36 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



their positions because they fomi ridges on the inner surface (B) 

 which have definite relations to the muscle attachments of the proter- 

 gum. The first (c) is a short curved line on the upper lateral part of 

 the tergum ; the second (d) is a longer line extending from the back 

 almost to the ventral margin of the tergum; the third (e) begins at 

 the dorsal notch and reaches ventrally just before the second non- 

 impressed line (b) to the middle of the side. Connecting the three 

 vertical grooves are four short longitudinal grooves, one (/) lying be- 

 tween the first and second vertical grooves (c, d), the other three 

 {g, h, i) between the second and third vertical grooves (d, e). 



A study of the inner surface of the tergum (fig. 20 B) will show 

 the endoskeletal ridges (c-i) formed by the external grooves. There 



Acx Ps 



Fig. 21. — The prothoracic sternum and pleura of Dissosteira. 



A, ventral view of sternum and lower edges of pleura, showing the spina- 

 sternum (Ss) united with the segmental sternal plate (S), the latter continuous 

 with the episterna (Eps) by the antecoxal bridges (Acx). B, dorsal surface of 

 sternum, showing bases of sternal apophyses (SA, SA) united by a furcal 

 ridge (k). 



will also be noted the anterior and posterior inflections of the tergal 

 walls. From the margin of the first inflection (a) the neck membrane 

 (Cv) is reflected forward, and from the margin (b) of the posterior 

 fold or reduplication (Rd) the intersegmental membrane (Mb) is 

 reflected posteriorly to the mesothorax. The first spiracle (Spz) is 

 located in this membrane. 



The proplcura. — We have already noted that each pleuron of the 

 prothorax appears externally only as a small plate projecting from 

 beneath the edge of the tergum anterior to the base of the leg (fig. 20 A, 

 Eps). It is to be seen on the internal surface of either half of the seg- 

 ment {B), however, that the pleural piece exposed externally is merely 

 the lower anterior corner of a much larger triangular sclerite (B, 

 Eps) extended upward within the lateral tergal wall by a deep inflec- 

 tion of the tergo-pleural membrane. The posterior margin of the 

 sclerite is turned inward, forming the pleural ridge, which gives oft' 



