THE D YTISCIDJE. 
297 
fly ; and they are frequently noticed to crawl out of the water, 
and to fly from one pond to another. They thus appear to lead 
two lives : one, the ordinary sub-aerial existence of beetles ; and 
the other the special life of water insects. They are made like 
land beetles ; and they are therefore obliged to come to the surface 
every now and then in order to breathe, and a very simple arrange- 
ment enables them to obtain a fresh supply of air. The spiracles, 
or breathing organs, are situated on the upper part of the body, 
and are protected by the elytra, or wing cases, which fit the upper 
part and sides of the body, so that no water can get underneath 
them. When the insect wishes to breathe, it lets itself float to 
the top of the water, so that the upper part of the back projects 
above the surface ; then the elytra are raised up, expiration takes 
place, and a supply of fresh air is inspired, and then the beetle 
shuts down the wing-cases, and includes beneath them a certain 
quantity of air, which it uses from time to time. 
The Dytiscidcz are carnivorous, and attack all kinds of aquatic 
insects, which they tear and injure with their strong mandibles. 
Their larvae live in the water, and have the same propensities 
as the adults ; and they are constantly chasing a number of insects, 
mollusca, and even young fish. They are long creatures, nar- 
rowed behind, and are clothed in such a hard integument that they 
may be said to be truly armoured. Their head is large and 
broad, and the mouthpieces project ; their mandibles are arched, 
and are perforated at the extremity, so that the insect can suck 
through them ; and their legs are long. They are very active, 
and swim readily by means of a flexible abdomen, which is ter- 
minated by two appendages, which are often leaf-shaped. 
These insects are provided with spiracles on the sides of all 
the abdominal segments ; nevertheless, when they come to breathe 
on the surface of the water, they lift up their tails where the last 
spiracles are, and appear to respire with them in particular. When 
about to undergo their metamorphosis, they come out of the 
water, and hide themselves in the mud or sand by its side, and 
make a cell according to their size, and are transformed into 
nymphs. 
The commonest kind is Dytiscus marginalis , and it abounds 
in all the stagnant waters of Europe. 
