The Story Book of Science  by Jean Henri Fabre


"S OONER or later, according to its species, a day comes when the larva feels itself strong enough to face the perils of metamorphosis. It has valiantly done its duty, since to stuff its paunch is the duty of a worm; it has eaten for two, itself and the matured insect. Now it is advisable to renounce feasting, retire from the world, and prepare itself a quiet shelter for the death-like sleep during which its second birth takes place. A thousand methods are employed for the preparation of this lodging.

"Certain larvæ simply bury themselves in the ground, others hollow out round niches with polished sides. There are some that make themselves a case out of dry leaves; there are others that know how to glue together a hollow ball out of grains of sand or rotten wood or loam. Those that live in tree-trunks stop up with plugs of sawdust both ends of the galleries they have hollowed out; those that live in wheat gnaw all the farinaceous part of the grain, scrupulously leaving untouched the outside, or bran, which is to serve them as cradle. Others, with less precaution, shelter themselves in some crack of the bark or of a wall, and fasten themselves there by a string which goes round their body. To this number belong the caterpillars of the cabbage butterfly and the swallow tail. But especially in the making of the silk cell called cocoon is the highest skill of the larvæ shown.

"An ashy white caterpillar, the size of the little finger, is raised in large numbers for its cocoon, with which silk stuffs are made. It is called the silkworm. In very clean rooms are placed reed screens, on which they put mulberry leaves, and the young caterpillars come from eggs hatched in the house. The mulberry is a large tree cultivated on purpose to nourish these caterpillars; it has no value except for its leaves, the sole food of silkworms. Large tracts are devoted to its cultivation, so precious is the handiwork of the worm. The caterpillars eat the ration of leaves that is frequently renewed on the screens, and from time to time change their skin, according to their rate of growth. Their appetite is such that the clicking of their jaws is like the noise of a shower falling during a calm on the foliage of the trees. It is true that the room contains thousands and thousands of worms. The caterpillar gets its growth in four or five weeks. Then the screens are set with sprigs of heather, on which the worms climb when the time comes for them to spin their cocoons. They settle themselves one by one amid the sprigs and fasten here and there a multitude of very fine threads, so as to make a kind of network which will hold them suspended and serve them as scaffolding for the great work of the cocoon.


Silk Worm
  Eggs, worm, cocoon, and butterfly  

"The silk thread comes out of the under lip, through a hole called the spinneret. In the body of the caterpillar the silk material is a very thick, sticky liquid, resembling gum. In coming through the opening of the lip, this liquid is drawn out into a thread, which glues itself to the preceding threads and immediately hardens. The silk matter is not entirely contained in the mulberry leaf that the worm eats, any more than is milk in the grass that the cow browses. The caterpillar makes it out of the materials of its food, just as the cow makes milk of the constituents of her forage. Without the caterpillar's help man could never extract from the mulberry leaves the material for his costliest fabrics. Our most beautiful silk stuffs really take birth in the worm that drivels them into a thread.

"Let us return to the caterpillar suspended in the midst of its net. Now it is working at the cocoon. Its head is in continual motion. It advances, retires, ascends, descends, goes to right and left, while letting escape from its lip a tiny thread, which rolls itself loosely around the animal, sticks itself to the thread already in place, and finishes by forming a continuous envelope the size of a pigeon's egg. The silken structure is at first transparent enough to permit one to see the caterpillar at work; but as it grows thicker what passes within is soon hidden from view. What follows can easily be guessed. For three or four days the caterpillar continues to thicken the walls of the cocoon until it has exhausted its store of liquid silk. Here it is at last, retired from the world, isolated, tranquil, ready for the transfiguration so soon to take place. Its whole life, its long life of a month, it has worked in anticipation of the metamorphosis; it has crammed itself with mulberry leaves, has extenuated itself to make the silk for its cocoon, but thus it is going to become a butterfly. What a solemn moment for the caterpillar!

"Ah! my children, I had almost forgotten man's part in all this. Hardly is the work of the cocoon finished when he runs to the heather sprig, lays violent hands on the cocoons and sells them to the manufacturer. The latter, without delay, puts them into an oven and subjects them to the action of burning vapor to kill the future butterfly, whose tender flesh is beginning to form. If he delayed, the butterfly would pierce the cocoon, which, no longer capable of being unwound on account of its broken threads, would lose its value. This precaution taken, the rest is done at leisure. The cocoons are unwound in factories called spinning mills. They are put into a pan of boiling water to dissolve the gum which holds the successive windings together. A work-woman armed with a little heather broom stirs them in the water, in order to find and seize the end of the thread, which she puts on a revolving reel. Under the action of the machine the thread of silk unwinds while the cocoon jumps about in the hot water like a ball of wool when one pulls the yarn.

"In the center of the threadbare cocoon is the chrysalis, scorched and killed by the fire. Later the silk undergoes divers operations which give it more suppleness and luster; it passes into the dyer's vats where it takes any color desired; finally it is woven and converted into fabric."