38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, 82 



are freely attached to the upper pleural margins of the pterothoracic 

 trough by the ample membranes of the wing bases, and they are mov- 

 ably joined to each other. As we shall later see, the wing mechanism 

 demands at least a limited freedom of movement in the wing-bearing 

 terga. 



In the grasshopper the back plates of the pterothorax (figs, 22, 24) 

 differ somewhat in shape and in details of form and proportion, but 

 the two have the same essential structure. They are relatively small, 

 and when the insect is at rest they are hidden beneath the folded wings. 

 The pleurites are defined externally by distinct grooves (fig. 26) 

 forming strong ridges internally (fig. 28), which slant posteriorly and 

 downward in a manner to suggest that they serve thus to brace the 

 pleural walls against the projectile force of the hind legs. The sterna 

 of the wing-bearing segments are wide plates fused laterally with the 

 pleura before the leg bases (fig, 30). 



The mesoterguni. — The tergum of the mesothorax (fig. 22 A) is a 

 rectangular plate ending posteriorly in a distinct, transverse fold (Rd), 

 the extremities of which are continued into the posterior thickened 

 margins, or axillary cords (Axe), of the wing bases. Close to the 

 anterior margin of the tergum is a deep groove (acs). This is the 

 antecostal suture, or primary intersegmental inflection which forms 

 the antecosta of the internal surface of the definitive tergum (B, Ac). 

 The antecosta bears laterally two wide, flat apodemal plates (iPh) 

 projecting into the cavity of the thorax (fig. 25), which are the first 

 pair of thoracic phragmata. 



On the external surface of the mesotergum, two sutures (fig. 22 A, 

 ps, ps) diverge laterally and posteriorly from the antecostal suture 

 (acs). They form internally a pair of strong ridges (B, PR) extend- 

 ing to the bases of the anterior wing processes (ANP) . The large, ir- 

 regular, triangular regions (A, Psc, Psc) forming the anterior lateral 

 angles of the tergum, set off by the divergent sutures {ps, ps), con- 

 stitute the prescutal areas of the tergum. In the metathorax the 

 prescutal sutures do not meet the antecostal suture, and the lateral 

 prescutal lobes are continuous by a narrow median bridge behind the 

 antecostal suture (fig, 24). In some other Acrididae, as in Melanoplus, 

 the continuity of the prescutal area is more pronounced. In other 

 orthopteran families the prescutum is narrow, but in the Blattidae and 

 Gryllidae there is a suggestion of its separation from the scutal area. 

 In any case, however, the prescutum of the Orthoptera must be re- 

 garded as a secondary differentiation of the anterior part of the tergum. 

 Its lateral parts become most sharply defined in the mesotergum of 

 the Acrididae by the strong development of the prescutal ridges (fig. 



