The order is an immense one, as to its range and variety, and hence there are in it many curious exceptions to the general conformation of a beetle. Some are utterly incapable of flight, owing to the non-possession of elytra, or wings; some have elytra only; and in some the elytra meet and unite along the suture; so that, if the insect had wings underneath, it would be impossible for it to use them.
The specimens of coleoptera, kept in the water-cabinet, are among the most interesting of the whole collection, whether in the larva or imago form; and to this order we are indebted for a large number of aquatic species, that may be kept in jars, and some few that may be introduced without danger to the tank.
Dytiscus Marginalis is one of the handsomest of water-beetles, and its habits are amusing and instructive. It possesses an insatiable appetite, and great muscular power, as we soon discover when removing it from one jar to another, for if it succeeds in planting its claws firmly on the edge of the vessel, it is difficult to move it. It belongs to the large tribe of aquatic carnivora, ranged in the section Pentamera, in which the tarsi of all the feet are five-jointed, the fourth being of ordinary size.
DYTISCUS AND LARVA (REDUCED).
The Dytiscus is a true water-beetle, being aquatic in both its larva and perfect forms. The larva, known as the water-tiger, is found in plenty in the muddy ditches round London, and is a strong, stubborn, ugly, and ravenous worm, with a tail formed for respiration, and curved mandibles to tear its prey to pieces. It is very active, and may be kept without difficulty; but nothing else should be placed in the same jar, unless intended as food for this savage. A small fish thrown is eagerly clutched, and held firm by the claws; and the larva then plunges its mandibles into it, and is soon buried head-deep in the mangled body of its prey. I have generally fed them on beef, but they prefer small fish, or larva of the dragon-fly, and do not go through their metamorphosis well without such food.
The imago is a handsome creature, with strong hooked claws, furnished with amber hairs, which, under a lens, resemble very closely the claws of a crab. The elytra are beautifully tinted with rich green and bronze, and the divisions of the head and thorax separated from the abdomen by sharp, whitish lines. Small fishes make the best diet for this beetle; but as this food fouls the water, it is best to keep them in clear jars, with a few pebbles and weeds, and once a week remove them to another vessel, to be fed. This plan preserves the brightness of the beetle jar, and prevents the annoyance of effluvia.
Hydrous Piceus.—This is the largest of our native aquatic beetles, and, with the exception of the stag-beetle, it exceeds in bulk any other species of indigenous Coleoptera. It is common in the brooks and ponds in southern counties, but becomes rare as we travel northwards. In the larva state this is a rapacious and bloodthirsty insect, and of so destructive a character as to deserve its French name of ver-assassin. In that early condition it resembles a large soft worm, of a somewhat conical form, provided with six feet, and having its large scaly head armed with two formidable jaws. The head moves with such freedom in all directions, that it can readily seize small shell-fish and other mollusca floating on the surface, without altering the horizontal position of the body maintained in swimming; and it is even bent backwards, and devours its prey more conveniently by using the back as a kind of support. These larvæ swim with facility, and have two fleshy appendages at the tail, by means of which they suspend themselves at the surface with their heads downwards, when they have occasion to respire (Cuvier). The beetle differs greatly in habit from the grub; it is by no means carnivorous, but quite harmless, docile, playful, and tameable. It is a noble creature for the cabinet, and may even be kept in the Aquarium safely. In its complete form it is as interesting for its gentleness as it is in the larva state for its rapacity and destructiveness. The female spins an elegant and waterproof cocoon for the reception of its eggs, and when they are deposited she watches them with a maternal solicitude not frequently exhibited by creatures of this family.
HYDROUS PICEUS.