham-shaped piece is always found just 
 beneath the clypeus. Stained and sectioned material shows 
 clearly that this piece is a structural modification of the ali- 
 mentary canal, fitted for sucking and homologous with the 
 pharynx of sucking insects (fig. 5, 6, 19, 27, 35, 38-46, 48, 49 
 and 52-59). Jordan figures this particular piece and calls it 
 the hypopharynx of the pharynx, while his epipharynx is what 
 we have interpreted to be the left mandible. 
 
 Structurally the pharynx consists of one piece except for a 
 small, chitinized, transverse area (t. a.) at the caudo-dorsal end 
 (fig. 46 and 48). However, in respect to size and shape the 
 pharynx is divided into two regions. The ventral half is a 
 small, chitinized tube which opens within the ental groove 
 of the labrum, while the dorsal half widens out into a broad, 
 thick region. The entire pharynx resembles a gourd dipper 
 which has had removed from one side almost one-half of its 
 enlarged portion, and across this cavity there has been placed 
 a concave, elastic membrane (e.) to which muscles (d. m.) 
 are attached. On the caudal side of the enlarged portion there 
 are two small openings. Glandular ducts (d.) pass thru these 
 openings and pour their secretions into the sucking chamber. 
 
 When the pharynx is sectioned in one of three planes, 
 frontal, sagittal and transverse, it shows a centrally located 
 canal extending thru its entire length. This lumen starts with 
 
1915] The Head and Mouth-Parts of Thysanoptera 49 
 
 the mouth and continues dorsad of the pharynx as the oesopha- 
 gus (oe.). Transverse sections of the pharynx show that it is 
 firmly connected on its caudo-lateral aspects with the maxillary 
 guides (mx. g.) and the mesal extensions (et. and 1. et.) from the 
 lateral margins of the maxillary sclerites and the labium. 
 Over the lateral extensions (et.) the maxillary setae (mx.) pass. 
 The left mandible (1. md.) passes over an extension on the 
 left lateral margin of the pharynx cephalad of the above 
 extensions and the right, rudimentary mandible (r. md.) 
 unites with the right margin of the pharynx. The above 
 extensions and supports that connect with the pharynx serve 
 to hold it in position while the large muscles are dilating its 
 elastic membrane (e.). The above facts in regard to the 
 pharynx hold for all thrips as far as observed. 
 
 Heliothrips femoralis. — Figures 38-46 and 58 give a much 
 better idea of the general structure of the pharynx of Helio- 
 thrips than a lengthy discussion, consequently only the im- 
 portant and exceptional facts will be pointed out. The trans- 
 verse sections (fig. 37-44) begin with a section thru the com- 
 missures between the supraoesophageal and suboesophageal 
 ganglia and end at the tip of the mouth-cone. Every second or 
 third section, ten microns in thickness, has been figured. In 
 this series the lumen (1.), lateral extensions (et. and 1. et.), 
 piercing organs (mx., 1. md. and r. md.), elastic membrane 
 (e.), and muscles • (d. m.) are shown. The connection of the 
 muscles along the meson of the elastic membrane is character- 
 istic of the Terebrantia. The large nucleated and cross- 
 striated muscles, the dilators of the pharynx (fig. 58, d. m.), 
 extend cephalad into the cavity of the head-capsule and unite 
 with the ventral and cephalic areas. Two or more of these large 
 muscle bands unite with a more or less chitinous tendon 
 (c. t.) which arises from the elastic membrane. Besides these 
 long muscles a number of short muscles extend between the 
 small ventral portion of the pharynx and the clypeus. 
 
 The food of thrips is of a liquid nature and is sucked into the 
 oesophagus in the following manner, judging from the structure 
 of the parts. The muscles along the meson of the elastic 
 membrane contract and dilate the lumen of the pharynx so 
 that a partial vacuum is formed, and into this cavity is sucked 
 the juice in which the tip of the mouth-cone is immersed. On 
 
50 
 
 Annals Entomological Society of America [Vol. VIII, 
 
 the relaxation of the dilating muscles the elastic membrane forces 
 the food dorsad thru the open valve (v.) into the oesophagus. 
 A more detailed account will be given of this process in the 
 discussion of the pharynx of Cephalothrips. 
 
