It has been calculated, let us say by the way, that forty thousand cocoons would suffice to surround the earth at the equator with one thread of silk. Folded on itself almost like a horse-shoe, its back within, its legs without, the worm arranges its thread all round its body, describing ovals with its head. It approximates gradually the points of attachment of the thread. As long as the cocoon is not very thick one can watch it through the meshes of the web applying and fixing its thread, still to a certain degree soft, in such a manner as to make it adhere closely to the parts already formed. "We can state," says M. Robinet, "that the silkworm makes every second a movement extending over about five millimètres. The length of the threads being known, it follows that the worm moves its head three hundred thousand times in making its cocoon. If it employs seventy-two hours at its work, it is a hundred thousand movements every twenty-four hours, four thousand one hundred and sixty-six an hour, and sixty-nine a minute, that is to say, a little more than one a second." About the fourth day, after having expended all its silk,[62] the worm shut up in the cocoon becomes of a waxy white colour, and swollen in the middle of its body. The abdominal legs wither away; the six fore legs approach each other and become black. The parts of the mouth tend downwards; the skin wrinkles. Very soon it is detached and pushed down towards the hinder part, and the chrysalis appears under the rents in the skin. It is at first white, but speedily becomes of a brown red. [62] "Manuel de l'Educateur du Ver à Soie," p. 37. The silkworm remains in general from fifteen to seventeen days in the pupa state. At the moment of hatching, the moth begins by breaking the skin in which it is shut up, and which is pretty thin. But how can it come out of the silky prison which it has itself built? To effect this it makes use of a peculiar liquid contained in a little bladder with which its head is provided, and which was discovered by M. Guérin-Méneville. It moistens the cocoon with this liquid; which soaks through and penetrates the whole thickness of the silken wall which confines it. The threads of silk of which it is composed are moistened and disunited, but not broken. The moth opens a passage for itself through the threads thus separated, and makes its appearance in the light of day. Its wings are folded back on themselves, and it is still quite wet, but it seeks immediately for a good place in which to dry itself, and in a little time assumes its final appearance (Figs. 209, 210). The female (Fig. 210) has whitish wings, the antennæ only slightly developed and pale, the abdomen voluminous, cylindrical, and well filled. It is quiet, heavy, and stationary. The male is smaller; its wings are tinged with grey, its antennæ blackish; it moves about, beats its wings together, and is lively and petulant.