134 STATURAL HISTORY OF 
lighter than water, it obeys the tendency to float 
upwards as 9oon 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 JDytiscus 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 
