Silkworm, the larva of a lepidopterous insect of the moth division, family bombycidae, and genus bombyx (Schrank). Of all the silk-producing larvae, that of the common silkworm (B. mori, Schr.) is the most important, as from it is obtained all the European and most of the Chinese silk. The moth is about an inch long and 2 in. in alar extent, of a whitish or pale yellowish color, with two or three obscure streaks and a lunate spot on the upper wings; the trunk is very short; the superior wings decumbent, and the inferior extending almost horizontally beyond them; the antennae of the males are pectinated; the males fly swiftly in the evening and sometimes by day, but the females are inactive; the latter live but a few hours after the eggs are deposited on the mulberry trees. The eggs are about the size of mustard seeds, and the young emerge in a few days if the weather or air of the breeding room is warm and dry; when first hatched they are one or two lines long, of a dark color, and very soon begin to eat voraciously, with short intervals of abstinence during the moultings, until full grown, when they are about 3 in. long, light green with darker marks, with blackish head, and fleshy protuberance on the last joint but one; there are 12 segments to the body, 9 stigmata or breathing holes on each side, and 10 legs, of which the anterior 6 are hooked, and the others, including the 2 on the last segment, end in disks; the mouth has a vertical opening, with strong and serrated jaws; the stomach is very large, as would be expected in such a voracious larva.
It lives exposed in the wild state, but none of the Chinese or European worms are allowed to incur the risks of life in the open air. According to the experiment of Count Dandolo, 100 newly hatched silkworms weigh 1 grain, after the first moult 15, after the second 94, after the third 400, after the fourth 4,628, and at full size 9,500 grains; each consumes an ounce of mulberry leaves during these stages, about 60,000 times its primitive weight, and its length increases from 1 to 40 lines during the same period; by calculation the product of an ounce of eggs eats upward of 1,200 lbs. of leaves, and should furnish 120 lbs. of cocoons. Like most other caterpillars, it changes its skin four times, at intervals depending on the temperature and on the quantity and quality of the food; if kept at 80° to 100° F. it moults in half the time required at ordinary temperatures. As usually treated, the first moult takes place on the 4th or 5th day after hatching, the second begins on the 8th, the third takes up the 13th and 14th, and the last happens on the 22d or 23d day; after this the fifth age lasts 10 days, making about 32 days for the whole process to maturity.
The appetite increases with the size till after the fourth moult; during the last 10 days the silk gum is elaborated, the appetite diminishes, and the larva begins to spin its cocoon. The spinning apparatus is near the mouth and connected with the silk bags, which are long, slender, and convoluted, containing a liquid gum; they are closed below, and end above in slender tubes, one on each side, which unite to form the single spinning tube; the gum from which the silk is produced on contact with the air is elaborated by the long glandular organs; every thread of silk is made up of two strands. It is customary to supply to the worms a piece of rolled paper or some hollow substance into which they can retire, or a convenient twig, for the formation of the cocoons. They first make an outer covering of floss silk to keep off the rain; within this they spin fine silk, bending the head and body up and down and crossing to every side, entirely surrounding the body as a protection against wind and cold; and within this is a more delicate silk, glued firmly together for the inner chamber, resisting both cold air and water. After building the cocoon the larva is transformed into a chrysalis, and comes forth a moth, easily bursting through the case, the silk, and the floss.
The cocoon resembles a pigeon's egg, and is from 1 to 1½ in. long, and bright yellow; the moth emerges from it in from 15 to 56 days, according to temperature, the former being the time in the southern United States; 18 to 20 days is the time in Connecticut, three weeks in France, and five to six weeks in England; the cocoon is made in from a few hours to three days, and is more pointed at one end than the other; the silk is not interwoven nor the glue applied at the pointed end, toward which the head is always placed. The chrysalis has no spines nor serrations on the edge of the abdominal rings, has a leathery skin, and the stomach filled with a yellowish nutritive fluid; the organs of the moth are gradually developed, and in two or three weeks the skin of the chrysalis gives way, the moth escapes into the cocoon chamber, and readily sets itself free, leaving within the remains of its former covering. In the wild state the cocoon is made about the middle of June. The silk from the cocoons containing males is finer and more tenacious than that from the female cocoons.
It is fortunate that the threads do not adhere as they do in the cocoons of many other larvae, else the operation of unwinding would be very difficult if not impracticable; even in the B. mori the silk is sometimes coarse and adherent, when the quality of the food has not been good. Like other caterpillars, the silkworm sometimes makes mistakes, and two or three are occasionally shut up in a single cocoon, in which they undergo metamorphosis perfectly well. The usual way of throwing the cocoons into boiling water kills the chrysalis; but merely steaming them over boiling water softens the glue sufficiently to allow the unwinding of the silk, and permits the moth to come forth alive from the interior layer and deposit the eggs or prepare for a new brood. - The whole secret in raising the silkworm consists in securing for it warmth, dryness, plenty of proper food, and pure air. The mulberry tree, the leaves of which constitute the food of the silkworm, requires for its perfect growth long continued dry and warm weather, and suffers in the rainy seasons of England and France; it is said to have no insect feeding upon it but the bombyx; it exhausts the earth where it is planted, as far as any other vegetation is concerned; one tree of the M. multicaulis, it is computed, will feed as many silkworms as would produce annually 7 lbs. of silk.
Silkworms are very tender and liable to perish from slight changes of temperature and dampness, from foul air, and improper or insufficient food; the periods of the moultings are times of sickness and danger; great destruction is caused by a disease called muscadine, which is a minute fungus (botrytis Bassiana) occupying the interior of the body and bursting through the skin. The disease called the " reds," manifested by red stains and blotches on the skin, is ascertained to be due to some acid, resulting from disordered digestion; the larvae seem cramped and stupefied, the rings dry up, and they look like mummies. - The larvae of several large moths of the genus saturnia (Schr.) form cocoons from which silk is obtained; among these are the arrindi silkworm, 8. [Samia] Cynthia (Schr.), of India, and the S. mylitta (Schr.), whose moths have an alar expanse of about 8 in., and appear to be the wild silkworms of the East. The S. mylitta abounds in Bengal, and yields much coarse and dark-colored silk, highly prized by the Hindoos; it cannot be domesticated; the natives catch the caterpillars, put them on the asseem trees, and guard them from birds by day and bats by night; the natural food is the rhamnus jujuba.
The S. Cynthia is domesticated in the interior of Bengal, on leaves of the castor oil plant (ricinus communis or palma Christi) and of the ailantus glandulosa; the cocoons are generally about 2 in. long and 3 in. in circumference, whitish or yellowish, of soft and delicate texture. There are eight or ten species of American silkworms; the cal-losamia Promethea and C. angulifera feed on the lilac and wild cherry; others are platysamia Euryale, P. Columbia,, P. Cecropia, and tropoza lima; but practically the larva of telea Polyphemus is the only important one. This feeds on the leaves of the oak, maple, elm, willow, and several other trees. For descriptions and figures of this species, in all its stages, and the method of rearing the larvaa, see "American Naturalist," vol. i., 1867.
Silk Spider, Male and Female, one half the natural size.
Larva, Pupa, Cocoon, and Moth of Bombyx mori.
Silkworm Moth, Male.
Silkworm Moth, Feinale.