i8 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



The usual trochantin of the pterygote pleuron (fig-. 13 B, Tn) is 

 clearly a remnant of a more extensive, primitive, supra-coxal scleroti- 

 zation (fig. i, Eiiiu) carrying the dorsal articulation of the coxa (c), 

 which Crampton (1914) has named the cutrochantin, and which is 

 best preserved in the ventral arch of the apterygote pleuron (fig. 

 12). The cutrochantin is retained as an independent sclerite also 

 in the prothorax of the Plecoptera, but in all other Pterygota (fig. 13, 



Pc acs 



Pc acs 



Sex -H-^- 



'PIS 



WP 



Tn — ff 



Stn 



sa 



1st 



Ls Stn Ss(=Ist) 



Fig. 13. — Diagrams suggesting the development of the pterygote pleuron from 

 the subcoxa of the leg basis. (Compare with Fig. i.) 



A, subcoxal sclerotization (Sex) united ventrally with edge of primary ster- 

 nite (Stn), its dorsal extremity prolonged upward as a wing support (B, WP), 

 posterior part of entrochantin (fig. i, Eutn) fused with eupleural arch (fig. i, 

 Eiipl) of subcoxa. 



B, a fundamental structural condition of pleuro-sternal region of a wing- 

 bearing segment: the area of subcoxa differentiated into an episternum (Eps), 

 an epimeron (Epm), a precoxal bridge (Acx), a postcoxal bridge (Pcx), a 

 laterosternite (Ls) , and a trochantin (Tn) ; the definitive sternum includes the 

 primary segmental sternite (Stn), the following intersegmental intersternite, or 

 spinasternite (Ss), and a subcoxal laterosternite (Ls) on each side; the ventral 

 coxal articulation (\, d) is lost, and coxa has a secondary anterior articulation 

 with trochantin (e). 



A, B) its dorsal and posterior parts unite or fuse with the upper arch 

 of the pleuron (eupleuron), and only its anterior part remains as a 

 free sclerite (fig. 13 B, Tn) carrying the anterior coxal articula- 

 tion {e). 



The elaborate pterygote pleuron has evidently been developed to 

 give support to the paranotal lobes, or to the wings evolved from the 

 latter. It is therefore strengthened by an internal ridge formed from 

 a linear inflection of its wall, the pleural suture (fig. 13 A, PIS), ex- 

 tending from the dorsal articulation of the coxa (c) upward to the 

 wing support ( B, WP). The area lying posterior to the pleural suture 

 is the epimeron (B, Epm), that situated anterior to it and dorsal to 



