528 
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 Hydrophilidae 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 
Hydrophilus triangularis (H. tri-an-gu- 
la'ris) (Fig. 637). 
The beetles of the genus Tropister- 
nus (Trop-i-ster'nus) agree with Hy - 
drophilus 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 Hydrophilus 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 
