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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



is divided into scutal and scutellar areas (Set, Sd) by the suture (vs) 

 of an internal V-shaped ridge (B, VR), the arms of which are con- 

 vergent forward from the posterior lateral angles of the tergum. 



In the Acrididae the V-shaped endotergal ridge (fig. 22 B, VR) 

 is almost obliterated, and the tergum is braced by two secondary ridges, 

 one on each side (s), that converge posteriorly from the posterior 

 lateral margins of the scutal area and intercept the arms of the rudi- 

 mentary V-ridge (VR). The altered structure of the acridid tergum 



Psc 



Rd 



Fig. 23. — Diagram of the structure of a wing-bearing tergum of Acrididae. 



The prescutal suture (ps) is either continuous, or suppressed medially; the 

 usual V-ridge and its suture (vs) are partially suppressed and subordinated to a 

 secondary ridge of similar shape but having its arms (s, s) convergent posteriorly. 



may be expressed diagrammatically as in figure 23, where the sup- 

 pressed suture (zv) of the obsolete V-ridge is crossed by the dominant 

 suture (s) of a secondary ridge of similar shape but having its arms 

 convergent posteriorly. Thus the scutum consists of a principal an- 

 terior scutal area (Set) and of two small postero-lateral scutal areas 

 (set, set) ; and the scutellum is divided into a median scutellar area 

 (Set) and two lateral scutellar areas (set, set), including the posterior 

 fold of the tergum (Rd). The evolution of this condition can be traced 

 in other Orthoptera from the primary structure which occurs in the 

 Blattidae. A similar modification has taken place in the mesotergum 

 of Hemiptera and Coleoptera, producing the triangular elevated shield 

 of the scutellum that lies between the bases of the folded wings. 



