Bombyx mori

"Silkworm" redirects here. For other uses, see Silkworm (disambiguation).

Domesticated Silkmoth Silkworm
Paired male (above) and female (below)
Fifth instar silkworm larvae.
Conservation status
Domesticated
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Lepidoptera
Family:	Bombycidae
Genus:	Bombyx
Species:	B. mori
Binomial name
Bombyx mori Linnaeus, 1758
Synonyms
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 any other mulberry tree (ie, Morus rubra or Morus negra) as well as the Osage Orange. It is entirely dependent on humans for its reproduction and does not occur naturally in the wild. Sericulture, the practice of breeding silkworms for the production of raw silk, has been underway for at least 5,000 years in China, Korea, 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 the eastern regions of Russia. The domesticated silkworm derives from Chinese rather than Japanese or Korean stock. It is unlikely that silkworms were domestically bred before the Neolithic age. It wasn't until then that the tools required to facilitate the manufacturing process of larger quantities of silk thread, had been developed. 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.

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. After molting, the instar phase of the silkworm emerges white, naked, and with little horns on the backs.

After they have molted four times (i.e., in the fifth instar phase), their bodies become slightly yellow and the skin become tighter. The larvae will then enter the pupa phase of their life cycle and enclose themselves in a cocoon made up of raw silk produced by the salivary glands. The cocoon provides a vital layer of 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 or gums such as the sericin, a water water-soluble protective layer that solidifies immediately when exposed to air. gum. It is sometimes subsequently dyed.

If the animal is allowed to survive after spinning its cocoon and through the pupa phase of it's life cycle, it will release proteolytic enzymes to make a hole in the cocoon so that it can emerge as a 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, and ruins the silk threads. 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).

During the adult phase of the life cycle (the moth) cannot fly. Silkmoths have a wingspan of 3–5 cm (1.5–2 inches) and a white hairy body. Females are about two to three times the bulkier than males (for they are carrying many eggs), but are similarly colored. Adult Bombycidaes 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. One useful development for the silk industry are silkworms that can feed on food other than mulberry leaves, including an artificial diet. Research on the genome also raises the possibility of genetically engineering 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.

Silkworm breeding: Silkworm is one of the most genetically exploited animals. Silkworms were first domesticated during the Han Dynasty in China about 2000 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. A draft sequence was published in 2004.

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. 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.

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.

Silkworm legends

In China, there is a legend that the discovery of the silkworm's silk was by an ancient empress Lei Zu, the wife of the Yellow Emperor and the daughter of XiLing-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 above legend is a nice story, however, the drinking of tea did not become common in China until about AD 500.

The Chinese guarded their knowledge of silk. It is said that about AD 550, a Catholic monk smuggled silkworms, in a hollow stick, out of China and sold the secret to the Byzantine Empire.

Silkworm diseases

Nosema bombycis: is a microsporidium that kills 100% silkworms hatched from infected eggs. Nosema bombycis is a microsporidium that can be carried over from worms to moths then eggs and worms again. This microsporidium comes from the food that silkworms eat. If silkworms get this microsporidium in their worm stage, there are no visible symptoms. However, mother moths will pass the disease onto the eggs, and 100% of worms hatching from the diseased eggs will die in their worm stage. Therefore, it is extremely important to rule out all eggs from infected moths by checking the moth’s body fluid under a microscope.

Botrytis bassiana: is a fungus that destroys the entire silkworm body. This fungus usually appears when silkworms are raised under cold and high humidity. This disease will not be passed on to the eggs from moths. Actually, the infected silkworms cannot survive to become moths and lay eggs. This fungus can extend to other insects.

Grasserie:If grasserie is observed in chawkie stage, then the chawke larvae must have been infected while hatching or during chawkie rearing 2. Chawkie larvae may get infected with the silkworm egg surface is not disinfected 3. The larvae also get infected, when the silkworm rearing hous is not disinfected and hygine is not practiced effectively during chawkie rearing 4. The disease development in early instar rearing is faster

Pebrine:Pebrine is a disease caused by a parasitic microsporidian, Nosema bombycis Nageli1. Diseased larvae show slow growth, undersized body and poor appetite. 2. Diseased larvae reveal pale and flaccid body. Tiny black spots appear on larval integument. 3. Dead larvae remain rubbery and do not undergo putrefaction shortly after death.

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
  Male adult. Scale is 15 mm
• Female adult. Scale is 20 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.
  Silkworm moth female reproductive organ.
• A female silkworm moth perched on a cocoon.
  A female silkworm moth perched on a cocoon.
• Silkworm eggs laid on another cocoon.
  Silkworm eggs laid on another cocoon.
• Cocoons
  Cocoons
• A female silkworm moth laying eggs.
  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.
• 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.

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
• Silkworm inform silkworm egg supply
• Life Cycle Of A Silkworm 1943 article with first photographic study of subject