THE MANDIBLES 



39 



mandibles differs in important features from that of the higher 

 insects {vide Snodgrass, 1931, 1935). In their musculature the 

 mandibles of Japyx^ for example, show a very evident resemblance 

 to that displayed in the jaws of the Chilopoda and some of the 

 lower Crustacea. The mandible of Japyx is an elongated organ 

 which is hinged to the head by means of a single dorsal articulation. 

 It is moved by means of four muscles, two being dorsal and two 

 ventral (Fig. 23, A). The dorsal muscles are the promotor and 



—ta 



^za 



Fig. 23. Diagram of typical A, apterygote, and B, pterygote 

 mandibles, a, primary articulation with cranium ; a-c, 

 secondary longitudinal axis of movement on cranium ; ha, 

 hypopharyngeal apophysis ; p, dorsal promotor ; r, dorsal 

 remotor ; ta, tentorial adductor ; za, zygomatic adductor. 

 (After Snodgrass, 1931.) 



remotor muscles, while the ventral muscles comprise two groups 

 of fibres that are, functionally, adductors — the zygomatic and 

 tentorial adductors of Snodgrass. As the latter author has pointed 

 out, in the Pterygota (Fig. 23, B) each mandible has developed a 

 long axis of attachment and instead of having a single articulation 

 it has developed two. One of these is the condyle which represents 

 the single articulation of the Apterygota, while the other, the 

 facet or ginglymus, is a secondary acquisition. Owing to this 

 change in the articulation, the muscles undergo correlated modifica- 

 tions. The promotor and remotor muscles become respectively 



