CHAPTER II 

 THE INSECT : ITS EXTERNAL STRUCTURE 



Bringing together the facts about insects already stated, we find that 

 an adult insect is a bilaterally symmetrical animal consisting of a series 

 of segments one behind another, and that these segments are grouped 

 into three regions, the head in front, followed in order by the thorax and 

 the abdomen (Fig. 10). Covering the animal is a skeleton, shell-like in 

 that it encloses the body, but horny in its nature. Attached to the seg- 



antennae 



"tarsus 



FIG. 10. Side view of Grasshopper with parts named. (From W olden, Conn. Geol. & 



Nat. Hist. Surv., Bull. 16.) 



ments are three pairs of jointed legs, a pair of antennae, mouth parts and 

 usually two pairs of wings. It breathes through air tubes, and the 

 reproductive organs open near the hinder end of the body. 



The adult insect does not show all the segments of which its body is 

 composed. In the embryo evidences of 21 have been found, 1 but as the 

 animal progresses toward maturity some of these fuse with others. The 

 head of the adult, though apparently consisting of only one segment, is 

 now believed to be the product of the fusion of six: the three found in 

 the adult thorax seem to have always been that number; and the abdo- 

 men, composed of 12 segments in the embryo appears to have been re- 

 duced in the adult to a number varying from three to 11, partly by a 



1 Some investigators believe that 22 segments are present, the head consisting of 

 seven, but this view is not universally accepted. 



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