30 



BULL-ETII?- 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 air. 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 air 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 sm*face of the water and inclines the 

 body sidewise, so as to bring the angle between the head and prothorax 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 prothorax and in the space between the prothorax and the head. 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 larvae 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 hunting 

 ground from whence to obtain their food, as a hiding place to escape^their 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 larvae are developed 

 wherever the adults may choose. 



