The domestic silk moth (Bombyx mori) is an insect from the moth family Bombycidae. It is the closest relative of Bombyx mandarina, the wild silk moth. The silkworm is the larva (or caterpillar) of a silk moth. The silkworm is of particular economic value, being a primary producer of silk. A silkworm's preferred food is white mulberry leaves, though they may eat other species of mulberry, and even leaves of other plants like the osage orange. Domestic silk moths are entirely dependent on humans for reproduction, as a result of millennia of selective breeding. Wild silk moths (other species of Bombyx) are not as commercially viable in the production of silk.
Sericulture, the practice of breeding silkworms for the production of raw silk, has been underway for at least 5,000 years in China, whence it spread to India, Korea, Nepal, Japan, and then the West. The domestic silk moth was domesticated from the wild silk moth Bombyx mandarina, which has a range from northern India to northern China, Korea, Japan, and the far eastern regions of Russia. The domestic silk moth derives from Chinese rather than Japanese or Korean stock.
Silk moths were unlikely to have been domestically bred before the Neolithic period. Before then, the tools to manufacture quantities of silk thread had not been developed. The domesticated B. mori and the wild B. mandarina can still breed and sometimes produce hybrids. It is unknown if B. mori can hybridize with other Bombyx species. Compared to most members in the genus Bombyx, domestic silk moths have lost their color pigments as well as their ability to fly.
Types
Mulberry silkworms can be divided into three major categories based on seasonal brood frequency. Univoltine silkworms produce only one brood a season, and they are generally found in and around Europe. Univoltine eggs must hibernate through the winter, ultimately cross-fertilizing in spring. Bivoltine varieties are normally found in East Asia, and their accelerated breeding process is made possible by slightly warmer climates. In addition, there are polyvoltine silkworms found only in the tropics. Their eggs typically hatch within 9 to 12 days, meaning there can be up to eight generations of larvae throughout the year.
Description and life cycle
Larvae
Eggs take about 14 days to hatch into larvae, which eat continuously. They have a preference for white mulberry, having an attraction to the mulberry odorant cis-jasmone. They are not monophagous, since they can eat other species of Morus, as well as some other Moraceae, mostly Osage orange. They are covered with tiny black hairs. When the color of their heads turns darker, it indicates they are about to molt. After molting, the larval phase of the silkworms emerge white, naked, and with little horns on their backs.
Pupae (cocoon) After they have molted four times, their bodies become slightly yellow, and the skin becomes tighter. The larvae then prepare to enter the pupal phase of their life cycle, and enclose themselves in a cocoon made up of raw silk produced by the salivary glands. The final molt from larva to pupa takes place within the cocoon, which provides a layer of protection during the vulnerable, almost motionless pupal state. Many other Lepidoptera produce cocoons, but only a few—the Bombycidae, in particular the genus Bombyx, and the Saturniidae, in particular the genus Antheraea—have been exploited for fabric production.
The cocoon is made of a thread of raw silk from 300 to about 900 m (1,000 to 3,000 ft) long. The fibers are fine and lustrous, about 10 μm (0.0004 in) in diameter. About 2,000 to 3,000 cocoons are required to make 1 pound of silk (0.4 kg). At least 70 million pounds (32 million kg) of raw silk are produced each year, requiring nearly 10 billion cocoons.
If the animal survives through the pupal phase of its life cycle, it releases proteolytic enzymes to make a hole in the cocoon so it can emerge as an adult moth. These enzymes are destructive to the silk and can cause the silk fibers to break down from over a mile in length to segments of random length, which reduces the value of the silk threads, although these damaged silk cocoons are still used as "stuffing" available in China and elsewhere in the production of doonas, jackets, and other purposes. To prevent this, silkworm cocoons are boiled in water. The heat kills the silkworms, and the water makes the cocoons easier to unravel. Often, the silkworm itself is eaten.