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 these regions; it bears the mouth-parts, 

 the eyes, and the antennas. The thorax 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. 



II. 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 ommatidiuni. 

 The number of ommatidia of which a compound 



Fig ^44 -^Part*of a e y e ^ s com P ose d varies greatly; there may be not 

 cornea of a com- more than fifty, as in certain ants, or there ma}' 

 be 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, 



