HEMIPTERA. «29 



Family CORISID^E (Co-ris'i-dae). 

 The Water-boatmen. 



These are oval, gray and black, mottled bugs, usually 

 less than half an inch in length, which occur in the streams, 

 ponds, and lakes of the whole United 

 States. The characteristic form and 

 markings of these insects are shown in 

 Figure 148. These Water-boatmen 

 have the body flattened above, and 

 swim upon the ventral surface ; in 

 these respects they differ from the mem- 

 bers Of the next family. F,G - m8— Corisa undulata. 



The body of these insects, as they swim through the 

 water, is almost completely enveloped in air, which glitters 

 like a silver armor. This air is breathed by the insects while 

 they remain under water, and in good water it is purified 

 by contact with the fine particles of air scattered through 

 the water; so that the insects can breathe their coats of air 

 again and again indefinitely. This is the case especially 

 when they are in their favorite attitude, anchored near the 

 bottom of a pond or aquarium. Here they will remain for 

 a long time clinging to some object by their fore legs, and 

 with their hind legs stretched out sidewise ; these they move 

 frequently as when swimming. The result of this movement 

 is to cause a current of water to pass over the coat of air, 

 purifying it. 



The body of these insects with the air clinging to it is 

 much lighter than water; consequently, whenever they loose 

 their hold upon the object to which they have been clinging, 

 they rise quickly to the surface, unless they prevent it by 

 swimming. They occasionally float on the surface of the 

 water, and can leap into the air from the water and take 

 flight. They feed upon other insects and lay their eggs 

 upon water-plants. All of the North American species of 

 this family belong to the genus Corisa (Cor'i-sa). 

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