THE LABIUM 37 



more, undergo no modification whether the latter region be in its 

 undivided or subdivided condition. 



The homologies of the basal region of the labium (postmentum 

 or mentum plus submentum, as the case may be) have given rise 

 to considerable discussion. The lateral articulation of the basal 

 angles of this plate with the margin of the cranium, in Orthopteroid 

 insects, shows considerable similarity with the articulations of 

 the cardines of the maxillae. Snodgrass (1935) maintains that the 

 postmentum corresponds with the cardines of the first maxillae 

 which have become completely amalgamated with the primitive 

 sternum of the labial segment (Fig. 21). Other writers have 

 claimed that the postmentum, or at least the submentum, is 

 wholly a sternal derivative. The recent studies of Roonwal 

 (1937), on Locusta, show that the postmentum contains no sternal 

 component. If this conclusion be substantiated it would appear 

 that the postmentum has been formed entirely by the fusion of the 

 originally separate parts comparable with the cardines of the 

 first maxillae. Those who wish to follow recent work on the 

 morphology of the labium should consult the writings of Snodgrass 

 (loc. cit.) and Walker (1931). 



Reference needs to be made to the gula, which is most usually 

 developed in insects in which the heads are prognathous (Walker, 

 1932). In such cases the ventral (or, morphologically, the posterior), 

 side of the head becomes lengthened by an extension of the post- 

 genal areas. The result is that articulations of the maxillae and 

 labium become carried forwards so as to become separated by a 

 more or less variable distance from the occipital foramen (Fig. 22). 

 It is in the intervening area thus formed that the gula is developed. 

 The researches of American morphologists, notably Crampton 

 (1921, 1928), Stickney (1923) and Snodgrass (1928), have done 

 much to establish the morphological identity of the gula upon a 

 sound basis. It is more frequently developed in Coleoptera than 

 in any other order, but it also occurs in Neuropterous larvae. 

 The gula is a sclerite which has become demarcated by the forward 

 migration of the tentorial pits, or mouths, of the invaginations 

 of the posterior arms of the tentorium (Fig. 22). According to 

 Snodgrass, the ventral parts of the post-occipital sutures terminate 



