134 NATURAL HISTORY OF 



lighter than water, it obeys the tendency to float 

 upwards as soon as the counteracting force ceases 

 to be applied. When resting on the surface the 

 head is turned downwards, and the legs extended 

 at right angles with the body, of which the only 

 portion that appears above the water is the extre- 

 mity of the abdomen, where the air is admitted by 

 the tips of the elytra being slightly raised; and 

 when the insect descends, it carries along with it 

 a bubble of air resembling a globule of quick- 

 silver. If they wish to remain below the surface 

 with their legs unemployed, they can do so only 

 by adhering to the roots or stem of some aquatic 

 plant. 



They are strictly an amphibious family ; for, al- 

 though water is their favourite element, they sur- 

 vive for a long time on moist land, and usually fly 

 about in the evening twilight, or even during the 

 darkness of the night. Their flight is rapid and 

 sustained, and may be assisted in certain species 

 of Dytiscus by a kind of winglet, similar to that 

 found in the majority of two-winged flies, which is 

 placed under the base of the elytra. All the spe- 

 cies are extremely voracious, as might be inferred 

 from the structure of the masticatory and other or- 

 gans, which are very fully developed. Their usual 

 prey are the larvae of gnats, ephemerae, and dragon- 

 flies ; and the more powerful kinds are said to at- 

 tack with success animals considerably exceeding 

 themselves in size. By far the most conspicuous 



