XIII. INSECTS 



The Insects comprise one of the chief classes of the Arthropoda, 

 and from many standpoints are to be regarded as one of the most 

 remarkable groups in the entire animal kingdom. Numerically, 

 about four-fifths of all the known animal species belong to the Class 

 Insecta. Structurally they are at once one of the most highly 

 specialized groups and one of the most adaptive to nearly every 

 niche in nature. Economically they affect almost every phase of 

 Man's existence, for they include some of his important allies, as 

 well as many of his most destructive enemies which pillage his 

 possessions and ravage by disease. Thus, Insects are of funda- 

 mental importance from many standpoints. 



The structure of the Insect body presents a number of well- 

 defined characteristics. The body of the adult is clearly divided 

 into head, thorax, and abdomen. Three pairs of legs are present 

 on the thorax and, generally, two pairs of wings. Insects are the 

 only Invertebrates which possess wings. Typically the head 

 consists of six segments, the thorax of three segments, and the 

 abdomen of ten or eleven segments. The segmentation of head 

 and abdomen is subject to some variation. Insects as a group 

 possess a noteworthy and characteristic respiratory system, 

 adapted for air breathing, which consists of a network of fine tubes 

 ramifying throughout the body tissues, designed to carry oxygen 

 directly to the cells without the aid of the blood stream. 



The extraordinary number and variety of Insect species make it 

 impossible to gain an adequate conception of the various types in a 

 general course. We shall briefly consider only two types, namely, 

 the Grasshopper and Honeybee, which, on the whole, have been 

 found most satisfactory for an introduction to this field. The 

 Grasshopper may be regarded as a fairly typical Insect in its gen- 

 eral structural and functional features. The Bee, on the con- 

 trary, is a very highly specialized Insect and may be said to repre- 

 sent the acme of Insect development. It presents a number of 

 extraordinary structural adaptations, a highly organized com- 

 munal life, and, at the same time, a considerable importance from 

 the economic standpoint. 



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