THE INSECT HEAD SNODGRASS 



461 



spending parts of the subgenal strip may be distinguished as the 

 pleurostoma (Plst) and the hypostoma {Est). In conformity with 

 this nomenclature, the upper part of the clypeus is sometimes called 

 the epistoma^ though, when the clypeus is divided, its parts are more 

 commonly termed the anteclypcus and the postclypeus. The pleuro- 

 stoma is usually a small but distinct subgenal area above the mandible 

 (fig. 9 A, Plst) ; the hypostoma is typically a narrow marginal band 

 of the postgenal area of the cranium (figs. 9 B, 13, Est) , but in some 

 insects, as in lepidopterous larvae and in adult Hymenoptera and 

 Diptera, the hypostomata are greatly enlarged and extended medi- 

 ally on the ventral wall of the head, where, in the higher Hymenop- 

 tera and Diptera, they are united into a continuous hypostomal 

 bridge. The epistomal-pleurostomal-hypostomal marginal area of 

 the cranium constitutes the peristome. 



The internal skeleton of the head. — The ventral edges of the 

 cranium are usually braced by an internal skeletal structure known 



Figure 11. — Diagrams showing progressive modiflcations of the tentorium from 

 the generalized condition at A, through B, to specialized structure at C 



At, anterior arm ; at, anterior tentorial pit ; CT, corpotentorium ; Cv, neck ; 

 DT, dorsal arms ; PT, posterior arms ; pt, posterior tentorial pits ; Foe, post- 

 occiput ; TB, posterior tentorial bar. 



as the tentorium., which, in the absence of a substantial floor to the 

 cranium, gives attachment to the ventral muscles of the mouth 

 appendages. The tentorium, in its typical form (fig. 11 A), con- 

 sists of a transverse posterior tentorial har {TB) extending through 

 the back part of the head between the posterior tentorial pits {pt., pt) , 

 and of two longitudinal anterior tentorial arms {At., At), arising 

 at the anterior tentorial pits {at, at) and uniting posteriorly with 

 the transverse bar near its lateral extremities. The whole structure 

 is formed by four cuticular invaginations, the roots of which are 

 marked by the external pits. In many insects the posterior ends 

 of the anterior arms are approximated (fig. 11 B), and they may 

 be united in a broad median plate (C). By such modifications the 

 tentorium in appearance often departs radically from its more prim- 

 itive structure. The central plate is called the corpotentorium 

 (C, CT), and the lateral parts of the transverse bar become the 

 posterior tentorial arms (B, C, PT, PT). Branches from the ante- 



