The name moult has been given to a sort of crisis during which the renewing of the skin of larvæ takes place. When it approaches, the silkworm changes its colour. Its robe, which was white or grey, and opaque, becomes yellow and somewhat transparent. The head swells considerably, especially above, and the skin becomes wrinkled (Fig. 202). The worm then fasts, and prepares to cast its skin. It places here and there some silk threads on the surrounding objects. It then slips under these threads, so that during its movements the old skin which it will abandon is, so to speak, gathered up. It then assumes a peculiar position, that represented in Fig. 203, and remains in it in a state of immobility which has been called sleep (_sommeil_). During this sleep the new skin is formed under the old. A liquid oozes forth between the two membranes which separates them, and allows the silkworm to leave its old skin. To effect this, the worm begins by raising its head, and by making contortions. The old skin splits round the muzzle, or snout, on the head and back; then by different movements the animal emerges from its skin, which remains held up by the silken threads. The duration of the time occupied in moulting varies with the degree of the heat or humidity of the atmosphere; but in general the state of _sleep_ lasts from twelve to twenty-four hours. One hour after the crisis the worm begins again to eat. [Illustration: Fig. 204.--Egg and first age.] The _ages_ of the silkworm are the periods of time which elapse between one moult and another. If we observe some silkworms when the temperature is favourable, we shall find that there are four moults, and consequently five ages. At the first age (Fig. 204), the silkworm is black and hairy; then of a nut colour at the moment when the first moult is going to take place. "The appearance presented by these worms collected together on a leaf," says Dandolo, "is that of a downy surface of a dark chestnut colour, in the midst of which one sees nothing but a movement of little animals having their heads raised, working them about, and presenting black, shiny muzzles. Their bodies are completely covered with hairs arranged in straight lines, between which one perceives along the whole length of the body other longer hairs."[60] [60] "L'Art d'élever les Vers à Soie," par le Comte Dandolo. In 8vo. 2e édition. Lyon, 1825. [Illustration: Fig. 205.--Second age.] [Illustration: Fig. 206.--Third age.] [Illustration: Fig. 207.--Fourth age.]