38 



SOME ASPECTS OF MORPHOLOGY 



around the tentorial pits, but when the latter pass forwards the 

 sutures just named are carried with them, and are termed the 

 gular sutures. The median cranial area, thus delimited by these 

 sutures, is the gula, and it partly or wholly occupies the region 

 between the submentum and the occipital foramen. A graduated 



series of stages, from the 



B 



Fig. 22. Diagrammatic figures of the 

 ventral region of the head illustrating 

 the origin and development of the 

 gula among Coleoptera. Excepting 

 the submentum, the mouth-parts are 

 not shown. A, generalised condition ; 

 B, rudimentary gula ; C, complete, 

 well-developed gula ; D, condition 

 found in weevils and a few other 

 Coleoptera in which the gular sutures 

 are confluent. e, compound eye ; 

 g, gula ; gp, tentorial (or gular) pit ; 

 gs, gular suture ; o, occipital foramen ; 

 OS, post-occipital suture ; s, sub- 

 mentum. 



beginnings of the formation 

 of the gula up to its evolution 

 as a large chin plate, can 

 be traced among different 

 Coleoptera. When it is 

 fused with the submentum, 

 or its frontal margin is ill- 

 defined, the anterior limit of 

 the gula can be generally 

 fixed by an imaginary line 

 between the two tentorial 

 pits. Among termites, the 

 postmentum and gula are 

 fused, in the soldiers, to 

 form one continuous and 

 greatly elongated ventral 

 plate. In the Rhynchophora 

 (Fig. 22, D) and a few other 

 beetles, the gula is suppressed 

 owing to the meeting of the 

 two gular sutures in the 

 mid-ventral line of the head. 

 It is by no means certain, 

 however, whether the gula is 



invariably formed in the manner thus described. Thus in the 

 Embioptera there is no evidence to show how the gula region in 

 those insects has been formed, since there are no traces of gula 

 sutures present {vide Walker, 1933). The so-called gula region 

 of Hemiptera requires investigation, especially in view of the 

 classificatory importance attached to it by some systematists. 

 (c) The Mandibles. In the Apterygota the prevailing type of 



