The Book of the Aquarium/Part 3/Chapter 5
The beetle tribe are distinguished from other insects by the possession of elytra, or wing-cases; which wing-cases are, with regard to the typical structure of an insect, to be regarded as really the first pair of wings hardened into a horn-like consistence to protect the others. The wing-cases are of little or no use in flight, this action being accomplished by means of the second pair, or the true wings, which are generally of large size, and when not in use are neatly folded up beneath the elytra. The division of the body into three parts—head, thorax, and abdomen—is very plain to the eye; but the segments, of which the several parts are composed, are frequently so consolidated that it is difficult to detect or count them. For instance, the thorax, theoretically, consists of three segments; but, practically, the first of these is usually so largely developed as to appear to constitute the thorax in itself. The (theoretical) nine segments of the abdomen are, in like manner, reduced to six or seven, in consequence of the last two or three being consolidated into one.
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 live-jointed, the fourth being of ordinary size.
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
DYTISCUS AND LARVA (REDUCED).
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 blood-thirsty 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.
The hydrous piceus is here represented of the natural size; the ground colour is black, with a shade of bronze, and the margins of the elytra are tinged with green and purple. Each wing case is marked with dotted lines, the breast is dingy yellow, and the antennæ and organs of the mouth dull red. The legs are black, and the hairs which fringe the tarsi reddish brown.
Colymbetes, of several species, maybe obtained in plenty from clear brooks in every part of the country.
These are elegantly-formed and lively beetles; their elytra, legs, thorax, head, and breast of the same jet-black hue, and highly burnished, though, when immersed, the breast and abdomen glisten with an intensely metallic silvery lustre, owing to the film of atmospheric air which the beetle obtains from the surface, and which adheres to the hairy covering of the abdomen. This silvery species is here represented in its natural size. They are comparatively harmless, though I have just witnessed the demise of one that was attacked and eaten by his pretty kindred, the wing-cases and head only remaining to testify of his former individuality. This is a delicate beetle, that requires very pure water and a neat arrangement of aquatic plants, to give full effect to its beauty as a cabinet specimen, and, when carefully preserved, a jar of them forms a conspicuous and attractive object. I have several specimens of a smaller species in a very fine jar of confervoids; they are incessantly in motion, threading their way through the interstices of the delicate vegetation in a business-like way, that seems to say, “I’m on an important errand—have not a moment to lose, and, above all things, I must take care of the bubble of air that sticks to my tail.”
Gyrinus Natator.—This is a member of an interesting and pretty family. Gyrinus takes its name from the tendency most of the beetles of the tribe have to move in circles, and this particular species exemplifies the habit of the tribe in a very striking manner. It is a very sociable beetle, always found in company with others of its kindred, forming little communities, which pass their time in whirling and spinning upon the surface of still pools, like congregations of dancing dervishes. They are as shy as they are nimble, and it requires some dexterity to net them, for they dive and scatter on the approach of a foot-step; but if the observer remains quiet a few minutes by the water’s edge, or on a plank or bridge above the pool, they soon resume their gambols under his eye, and in the sunshine appear like minute buttons of burnished metal rapidly revolving or darting to and fro upon the surface, and for a length of time, that proves them to be by no means subject to giddiness.
The species of Gyrinus are not numerous in Britain, not more than eight being known to naturalists; but the paucity of species is compensated by the profuseness of the individuals, and of these natator is the most abundant.
When placed under a lens, this beetle bears the form represented in the next page. The colour is blue-black, with a resplendent metallic lustre, in which shades of copper, silver, and bronze occur, as we view it at various angles to the rays of light. The thorax is marked with three transverse lines on each side, of which the anterior one is punctured, and runs parallel with the margin. The elytra are turned in at the sides, and the surface of each is marked with eleven striæ or longitudinal lines, composed of minute punctures. The terminal segment of the abdomen, together with the legs, are of a rust-red.
If we inquire by what means it is enabled to perform its elegant performances on the water, and which very closely resemble those of a skilful skater on the ice, we
find that its structure is eminently fitted for such peculiar movements. In the first place, the antennæ are short and thick; if they protruded forward to a great length, as in Longicornes, they would seriously impede the freedom of action, which is the life and joy of this nimble fellow. They are clavate, and consist of seven closely-jointed rings, each antenna being attached by a slender peduncle to the upper and internal edge of a large radical joint furnished with an auricle at its outer side, which, like the lid of a box, shuts in the antennæ when unemployed, and protects them from the water (Kirby). The anterior legs are long, and formed for walking or to act as instruments of prehension; the four hinder ones are very short, and ciliated externally, bearing a strong resemblance to the paddle of an oar. “ The head is sunk in the thorax as far as the eyes, and the latter are divided by a process from the anterior part of the head, in such a manner that there appear to be two eyes above and two below—a mode of structure admirably adapted to the wants of the insect, which requires at the same time to observe objects both in the air and water.” The hinder legs are capable of a free oar-like action, which render this gyrinus the most expert of swimmers, and the circular movements are accomplished by the more rapid action of the oars on one side than on the other. So equipped, the whirlwig leads a merry life; he skates away from morning to night, never in fear of being drowned, and seemingly never tired.