The great insect-class presents us with the most familiar examples of these changes, and the butterflies and moths exemplify metamorphosis in its most typical aspect. Thus we know that from the egg of the butterfly, deposited by the short-lived parent upon the leaves of plants, a crawling grub-like creature is first developed. This form we name the “larva” or “caterpillar”; and if we might fail to recognize its relationship to the bright denizen of the air as far as outward appearance is concerned, we might also be at a loss to reconcile its internal structure with that of the perfect butterfly. Thus the latter is winged; possesses a mouth and digestive system, adapted for the reception and assimilation of flower-juices; and wholly differs in structure and habits from its worm-like progeny. The caterpillar is provided with a mouth furnished with jaws, and adapted for biting or mastication; its digestive system presents a type differing widely from that of the perfect form; and its crawling, terrestrial habits appear in strong contrast to the light and ethereal movements of its parent. The life of this larva may be accurately described as one devoted solely to its nourishment. Its entire existence, while in the caterpillar state, is one long process of continuous eating and devouring. By means of its jaws it nips and destroys the young leaves of plants, much to the gardener’s annoyance; and so rapidly does its body increase in size, that the first skin with which its body is provided soon cracks and bursts like a tight-fitting coat, and a process of moulting ensues. As the result of this process the larva emerges, clad in a new skin, adapted to the increased size of its body. This second skin may similarly become inadequate to accommodate its ever-increasing growth, and a second process of moulting produces in turn a new investment. In this way the caterpillar may change its coat many times--twenty-one moultings have been counted in the development of the Mayflies--and on arriving at the close of its larval stage of existence, may present a very great increase in size, as compared with the dimensions it presented at the beginning of its life. But, sooner or later, the caterpillar appears to sicken, and to become quiescent. Its former state of activity is exchanged for one of lethargy, from which it awakes to begin an operation of a novel and different nature from that in which it has been previously engaged. It begins to spin a delicate silky thread by means of a special apparatus, situated in the head, and which consists of silk-glands and of an organ named the “spinneret.” Within the silken case or “cocoon” which it thus constructs with the thread of the spinneret, the caterpillar-body is soon inclosed; the first stage of its existence comes to an end; and the second or cocoon stage, marked by outward quiescence and apparent rest, becomes known to us as that of the “pupa,” “chrysalis,” or “nymph.”