NO. 3 



INSECT HEAD — SNODGRASS 



43 



wall of the stomodeum, however, is direct evidence that they do not 

 belong to the system of the ventral nerve cord. 



The stomodeum (fig. 19) is usually differentiated into several parts 

 in the mature insect, which may include a buccal cavity (BuC), a 

 pharynx (Pliy), an oesophagus (OE), a crop {Cr), and a proven- 

 trkulus {Pvent). The entire length of the tube, except the extreme 

 anterior end, is surrounded by circular and longitudinal muscles. In 

 general the circular muscles form an external layer, the longitudinals 

 an internal layer, but the arrangement and relative development of 



Vx Oc Poc Cv ,T, 



Ao 



Cr Pvent \ent- 



FrGne - 



PrC Epby Hphy 



Fig. 19. — The stomodeum of an insect, and its relation to associated organs 

 in the head, diagrammatic. 



Ao, aorta; Br, brain; BnC, buccal cavity; Clp, clypeus ; Cr, crop; Ephy, 

 epipharynx ; es, epistomal suture; Fr, frons; FrGng, frontal ganglion; Hphy, 

 hypopharynx; Lb, labium; Liii, labrum ; LNv, lateral stomodeal nerve; Mth, 

 mouth ; Oc, occiput ; CE, oesophagus ; Pliy, pharynx ; Poc, Postocciput ; PoR, 

 postoccipital ridge ; PrC, preoral cavity ; Pvent, proventriculus ; SID, salivary 

 duct ; SIO, orifice of salivary duct ; Ti, tergum of prothorax ; Tent, body of 

 tentorium ; Vent, ventriculus ; J '.1% vertex ; 2, anterior labral muscle ; j, posterior 

 labral muscle ; 3S, retractor muscle of the mouth angle. 



the two layers varies much in different insects, as will be illustrated 

 in the grasshopper and the caterpillar (pages i [5 and 145). The buccal 

 cavity, the pharynx, the oesophagus, and the crop are provided with 

 dilator, or '' suspensory " muscles arising on the walls of the head, 

 on the tentorium, and on the walls of the thorax (figs. 41, 44, 55)- 



The parts of the stomodeum can not be concisely defined, because 

 they are functional adaptations of structure varying in different in- 

 sects, rather than strictly morphological regions of the stomodeal tube. 

 The buccal cavity is the anterior, or ventral, end of the stomodeum, in- 

 cluding the region of the mouth opening (fig. 19, BuC). The dilator 

 muscles of the stomodeum that have their insertion on the dorsal wall 