 Cephalothrips yuccce. — In a similar manner, as with Helio- 
 thrips, figures 48-57 and 59 show the chief characteristics of 
 the pharynx of the Tubulifera. In its main features it cor- 
 responds with the general description of the pharynx but in a 
 few details it shows a greater degree of specialization than the 
 pharynx of Heliothrips. In the first place it is strikingly ham- 
 shaped and comparatively smaller. This difference in size 
 is probably due to the reduction of the mouth-cone. Trans- 
 verse sections show that the maxillary guides (mx. g.) unite 
 with the pharynx for a short distance, and the extension over 
 which the left mandible passes is the cephalic margin of the 
 left, maxillary guide. The right, rudimentary mandible (r. md.) 
 is not as large as in Heliothrips, however, it still retains its con- 
 nection with the right side of the pharynx. The lumen (1.) 
 of the pharynx is straight. The most striking difference between 
 the pharynx of Heliothrips and Cephalothrips is in the arrange- 
 ment of the dilating muscles. 
 
 A lateral view and transverse sections of the pharynx 
 show a distinct chitinized plate (pt.) standing on edge along the 
 meson of the elastic membrane (e.). The muscles are confined 
 to the ventral and dorsal ends of this plate and the majority 
 of them are connected to the chitinous tendon (c. t.) at the 
 ventral end. These muscles, so-called ventral dilators (v. d. m.), 
 extend into the head and unite with the vertex. There is a 
 small band of muscles, the so-called dorsal dilators (d. d. m.), 
 which extend between the dorsal end of the plate and the caudo- 
 ventral area of the front. This arrangement of muscles is 
 easily derived from the more generalized type found in Helio- 
 thrips. On the whole the form and arrangement of the parts 
 in the pharynx of Cephalothrips would suggest that it is a 
 more efficient organ than that of Heliothrips. 
 
 The lumen (1») of the alimentary canal is cut off by a valve- 
 like structure at the dorsal end of the pharynx (fig. 58a.). A 
 prominent , projection (p.) extends from the caudo- dorsal end 
 of the pharynx into the lumen. This projection fits into a 
 pocket (po.) on the opposite side. Under normal conditions 
 
1915] The Head and Mouth-Parts of Thysanoptera 51 
 
 this valve is closed, consequently the lumen of the oesophagus 
 is cut off from the pharynx while the elastic membrane is dilated. 
 In suction the ventral dilating muscles (v. d. m.) contract and 
 pull out the ventral portion of the elastic membrane. This 
 results in the formation of a partial vacuum, and into this space 
 the liquid food is drawn. The dorsal dilators (d. d. m.) now 
 contract and dilate the dorsal end of the elastic membrane 
 (e.). This dilation opens the valve separating the lumen of the 
 pharynx and the oesophagus. While the dorsal dilators are 
 contracting the ventral dilators relax and the ventral portion 
 of the elastic membrane presses upon the enclosed food. This 
 pressure forces the food dorsad in the lumen and then on the 
 relaxation of the dorsal dilators the elastic membrane falls 
 back into its normal position and the food is forced on and into 
 the oesophagus. This completes one stroke or dilation of the 
 pharynx of Cephalothrips. 
 
 The dilating muscles of the pharynx (d. m.) of Heliothrips, as 
 before noted, are arranged along the meson of the elastic mem- 
 brane (e.). This fact, along with the large size of the pharynx 
 and its peculiar bent condition, would indicate that the elastic 
 membrane does not have as distinct a ventro-dorsal dilation 
 as that of Cephalothrips. Possibly the entire central portion 
 of the membrane is dilated at one time and then as the large 
 muscles relax the few strands of muscles uniting with the 
 pharynx in the region of the valve contract and open the lumen. 
 With the valve open and the elastic membrane pressing upon 
 the enclosed food, the plant juice is forced into the oesophagus. 
 
 SALIVARY GLANDS. 
 
 Two or three kinds of salivary glands (fig. 15, 16, 23, 24, 
 30, 31, 38-44, 45, 49 and 51-55) are present in thrips and 
 these are all located in the thorax and abdomen. Uzel has 
 figured the glandular portion of the salivary glands located 
 in the thorax and abdomen but does not figure the course 
 of the ducts. In Uzel's figures two kinds of salivary glands are 
 present in the thorax of Aeolothrips fasciatus, a form belonging 
 to the suborder Terebrantia, and three kinds of salivary glands 
 are found in the thorax of Trichothrips copiosa, a form belonging 
 to the suborder Tubulifera. The species figured in this paper 
 have only two kinds of salivary glands, which extend into the 
 thorax and abdomen. These two kinds are paired and distinct. 
 
