5 28 



THE STUDY OF INSECTS. 



plants ; in others they are provided with floats and let loose 

 in the water ; and in still other species the cases are carried 

 by the mother underneath her body and steadied with her 

 hind legs. Frequently some of the young larvae devour 

 their companions ; in this way the size of the family is de- 

 creased before it escapes from the egg-case. Later they 

 live upon insects that fall into the water and upon snails. 



These larvae resemble somewhat those of the Dytiscidae; 

 but the body is much more plump, and the mandibles are of 

 moderate size. A very interesting observation on the mode 

 of feeding of one of these larvae has been published. It cap- 

 tured a fly, and swam with it rapidly through the water to a 

 leaf near the surface. It then rested on this leaf, and, rais- 

 ing its head out of the water, crushed the fly to pulp with its 

 jaws, letting the blood run into its open mouth. 



The Hydrophilidse are represented in North America by 

 about one hundred and fifty species. Our three largest be- 

 long to the genus Hydrophilus (Hy-droph'i-lus). In this 

 genus the metasternum is prolonged backward into a spine 



between the hind legs, and the ster- 

 num of the prothorax bears a deep 

 furrow. Our most common species is 

 HydropJiilns triangularis (H. tri-an-gu- 

 la'ris) (Fig. 637). 



The beetles of the genus Tropister- 

 nus (Trop-i-ster'nus) agree with Hy- 

 dropliilus in the form of the proster- 

 num and metasternum, but differ in 

 size, our species measuring less than 

 one-half inch in length. The most 

 common species in the East is Tropi- 

 sternus glaber (T. gla'ber), and, on the 

 Pacific coast, T. californicus. 



Next in size to HydropJiilns are several species of Hy- 

 drocharis (Hy-droch'a-ris). In this genus the metasternum 

 is prolonged somewhat, but does not form a long, sharp 



FIG. 637. 



