NO. 3 INSECT HEAD SNODGRASS I37 



being no perceptible chitinization beyond it to form a postoccipital 

 rim in the neck region. The postoccipital ridge gives origin to plate- 

 like apodemes that constrict the actual opening of the head cavity into 

 that of the neck. Usually there is a pair of dorsal apodemes (A, B. 

 dap) in the notch of the vertex, and a pair of larger ventral apodemes 

 (A, D, E, vap) arising from the postgenal and hypostomal parts of 

 the postoccipital ridge. The apodemes vary much in size and shape in 

 different species, but those of the ventral pair are usually the larger 

 and the more constantly developed. The apodemes furnish surfaces 

 of attachment for the anterior ends of prothoracic muscles inserted 

 on the back of the head (fig. 57 A, C). In the caterpillars the foramen 

 magnum is crossed laterally by oblique foraminal muscles, which are 

 the following : 



2. — Muscles of the foramen magnum (figs. 51 E, 57 A). — Attached 

 below on each side to ventral postoccipital apodeme (fig. 51 E, vap) 

 laterad of posterior root of tentorium ; spreading dorsally and laterally, 

 sometimes as a broad fan (fig. 57 A), to the dorso-lateral parts of 

 postoccipital ridge. The foraminal muscles are of the nature of the 

 transverse muscles of the intersegmental folds in the body of the 

 caterpillar. From their position it would appear that they must pro- 

 duce a tension on the hypostomal regions of the head wall. Foraminal 

 muscles are not present in insects generally. 



The tentorium of the caterpillar is a simple structure consisting of 

 two slender longitudinal bars, and of a delicate transverse posterior 

 bridge. The longitudinal bars, which represent the anterior arms of 

 the tentorium (fig. 53 D, E, AT), arise from the lateral parts of the 

 epistomal ridge at the sides of the clypeus (fig. 50 E, I, AT). They 

 extend horizontally through the head (fig. 53 E), and are united 

 posteriorly with the ends of the posterior bridge (figs. 51 A, C, E, 

 53 D, Tnt). The bridge represents the united median parts of the 

 posterior tentorial arms (fig. 51 A, C, PT), the origins of which (E, 

 pt) are at the posterior angles of the hypostomal plates in the deep 

 inflections that form the inner ends of the ventral postoccipital apo- 

 demes {vap). The positions of all the tentorial roots in the caterpillar, 

 thus, are identical with those of the tentorial roots in an orthopteroid 

 head, notwithstanding the considerable alterations which the surround- 

 ing parts have suffered. 



THE ANTENNAE 



The antennae are much reduced in all caterpillars, being so small 

 by comparison with the adult organs that the latter are forced to de- 

 velop by recession, and during the propupal stage their tips only lie 

 within the antennae of the larva. The antennae of the caterpillar are 



