22 ENTOMOLOGY 



flexure, i. e., the crests of the folds. This explanation, which has been 

 elaborated in some detail by the Neo-Lamarckians, applies also to the 

 segmentation of the limbs, as well as the body. 



Head. In an insect several of the most anterior pairs of primary 

 appendages have been brought together to co-operate as mouth parts and 

 sense organs, and the segments to which they belong have become com- 

 pacted into a single mass the head in which the original segmentation 

 is difficult to trace. The thickened cuticula of the head forms a skull, 

 which serves as a fulcrum for the mouth parts, furnishes a base of attach- 

 ment for muscles and protects the brain and other organs. 



While the jaws of most insects can only open and shut, transversely, 

 their range of action is enlarged by movements of the entire head, which 

 are permitted by the articulation between the head and thorax. 



As a rule, one segment overlaps the one next behind; but the head, 

 though not a single segment of course, never overlaps the prothorax in the 

 typical manner, but is usually received into that segment. This condi- 

 tion, which may possibly have been brought about simply by the back- 

 ward pull of the muscles that move the head, has certain mechanical 

 advantages over the alternative condition, in securing, most economically, 

 freedom of movement of the head and protection for the articulation itself. 



The size and strength of the skull are usually proportionate to the 

 size and power of the mouth parts. In some insects almost the entire 

 surface of the head is occupied by the eyes, as in Odonata (Fig. 20, B) 

 and Diptera (Fig. 39). In muscid and many other dipterous larvae, or 

 "maggots," the head is reduced to the merest rudiment. 



Though commonly more or less globose or ovate, the head presents 

 innumerable forms; it often bears unarticulated outgrowths of various 

 kinds, some of which are plainly adaptive, while others are apparently 

 purposeless and often fantastic. 



Sclerites and Regions of the Skull. The dorsal part of the skull 

 (Fig. 33) consists almost entirely of the epicranium, which bears the com- 

 pound eyes; it is usually a single piece, or sclerite, though in some of the 

 simpler insects it is divided by a Y-shaped suture. The middle of the 

 face, where the median ocellus often occurs, is termed the front; ordinarily 

 this is simply a region, though a frontal sclerite exists in some insects. 

 Just above the front, and forming the summit of the head, is the region 

 known as the vertex; it often bears ocelli. The clypeus is easily recog- 

 nized as being the sclerite to which the upper lip, or labrum, is hinged, 

 though the clypeus is not invariably delimited as a distinct sclerite. 

 The cheeks of an insect are known as the gence, and post-gence sometimes 