52 Annals Entomological Society of America [Vol. VIII, 
 
 Considerable variation occurs in regard to the exact location of 
 these glands and the position and points of union of their ducts. 
 
 One of the two pairs of salivary glands (fig. 15 and 24, 31) 
 in the thorax and .abdomen of the two species here considered 
 is long, tubular and more or less homogenous thruout its length 
 1. s. g.). This pair is located laterad or dorsad of the alimentary 
 canal and continues caudad into the abdomen. Longitudinal 
 sections thru these glands show that they are homogenous, 
 nucleated and apparently syncitial. 
 
 The second pair of salivary glands (fig. 16 and 30) are short, 
 thick and more or less irregular in outline and usually confined 
 to the thorax (s. s. g.). The cells of these glands are large and 
 the cell walls are distinct. The nuclei are prominent and 
 stain deeply. The protoplasm of the tissue stains un- 
 evenly and is more or less filled with vacuoles. A distinct 
 centrally located lumen extends thruout the entire length of the 
 gland. 
 
 The above two pairs of salivary glands give rise to ducts 
 (1. d. and s. d.) at their cephalic ends. The four ducts extend 
 into the head laterad and dorsad of the oesophagus. In the 
 region between the supraoesophageal and suboesophageal 
 ganglia the ducts turn ventrad and continue to the y-shaped 
 chitinous structure (y.) caudad of the pharynx (fig. 23, 45, 49, 
 58 and 59.). Within this structure or before entering it the 
 four ducts unite into a common duct and this common duct 
 (c. d.) continues ventrad to the apex of the mouth-cone. The 
 secretions from the salivary glands are thus* poured into the 
 punctures in the host plant which are made by the mouth-parts. 
 When thrips puncture green leaves or colored flowers the area 
 about the puncture becomes light in color. This discoloration 
 is undoubtedly due to the action of the salivary secretion on 
 the plant tissue. 
 
 The y-shaped piece (y.) caudad of the pharynx in all thrips 
 is a characteristic structure. In both suborders this piece is 
 somewhat different in shape (fig. 35 and 47) but it has the same 
 relation to the mouth-parts. There is a distinct muscle band 
 or there may be several bands (mu.) extending between the 
 base of the arms of the y and the dorsal part of the caudal aspect 
 of the pharynx (ph.). Possibly, on contraction, these muscles 
 move the y-shaped piece in such a manner as to control the flow 
 from the salivary glands. The exact homology of the y-shaped 
 
1915] The Head and Mouth-Parts of Thysanoptera 53 
 
 structure is not clear but its general position and structure, 
 its opening into the mouth-cavity and its relation to the sali- 
 vary ducts would indicate that it is homologous with the salivary 
 syringe of the Hemiptera. 
 
 Heliothrips femoralis. — The long, narrow, tube-like glands 
 (fig. 31 and 24) are located laterad of the alimentary canal 
 (1. s. g.) and extend from the metathorax caudad to about the 
 fourth abdominal segment. Figure 24 shows that the gland is 
 narrow and has a straight lumen extending thruout its length. 
 The ducts from these glands (fig. 31, 1. d.) are enlarged and 
 located dorsad of the oesophagus in the region of the pro- 
 thorax and mesothorax. The enlarged portions of the ducts 
 serve as reservoirs for the secretion. The ducts from these 
 similar right and left glands unite with each other before 
 entering the y-shaped structure caudad of the pharynx. 
 
 The short, thick, oval glands (fig. 30, s. s. g.) in the thorax 
 are about the length of two segments and located laterad of the 
 oesophagus. The right gland is usually confined to the meta- 
 thorax and the first abdominal segment. The cells of these 
 glands are exceedingly large and distinctly differentiated. 
 One or two deeply staining, irregular nuclei can be seen in each 
 cell, and the protoplasm of the cells stains more or less unevenly. 
 The ducts from these glands (s. d.) have about the same thick- 
 ness thruout. In the region of the head they are somewhat 
 larger than the ducts from the above glands in the same region. 
 The duct from the left gland unites with the duct from the 
 right similar gland before entering the y-shaped piece (fig. 40, 
 45 and 58). Within the y-shaped piece the united ducts from 
 the two kinds of glands join and form a common duct which 
 continues to nearly the apex of the mouth-cone. 
 
