28 ENTOMOLOGY 



greatest flexure, i. e., the valleys of the folds, and greatest at the places 

 of least 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 

 compacted into a single mass the head in which the original seg- 

 mentation 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 attachment 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 

 condition, which may possibly have been brought about simply by the 

 backward 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. 21, B) 

 and Diptera (Fig. 40). 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. 34) consists almost entirely of the epicranium, which bears the 

 compound 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 epicranial 

 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 between the branches of the epicranial 

 suture. 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 



