NO. 2 THORACIC MFXHANISM OF A GRASSHOPPER — SNODGRASS 4I 



It is clear that the external " divisions " of the wing-bearing ter- 

 gum are incidental to the development of the internal ridges, which 

 are adaptations to the part the tergum plays in the mechanism for 

 moving the wings. The old idea that the tergum is " composed of " 

 sclerites gave undue emphasis to surface features. Though a study 

 of the latter may have a value for descriptive purposes, the student 

 must look to the internal characters for a true understanding of the 

 skeleton of insects. 



There is no postscutellar plate in the mesothorax of the grasshopper. 

 The posterior deflected margin of the scutellum ends in a narrow inter- 

 segmental membrane (fig. 25, 2Mb) uniting the mesotergum with 

 the anterior margin of the precosta of the metatergum. The tergum 

 of the mesothorax of the grasshopper, therefore, is a typical dorsal 

 plate of a secondary segment, comprising the primary segmental 

 sclerotization and the preceding primary intersegmental sclerotization 

 of the back. In the latter the primary intersegmental fold is marked 

 by the antecosta (fig. 25, Ac) and the antecostal suture (acs). 



The lateral margins of the mesotergum are very irregular (fig. 22). 

 The wings are extended from the tergal edges between the middle of 

 the prescutal borders and the posterior reduplication of the scutellum. 

 Anterior to the wing bases the anterior angles of the tergum are ex- 

 tended as short prealar arms (fig. 22 A, Aw) to which are articulated 

 the dorsal processes of the first basalar plates (fig. 26, Ba). The lat- 

 eral margin of the prescutal area forms posteriorly a small process 

 bearing a socket-like surface (n) in which the base of the subcostal 

 wing vein turns when the wing is flexed or extended. Posterior to this 

 process the anterior angle of the scutum is produced to form the large 

 anterior notal zving processes (ANP), which support the neck of the 

 first axillary sclerite of the wing base (lAx). The inner edge of the 

 first axillary bridges the lateral cmargination of the tergum (Em) 

 and articulates with a marginal lobe (o) behind the latter. There is 

 no posterior notal wing process in the mesotergum of the grasshopper ; 

 the fourth axillary {4 Ax), which is itself probably a detached piece 

 of the tergal margin, articulates with the edge of the scutellum. 



The metatergum. — The tergal plate of the metathorax (fig. 24) is 

 somewhat longer than that of the mesothorax, since it must support 

 the wider bases of the hind wings ; but in many respects it is more 

 weakly develo|>ed than the mesotergum, there being extensive non- 

 sclerotized areas in the posterior part of the scutal region. 



The precostal rim of the metatergum (fig. 24, Pc) is narrow, except 

 medially where it fomis a conspicuous lip before the deeply inflected 



