30 
BULLETIN OF THE BUREAU OF FISHERIES. 
of the body is filled with a large flattened bubble of air. On the under surface 
also the hairy tracts already noted on the thorax and first abdominal segment and 
on the sides of the other abdominal segments retain a film of ah. This film joins 
the dorsal air bubble at the groove between the pro and meso thorax and along the 
margins of the elytra. The spiracles open into this combined ah supply, the larger 
ones being nearer the groove just mentioned. 
When the Hydrous wishes to renew its air supply, it does not thrust the poste- 
rior end of the body out of the water as does Dytiscus. On the contrary, it assumes 
a position approximately parallel with the surface of the water and inclines the 
body sidewise, so as to bring the angle between the bead and protborax on one side 
above the surface. At this angle a funnel appears leading to the air film on the 
thorax below. At the same time the antenna on this side is moved outward. 
Its four enlarged terminal joints lie in the air film beneath the overhanging edge 
of the protborax and in the space between the protborax and the bead. These 
joints are fringed with long bristles and covered with fine short hairs, which can not 
be wetted by water. Hence, when they move outward the air film is carried along 
with them, clinging to the hairs and bristles. As soon as any part of these unwetted 
joints comes to the surface of the water the air film breaks and a passage is thereby 
opened through the funnel from the outside air to the air film on the ventral surface 
of the thorax. The terminal joints are drooped downward at an angle with the 
rest of the antenna, and the air passage is thus inclosed between the hairy side of 
the thorax and the vertically arranged antennal joints, also covered with hair. 
The. size of the opening is determined by the distance to which the antennal joints 
are carried from the side of the body. The vitiated air is expelled and the fresh 
air drawn in by rhythmical movements of the abdominal segments and the elytra. 
Especial attention is called to the fact that the air enters and leaves the dorsal 
bubble only by way of the air film on the ventral surface of the thorax and is never 
taken directly into the space beneath the elytra. 
ECONOMIC RELATIONS. 
Having shown that both the larvas and adults are a considerable menace to 
young fish and having gained a fairly complete knowledge of the life history of this 
beetle, its relations to pond fish culture may be discussed more intelligently. 
RELATION TO VEGETATION. 
The kind as well as the amount of vegetation in a pond is of great importance. 
Both the adult and the larva of Hydrous use various water plants as a bunting 
ground from whence to obtain their food, as a biding place to escape^tbeir enemies, 
and as a means of reaching the surface to obtain oxygen. The adult beetle also 
feeds upon living plant tissue, and the female attaches her egg case to floating 
leaves or bits of dead vegetation. Accordingly, the inference would be that the 
greater the amount of vegetation in a pond the better it would be adapted to the 
breeding of these beetles, but this does not prove to be true. The adults seem to 
manifest a decided choice in the kind of vegetation and are easily discouraged by 
a superabundance, and of course the eggs are laid and the larva; are developed 
wherever the adults may choose. 
