NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 47 



There can be little doubt that the epipleurites are derived from the 

 upper parts of the pleura. In a nymphal orthopteron the muscles that 

 are inserted on the epipleurites in the adult (fig. 49) are attached 

 directly to the upper edges of the episternum and epimeron (fig. 27 C, 

 M' , M"). In many adult insects the basalare remains as an undetached 

 lobe of the episternum (fig. 14 A, Ba). 



In the membranous corium at the base of each leg there is a small 

 plate (fig. 26, Tn) situated before the coxa and loosely attached by its 

 lower end to the rim of the coxa. These sclerites are evidently rem- 

 nants of the trochantins (fig. 13 A, B, Tn) since they exactly cor- 



Epm^ 



Fig. 29. — Upper edge of the metathoracic pleuron and epipleurites of Dissosteira, 



inner view. 



iBa, first basalare ; 280, second basalare ; Epin, epimeron ; Eps, episternum ; 

 Sa, subalare ; IV P, pleural wing process. 



respond with the small trochantin of the prothorax (fig. 20 A, Tn), 

 which is identified as such by the attachment of the promotor leg 

 muscle upon it (fig. 33 A, 62). 



The pterothoracic sterna. — The sternal plates of the mesothorax and 

 metathorax are united in a broad plastron covering the ventral surface 

 of the pterothorax, and continuous laterally, in the adult, with the 

 pleura by a fusion with the precoxal parts of the latter (fig. 30 A). 

 In the nymph of Dissosteira and of other Acrididae, as already noted, 

 the pleural plates of the mesothorax and metathorax (fig. 27 A, Pin, 

 P/3) are distinctly separate from the sterna (S2, S3), and the pre- 

 coxal part of each pleuron is extended ventrally and posteriorly as a 

 slender arm (Ls, Ls) between the sternum and the coxal corium. 

 These arms are clearly remnants of the infra-coxal arcs of the sub- 



