Silkworm:

The silkworm is the larva or caterpillar of the domesticated silkmoth, Bombyx mori (Latin: "silkworm of the mulberry tree"). It is an important economic insect since it is the producer of silk. A silkworm's preferred food is white mulberry leaves, but it may also eat the leaves of the Osage Orange or the Tree of Heaven. It is entirely dependent on humans for its reproduction and no longer occurs naturally in the wild. Sericulture has been practised for at least 5,000 years in China.

It was domesticated from the wild silkmoth Bombyx mandarina which has a range from northern India to northern China, Korea, Japan and far eastern Russia. It derives from Chinese rather than Japanese or Korean stock.^[1] The 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. The domesticated and wild species can still breed and so hybridize together.

The full genome of the silkworm was published in 2008 by the International Silkworm Genome Consortium.^[2]

Development

Eggs take about ten days to hatch. They eat continuously. They have a preference for White Mulberry having an attraction to the mulberry oderant cis-jasmone. They are not monophagous since they can eat other species of Morus as well as some other Moraceae. Hatchlings and second-instar larvae are called kego & chawki in India. They are covered with tiny black hairs. When the color of their heads turns darker, it indicates that they are about to molt. Later instars are white, naked, 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 thread of raw silk from 300 to about 900 meters (1,000 to 3,000 feet) long. The fibers are very fine and lustrous, about 10 micrometers (1/2,500th 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 1,100 yards) per cocoon, ten unraveled cocoons could theoretically extend vertically to the height of Mount 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 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. To prevent this, 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 Cuisine).

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, though a human caretaker can also feed them.

Research

Due to its large size and ease of culture, the silkworm has become a model organism in the study of Lepidopteran and arthropod biology. Fundamental findings on pheromones, hormones, brain structures and physiology have been made with the silkworm. One example of this was the molecular identification of the first known pheromone, bombykol which required extracts from 500,000 individuals due to the very small quantities of pheromone produced by any individual worm.

Currently, research is focusing on genetics of silkworms and the possibility of genetic engineering. Many hundreds of strains are maintained, and over 400 Mendelian mutations have been described. Another source suggests 1000 inbred domesticated strains are kept worldwide.^[2] One useful development for the silk industry are silkworms that can feed on food other than mulberry leaves, including an artificial diet. Also research upon the genome raising the possibility of genetic engineering of silkworms to produce proteins including pharmacological drugs in the place of silk proteins.

Domestication

The domesticated variety compared to the wild form has increased cocoon size, growth rate and efficiency of its digestion. It has also gained tolerance to human presence and handling and living in crowded conditions. It also cannot fly and lacks fear of potential predators. These changes have made it entirely dependent upon humans for their survival.^[3]

Genome

The genome of the silkworm is mid-range with a genome size of ~432 Mb. It was published in 2008 by the International Silkworm Genome Consortium.^[2] A draft sequence was published in 2004.^[4]

High genetic variability has been found in domestic lines of silkworms though this is than that among wild silkmoths (~83%). This suggests that was a single event of domestication and that this happened over a short period of time with a large number of wild worms being collected for domestication.^[5] However major questions remain unanswered: “Whether this event was in a single location or in a short period of time in several locations cannot be deciphered from the data,” Research also has yet to identify the area in China where domestication arose.^[6]

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. Silkworms have also been proposed for cultivation by astronauts as space food on long-term missions.^[7]

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 silk cocoon fell into her tea. She picked it out and started to wrap the silk thread around her finger, she slowly felt a warm sensation. When the silk ran out, she saw a small larva. In an instant, she realized that this caterpiller larva 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.

Traditional Chinese medicine

Silkworm is the source of the "stiff silkworm". This is made from 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.

Gallery

Silkworm

Contents

Development

Research

Domestication

Genome

Cuisine

Silkworm legends

Traditional Chinese medicine

Gallery

See also

References

Footnotes

External links