 Cephalothrips yucca. — The long, tube-like glands of Cephalo- 
 thrips (fig. 15, 1. s. g.) are located laterad of the alimentary 
 canal and extend from the prothorax into the first or second 
 abdominal segment. These glands are homologous with the 
 long, tube-like glands in Heliothrips, but they are thicker 
 and : have a sinuous lumen thruout their length. The cell 
 consistency is about the same as in Heliothrips. The ducts 
 extending from these glands into the head are very small and 
 not dilated in the thorax. 
 
54 Annals Entomological Society of America [Vol. VIII, 
 
 The short, thick glands (fig. 16, s. s. g.) in the thorax of 
 Cephalothrips are usually confined to the mesothorax and 
 metathorax and located laterad of the alimentary canal. These 
 glands are homologous with the short, thick glands of Helio- 
 thrips. In Cephalothrips these glands are longer and made 
 up of smaller cells filled with vacuoles and granulated areas. 
 The ducts from these glands (fig. 23) are of the same consistency 
 thruout and only slightly larger than the ducts from the long 
 tubular glands. The union of the salivary ducts in the head 
 is somewhat different from that of Heliothrips. The ducts 
 (fig. 23, 49, 52-55 and 59) of one side (1. d. and s. d.) unite and 
 these united ducts meet in the center of the mouth-cone and 
 form a common duct (c. d.) before entering the y-shaped piece. 
 
 HEAD- GLANDS. 
 
 A distinct, multinucleated and deeply staining tissue (h. g.) 
 is present in definite parts of the head and mouth-cone of 
 thrips and so far as known is described here for the first time 
 (fig. 26, 38-43, 45, 49, 51-55, 58 and 59). This tissue, as 
 far as can be determined, is of a glandular nature and it will 
 be here designated as the head-gland. Its histological struc- 
 ture is different from that of the thoracic glands. Num- 
 erous and coarsely granulated nuclei are present; the cell walls 
 cannot be differentiated; the protoplasm stains unevenly and 
 no lumen could be identified. This tissue is most abundant 
 in the members of the suborder Terebrantia. In Heliothrips 
 (fig. 26, 38-43, 45 and 58) it occurs in three distinct regions. 
 The most prominent massing occurs cephalad of the pharynx 
 (h. g.) on the two sides of the dilating muscles. The extent 
 and shape of these two masses varies considerably but the 
 figures of the transverse sections and the lateral views of the 
 pharynx show the usual distribution in this area. Distinct 
 ducts (d.) arise from the dorsal ends of these masses and ex- 
 tend around the lateral sides of the oesophagus and turn ven- 
 trad and enter the two small openings on the caudal aspect 
 of the pharynx (fig. 27, 45, 46 and 58). If the above masses are 
 glandular then one can readily see how their secretions may be 
 poured into the pharynx and aid in digestion. Besides the 
 above glandular masses cephalad of the pharynx, similar tissue 
 is present at the proximal end of the left and right mandibles. 
 
1915] 
 
 The Head and Mouth-Parts of Thysanoptera 
 
 55 
 
 The small mass on the left side (fig. 26) projects for a short 
 distance into the hollow cavity of the left mandible. This 
 is not the case with the mass adjacent to the right, rudimentary 
 mandible. These two small masses are connected with the 
 masses above described by means of fine ducts (du.). The 
 exact extent of these ducts has not been definitely determined. 
 It is possible that they unite with the ducts entering the pharynx. 
 It is also possible that the secretions from the small mass within 
 the left mandible passes out thru the minute lumen of the 
 mandible. In the concave portion of the labrum small masses 
 of tissue are present which resemble the above so-called gland- 
 ular tissue. The extent of these masses varies and as yet no 
 ducts have been observed in connection with them. 
 
