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 ; 
Fic. 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. 
