524 THE STUDY OF INSECTS, 



the air becomes impure the beetle rises to the surface, 

 forces it out, and takes a fresh supply. 



These beetles are very voracious. They destroy not 

 only other insects, but some of them will attack larger ani- 

 mals, as small fish. When kept in aquaria they can be fed 

 upon any kind of meat, raw or cooked. They fly from pond 

 to pond, and are often attracted to lights at night. Many 

 of the species make sounds, both under the water and in 

 the air. In some cases this is done by rubbing the abdom- 

 inal segments upon the elytra; in others, by rubbing the 

 hind legs upon a rough spot on the lower side of the 

 abdomen. 



The females deposit their eggs at random in the water. 

 The larvae are known as water-tigers, because of their blood- 

 thirstiness. They are elongated, spindle-form grubs (Fig. 

 632). The head is large, oval or rounded, and flattened ; 



FIG. 632. 



the mandibles are large, sickle-shaped, and hollow, with a 

 slit-like opening near the tip ; they are admirably fitted for 

 holding the prey, and at the same time sucking the juices 

 from its body, the hollow of the mandibles communicating 

 with the oesophagus. The true mouth is quite small. The 

 thorax is furnished with six well-developed legs. With 

 many of these larvae the body ends in a pair of breathing- 

 tubes, which they protrude into the air at intervals. 



When a larva is fully grown it leaves the water, burrows 

 into the ground, and makes a round cell, within which it 

 undergoes its transformations. The pupa state lasts about 

 three weeks in summer ; but the larvae that transform in 

 autumn remain in the pupa state all winter. 



This is the largest of the families of water -beetles; 

 nearly three hundred North American species are known. 



