HEMIPTERA . 
129 
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. Fig - *4 B.—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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