10 Bulletin American Museum of Natural History [Vol. XLII 



II.— LIFE HISTORY AND BIOLOGY 

 General Survey 

 Without doubt, the water-scavengers are the most abundant of our 

 water beetles. In this respect they are approached only by the Dytiscidse 

 which are better adapted to aquatic life. The smaller members of this 

 group are very numerous, and yet are often overlooked. 



Their most common habitat is within the confines of some little 

 pool which, as the seasons progress, becomes filled with algae and other 

 aquatic vegetation. Nevertheless, all the species may be taken in 

 rapidly flowing streams but usually near the bank in debris, moss, etc., 

 or in protected bays, where the water flows the slowest. One isolated 

 record shows the capture of Cymbiodyta fimbriata beneath some falls in 

 a bed of moss. The genera which are best adapted for living at some 

 depth and under harsher conditions than the others are Hydrous, Tropi- 

 sternus and Berosus. 



The adult is chiefly herbivorous. It feeds mostly on the lower plant 

 forms, such as alga?, but does not seem to be restricted to this diet. 

 Decaying vegetation is its most popular food and it feeds readily on 

 dead animal tissue (earthworms, larvae, etc.). 



It is an air breather, though much of its time is spent below the 

 water. To obtain air 1 the beetle comes to the surface and extends its 

 pubescent antennae above water. They are then folded under the head 

 and the air which has gathered on the antennae is passed on to the hairs 

 of the prothorax. Inspiration takes place by means of the pro-meso- 

 thoracic spiracles only. The air passes through the tracheae and, as 

 fast as used, is expelled through the other seven pairs of spiracles. It 

 then spreads over the ventral side of the abdomen, where it is held as a 

 film by the hydroscopic hairs. Thus the characteristic air film is formed. 

 When a fresh supply is needed, the used air is passed along the prothorax 

 to the antennae again and the process repeated. Continual contraction 

 and expansion of the body accompanies this breathing process. 



Most of the Hydrophilidae hibernate as imagines. It would be 

 hard to tell where all of them spend their winter but a number of the 

 species have been found in the bank a few feet from the water. They do 

 not burrow down very deeply but remain near the surface, apparently 

 in readiness to enter the water during any warm spell. An examination 

 of the beetles which were taken from the bank or in early spring often 



JBrocher in his excellent paper (1913) carefully explains the respiration of Hydrophilus. 



