silkworm

The Columbia Encyclopedia, 6th ed.

silkworm

silkworm, name for the larva of various species of moths, indigenous to Asia and Africa but now domesticated and raised for silk production throughout most of the temperate zone. The culture of silkworms is called sericulture. The various species of silkworms raised today are distinguished by the quality of the silk they produce, the type of leaves on which they feed, and the number of breedings per year. The most widely raised type and the producer of the finest silk is the larva of Bombyx mori, of Asian origin. After centuries of domestication, Bombyx mori is no longer found anywhere in a natural state. The legs of the larvae have degenerated, and the adults do not fly. Hatched from eggs so small that about 35,000 of them weigh only an ounce, these silkworms are immediately quite active and feed voraciously on mulberry leaves. At the end of the larval stage (32 to 38 days after hatching) they are about 3 in. (7.5 cm) long. A mature larva attaches itself to a twig and, with a weaving motion of its head and a slow, circular motion of its body, begins to spin its cocoon (see pupa). A moist substance, fibroin, is manufactured in two silk glands located on the underside of the larva's body; mixed with a small amount of wax, it is emitted from an orifice called the spinneret, in the lip of the larva. The fibroin dries quickly in the air, hardening into a half-mile-long thread of silk that makes up the cocoon. The adult moth, with a wingspread of 1.75 in. (4.5 cm), emerges from the cocoon in about two weeks. The moths mate and lay their eggs (several hundred from each female) within a week; the eggs hatch in about ten days. Only enough cocoons to ensure adequate reproduction are allowed to hatch; the rest are unwound after developing for a week, and the silk is processed. The giant silkworms used in some Asian and South American sericulture are the larvae of the closely related saturnid moths (family Saturniidae). They include the tussah moth (Antherala pernyi), the producer of tussah silk. The ailanthus moth (Samia walkeri), a large, olive-green saturnid moth used in China to produce a coarse grade of silk, was imported to the United States along with its food plant, the Chinese ailanthus tree, as the basis of an industry that never materialized; the moth has been firmly established in the New York City area since 1861. Diseases of silkworms have occasioned important scientific work. When Pasteur saved the French silk industry from destruction by pébrine, a protozoan disease of insects, in the mid-18th cent., he also made an important contribution to the germ theory of disease. The common silkworm, Bombyx mori, is classified in the phylum Arthropoda, class Insecta, order Lepidoptera, family Bombycidae.

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