78 NATURAL HISTORY OF AQUATIC INSECTS CH. 



from the atmosphere. The Hydrophilus creeps or 

 swims to the surface, and without difficulty assumes 

 a nearly horizontal position. Then the body is in- 

 clined a little to one side, so as to bring the angle 

 between the head and the prothorax on one side of 

 the body to the surface. A funnel-shaped depression 

 instantly appears at this angle. If the Beetle is 

 alarmed or suspicious, it sinks after a momentary 

 gulp of air, but if perfectly tranquil it may remain 

 for several minutes at the surface, drinking in air 

 energetically. The funnel-shaped depression remains 

 unchanged throughout the whole operation ; the 

 abdominal segments appear to move in and out, and 

 are alternately flattened and arched. There is also a 

 corresponding depression or elevation of the wing- 

 covers. During these various movements, which all 

 co-operate to enlarge or contract the air-spaces en- 

 closed within the body or lying beneath the wing 

 covers, the air-film on the ventral side may be seen to 

 dilate and contract alternately, and the body of the 

 Insect rises and falls a little. The channel of com- 

 munication between the spiracles and the funnel of 

 admission is bounded only by water on the under 

 side, yet the free passage of air is never checked. 



We have next to consider how the funnel-shaped 

 depression by which the air passes in and out is 

 formed and maintained. The mere rising of the 

 Insect to the surface is not enough, nor is it enough 

 to bring one part of its air-bubble, where it is enclosed 

 by a mere film of water, to the surface. A film of 

 small extent might preserve its continuity for some 

 time, and so exclude the air, especially if the water is 



