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 [Pg 362] 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]