NO. 3 INSECT HEAD SNODGRASS I35 



but the error of this interpretation is shown by the fact that none 

 of the stomodeal muscles arise upon the membrane, the clypeal dila- 

 tors having their origin on the triangular plate above. The caterpillar 

 labrum has but a single pair of muscles : 



/. — Retractor muscles! of the lahrmn (figs. 50 E, G, 53 E). — A pair 

 of long slender muscles arising on the inflected frons (figs. 50 E, 53 E, 

 Fr) ; inserted by long tendons on bases of tormae (figs. 50 G, 53 E) 



The ventral surface of a caterpillar's head presents a number of 

 secondary modifications that, at first sight, somewhat obscure the basic 

 structure ; but, when the general head " landmarks " are once recog- 

 nized, it is not difficult to see that the fundamental structure is no 

 difl:'erent from that in an orthopteroid head. 



As we have noted, the caterpillar head is characterized by an elon- 

 gation of the postgenal regions between the foramen magnum, or the 

 end of the neck membrane (fig. 51 E, NMh) , and the posterior articu- 

 lations of the mandibles (a). On each side, a posterior median part 

 of the postgena (A, E, Hst) is separated from the more lateral post- 

 genal region (Pge) by a suture (/). 



The median area thus set off is called the hypostoma {Hst), and the 

 inner angles of the two hypostomal areas are approximated and 

 sometimes united on the median line behind the base of the labium, 

 which is thus separated from its usual basal connection with the neck 

 membrane, or with the postoccipital rim of the cranium. In this 

 manner a condition has been evolved which is almost a replica of that 

 in the head of adult Hymenoptera (fig. 48 B, C), except that in the 

 latter the hypostomal areas are not separated from the rest of the 

 postgenal regions. 



In some caterpillars a well-developed subgenal ridge (fig. 51 D, 

 SgR) follows the outer margin of the membranous area of the an- 

 tennal base from the anterior articulation of the mandible (c) to the 

 posterior (a), and is then continued along the anterior mesal margin 

 of the hypostoma (Hst). Some entomologists distinguish the part 

 of the subgenal ridge that skirts the mandibular area as the " pleuro- 

 stomal ridge," or " pleurostoma," and that part which follows the 

 hypostomal margin as the " hypostomal ridge." The external suture 

 that defines the hypostomal area on each side (E, ;) forms internally 

 a strong ridge (D, ;) extending from the subgenal ridge at the pos- 

 terior mandibular articulation (a) to the postoccipital ridge (PoR). 

 The subgenal ridge, especially its hypostomal part, is lacking or but 

 weakly developed in some caterpillars (C), but the ridge of the 

 hypostomal suture (;) is always well developed, and apparently serves 

 to brace the genal area between the mandible and the posterior rim 



