NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS I9 



the trochantin is the episternum (Eps). The sclerotized parts of the 

 suhcoxa lying anterior and posterior to the coxa are the precoxal and 

 postcoxal bridges {Acx, Pcx), or precoxalia and postcoxalia. The 

 ventral wall, or infra-coxal arc, of the subcoxa, as already noted, prob- 

 ably unites in many cases with the edge of the primary sternum {Stn) 

 to form a laterosternal element {Ls) in the definitive sternum, though 

 it may be reduced to a separate sclerite or form a membranous fold 

 between the coxa and the sternum. 



All parts of the pleuron are subject to innumerable secondary modi- 

 fications taking the form of sutures that subdivide the primary areas, 

 or of membranous lines and spaces that break them up into separate 

 sclerites. Such modifications are not necessarily homologous between 

 different orders, but within an order or group of orders they may give 

 valuable evidence of the evolution and interrelationships of the fami- 

 lies and genera. An example of this is given by Shepard (1930) in a 

 study of the secondary pleural sutures of Lepidoptera. 



In the wing-bearing segments of the Pterygota two large pleuro- 

 coxal muscles (fig. 11, M' , M") become important muscles of the 

 wings. These muscles evidently are derived from the abductor system 

 of the coxa. In nymphal Orthoptera (fig. 27 C) the anterior muscle 

 {M') has its origin on the dorsal part of the episternum, the posterior 

 muscle {M") on the dorsal edge of the epimeron. In adult insects, 

 however, the areas upon which these muscles are attached become par- 

 tially or entirely separated from the pleuron and intimately associated 

 with the base of the wing, the first lying before the pleural wing 

 process, the second behind it. In this way the muscles come to func- 

 tion as wing muscles, though each retains its ventral attachment on the 

 coxa. 



The epipleurites (" paraptera "), or sclerites detached from the 

 pleuron in connection with the coxo-alar muscles, include one or two 

 episternal sclerites, or basalares, and usually a single epimeral sclerite, 

 or subalare. The subalare is always completely detached from the 

 epimeron in adult insects (fig. 14 A, B, Sa). A basalar plate, how- 

 ever, is not always present as a distinct sclerite ; it frequently occurs as 

 but an imperfectly separated lobe of the upper end of the episternum 

 (fig. 14, Ba), and its area is sometimes marked only by the insertion 

 of the anterior coxo-alar muscle (M') . Even when the basalare is dis- 

 tinct from the episternum, it is generally hinged to the upper edge of 

 the latter in such a manner that it is deflected by the contraction of its 

 muscle. Frequently there is present a second basalar muscle (figs. 27 C, 

 E) having its origin on the pleuron or on the sternum. 