 In Cephalothrips (fig. 48, 49-51, 55 and 59) the above sup- 
 posed glandular tissue of the head is present but it is not so exten- 
 sive. Two distinct masses can be identified cephalad of the 
 pharynx and these give rise to ducts (d.) that empty into the 
 pharynx as in Heliothrips. Also a small mass of similar tissue 
 is located at the base of the left mandible but no such mass 
 is found on the right side near the right mandible. No glandu- 
 lar tissue was found in the concave portion of the labium as 
 in Heliothrips. 
 
 SUMMARY. 
 
 1. The general arrangement of the head and mouth-parts 
 of the Thysanoptera and the Hemiptera is similar. 
 
 2. The resemblance between the mouth-parts of Thysan- 
 optera and Orthoptera is apparently closer than the resemblance 
 between the mouth-parts of Hemiptera and Orthoptera. 
 
 3. A comparison of the different structures of the two 
 suborders shows clearly that the suborder Terebrantia is more 
 generalized than the suborder Tubulifera. 
 
 4. The mouth-parts of thrips are fitted for sucking. 
 
 5. The parts of the mouth are in the form of a cone which 
 encloses the piercing organs. The cone is composed of the 
 clypeus (cl.) labrum (lr.), maxillary sclerites (mx. s.) and labium 
 
 db.). 
 
 6. The mouth-parts of thrips are asymmetrical. 
 
 7. The asymmetry of the clypeus (cl.) and the maxillary 
 sclerites (mx. s.) in the two suborders is of an entirely different 
 nature. 
 
56 Annals Entomological Society of America [Vol. VIII , 
 
 8. The left asymmetrical piercing organ (1. md.) is the left 
 mandible and the right, rudimentary piece (r. md.) extending 
 between the pharynx (ph.) and the front (fr.) is the right, 
 rudimentary mandible. These mandibular parts are present 
 in all thrips as far as observed. 
 
 9. A pair of maxillae are present in all thrips. Each 
 maxilla is divided into two parts; the asymmetrical or sym- 
 metrical, palpus-bearing, maxillary sclerite (mx. s.) and the 
 symmetrical maxillary seta (mx.). 
 
 10. The tentorial structures in the nymph of Heliothrips 
 can be homologized with the tentorium of Cicada. On the 
 basis of this homology , the greatly reduced tentorium of adult 
 thrips can be interpreted. 
 
 11. The usual association between the tentorium and the 
 mouth-parts in generalized insects is present in thrips and aids 
 in interpreting the piercing organs. The maxillary setae are 
 closely associated with the posterior arms, while the left and 
 right mandibles are associated with the invaginations of the 
 anterior arms of the tentorium. 
 
 12. A comparison of the head of the nymph of Cephalo- 
 thrips and the head of the adult of Heliothrips shows clearly the 
 homology existing between the two suborders in respect to the 
 piercing organs. 
 
 13. The semi-pupal stage of Cephalothrips shows a distinct 
 reduction in the mouth-corie. The peculiar shape of the left 
 mandible and the point of attachment of the maxillary setae 
 in the adult is in part due to this reduction. 
 
 14. The pharynx at the anterior end of the alimentary 
 canal is modified into a distinct and characteristic sucking 
 apparatus. 
 
 15. Two kinds of paired salivary glands are found in the 
 thorax and abdomen of the two species here figured. The 
 ducts from these glands unite and pass into a y-shaped structure 
 caudad of the pharynx. 
 
 16. The tissue in the head, here called head-glands, has a 
 similar structure and a definite arrangement. Ducts can be 
 traced from the majority of these glands to the two small open- 
 ings on the caudal aspect of the pharynx. 
 
1915] 
 
 The Head and Mouth-Parts of Thysanoptera 
 
 57 
 
 BIBLIOGRAPHY. 
 
 Berlese, Antonio. — Gli Insetti, loro organizzazione, sviluppo, abitudini e rapporti 
 
 coll'uomo. Vol. I., Milan, 1909, 153. 
 Bugnion, E. — L'Appareil Salivaire des Hemipteres. Arch. D'Anat. Mic. Vol. 10, 
 
 1908, 227-268; Vol. 11, 1910, 435-456. 
 Faure-Fremiet, E. — Contribution a L'Etude des Glandes Labiales des Hydrocor- 
 
 ises. Ann. des Sci. Nat. Zool., N. S. 9, Vol. 12, 1910, 217-239. 
 Garman, H. — The Mouth-parts of the Thysanoptera. Bui. of the Essex Inst. 
 
