Fig. 216.—Larva, pupa, cocoon, and moth of Bombyx mori.
When the cocoons are completed, the people in charge of the rearing establishments separate them from the heather and sell them to the silk-spinners. But they must manage to get these cocoons into a state in which they will remain entire during a long time. They must, in other words, kill the chrysalis, to prevent the cocoons being pierced by the moth. To kill the chrysalis so as to prevent the development of the imago is an operation which is called the étouffage, or stifling.
Fig. 217.—Apparatus for stifling the chrysalides in the cocoons.
To effect this stifling, the cocoons are exposed to a high temperature. Formerly, in the Cévennes, the cocoons were placed in a baker's oven, heated for baking bread. But they ran the risk thus of being burnt, or of a certain number of chrysalides remaining alive. Now, to kill the chrysalides, they make use of steam at 100°, produced by water boiling in a vessel, and which passes through wicker baskets filled with cocoons.
The rearer must also take care at the time he gathers them, to separate the cocoons which are to provide eggs for the next year. As the female cocoons are heavier than the male cocoons, they are easily separated by weighing them in a pair of scales.
To obtain the eggs, or grain, the cocoons are fixed on sheets of brown paper, covered with a slight coating of paste made of flour. They are arranged in such a manner that the moths shall find no obstacle when they come out of them, head foremost; and, moreover, so that they may be able to reach with their legs the cocoon which is opposite them, so as to hang on to it, and to facilitate their exit from their own cocoon ([Fig. 218]). The male and female cocoons are pasted on separate sheets.