CHAPTER XLV 

 CLASS INSECTA 



The fourth class of Arthropoda is Insecta (in sek' ta; L., insedum, 

 having been cut into). The number of described species of insects is 

 enormous; some of the more recent estimates place it as high as 600,000. 

 It is also certain that a very large number have not yet been described. 

 In the number of known species the class far surpasses all other animal 

 groups combined, and the number of individuals is correspondingly large. 

 Insects are represented everywhere on the land surface of the earth, 

 except at the poles and at the glaciated summits of the highest mountains. 

 They are also numerous in fresh water but are almost entirely absent from 

 the oceans, though one type of true bug is known which occurs on the 



Hsacf 

 Anfennae 



Fig. 169. — A locust, Schistocerca americana Drury, which may serve as a typical insect. 

 {From Lutz, " Fieldbook of Insects," by permission.) Natural size. 



surface of the sea even at a considerable distance from land. The 

 largest insects are certain beetles, the bodies of which reach a length 

 of more than 6 inches, and certain moths, the wing spread of which may 

 be as great as 10 inches. On the other hand, the most minute insects 

 known are no longer than 0.01 inch. 



310. External Characteristics. — Insects agree in having three divisions 

 of the body— head, thorax, and abdomen (Figs. 169 and 170). The 

 metameres represented in the head are so fused as to make it difficult to 

 determine the exact number, but the full number is considered to be six. 

 The thorax contains three, termed in order prothorax, mesothorax, and 

 metathorax, or, sometimes, called pro-, meso-, and metanotum. The 

 first is freely movable, but the two others are fused. There is much 

 variation in regard to the number of metameres present in the abdomen, 

 where the posterior ones are variously modified, but the full number is 

 considered to be 11. 



The head bears a pair of antennae, several mouth parts, and a pair 

 of compound eyes (Figs. 170 and 171). The antennae usually consist of 



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