NO'. I 



INSECT THORAX — SNODGRASS 



29 



the Apterygota. In the Pterygota, on the other hand, the pleuron 

 has evolved along a definite, constructive line. In the lower orders 

 of winged insects it may consist, as in the Apterygota, of a number 

 of separate sclerites, but in the higher orders it becomes a continuous 

 plate, resting below upon the sternum, giving a solid point of suspen- 

 sion for the leg, and, in the wing-bearing segments, a support for the 

 wing, and for the tergum also, since the latter must be braced against 

 the downward pull of the tergo-sternal muscles in order that these 

 muscles may function as elevators of the wings. 



^P2>^ ^/^ ^^ '^^ Sa 



Pcx 



Acx 



Fig. 14. — Mesopleuron and metapleuron of Psocus I'owsits. 



The dorsal part of trochantin (Tn) of mesothorax here united with episternal 

 part (Eps) of anapleurite (fig. 13, Apl) ; postarticular part of trochantin 

 (fig. 13, Tn) either absent or fused with epimeron (Epnt) ; basalares {Da, Ba) 

 not separated from episterna. 



The evolution of the pterygote pleuron has included principally a 

 development of the eupleural arch, and particularly of its anapleural 

 region. With the latter has been united the dorsal part of the tro- 

 chantin, as shown by Crampton (1914) and by Weber (1924). The 

 resulting plate, bearing the dorsal articulation of the coxa, has been 

 strengthened by an internal ridge extending upward from the coxal 

 condyle, the external suture of which separates the definitive epister- 

 num and epimeron. There is perhaps no evidence as to whether the 

 part of the trochantin .posterior to the coxal articulation (fig. 13, 

 A, B, C) has been absorbed by the epimeron, or has been independently 

 lost, but there is abundant evidence of the fate of its pre-articular 



