NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 39 



22 B, PR) that brace the anterior wing processes. Upon the irregular 

 surfaces of the prescutal lobes are attached the tergo-sternal muscles 

 which are the principal elevators of the wings. 



A prescutum similar to that of the Orthoptera occurs also in certain 

 other insects, though very likely it may be an independent differen- 

 tiation formed as an adaptation to similar demands. In many insects 

 of the higher orders, however, such as the Hemiptera, Diptera, and 

 Hymenoptera, a prescutal area of quite a different nature is set off in 

 the anterior median part of the tergum by the development of two 



-AxC 



,AxC 



Rd Mb 



Fig. 22. — The mesothoracic tergum of Dissosteira. 



A, dorsal surface. B, ventral surface. Ac, antecosta ; acs, antecostal suture ; 

 ANP, anterior notal wing process ; Aw, prealar arm of tergum ; lAx, first 

 axillary ; 4Ax, fourth axillary ; AxC, axillary cord ; Em, lateral emargination 

 of tergum ; Mb, secondary intersegmental membranes ; n, lobe of prescutum 

 articulating with base of subcostal wing vein; o, lobe of scutum articulating 

 with posterior part of first axillary ; Pc, precosta ; iPh, first phragma ; ps, pre- 

 scutal suture ; Psc, Psc, lateral prescutal areas ; Rd, posterior reduplication of 

 tergum ; Scl, scutellum ; Set, principal part of scutum ; set, set, posterior lateral 

 subdivisions of scutum ; s, s, secondary ridges of tergum ; tg, tegular rudiment ; 

 VR, remnant of V-ridge of tergum. 



lateral ridges, the parapsidal gradients (fig. 5 B, PaR), which extend 

 a varying distance posteriorly from the anterior tergal margin, and 

 usually converge. These ridges and their sutures apparently lie in the 

 scutal region of the tergum, for there is sometimes present a narrow 

 transverse prescutal band anterior to their bases. Parapsidal ridges are 

 absent in the Orthoptera. 



The area of the mesotergum of Dissosteira posterior to the ante- 

 costal and prescutal sutures is differentiated topographically into a 

 large anterior scutal region (fig. 22 A, Set), a median, posterior, tri- 

 angular scutellar region (Scl), two small, lateral, posterior scutal re- 

 gions (set J set), and a posteriormost, deflected marginal region (Rd). 

 The structure here presented is quite different in appearance from 

 that of a typical wing-bearing tergum (fig. 5 A) in which the surface 



