THE HEAD CAPSULE. 119 



occipital foramen, which articulates with one of the thoracic 

 (cervical) sclerites. This I term the ' condyle.' 



The frontal region (Fig. 22, /) extends from the epifrontal 

 sutures to the antennal ridge, which is sometimes produced 

 as a salient rostrum — the antennal rostrum. The face (fa) 

 extends from the antennal ridge to the epistome (cp). 



The frontal and facial regions are often covered by secondary 

 scleritic plates, but are developed respectively from the posterior 

 and anterior cephalocele. Both regions are represented by 

 hemispherical swellings in the Dragon-flies, and one or other 

 retains this embryonic character in many insects. 



The Internal Skeleton of the Head. — The gula often supports a 

 saddle -shaped endocranium, on which the infra -oesophageal 

 ganglia lie, corresponding with the furca of the thoracic sterna ; 

 it is, however, frequently rudimentary or entirely absent. 

 The remaining endocranial sclerites are the fulcrum and the 

 scleral ring, which will be described hereafter, and the jugum 

 (Figs. 217 and 24 jtt), a chitinous bar, formed of two lateral 

 halves, springing from the inner edge of the occipital foramen. 

 The jugum assists in supporting the tentorium, on which the 

 supra-cesophageal nerve centres rest. 



c. The Head Capsule of the Blow-fly. 



The head capsule of the adult imago (Fig. 23, / and :^) of the 

 Blow-fly is comparatively thin — a characteristic shared by the 

 majority of the Diptera. It exhibits a convex anterior and a 

 conically concave posterior surface. The proboscis hangs from 

 its inferior surface, and is capable of being withdrawn into the 

 lower part of the head capsule. 



The line of junction between the anterior and posterior 

 surfaces of the head capsule I term the ' crista,' and its central 



Bibliography : — 



49. ROBiNEAU-DESVOmv, J. B., ' Essai sur les Myodaires.' Mem. de 

 rinstitut de France, Sc. Math, et Physiques, torn. :i., 1830. 



50. Menzbier, M. a.. 'Ueberdas Kopfskelet und die Mundweikzeuge 

 der Zweifliisler.' Bull, de la Soc. Imp. des Nat. de Moscou, torn. Iv , 

 1880. Abst. in Journ. Roy. M. Soc, ser. ii., vol. i., p. 236. 



