NO. 2 THORACIC MECHANISM OF A GRASSHOPPER — SNODGRASS 75 



but they are inconspicuous externally loecause each is bent outward and 

 flattened upon the dorsal wall of the coxa. The articular surface (c) 

 by which the coxa is hinged to the internal coxal process of the pleuron 

 (D) is strongly inflected mesally in such a manner as to bring the 

 point of suspension near the central axis of the coxa, thus giving the 

 coxal muscles a leverage on all sides of it (fig. 43). Each coxa has a 

 dicondylic hinge with the trochanter (fig. 36 D, /, g), the axis of which 

 is horizontal and transverse to the length of the leg. The dorsal sur- 

 face of the coxa is deeply emarginate between the hinge points, and 

 the ample articular membrane that occupies the notch allows a free 

 upward movement of the telopodite. When the latter is deflexed, the 

 ventral lip of the trochanter passes inside the lower edge of the coxa. 



The anterior wall of the prothoracic coxa is marked by a coxal 

 suture (fig. 41 A, cxs) which extends from the anterior trochanteral 

 articulation (/) to the basicostal suture at the articulation of the tro- 

 chantin (Tn). The middle coxa (B) has a similar suture (cxs) ending 

 at the trochanteral articulation (/) , but it begins basally at the pleural 

 articulation (c) and thus falls in line with the pleural suture (PIS) 

 of the mesopleuron. The suture is absent in the hind coxa (C). The 

 coxal suture, when present, forms a ridge on the inner surface of the 

 coxal wall (fig. 36 A), the purpose of which is evidently to strengthen 

 the latter. 



A coxal structure such as that of the middle leg of the grasshopper 

 (fig. 41 B), in which the anterior wall is divided by a groove (cxs) 

 continuous with the pleural suture, is likely to be confused with the 

 quite different structure illustrated by the coxa of Panorpa (fig. 14 A) , 

 in which the postarticular part of the basicostal suture (bcs) bends 

 distally in the coxal wall and also falls in line with the pleural suture 

 (PIS). The fundamental differences in the two cases, however, are 

 quite apparent: in the grasshopper (fig. 41 B) the outer wall of the 

 coxa itself is divided; in Panorpa (fig. 14 A) the meron (Mer) is 

 greatly enlarged and is extended into the posterior coxal wall. 



The internal ridge of the coxal suture in the prothoracic and meso- 

 thoracic legs of Dissostcira is continued through the anterior coxo- 

 trochanteral articulation (fig. 36 A, /), giving a firm but flexible union 

 between the two articulating segments. The posterior articulation be- 

 tween the coxa and trochanter of the first and second legs consists of 

 a condyle on the trochanter opposed by a concave surface on the coxa, 

 but the two are united by membrane. In the hind leg both coxo- 

 trochanteral articulations consist of opposing processes united by 

 ligament-like thickenings of the articular membrane (fig. 36 D, /, g). 



