54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



even in segments having an anterior phragma, the prescutum entirely 

 loses its individuality, as in the cicada and in the higher Hymenoptera 

 (fig. 24 F, J). In the higher Diptera, remnants of the prescutum are 

 distinguishable at the sides of the scutum (E, Psc) terminating in 

 prealar lobes (u) corresponding with those of the Tipulidae (D), 

 but anteriorly the prescutum is not discernible in those Diptera in 

 which the protergum (E, Ti) is fused with the mesotergum. 



A prescutum is generally lacking as a defined area in metathoracic 

 terga that have parted with the middle phragma (fig. 22, T3), but this 

 condition probably does not mean necessarily that the entire prescutal 

 region has become detached with the phragma. 



The separation of the scutellum from the scutum by the V-ridge 

 and its suture is usually a very distinct one (fig. 24 D-J), but in 

 some groups the boundary between these two areas of the tergum also 

 is difficult to determine. In Blattidse, the scutellum is of typical form, 

 though its median part extends forward in a long narrow triangle 

 almost to the prescutum. In Gryllus (fig. 24 B) the lateral arms of 

 the V-ridge do not meet, and the scutum and scutellum are continuous 

 medially. In Acrididae, the V-ridge (C, vs) is interrupted by another 

 ridge of similar shape but turned in the opposite direction. The arms 

 of this second ridge, converging medially and posteriorly, bound the 

 posterior margins of the shield-shaped elevation of the scutellum 

 (Scl) that lies between the bases of the folded wings. A similar 

 condition exists in the mesotergum of Hemiptera and Coleoptera. 

 In the higher Diptera, the position of the V-ridge is less noticeable 

 externally (E, vs) than is the secondary suture (s) that forms a 

 deep groove across the anterior end of the scutellum. In the Ten- 

 thredinidae a pair of sutures extends inward and posteriorly from the 

 lateral emarginations of the mesotergum (I, s), which sutures be- 

 come a continuous cleft in some of the higher Hymenoptera (J, ^) 

 that cuts the tergum into two pieces, the second of which includes 

 the scutellum (Sd) and the posterior lateral parts of the scutum (set, 

 set). Systematists, for convenience, usually designate this secondary 

 suture as the division between scutum and scutellum, a disposition 

 entirely wrong from a morphological standpoint. 



While, then, in terga of simple construction, it is not difficult to 

 identify homologous regions if the internal structure is examined, 

 complications are encountered within some of the orders, especially in 

 the scutum (fig. 24 D-J), which have led to widely different inter- 

 pretations. The following considerations, however, suggest an ex- 

 planation of these tergal modifications which gives a simplified con- 



