9 8 THE STUDY OF INSECTS 



their coats of air again and again if they are in clean water. If the insects 

 are in stagnant water they have to come to the surface at intervals for a 

 fresh supply of air. 



In their favorite attitude the water-boatmen are anchored to some 

 object near the bottom of the water by their long middle legs. The body 

 of these insects, with the air which clings to it, is much lighter than water; 

 consequently whenever they lose 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. 



These insects feed upon the vegetable matter in the ooze at the bot- 

 toms of ponds and at the same time consume the minute animals which 

 are present in this plant material. 



Both the adults and eggs of species of the genus Corixa are used as 

 food for man and for birds. It is said that in Mexico the natives bind 

 the stalks of a sedge into bundles which are then floated on the water of 

 a lake where the bugs will deposit their eggs on them in great abundance. 

 The bundles are then removed and dried and the eggs beaten off on to a 

 cloth and then ground into flour for baking. The adults and eggs of 

 Corixa mercendria are said to be imported into England by the ton as 

 food for birds, game, and fish. 



Family Notonectid^; 



The Back-swimmers 



The back-swimmers have the back shaped like the bottom of a boat 

 instead of being flat like that of the water-boatmen and they differ from 

 all other aquatic bugs in that they always swim on their backs. 



The favorite attitude of the back-swimmers is floating on the surface 

 of the water, back downward, with the hind end of the body projecting 

 sufficiently to admit of air being drawn into two air chambers on the 

 ventral side of the body. There are two longitudinal furrows on the 

 ventral side of the abdomen arched over by long hairs thus forming two 

 tubes into which air is taken. The spiracles open into these tubes. 



The hind legs are long and act as oars and when the back-swimmers 

 are disturbed they dart away toward the bottom of the 

 pond. They do, however, lie right side up on the sur- 

 face of the water occasionally and often take flight into 

 the air. 



fig. 152. — a back- The species of the genus, Notonecta, are most com- 

 swimmer - mon (Fig. 152). They have sharp piercing mouth-parts 



and sometimes sting with them unless handled carefully. 



Family Nepid^e 



The Water-scorpions 



The water-scorpions have two long filaments on the end of the body, 

 which are grooved on the inner side. By putting these filaments together 

 a long tube is formed, which the insects can project to the surface of the 

 water, and thus obtain air for breathing, while resting on the bottom of 





