36 AN INTRODUCTION TO ENTOMOLOGY 



g. THE REGIONS OF THE BODY 



The segments of the body in an adult insect are grouped into three, 

 more or less well-marked regions: the head, the thorax, and the 

 abdomen. Each of these regions consists of several segments more or 

 less closely united. 



The head is the first of the«^e regions ; it bears the mouth-parts, 

 the eyes, and the antennae. Tiu: I horax is the second region; it bears 

 the legs and the wings if they are present. The abdomen is the third 

 region; it may bear appendages connected with the organs of repro- 

 duction. 



11. THE HEAD 



The external skeleton of the head of an insect is composed of 

 several sclerites more or less closely united, forming a capsule, which 

 includes a portion of the viscera, and to which are articulated certain 

 appendages. 



a. THE CORNEAS OF THE EYES 



The external layer of the organs of vision, the corneas of the eyes, 



is, in each case, a translucent portion of the cuticula. It is a portion 



of the skeleton of the head, which serves not merely for the admission 



of light but also to support the more delicate parts of the visual 



apparatus. 



The corneas of the compound eyes. — ^The compound eyes are the 



more commonly observed eyes of insects. They are situated one on 



each side of the head, and are usually conspicuous. Sometimes, as in 



dragon-flies, they occupy the larger part of the surface of the head. 



The compound eyes are easily recognized as eyes; but when one 



of them is examined with a microscope it is found to present an 



appearance very different from that of the eyes of higher animals, its 



surface being divided into a large number of six-sided divisions (Fig. 



44) ; hence the term compound eyes applied to them. 



A study of the internal structure of this organ 



has shown that each of these hexagonal divisions 



is the outer end of a distinct element of the eye. 



Each of these elements is termed an ommatidium. 



The number of ommatidia of which a compound 



„. T^ . J- eye is composed varies greatly; there may be not 



Fig. 44.— Part of a -^ , ^ , . . ^, 



cornea of a com- more than fifty, as m certam ants, or there may 



pound eye. -[^g many thousand, as in a butterfly or a dragon-fly. 



As a rule, the immature stages of insects with a gradual metamor- 

 phosis and also those of insects with an incomplete metamorphosis. 



