3 AQUATIC BEETLES 79 



in any degree impure, as must often be the case with 

 the water of a ditch or pond. The convenience, and 

 at times the safety, of the Hydrophilus require that 

 the moment it rises to the surface the film should 

 burst, and place the enclosed air in free communica- 

 tion with the atmosphere. Hydrophilus possesses in 

 its antenna an instrument for breaking the film cer- 

 tainly and without delay. 



This antenna consists of nine joints. The basal 

 joint is large and curved, the next four joints smaller 

 and forming a flexible stalk. All these are readily 

 wetted by water, and serve, so far as is known, 

 merely as a handle to direct the movements of the 

 terminal joints. The terminal joints are four in 

 number. Each is dilated, except just where it is 

 attached to the joint beneath, and curved into a 

 semicircular plate. The enlarged joints form what 

 looks like the clubbed end of the antenna, which is 

 not really by any means so solid as it appears at first 

 sight. They are fringed on their concave sides with 

 long bristles, and are uniformly covered with fine 

 short hairs, which cannot be wetted by water. 



The ordinary position of this antenna in the sub- 

 merged Beetle is horizontal, and extended in a gentle 

 curve. Springing just in front of the compound 

 eye, it passes backwards to the overhanging edge of 

 the prothorax. The four terminal joints lie in the 

 air-space ; the smooth basal joints are immersed and 

 wetted by the surrounding water. 



When the Hydrophilus comes up to breathe and 

 directs upwards the very adjustable cleft between its 

 head and prothorax, the antenna of the same side 



