Bombyx mori

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. 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, and Japan.

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 B. mori and the wild B. mandarina can still breed and sometimes produce hybrids.

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

Contents 1 Development 2 Research 3 Domestication 4 Genome 5 Cuisine 6 Silkworm legends 7 Traditional Chinese medicine 8 Gallery 9 See also 10 References 11 Footnotes 12 External links

Development

Eggs take about fourteen days to hatch into larvae, which 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 and 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 horns on the backs.

After they have molted four times (i.e., in the fifth instar), their bodies turn slightly yellow and their skins become tighter. The larvae enclose themselves in cocoons of raw silk produced in the salivary glands that provide protection during the vulnerable, almost motionless pupal state. Many other Lepidoptera produce cocoons, but only a few—the Bombycidae (in particular the Bombyx genus) and the Saturniidae (in particular the Antheraea genus) -- 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. Silk produced yearly is not only consumed in the fabric industry for clothing, but is also used in the medical industry. The suture material mersilk, a non-absorbable, polyfilament, braided suture is composed of the organic protein fibroin produced by larvae of Bombyx mori. This silk is usually processed to remove natural waxes and gums, for example sericin gum. It is sometimes subsequently dyed.

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 on the genome raises 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 survival.^[3]

Silkworm breeding: Silkworm is one of the most genetically exploited animals. Silkworms were first domesticated during the ‘Han Dynasty’ in China about 5000 years ago. Since then, the silk production capacity of the species has increased nearly tenfold. Silkworm is one of the few organisms wherein the principles of genetics and breeding were applied to harvest maximum output. It is next only to maize in exploiting the principles of ‘heterosis’ and ‘cross breeding’ Silkworm breeding is aimed at the overall improvement of silkworm from a commercial point of view. The major objectives of silkworm breeding are improving fecundity, healthiness of larvae, quantity of cocoon and silk production, disease resistance, etc. Fecundity refers to the egg laying capacity of a breed. It is a very important factor, since commercial sericulture is strongly dependent on silkworm egg availability. Healthiness of larvae leads to a healthy cocoon crop. Healthiness is dependent on factors such as better pupation rate, less number of dead larvae in the mountage, shorter larval duration (the shorter the larval duration, the lesser the chances of infection) and bluish tinged fifth instar larvae (it is observed that bluish colored fifth instar larvae are healthier than the reddish brown ones). Quantity of cocoon and silk produced is directly related to the pupation rate and larval weight. Healthier larvae have greater pupation rates and cocoon weights. Quality of cocoon and silk depends on a number of factors including genetics. Specific purposes apart from commercial purpose are given attention by advanced countries to breed development for specific purposes like sericin production, sex limited breeds, thin/thick filament production etc. Disease resistance breeding is important, as the major reason for crop losses is pathogen infection. Efforts are being made to select breeds which are tolerant or resistant to various pathogens. [1]

Silkworm Raising For Entertainment: The silkworm has been raised for entertainment in China. Children often pass on the eggs, creating a noncommercial population. The experience provides children with the opportunity to witness the lifecycle of silkworms.

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 less than that among wild silkmoths (~83%). This suggests a single event of domestication, and that it happened over a short period of time, with a large number of wild worms being collected for domestication.^[5] Major questions, however, 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 caterpillar 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 Catholic priest 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", which is made from dried 4-5th instar larvae which have died of white muscardine disease. Its uses are to dispel flatulence, dissolve phlegm and relieve spasms.

Gallery

Females laying eggs	7-day (second instar) kego	Male adult. Scale is 15 mm	Female adult. Scale is 20 mm
Silkworm in action, spinning a thread	Silkworm, 5th instar.	Fifth instar silkworm larvae, clustered on a leaf.	Silkworm cocoons.
Silkworm moth female reproductive organ.	A female silkworm moth perched on a cocoon.	Silkworm eggs laid on another cocoon.	Cocoons
A female silkworm moth laying eggs.

See also

• History of Silk
• Silk Road
• Sericulture

References

• Grimaldi & Engel (2005): Evolution of the Insects. Cambridge University Press.
• Johnson, Sylvia (1989): Silkworms. Lerner Publications. Children's book with lots of photos.
• Maekawa, H.; Takada, N.; Mikitani, K.; Ogura, T.; Miyajima, N.; Fujiwara, H.; Kobayashi, M. & Ninaki, O. (1988): Nucleolus organizers in the wild silkworm Bombyx mandarina and the domesticated silkworm B. mori. Chromosoma 96: 263–269. doi:10.1007/BF00286912 (HTML abstract)
• Scoble, M.J. (1995): The Lepidoptera: Form, function and diversity. Princeton University Press.
• Yoshitake, N. (1968): Phylogenetic aspects on the origin of Japanese race of the silkworm, Bombyx mori L.. Journal of Sericological Sciences of Japan 37: 83–87.

Footnotes

External links

• Student page on silkworm
• WormSpit, a site about silkworms, silkmoths, and silk
• Information about silkworms for classroom teachers with many photos
• SilkBase Silkworm full length cDNA Database
• Silkworm breeding-certain fundamental thoughts