 Vol. 22, 1890, 24-27. 
 
 Hinds, W. E. — Contribution to a Monograph of the Insects of the Order Thy- 
 sanoptera Inhabiting North America. Proc. of the U. S. Nat. Mus. Vol. 26, 
 1903, 79-242. 
 
 Jordan, Karl. — Anatomie und Biologie der Physapoda. Zeit. fur Wiss. Zool. Vol. 
 47, 1888, 541-620. 
 
 Muir, F. and Kershaw, J. C. — On the Homologies and Mechanism of the Mouth- 
 parts of Hemiptera. Psyche. Vol. 18, 1911, 9-10. 
 Uzel, Heinrich. — Monographic der Ordnung Thysanoptera. 1895. 
 
 EXPLANATION OF PLATES. 
 
 Plate I. 
 
 Fig. 1. Ventral aspect of the head of Heliothrips femoralis. 
 
 Fig. 2. Left later 1 aspect of the head of leliothrips femoralis. 
 
 Fi [. 3. Left lateral aspect of the mouth-cone of Thrips physapus. 
 
 Fig. 4. Antenna of Heliothrips femoralis. 
 
 Fig. 5. Right lateral aspect of the head of Heliothrips femoralis. 
 
 Fig. 6. Right lateral aspect of the mouth-cone of Thrips physapus. 
 
 Fig. 7. Dorsal aspect of the head-capsule of Heliothrips femoralis. 
 
 Plate II. 
 
 Fig. 8. Ventral aspect of the head of Cephalothrips yuccas. 
 
 Fig. 9. Dorsal aspect of the head-capsule of Cephalothrips yuccas. 
 
 Fig. 10. Antenna of Cephalothrips yuccae. 
 
 Fig. 11. Left lateral aspect of the head of Cephalothrips yuccae. 
 
 Plate III. 
 
 Fig. 12. Left lateral aspect of the head of the nymph of Heliothrips femoralis. 
 Fig. 13. Ventral aspect of the head of the nymph of Heliothrips femoralis. 
 Fig. 14. Ental view of a maxillary sclerite of Thrips physapus. 
 Fig. 15. Longitudinal section of the proximal and distal portions of a long, 
 
 salivary gland of Cephalothrips yuccas. 
 Fig. 16. Longitudinal section of a short, salivary gland of Cephalothrips yuccae. 
 Fig. 17. Ventral aspect of the head of the nymph of Cephalothrips yuccae. 
 Fig. 18. Left lateral aspect of the head of the nymph of Cephalothrips yuccae. 
 Fig. 19. Right lateral aspect of the head of the nymph of Heliothrips femoralis. 
 Fig. 20. Ventral aspect of the head of the semi-pupa of Cephalothrips yuccae. 
 Fig. 21. Ectal aspect of a maxillary sclerite of Frankliniella tritici. 
 
58 
 
 Annals Entomological Society of America [Vol. VIII, 
 
 Plate IV. 
 
 Fig. 22. Ental view of the ventral aspect of the mouth-cone and a portion of 
 
 the head-capsule of the nymph of Cephalothrips yuccae. The parts 
 
 have been spread and the pharynx omitted. 
 Fig. 23. The union of the salivary ducts in Cephalothrips yuccae. The solid 
 
 lines indicate the structures in a sagittal section, ten microns thick. 
 Fig. 24. Distal portion of a longitudinal section of the long salivary glands of 
 
 Heliothrips femoralis. 
 Fig. 25. Manner of interlocking of the grooved maxillary setae of Cephalothrips 
 
 yuccae when projected from the mouth-cone. 
 Fig. 26. Sagittal section of the left, asymmetrical mandible of Heliothrips 
 
 femoralis. 
 
 Fig. 27. Ental view of the ventral aspect of the mouth-cone and a portion of the 
 
 head-capsule of Heliothrips femoralis. The parts have been spread 
 
 and the maxillary extensions (et.) omitted. 
 Fig. 28. Ental view of the ventral aspect of the mouth-cone and a portion of the 
 
 head-capsule of the nymph of Heliothrips femoralis. The parts have 
 
 been spread and the pharynx omitted. 
 Fig. 29. The tentorium of Cicada septendecim. 
 
