8o NATURAL HISTORY OF AQUATIC INSECTS CH. 



is bent outwards. At the same time the enlarged 

 terminal joints hang downwards and form a well-- 

 marked angle with the rest of the antenna. The 

 film which bounds the imprisoned air is thus drawn 

 upwards and outwards in the cleft, following the 

 movement of the antenna to which it clings. As 

 soon as any part of the unwetted terminal joints 

 comes to the surface, the film breaks, and a passage 

 is instantly opened from the air above to the air-space 

 beneath. The air-passage is enclosed between the 

 hairy side of the body and the vertically arranged 

 antennal joints, neither of which can be touched by 

 water. The size of the opening is determined by the 

 relative position of the antenna and the side of the 

 body ; the shape of the opening mainly by the tension 

 of the surface film and the varying pressure of the 

 water. It may give a better idea of the action of the 

 antenna to compare it to an arm, bent at the elbow 

 and with the hand hanging down, held at a little 

 distance from the side of the body, to ward off, not 

 a blow, but the water which would otherwise enter 

 the wider part at least of the cleft between the head 

 and prothorax. 



A Hydrophilus may take in air so energetically 

 that the film enclosing the ventral air-space becomes 

 over-distended. Then bubbles escape at the base 

 of the third pair of legs every time the wing-covers 

 descend. Sometimes the Beetle becomes much 

 inconvenienced by the large quantity of air which it 

 has attached to its body, and has to drag itself down- 

 wards by clutching the stems of aquatic plants. I 

 once observed a Beetle thus rendered over-buoyant, 



