4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



but as a rule lateral to the latter. They insert on the dorsal (anterior) 

 surface of the prementum near the bases of the paraglossae. In the 

 large majority of cases their points of insertion are distal to those of 

 the ventral adductors. 



The ventral (posterior) adductors {2adlh) usually originate, as 

 stated above, on the tentorium medially to the dorsal adductors. They 

 insert on the base of the prementum, usually on the main sclerite but 

 sometimes (as in Periplaneta) on a pair of smaller sclerites (B, h, h). 

 This pair of muscles and that described in the preceding paragraph 

 may work together or oppose one another. 



The retractors (or flexors) of the prementum {rst) are median 

 muscles which originate from the submental subdivision (A, Smt) 

 of the postmentum (Pmt). They insert on the base of the prementum, 

 and in nearly all cases their points of insertion are posterior to those 

 of any other pair of labial muscles. 



From the preceding descriptions it can be seen that the typical 

 labium may be separated into its two major divisions, the prementum 

 and the postmentum, on the basis of the insertions of the dorsal and 

 ventral adductors of the labium, and the retractors of the prementum. 

 It has become well established by previous investigators that the 

 retractors and adductors insert in all cases only on the prementum. 

 That the mentum, a subdivision of the postmentum, contains no 

 muscle insertions is shown by examples: Periplaneta (pi. i, B) and 

 the labium of adult Harpalus (pi. i, C). It is necessary to accept this 

 interpretation if we are to gain the consistency essential to a compre- 

 hensive understanding of the labial structure. 



In the larvae of Coleoptera the prementum and the postmentum 

 are again the principal regions of the labium. The postmentum is 

 made up of two distinct subdivisions in Orthosonia (pi. 7, A) and in 

 Ptilodactyla (pi. 6, B), but it may be a single area as in Byrrhus 

 (pi. I, D). The prementum in coleopterous larvae is very often sub- 

 divided so that confusion has arisen in giving the proper connotation 

 to the parts. Taking Byrrhus as a typical example of this condition, 

 we find the prementum made up of two sclerites or divisions which 

 are here called the first prementum {I Print) and the second pre- 

 mentum (IlPriiit). Neither one of these is to be considered as secon- 

 dary to the other since both have the same value morphologically. The 

 second prementum, in all species studied, serves as the area on which 

 the retractors of the prementum insert. In general this subdivision 

 has been considered by previous investigators to be the mentum, but 

 the fact that muscles do insert on it shows that it is impossible cor- 

 rectly to call it the mentum in the sense of the term as applied to the 



