Fig. 224.—A beetle of the Dytiscus family.
A, larva; B, adult insect (male).
The thorax consists of the usual three segments, but only the first and a small triangular area of the second are to be seen until the wing-covers are pulled aside. These last differ from the wing-covers of the cockroach not only in being much harder and stronger, but also in their inner edges meeting accurately along the middle line of the body. The delicate, filmy wings, which alone are of use in flight, are folded transversely as well as longitudinally. On the lower side of the thorax are three pairs of legs; they are very interesting and are worth examining in some detail. In the male, the first leg on each side is furnished with little circular areas which were at one time believed to be suckers. It is now known, however, that they give off a sticky substance which adheres firmly to any object clasped by the legs. The middle pair of legs seems to be used chiefly for steering. In both sexes the third or last pair of legs is modified to form a pair of sculls. Ordinary land-beetles can move their legs in a vertical direction, as well as in a horizontal one; but the hind legs of water-beetles are jointed to the thorax in such a manner that they can only move backwards and forwards, not up and down. The resemblance of the hind leg to an oar does not stop here, however. On one side there is a fringe of stiff hairs, forming the blade of the oar. The joint carrying the hairs is so arranged that the beetle can “feather” its oar—by turning the edge of the blade to the water—at each stroke.
Occasionally, usually after sunset, the beetle quits his watery home, and “wheels his droning flight” in search of pastures new. His flights are, however, only temporary and merely from one pond to another. Although Dytiscus thus normally lives in water, very cursory observation only is needed to see that he cannot exist without a regular supply of fresh air. He no sooner stops paddling than his body rises naturally to the surface, and as the tail is lighter than the head, it rises out of the water. The wing-covers are now raised a little, so that the space between them and the wings is put into communication with the outside air. The impure gas contained in this space is soon replaced by a bubble of fresh pure air, the wing-cases are lowered, and the “little diver” plunges once more into the depths. The water is prevented by hairs from getting into the air-space below the wing-cases, and the true wings are thus kept always dry. The spiracles ([p. 355]) are in communication with the air-space, so that the animal is enabled to remain below the surface for a relatively long time.
The life-history of Dytiscus.—In March or April the female Dytiscus lays her eggs in slits which she cuts in the submerged stems of pond-weeds, and the eggs hatch in about three weeks. The creature which emerges from the egg is of active habits, but is not at all like the parent in appearance. A young animal which leads an independent and self-supporting life, and differs markedly in structure from the adult, is called a larva. Thus a tadpole is a larval frog, and a caterpillar is the larva of a butterfly or moth. The larva of Dytiscus when of full size is about 2 inches in length. Like other larvae of its family ([Fig. 224], A), it has six slender legs, which serve both for swimming and for crawling over the bottom of the pond, and its head is provided with a pair of sickle-shaped mandibles, with which it seizes its prey. Each mandible is grooved on its inner side, the groove being converted into a tube by a membrane which covers it in. The savage larva sucks the blood of its victim until literally nothing is left but the shrivelled husk. At the end of the Dytiscus larva’s tail are two appendages which are fringed with hair. When the creature wishes to breathe it comes to the surface, and the tip of its tail protrudes out of the water. As each of the appendages just mentioned is pierced with a hole which leads into one of the two main air-tubes of the body, an interchange of vitiated for pure air readily takes place.
When the larva is about six weeks old, it leaves the pond and buries itself in the soil on the banks. Its exoskeleton is shed, and a thin, transparent layer of chitin—the “pupa-skin”—takes its place. In this condition the animal sinks into a state of torpor, and apparently becomes as motionless as a mummy. In this resting stage it is called a pupa. The pupal stage is necessary for the completion of the great changes—commenced some time previously—which must take place before the larva can acquire the structure of the adult. The pupal stage lasts two or three weeks, and when at last the creature emerges from its cell it is a beetle like its parents.
In consisting of three well-marked stages, the life-history of a beetle thus differs essentially from that of a cockroach. All beetles agree with Dytiscus in this respect, though in manner of life almost every conceivable variation is found. The beetle-order of insects receives its scientific name—Coleoptera[30]—from the sheathing character of the strong and closely-fitting wing-covers. The wings themselves are large, and folded in a somewhat complex manner. The mouth parts greatly resemble those of the cockroach.
64. BUTTERFLIES AND MOTHS.[31]
1. Cabbage-white butterflies.—(a) The eggs.—In May or September, search the leaves of cabbages, turnips, and other crucifers ([p. 95]) for the tiny eggs of cabbage-white butterflies. Do the eggs occur singly or in clusters? Are they found on the upper or the lower surface of the leaf? Cut off a piece of leaf which carries eggs, put it under a tumbler, and examine it every day until the larvae (caterpillars) emerge from the eggs.