Bombyx mori

The silkworm is the larva or caterpillar of Bombyx mori (Latin: "silkworm of the mulberry tree"), the domesticated silkmoth. A moth in the family Bombycidae, it is very important economically as the producer of silk. It is entirely dependent on humans for its reproduction and no longer occurs naturally in the wild. Silk culture has been practiced for at least 5,000 years in China (Goldsmith et al. 2004). A silkworm's preferred food is White Mulberry leaves. It is native to northern China.

Its nearest wild relative is Bombyx mandarina which is able to hybridize with the domestic taxon (Goldsmith et al. 2004), and which ranges from northern India to northern China, Korea and Japan. It is not known when the domestic silkmoth diverged from its wild relatives, only that the domestic population originated from inland Chinese rather than Japanese or Korean stock (Maekawa et al. 1988, Arunkumar et al. 2006). Molecular clock studies suggesting an age of many millions of years cannot be taken seriously, as they assume that both species have evolved with constant speed since their divergence.

This is not correct however due to the domestication process having accelerated the pace of evolution (a similar problem affects the attempt to resolve the phylogeny of domestic Western honey bee subspecies). In fact, the domestic silkworm has undergone such strong artificial selection that it is completely unable to survive in the wild for any length of time. It is probably the most heavily domesticated animal known apart from domestic hybrids such as mules. Regardless whether the domestic silkworm is derived from a wild species that has since gone extinct, or from a stock of Bombyx mandarina that was taken into human care some 4,600 years ago (Yoshitake 1968), breeding of silkworms cannot have originated before the Neolithic as the tools necessary to make use of the silk thread on a large scale only have become available since then.

Sometimes, the Wild Silkmoth is considered a subspecies of Bombyx mori (the older specific name is used as per ICZN rules) as they are theoretically capable of full hybridization. However, due to the domesticated moth's requirement for human care to survive, gene flow is all but nonexistent and thus, despite its apparently recent origin, the domestic animal is generally treated as a distinct monotypic species today.

Development
Eggs take about ten days to hatch. Silkworms have a strong appetite, as do all lepidopteran larvae. They eat day and night, preferring White Mulberry but not being strictly monophagous they also take other species of Morus and some other Moraceae. Hatchlings and second-instar larvae are called kego (毛蚕, "hairy silkworm") in Japan, or chawki in India. They are covered with little black hairs. When the color of their heads turns darker, it means that it is time for them to molt. Later instars are white, nude, and have a horn on the back.

After they have molted four times (i.e., in the fifth instar), their bodies turn slightly yellow and their skin becomes tighter. The larvae enclose themselves in a cocoon of raw silk produced in the salivary glands that provides protection during the vulnerable, almost motionless pupal state. Many other Lepidoptera produce cocoons, but only a few large Bombycidae and Saturniidae have been exploited for fabric production.



The cocoon is made of a single continuous thread of raw silk from 300 to 900 meters (1000 to 3000 feet) long. The fibers are very fine and lustrous, about 10 micrometers (1/2500th of an inch) in diameter. About 2,000 to 3,000 cocoons are required to make a pound of silk. Based on 1 kilometer (about 1100 yards) per cocoon, ten unraveled cocoons could theoretically extend vertically to the height of Mt Everest. At least 70 million pounds of raw silk are produced each year, requiring nearly 10 billion pounds of mulberry leaves. According to E. L. Palmer (Fieldbook of Natural History 1949), one pound of silk represents about 1,000 miles of filament. The annual world production represents 70 billion miles of silk filament, a distance well over 300 round trips to the sun.

If the animal is allowed to survive after spinning its cocoon, it will release proteolytic enzymes to make a hole in the cocoon so that it can emerge as a moth. This would cut short the threads and ruin the silk. Instead, silkworm cocoons are boiled. The heat kills the silkworms and the water makes the cocoons easier to unravel. Often, the silkworm itself is eaten (see also below).

The adult phase (the moth) cannot fly. The silkmoths have a wingspan of 3-5 cm (1.5 - 2 inches) and a white hairy body. Females have about twice to three times the bulk of males (for they are carrying many eggs), but are similarly colored. Adults in the Bombycidae have reduced mouth parts and do not feed.

Scientific and medical uses
Due to its large size and ease of culture, the silkworm has long been a model organism in the study of Lepidopteran and arthropod biology (Goldsmith et al. 2004). Fundamental findings on pheromones, hormones, brain structures and physiology were made with the silkworm (Grimaldi & Engel 2005). To characterize the first known pheromone, bombykol, extracts were needed from 500,000 individuals because only very small quantities are produced (Scoble 1995).

Currently, research is focusing on genetics of silkworms and genetic engineering. Many hundreds of strains are maintained, and over 400 Mendelian mutations have been described (Goldsmith et al. 2004). One useful mutant for the silk industry confers the ability to feed on food besides mulberry leaves, including an artificial diet (Goldsmith et al. 2004). The genome has been sequenced (Mita et al. 2004), and many projects have worked on genetic engineering of silkworms to produce desirable proteins in the place of silk. Such proteins include human drugs (Grimaldi & Engel 2005).

Silkworm is the source of the traditional Chinese medicine jiāngcán ("stiff silkworm",, trade name "Bombyx batryticatus"). It is the dried body of the 4-5th instar larva which has died of the white muscardine disease. Its uses are to dispel flatulence, dissolve phlegm and relieve spasms.

Cuisine
Like many insect species, silkworm pupae are eaten in some cultures (see Entomophagy). In Korea they are boiled and seasoned to make a popular snack food known as beondegi. In China street vendors sell roasted silkworm pupae. Some say they taste like chicken.

Silkworm legends
In China, there is a legend that the discovery of the silkworm's silk was by an ancient empress called Xi Ling-Shi. She was drinking tea under a tree when a cocoon fell into her tea. She picked it out and as it started to wrap around her finger, she slowly felt a warm sensation. When the silk ran out, she saw a small cocoon. In an instant, she realized that this cocoon was the source of the silk. She taught this to the people and it became widespread. There are many more legends about the silkworm.

The Chinese guarded their knowledge of silk. It is said that a Chinese monk smuggled silkworms, in a hollow stick, out of China and sold the secret to Europe, which was against the law and the punishment would be execution in the town square.