THE P LEU RON 



29 



teit 



has shown that the same general arrangement holds good, except 

 that the trochantin itself has apparently undergone division into 

 two pieces. In Eosentomon germanicum, Prell (1913) has 

 described eight small sclerites in the eupleural region, together 

 with a typically arched band-like trochantin. The ill-defined 

 and fragmented condition of the pleurites in this species renders 

 it scarcely possible to homologise the eupleural plates individually 

 with those of other insects. 



In the ^ Pterygota, notwithstanding the 

 diversity of the sclerites that are developed 

 in the pleural region, they appear to have 

 been evolved to some extent along a definite 

 line in response to the need for rigidity 

 occasioned by the presence of wings. As 

 Snodgrass remarks, they serve to brace the 

 tergum against the downward pull exerted 

 by the tergosternal muscles which function 

 as elevators of the wings. He advances 

 theoretical conclusions which suggest that 

 the Pterygote pleuron was derived from the j^j^. ^5 

 undivided subcoxa (Sex) which became 

 incorporated into the side wall of the body 

 (Fig. 16). According to his theory the 

 ventral part of the subcoxa has either 

 become membranous or has united with the 

 sternum {S), while the remainder has given 

 rise to the pleuron. In the early Pterygota 

 the subcoxa became divided into a narrow, arched trochantin 

 (Tn), bearing a dorsal articulation (a) with the coxa, and a 

 larger sclerite or eupleuron (Epl). The whole pleuron sub- 

 sequently became strengthened by an internal ridge formed by 

 a lateral inflexion of its wall, thus giving rise to the pleural 

 suture (P.S.). The areas lying anteriorly and posteriorly to 

 this suture become the episternum (E) and epimeron (Ep) 

 respectively. In the more primitive insects the trochantin 

 remains as a separate sclerite in front of the suture, while, 

 posteriorly, it becomes merged into the epimeron. The 



Prothorax 

 and leg of a Col- 

 lembolan (Isotoma). 

 ex, coxa ; fe, femur ; 

 h, base of head ; pt^ 

 pretarsus ; sex, sub- 

 coxa ; terg, tergum ; 

 ti, tibia ; tr, tro- 

 chanter. (After 

 Ewing.) 