 Fig. 30. Longitudinal section of the short salivary gland of Heliothrips femoralis. 
 Fig. 31. Longitudinal section thru a dilated duct of a long salivary gland of 
 
 Heliothrips femoralis. The distal portion of the gland shown in 
 
 figure 24. 
 
 Fig. 32. Ental view of the right maxillary sclerite of Heliothrips femoralis. 
 
 Plate V. 
 
 Fig. 33. Ental view of the ventral aspect of the mouth-cone and a portion of the 
 head-capsule of Cephalothrips yuccae. The parts have been spread 
 and the pharynx omitted. 
 
 Fig. 24. Ental aspect of the left maxillary sclerite of Cephalothrips yuccae. 
 
 Fig. 35. Ental view of the ventral aspect of the mouth-cone of Cephalothrips 
 yuccae. The parts have been spread. 
 
 Fig. 36. Ental aspect of the right maxillary sclerite of Cephalothrips yuccae. 
 
 Plate VI. 
 
 Fig. 37. Frontal section thru the commissures connecting the supraoesophageal 
 and suboesophageal ganglia of Heliothrips femoralis. 
 
 Fig. 38-44. Cross-sections of the pharynx from the dorsal end to the tip of the 
 mouth-cone of Heliothrips femoralis. Each second or third section 
 figured. 
 
 Fig. 45. Lateral view of the pharynx of Heliothrips femoralis. 
 
 Fig. 46. Caudal aspect of the pharynx of Heliothrips femoralis. The extensions 
 
 over which the maxillary setae pass, have been omitted. 
 Fig 47. Caudal aspect of the Y-shaped piece located caudad of the pharynx. 
 Fig. 48. Caudal aspect of the pharynx and labrum of Cephalothrips yuccae. 
 
 The extensions, over which the maxillary setae and left mandible 
 
 pass, have been omitted. 
 Fig. 49. Lateral view of the pharynx of Cephalothrips yuccae. 
 Fig. 50. Frontal section thru the commissures connecting the supraoesophageal 
 
 and suboesophageal ganglia of Cephalothrips yuccae. 
 Fig. 51. Cross-section of the oesophagus of Cephalothrips yuccae at the point 
 
 where ducts from the head-glands (d.) pass laterad of the oesophagus 
 
 in their extension from the head-glands to the two openings on the 
 
 caudal aspect of the pharynx. 
 Fig. 52-57. Cross-sections of the pharynx from the dorsal end to the tip of the 
 
 mouth-cone of Cephalothrips yuccae. Each second or third section 
 
 figured. 
 
1915] The Head and Mouth-Parts of Thysanoptera 59 
 
 Plate VII. 
 
 Fig. 58. Sagittal section thru the head and prothorax of Heliothrips femoralis. 
 
 The salivary glands have been omitted. 
 Fig. 58a. The valve at the dorsal end of the pharynx of figure 58 more enlarged. 
 Fig. 59. Sagittal section thru the head and a portion of the prothorax of Cephalo- 
 
 thrips yuccae. The salivary glands have been omitted. 
 Fig. 60. Caudal aspect of the labium of Heliothrips femoralis. 
 Fig. 61. Caudal aspect of the labium of Cephalothrips yuccae. 
 
 LIST OF ABBREVIATIONS. 
 
 a. a. 
 
 Anterior arms of the tentorium. 
 
 mx. g. 
 
 Maxillary guide. 
 
 al. c. 
 
 Alimentary canal. 
 
 mx. p. 
 
 Maxillary pillar. 
 
 ant. 
 
 Antennae. 
 
 mx. pi. 
 
 Maxillary palpus. 
 
 c. a. 
 
 Caudal projections or arms. 
 
 mx. s. 
 
 Maxillary scierite. 
 
 c. d. 
 
 Common duct of the salivary 
 
 n. 
 
 Notch. 
 
 
 glands. 
 
 ne. 
 
 Nerve. 
 
 cl. 
 
 Clypeus. 
 
 o. 
 
 Occiput. 
 
 CO. 
 
 Commissure. 
 
 oc. 
 
 Ocellus. 
 
 c. t. 
 
 Chitinous tendon. 
 
 oe. 
 
