HEMIPTERA. 185 



ing in the clypeus instead of being articulated to its tip. In the 

 males of Corisa the abdominal segments, especially the four caudal 

 ones, are very unsymmetrical, being upon one side broken into 

 irregular-shaped fragments. 



These water-boatmen have the body flattened above, and swim 

 upon their ventral surface ; they differ in these respects from the 

 members of the next family. They swim with a quick, darting mo- 

 tion ; they use for this purpose chiefly their long, oar-like posterior 

 legs. When in their favorite attitude, they are anchored to some 

 object near the bottom of the pond or aquarium by the tips of their 

 long, slender, intermediate legs, at such times the fore legs hang 

 slightly folded, and the posterior legs are stretched out horizontally 

 at right angles to the length of the body. The body of these in- 

 sects with the air which clings 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, un- 

 less 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. 



The bodies of these insects, as they swim through the water, are 

 almost completely enveloped in air. The coating of air upon the 

 ventral surface and sides can be easily seen, for it glistens like silver. 

 By watching the insects carefully, when they are bending their bod- 

 ies, air can be seen to fill the spaces between the head and prothorax, 

 and between the prothorax and mesothorax. The space beneath 

 the wings is also filled with air. When these insects are in impure 

 water, they must come to the surface at intervals to change this sup- 

 ply of air. But I have demonstrated that in good water it is not 

 necessary for them to do this. The air with which the body is 

 clothed is purified by contact with the fine particles of air scattered 

 through the water ; so that the insect can breathe its coat of air again 

 and again indefinitely.* 



The Water-boatmen are carnivorous, feeding upon other insects. 

 They attach their eggs to aquatic plants ; and the young are found 

 in the same situations as the adults. 



" In Mexico, the eggs of a species oicorisa are said to be gathered 

 from water-plants, and used as an article of food by the dwellers 

 near the lakes where they abound. The natives cultivate a sedge 

 upon which the insects will deposit their eggs. This sedge is made 



* See account of my experiments in American Naturalist, June, 1887. 



