NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 9 



The posterior gradient of an alar tergum usually has the form of a 

 V-sJiapcd ridge with the apex directed forward and the arms diverging 

 toward the posterior lateral angles of the tergum (fig. 5 B, VR). The 

 anterior gradient is less commonly developed than the posterior one, 

 but, when present, it generally consists of two ridges, the parapsidal 

 ridges (fig. 5 B, PaR), converging from the anterior margin of the 

 tergum toward the middle, where they usually terminate without meet- 

 ing. In some insects the anterior part of the tergum is strengthened 

 by a transverse prcscutal ridge (PR). In addition to these more gen- 

 eral endoskeletal structures of the tergum, there may be present also 



Aw 



A^C 



Fig. 5. — Structure of a wing-bearing tergum, not including a postnotum, 



diagrammatic. 



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

 notal wing process ; AxC, axillary cord ; Azv, prealar process of tergum ; Em, 

 lateral emargination of tergum ; PaR, parapsidal ridge ; pas, parapsidal suture ; 

 Pc, precosta ; Ph, phragma ; PNP, posterior notal wing process ; PR, prescutal 

 ridge ; ps, prescutal suture ; Psc, prescutum ; Rd, posterior fold, or reduplication, 

 of tergum ; Scl, scutellum ; Set, scutum ; VR, V-shaped ridge ; vs, suture of the 

 V-ridge, or scuto-scutellar suture ; W, base of wing. 



a variety of accessory ridges, or even lines of flexibility in the tergal 

 cuticula ; but all such features are highly variable in different groups 

 of insects, and homologies can be traced between them only within 

 limited groups. 



On the outer surface of the tergum the positions of the endoskeletal 

 ridges are marked by the lines, or " sutures," of their inflection 

 (fig. 5 A, ps, pas, vs). The tergal areas defined in this manner by the 

 more constant of the inner structures can be identified as homologous 

 in different insects, and some of them have been given distinctive 

 names used in descriptive works (fig. 5 A, Psc, Set, Sel). It is quite 

 impossible, however, to follow the lesser modifications consistently 

 through the various orders of winged insects, and attempts to do so 

 have only led to confusion. In any case, it must be recognized that 



