Because it may therein change from the caterpillar to the pupa state in greater safety. This cocoon it prepares from a gum or tenacious fluid contained in two pouches, placed along the back, beneath the stomach ; which fluid it spins into very fine threads, by means of a particular tube placed behind the mouth. It is the middle portion of the cocoon, after removing the floss or loose silk on the exterior, which is used in our manufactories.
Because it may dissolve any slight adhesions which may have occured when the caterpillar was spinning.
Because the cocoons may be generally unwound without breaking the thread. It is popularly supposed, however, that if the insect be disturbed during the operation by any sort of noise, it will take alarm, and break its thread; but Latreille says this is a vulgar error. The length of the unbroken thread in a cocoon varies from 600 to 1,000 feet; and as it is all spun double by the insect, it will amount to nearly two thousand feet of silk, the whole of which does not weigh above three grains and a half. Five pounds of silk from ten thousand cocoons is considerably above the usual average. When we consider, therefore, the enormous quantity of silk which is used at present, the number of worms in producing it will almost exceed our comprehension.
Because of the voracity of the animal, - a single caterpillar weighing, when first hatched, only the hundredth part of a grain - consuming, in thirty days, above an ounce of leaves; that is to say, it devours in vegetable substance about 60,000 times its primitive weight.
Count Dandolo, in his recent Treatise on Silkworm, thus estimates their progressive increase in weight:
100 worms, just hatched, weigh about
After the first moulting..
After the Second moulting..
After the Third moulting..
. . 400
After the fourth moulting..
On attaining their greatest size and weight.,
They have, therefore, in thirty clays, increased 9,500 times their primitive weight. The length of the silkworm also increases about forty times in twenty-eight days. Again, the Count calculates that the quantity of leaves drawn from the tree employed for each ounce of eggs, amounts to 1,6091b. 8oz.
In France, the scorzonera, or salsifis, has been advantageously substituted for the mulberry in rearing silkworms. The silk produced is equal to that of the worms fed upon mulberry leaves, and surpasses that obtained from worms fed upon lettuce-leaves ; in the latter case, the quantity has been doubled.
To prevent the jaundice common among silkworms, the Abbe Esseric, of Carpentras, used to powder them with quick-lime by means of a silk sieve; he then gave them mulberry-leaves moistened with a few drops of wine. It was at first supposed that the cocoons of silk were injured by this process; this, however is not the case, and his method is now adopted generally in the department of Vaucluse in the south of France.