36 



SOME ASPECTS OF MORPHOLOGY 



palpi, and from which the muscles of these parts take their origin. 

 In its most generalised condition it is a paired structure, as is 

 seen, for example, in the Machilidse, Blattida?, Plecoptera and 

 Isoptera (Fig. 20). It is probable that these paired components 

 are the equivalents of the stipites of the first maxillae. Further- 

 more, the muscles of the prementum appear to be directly 

 comparable with those of the palpi and the terminal lobes of the 



.Is 



t 



Fig. 21. Diagram expressing the origin of the labium from a pair of 

 united maxillae. Left, generalised labium. Right, theoretical 

 union of second maxillae incorporating, in part, the labial 

 sternum, c, cardo ; //, flexor of lacinia ; fl^, flexor of glossa ; 

 fg, flexor of galea ; fg', flexor of paraglossa ; g, galea ; g', para- 

 glossa ; /, lacinia ; /', glossa ; Is, labial suture ; o, levator of 

 palpus ; J), palpiger ; jaZ, palpus ; q, depressor of palpus ; r, 

 median or stipital refractors ; ,s', labial sternum. The broken 

 line (Is) leading to the labial suture divides the labium into 

 a proximal element or postmentum and a distal element or 

 prementum. (Adapted from Snodgrass, 1931.) 



first maxillae. The postmentum is, in many insects, subdivided 

 in mentum and suhmentum : much importance, from the morpho- 

 logical standpoint, has been given to these two subdivisions in 

 the past, but it now appears probable that their development is 

 solely due to a secondary division of an original single and entire 

 postmentum. The only muscles arising from the postmentum 

 are the median retractors of the labium : neither the mentum 

 nor submentum develops any muscles other than those alread}^ 

 evident in relation with the postmentum. These muscles, further- 



