INSECTS. 



169 



IXSECTS. 



The insects have been more often described than any other group in 

 the whole animal kingdom, except, possibly, the birds; and yet then; is 

 always something interesting to say concerning them. Insects are found 

 almost everywhere; even the sea has its inhabitants belonging to this 

 group, — curious bugs which live upon the surface of the ocean, miles and 

 miles from land. In Salt Lake, too, where the water contains a far larger 

 proportion of salt than does the ocean, the larvae of flies occur in laro-e 

 numbers. 



Insects are characterized by having but three pairs of legs in the adult, 

 and these legs are always borne on the middle of the three regions into 

 which the body is di- 

 vided 

 these 



head, and 

 the large 



The 



// ad. 



Thorax. 



Abdomen. 



regions 



Fig. 152. 



Diagram of a grasshopper, to show the three regions • 

 thorax, and abdomen — of an insect. 



-head, 



first of 

 is the 

 this bears 

 compound 

 eyes, so conspicuous in 

 the flies and dragon- 

 flies, a pair of jointed 

 antennae, and several pairs of jaws variously modified for the purposes of 

 eating. The middle region — the thorax — has in most insects, besides the 

 legs, one or two pairs of thin and delicate wings ; while the hinder division, 

 or abdomen, shows distinctly the segments of which it is composed. 



In most of the Crustacea respiration is effected by gills ; but in the 

 insects oxygen is conveyed to the tissues by means of delicate tubes, which 

 penetrate to all parts of the body. On the sides of the thorax and abdo- 

 men of any insect, careful observation will show a series of little holes, and 

 these are the openings through which the insect breathes. One can easily 

 see the process of breathing by holding some naked form, like a grass- 

 hopper, in the fingers. The abdomen will be seen to expand and con- 

 tract in a manner curiously like the enlargement and contraction of the 

 human thorax when one breathes. Indeed, the phenomenon has exactly 

 the same meaning in the two instances; when the body enlarges, air 

 rushes in through the openings (spiracles they are called) on the side of 

 the insect, distending the tubes and penetrating to every part of every 

 tissue. When the abdomen contracts, the air and the carbonic acid which 

 it has received from the animal is forced out again. Here, as in the higher 

 forms, respiration and the combustion of tissues involved produces heat, 

 and heat to a considerable amount. Bees at certain times breathe far 



