52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



MODIFICATIONS OF THE WING-BEARING TERGA 



The structural variations in the terga of the mesothorax and meta- 

 thorax of winged insects have furnished a problem that has caused 

 much vexation to those who would like to have the parts of the terga 

 in all insects conform with one simple plan of organization. The 

 tergal parts probably do represent one plan of structure, but they do 

 not always agree with the usual system of nomenclature applied to 

 them. The trouble is that the fundamental structure, easily recognized 

 in some cases, is so obscured in others that parts which appear to be 

 the same in different insects are really not so. 



The wing-bearing tergum, as we have seen (fig. 21), has an ante- 

 costa (B, Ac) on its front margin bearing a phragma (Ph), the base 

 of which is marked externally by the antecostal suture (A, ac). The 

 precosta (Pc), or anterior lip of the phragma base, before the meso- 

 thoracic tergum is but a narrow strip, often indistinguishable ; that 

 before the metatergum, however, is enlarged to form a postnotal plate 

 of the mesothorax if the fore wings are chiefly functional in flight, 

 and the precosta of the first abdominal tergum becomes usually a 

 postnotal plate of the metathorax. Behind the antecostal suture in 

 a tergum that retains the precosta and phragma preceding, there is 

 usually differentiated a narrow prescutum (Psc) ending laterally in 

 the prealar bridges or other lobes before the bases of the wings. A 

 triangular scutellum (Scl) is set off on the posterior part of the 

 tergum by the suture of the V-ridge (vs or VR), and the part of the 

 tergum between the prescutum and the scutellum is the scutum (Set). 

 The difificulties encountered in the study of the wing-bearing terga 

 of dift'erent insects arise from the obscuring of these landmarks, 

 either through a partial or complete suppression of the marks them- 

 selves, or from their being subordinated to characters of secondary 

 development. 



The prescutum occurs in typical form in most of the Orthoptera, 

 where it consists of a narrow transverse area of the anterior part of 

 the tergum (fig. 24 B, C, Psc), ending laterally in the prealar pro- 

 cesses, and usually expanded at the antero-lateral angles of the scutum. 

 In Blatta the prescutum is separated from the scutum by a faint suture 

 and internal ridge; in the Acrididae (fig. 24 C) it is demarked rather 

 by the line of declivity at the anterior margin of the scutum ; in Gryllus 

 (B) it is scarcely distinct from the scutum medially. In Lepidoptera 

 there is usually a small triangular prescutal plate in the mesothorax 

 (G, Psc) set into a notch in the anterior border of the scutum. In the 

 metathorax of Coleoptera the prescutum reaches its highest develop- 



