28 TEXT-BOOK OF ENTOMOLOGY 



is much the larger region, as it consists of ten to eleven, and per- 

 haps in the Dermaptera and Orthoptera twelve, segments, and con- 

 tains the mid- and hind-intestine, as well as the reproductive organs. 



When we compare the body of an insect with that of a worm, in 

 which the rings are distinctly developed, we see that in insects ring 

 distinctions have given way to regional distinctions. The segments 

 lose their individuality. It is comparatively easy to trace the seg- 

 ments in the hind-body of an insect, as in this region they are least 

 modified; so with the thorax; but in the head of the adult insect it 

 is impossible to discover the primitive segments, as they are fused 

 together into a sort of capsule, and have almost entirely lost their 

 individuality. 



In general it may be said that the head contains or bears the 

 organs of sense and of prehension and mastication of the food ; the 

 thorax the organs of locomotion; and the abdomen those of repro- 

 duction. 



When we compare the body of a wasp or bee with that of a worm, 

 we see that there is a decided transfer of parts headward; this pro- 

 cess of cephalization so marked in the Crustacea likewise obtains in 

 insects. Also the two hinder regions of the body are, in a much 

 greater degree than in worms, governed by the brain, the principal 

 seat of the intelligence, which, so to speak, dominates and unifies 

 the functions of the body, both digestive, locomotive, and repro- 

 ductive, as also those of the muscles moving the different segments 

 and regions of the body. To a large extent arthropodan morphology 

 and class distinctions are based on the regional arrangement of the 

 somites themselves. Thus in the* process of grouping of the seg- 

 ments into the three regions, some increase in size, while others 

 undergo a greater or less degree of reduction ; one segment being 

 developed at the expense of one or more adjoining ones. This 

 principle was first pointed out by Audouin, and is called Audouin's 

 law. It is owing to the greater development of certain segments 

 and the reduction of others, both of the body-segments and of the 

 segments of the limbs, that we have the wonderful diversity of 

 form in the species and genera, and higher groups of insects, as 

 well as those of other arthropods. 



/-. The integument (exoskeleton) 



The skin or integument of insects consists, primarily, as in worms 

 and all arthropods, of an epithelial layer of cells called the h >/)><>- 

 dermis. This layer secretes the cuticle, which is of varying thick- 



