NO. I INSECT THORAX SNODGRASS 27 



pterygote episternum and epimeron. It may be questioned, however, 

 whether these are cases of actual homology or of resemblances in 

 structures arising independently in response to similar demands. 

 There is no evidence of lineal or connected relationship between the 

 Protura, Machilis, and the Pterygota. 



A study of the pleura of Eoscntomon, Japyx, and Lcpisma leads to 

 the generalization that the pleurites of the Apterygota comprise a 

 eupleural series of chitinizations arched over the base of the coxa, 

 and a trochantinal sclerite in immediate contact with the coxa, while 

 with them there may be associated a sclerite derived from the coxal 

 base. A suggestion of the same pattern is to be found in the pleura 

 of the Chilopoda. In Machilis the pleurites are reduced to a single 

 plate, the identity of which is obscure — it might be the trochantin, or 

 it might be a sclerite of the eupleural series. In the Collembola the 

 thoracic pleurites are but poorly developed. Crampton (1926) finds 

 a trochantin closely associated with the coxa, and a pleural sclerite 

 lying above the coxa, but of the collembolan sclerites in general, he 

 says, they " are too weakly developed, and too unsatisfactory in 

 nature to afiford much evidence of relationships to other forms." The 

 line of descent of the Collembola, Crampton believes, " leads off in 

 a direction having no especial bearing on the evolution of the higher 

 forms." The present writer would add that the same may as truly be 

 said of any of the apterygote groups. There is no suggestion, for 

 example, in the Apterygota of the probable steps in the evolution of 

 the pterygote pleuron ; rather, each pleural pattern in the Apterygota 

 appears to have resulted independently from the disintegration of a 

 more concrete earlier structure. Reasons will later be given for believ- 

 ing that the structure of the pterygote pleuron is correlated with the 

 wings, and that it has been developed in the extinct and unknown line 

 of descent that led to the winged insects. 



The typical structure of the pterygote pleuron has already been 

 described and shown diagrammatically in figure 3. A more generalized 

 condition, however, is to be found in the prothorax of the Plecoptera 

 (fig. 13). Here there is a dorsal eupleural sclerite, or anaplcurite 

 (Apl), and a trochantinal sclerite (Tn) intervening between the 

 anapleurite and the coxa, and articulating both anteriorly and dorsally 

 with the coxa (C, a, b). The anapleurite is divided by an external 

 suture and an internal ridge into an anterior episternal region and a 

 posterior epimeral region. 



The more usual pleural structure in the Pterygota includes a pre- 

 coxal bridge from the episternal part of the anapleurite to the basi- 

 sternum, and often also a postcoxal bridge from the epimeral region 



