188s ENTOMOLOGY 
has some means of keeping its body dry. This is accom- 
plished usually by hairs, set together so thickly that water 
ies cannot penetrate between them. 
As the legs and body of Gerris 
are rendered water-proof by a vel- 
vety clothing of hairs, the insect, 
though heavier than water, is able 
to skate about on the surface. 
Gyrinus, by means of a_ similar 
adaptation, can circle about on the 
surface film, and minute collem- 
bolans leap about on the surface as 
readily as on land. 
The modifications of the legs 
for swimming have often impaired 
their usefulness for walking, so 
that many aquatic Coleoptera and 
Hemiptera can move but awk- 
wardly on land. When walking, 
it is interesting to note, Cybister 
and some other aquatic forms no 
longer move their hind legs simul- 
taneously as they do in swimming, 
but use them alternately, like ter- 
restrial species. 
The adaptations for swimming 
do not necessarily affect the power 
of flight. Dytiscus, Hydrophilus, 
Gyrinus, Notonecta, Benacus and 
many other Coleoptera and Hemip- 
tera leave the water at night and 
Larva (A) and pupa (B) of : ; 
mosquito, Culex pipiens. r, respi- fly around, often being found about 
ratory tube; ¢, tracheal gills. 
electric lights. 
Respiration.—Aquatic insects have not only retained the 
primitive, or open (holopneustic), type of respiration, charac- 
