THE INSECT HEAD SNODGKASS 



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tentorial pits. There is thus added to the submental region of the 

 basilabium (fig. 13, Smt) a proximal area of varying extent known 

 as the (jula {Gu). Though the gula is thus but an extension of 

 the base of the primary submentum, the two areas of the basilabium 

 separated hy an imaginary or real line hetimen the anterior ends 

 of the tentorial pits are now distinguished as " submentum " and 

 "gula" {Smt, Gu). It is clear that the evolutionary capacity of 

 the labium can not be made to conform with the niceties of nomen- 

 clatural consistency. In the Dermaptera (fig. 25 A) a gular plate 

 (Gu) is distinctly separated from the large basilabial plate (Bmt), 

 but there is no mentum present. 



_ -V^-^Sm-t 



Figure 25. — Examples of unusual labial structures 

 A, labium of an earwig, Anisolabls maritima, with distinct gular sclerite 

 iO-u) separated from a large basimental plate (Bmt) on which arises 

 the stipital retractors (rst), and ventral muscles of hypopharynx (mhv) . 

 B, labium of a May beetle, Phyllopliaga, in which the submentum (Smt) 

 is inflected between the mentum (Mt) and the gula (Ou) . 



The necessity of studying the muscles for identifying the parts 

 of the labium can not be overemphasized. The stipital region 

 (prementum), as we have seen, is the part of the labium (fig. 24, Lst) 

 that contains the muscles of the palpi and terminal lobes, and on 

 which are inserted the cranial muscles of the labium. The retractors 

 of the stipites {rst), when present, are always inserted on the base 

 of the prementum, never on the mentum. A good example of the 

 value of the muscles in determining the homologies of the labial 

 parts is furnished by the adult May beetle {Phyllophaga) , in which 

 the labium (fig, 25 B) at first sight appears to have no submentum, 

 though a distinct gula {Gu) and mentum {Mt) are present. An 

 examination of the inner surface of the labium, however, reveals 

 the submentum in the form of a deep inflection {Smt) between the 



