6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



COMPARISON OF THE VARIOUS TYPES OF LABIAL STRUCTURE 



As has been previously stated, the labium of insects in its primitive 

 State (pi. I, A) is made up of two major divisions, the prementum 

 (Print) lying distal to the insertions of all labial muscles, and the 

 postmentum (Pmt) lying proximal to the insertions of all labial 

 muscles. 



The roach, Periplaneta (pi. i, B), conforms to this scheme, although 

 the ventral adductors of the labium {2adlh) insert on small lateral 

 sclerites yh, h). These are, however, unquestionably but secondary 

 subdivisions of the prementum (Pnnt). The submental (Smt) divi- 

 sion of the postmentum (Pint), which is made up in the roach and 

 the adult Coleoptera (C) of the submentum and the mentum (Mt), 

 serves as the area from which the retractors of the prementum (B, rst) 

 originate. The ventral adductors of the labium (2adlb) arise in the 

 typical arrangement from the tentorium. 



In adult Coleoptera, of which Harpahis (C) serves us as an ex- 

 ample, the muscles again correspond to the primitive condition. The 

 retractors of the prementum {rst) arise from a short and low internal 

 ridge, which is common to the two components of the muscle. This 

 ridge is marked externally by a slender band of extra heavy sclerotiza- 

 tion lying in the submentum (Smt). The ventral adductors (2adlh) 

 again arise from the tentorium, but their point, of origin is nearly 

 contiguous with the submentum, being in the angle formed internally 

 between the latter and the tentorial arms. These muscles insert on 

 the prementum at the posterior margins of the segmentlike bases of 

 the palpi (Pip). 



As a typical example of the labium of coleopterous larvae, Byrrhus 

 was chosen (pi. i, D). " Typical " is used here in the sense that the 

 conditions of muscular arrangement and labial division, as shown in 

 Byrrhus, represent those found in the majority of the larvae studied. 

 That is, more larvae conform to this arrangement than to any other. 



The prementum of Byrrhus is made up of two divisions, the first 

 and second prementum (D, I Print, 1 1 Print) . The postmentum (Pmt), 

 however, is not subdivided and consists of a single sclerite. Either 

 the mentum, as found in the generalized forms and in some larvae, 

 has been lost or is inseparably fused with the submental sclerite to 

 form the postmentum. 



On the base of the second prementum are inserted the retractors 

 of the prementum (rst), which, in common with the generalized con- 

 dition, arise from the postmentum. The ventral adductors of the 

 labium (2adlh) insert on the base of the first prementum, close to the 

 midventral margin of the latter. They likewise conform with the 

 primitive labium in having their origin on the tentorium (Tent). 



