NO. I INSECT THORAX SNODGRASS I3 



has been developed the pleural sclerites in both the Apterygota and 

 the Ptengota. 



The key to the structure of the adult pterygote pleuron is the pleural 

 suture (fig. 3 A, PIS), a groove extending upward from above the 

 base of the coxa, and forming internally the pleural ridge (fig. 3 B, 

 PIR). The pleural ridge is to be identified by the pleural arm (PI A), 

 which extends inward and ventrally from its lower part, and by the 

 condyle at its ventral end, the pleural coxal process (CxP), which 

 forms the dorsal articulation of the coxa with the body. The part of 

 the pleuron dorsal to the coxa is divided by the pleural suture and 

 pleural apodeme into an anterior region, the episternum (fig. 3 A, B, 

 Eps), and a posterior region, the epimeron (Epm). These two 

 sclerites are thus secondary divisions of a primitive plate, there being 

 no evidence of their origin from separate centers of chitinization. 

 The relation of the pleural arm to the corresponding sternal arm 

 (fig. 3 B, PI A and SA) has already been noted. 



The anterior ventral angle of the episternum is usually extended 

 toward the sternum to form a precoxal bridge (fig. 3, Acx) before 

 the base of the leg, and often a similar extension from the epimeron 

 forms a post coxal bridge (Pcx) behind the leg, the first becoming 

 continuous with the basisternum, the second with the furcisternum. 

 The anterior bridge, however, in some of the lower insects is separated 

 from the episternum and constitutes an independent sclerite, the 

 anterior laterale, occasionally divided into an upper and a lower piece. 

 Less frequently is the posterior bridge an independent posterior 

 laterale. When all the regions of the pleuron thus far described — 

 episternum, epimeron, precoxal bridge, postcoxal bridge — are well 

 developed, they constitute an arch over the base of the coxa, braced 

 upon the sternum below, from which the leg is suspended by the 

 coxal process at the lower end of the pleural ridge. 



In the generalized pterygote pleuron a sclerite known as the 

 trochantin (fig. 3 A, Tn) lies before the base of the coxa, but behind 

 the precoxal bridge {Acx). The trochantin is usually triangular in 

 form, elongate dorsoventrally, with its upper end touching upon the 

 episternum or fused with the lower part of the latter. Its lower end 

 articulates by a trochantinal coxal process (a) with the anterior mar- 

 gin of the coxal base. The trochantin is a highly variable sclerite ; 

 it is best developed in the Apterygota and in the lower orders of the 

 Pterygota, though it may dift'er much in closely related species ; in 

 the higher pter}'gote orders it is rudimentary or absent. In some of 

 the Apterygota the trochantin forms an arch over the base of the 

 coxa ; only in rare cases does it extend dorsal to the coxa in pterygote 