 Oesophagus. 
 
 d. 
 
 Duct of the head-glands. 
 
 on. 
 
 Oenocyte. 
 
 d. a. 
 
 Dorsal arms of the tentorium. 
 
 p. 
 
 Pharyngeal projection. 
 
 d. d. m. 
 
 Dorsal dilating muscles of the 
 
 p. a. 
 
 Posterior arms of the tentorium. 
 
 
 pharynx. 
 
 ph. 
 
 Pharynx. 
 
 d. 1. 
 
 Dilated lumen. 
 
 P. 1. 
 
 Prothoracic leg. 
 
 d. m. 
 
 Dilating muscles of the pharynx. 
 
 po. 
 
 Pharyngeal pocket. 
 
 du. 
 
 Duct from glandular tissue near 
 
 pr. 
 
 Paraglossa. 
 
 
 mandible. 
 
 p. s. 
 
 Prothoracic scierite. 
 
 e. 
 
 Elastic membrane. 
 
 pt. 
 
 Chitinous plate. 
 
 et. 
 
 Ental extensions of the maxil- 
 
 r. md. 
 
 Right rudimentary mandible. 
 
 
 lary sclerites. 
 
 rt. 
 
 Reticulation. 
 
 f. 
 
 Fold in the clypeus. 
 
 s. 
 
 Suture. 
 
 fr. 
 
 Front. 
 
 s. c. 
 
 Sense cone. 
 
 g- 
 
 Gena. 
 
 s. d. 
 
 Duct of the short salivary gland 
 
 h. g. 
 
 Head-glands. 
 
 sm. 
 
 Submentum. 
 
 i. a. 
 
 Invaginations of the anterior 
 
 s. s. g. 
 
 Short salivary gland. 
 
 
 arms of the tentorium. 
 
 St. 
 
 Caudal head-strip. 
 
 i. d. 
 
 Invaginations of the dorsal 
 
 sub. 
 
 Suboesophageal ganglion. 
 
 
 arms of the tentorium. 
 
 supra. 
 
 Supraoesophageal ganglion. 
 
 i. p. 
 
 Invaginations of the posterior 
 
 t. 
 
 Thorax. 
 
 
 arms of the tentorium. 
 
 t. a. 
 
 Transverse area of the pharynx. 
 
 1. 
 
 Lumen. 
 
 v. d. m. 
 
 Ventral, dilating muscles of 
 
 lb. 
 
 Labium. 
 
 
 pharynx. 
 
 lb. pi. 
 
 Labial palpus. 
 
 vt. 
 
 Vertex. 
 
 L d. 
 
 Duct of the long, salivary gland. 
 
 X. 
 
 Ental thickening on margin of 
 
 1. et. 
 
 Ental extensions of the labium. 
 
 
 head-capsule. 
 
 1. md. 
 
 Left mandible. 
 
 xc. 
 
 Union of the anterior and pos- 
 
 lr. 
 
 Labium. 
 
 
 terior arms. 
 
 L s. g. 
 
 Long salivary gland. 
 
 y. 
 
 Y-shaped pharyngeal piece. 
 
 m. 
 
 Men turn. 
 
 z. 
 
 Ental thickening on margin of 
 
 me. 
 
 Membranous area. 
 
 
 head-capsule. 
 
 mu. 
 
 Muscle. 
 
 zc. 
 
 Piece connecting the united 
 
 mx. 
 
 Maxillary seta. 
 
 
 arms. 
 
Annals E. S. A. 
 
 Vol. VIII, Plate I. 
 
 Alvah Peterson. 
 
Annals E. S. A. 
 
 Vol. VIII, Plate II. 
 
 Alvah Peterson. 
 
Annals E). S. A. 
 
 Vol. VIII, Plate III. 
 
 Alvah Peterson. 
 
Annals E. S. A. 
 
 Vol. VIII, Plate IV. 
 
 Alvah Peterson, 
 
Annals E. S. A. 
 
 Vol. VIII, Plate V. 
 
 Alvah Peterson. 
 
Annals E. S. A. 
 
 Vol. VIII. Plate VI. 
 
 Alvah Peterson. 
 
Annals E- vS. A. 
 
 Vol. VIII, Plate VII. 
 
 Alvah Peterson. 
 
? 
 
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