NO. 13 LABIUM OF COLEOPTEROUS LARVAE — ANDERSON 3 



labium as made up of two main divisions, the prementum (Prmt), 

 and the postmentum (Pmt). The postmentum is in turn divided by 

 a suture into the mentum (Mt) and the submentum (Smt). These 

 are the parts in the usual three-part labium, although this seems infre- 

 quently to be the case in coleopterous larvae, as will be shown later. 



Kadic (1902) showed that the mentum and submentum are to be 

 considered as secondary subdivisions of the basal part of the labium 

 and that therefore the labium is fundamentally made up of two 

 regions. Walker (1931), after a study of the labial muscles, sub- 

 stantiates Kadic's view, namely that the mentum is in reality a sclerite 

 of the basal region. While holding to the same viewpoint as to the 

 importance of the divisions, it has been thought best in this paper to 

 use the terms mentum and submentum for the two basal subdivisions, 

 and adopt the term postmentum for the combined areas as suggested 

 by Snodgrass (1931). 



The prementum bears the palpi (pi. i, A, Pip) and the ligula (Lig). 

 The latter consists typically of four lobes, a median pair of glossae 

 (Gl) and a lateral pair of paraglossae {Pgl). The postmentum is 

 subdivided into two divisions, the mentum {Mt) and the submentum 

 {Smt). 



A description of the seven pairs of muscles found in the generalized 

 labium follows : 



The depressors of the palpus (pi. i. A, dplp) arise in the pre- 

 mentum near or on the margin of a median slit which in some cases 

 divides the prementum nearly to its base. These muscles insert on 

 the distal margin of the basal segment of the palpus (Pip). 



Opposing the muscles just described are the levators of the palpus 

 {I pip). These also originate in the prementum close to its postero- 

 lateral angles. In some cases {Periplaneta, pi. i, B) these muscles 

 arise from a pair of small separate sclerites which lie close to the base 

 of the prementum. In keeping with their function of opposing the 

 depressors, they insert on the proximal margin of the basal segment 

 of the palpus. 



The flexors of the glossae {fgl) have their origin ventrally (pos- 

 teriorly) on the prementum. They insert on the base of the glossae. 

 The flexors of the glossae and those of the paraglossae lie ventral 

 (posterior) to the depressors of the palpi. 



The flexors of the paraglossae (fpgl) originate also on the pre- 

 mentum and extend more or less parallel to the flexors of the glossae. 

 They insert on the base of the paraglossae. 



The dorsal (anterior) adductors of the labium (ladlb) arise on the 

 tentorium (Tent) close to the point of origin of the ventral adductors, 



