NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 37 



the pleural arm (PIA) and ends ventrally in the articular process of 

 the coxa (CxP). Behind the lower part of the pleural ridge is a small 

 epimeral piece (Epni) fused with the lower border of the tergum and 

 concealed just within the edge of the latter. The anterior ventral angle 

 of the episternum is continuous through the precoxal bridge (Ac.v) 

 with the anterior lateral angle of the prosternum (fig. 21 A). 



The prosternum. — The sternum of the prothorax in the grasshopper 

 consists of two distinct plates (fig. 21 A, S, Ss) separated by a trans- 

 verse suture. The anterior plate {S) is the larger and the more 

 strongly sclerotized. It is continuous laterally with the precoxal bridges 

 (Acx) from the episterna (Eps). The definitive sternal plate of the 

 prothorax, then, evidently includes laterosternal elements derived from 

 the pleura (subcoxae), but the true pleuro-stemal limits are entirely 

 obliterated. The anterior rim of the sternum is set oflf as a narrow 

 presternal strip (Ps) by a submarginal external suture (A, /) and 

 a corresponding internal ridge (B, /). Posteriorly the first sternal 

 plate is marked by a deep transverse groove which forms a strong 

 ridge on its inner surface (fig. 21 B, k) between the bases of the 

 sternal apophyses (B, SA, SA), which latter are indicated externally 

 by a pair of pits (A, sa, sa). The first prosternal plate in the grass- 

 hopper, therefore, is divided in the primitive fashion (fig. 18 C) into 

 a basisternal and a sternellar region by the suture of a ridge connecting 

 the bases of the apophyses. The prosternal apophyses are simple arms 

 (fig. 21 B, SA) diverging dorsally and laterally. Their distal ends are 

 solidly united with the corresponding pleural apophyses. 



The second prosternal plate (fig. 21, Ss) is a spinasternite, and is, 

 therefore, the intersternite between the prothorax and the mesothorax 

 which has become closely associated with the primary sternite of the 

 prothorax. It is mostly overlapped by the anterior margin of the 

 mesosternum. The spinasternite of Dissosteira is triangular in shape, 

 and is marked by a deep median impression (fig. 21 A) which fonns 

 the spina internally (B, Spn). 



THE PTEROTHORAX 



The united mesothorax and metathorax of the grasshopper consti- 

 tute a unit in the body mechanism. The pleural and sternal walls of 

 the two segments are solidly united, forming a trough-like structure 

 perforated only by the openings of the coxal cavities. The leaping 

 force of the hind legs is thus applied to a rigid middle section of the 

 body, which also bears the wings. The dorsum of this body section is 

 covered by the mesothoracic and metathoracic terga, but these plates 



