THE INSECT HEAD SNODGRASS 485 



in such insects as Apterygota (fig. 23 A), Isoptera (B), Odonata, 

 Diptera, the larvae of Lcpidoptera and Hymenoptera, and others. 

 When there are two sclerites in the basal region Walker would dis- 

 tinguish the proximal one as the 'pHmary suhmental plate^ and the 

 distal one (" Vorderplatte " of Kadic) as the secondary suhmental 

 plate. 



While agreeing in spirit, and in former usage (1928), with the 

 plan of labial nomenclature advocated by Kadic and by Walker, 

 the writer here follows the more common practise of giving the 

 names " mentum " and " submentum " to the two principal plates 

 that may be formed in the region of the basilabium. This usage, 

 however, leaves us without an appropriate name for a primitive 

 single sclerite occupying the basilabium, or for one that can not be 

 certainly identified with either the submentum or the mentum. For 

 a sclerite of this nature the term postmentum (fig. 23, B, Bmf) is 

 suggested. It would appear in some cases that the mentum and 

 submentum are differentiations from a primitive postmentum, and 

 in others that the mentum is formed from the membranous anterior 

 part of the basilabium. In any case, however, the mentum belongs 

 to the sub-stipital region of the labium, since it always lies proximal 

 to the insertions of the retractor muscles of the stipites (fig. 24, rst) 

 inserted on the base of the prementum. The mentum in some insects 

 evidently has suffered a secondary reduction, as in the Blattidae 

 (fig. 23 C, Mt.) ; in the Acrididae it is practically obliterated. 



The complete musculature of the labium (fig. 24) comprises four 

 groups of muscles. Those of the first group include two pairs of 

 muscles arising typically on the tentorium, both of which are in- 

 serted on the prementum, one pair proximally {ladlh), the other 

 distally {^adlh). These muscles correspond with the tentorial ad- 

 ductors of the first maxillae (fig 19, KLt). The second set of labial 

 muscles includes the muscles of the palpi (fig. 24, Iplp, dv^P)-! ^^^^1 of 

 the terminal lobes (/^?, fpgl)., all of which arise within the pre- 

 mentum, and have their exact counterparts in the maxillae (fig. 19, 

 O. Q, flcs, fga). The third group of labial muscles includes two 

 pairs (fig. 24, Is^ 2s) arising within the prementum and inserted on 

 or near the orifice of the salivary duct. These muscles are not rep- 

 resented in the maxillae. The labial muscles of the fourth set are 

 the stipital retractors (fig. 24, rst) arising medially on the basila- 

 bium (always proximal to the mentum when the latter is present), 

 and inserted on the extreme base of the prementum. These are im- 

 portant muscles of the labium, but they have no representatives in 

 the maxillae. 



Lastly, we must observe what happens to the labium in insects 

 having a prognathous type of head (fig. 12 B), and particularly in 



