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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 Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder Sym ...
of a silk moth. It is an economically important insect, being a primary producer of silk. A silkworm's preferred food are
white mulberry ''Morus alba'', known as white mulberry, common mulberry and silkworm mulberry, is a fast-growing, small to medium-sized mulberry tree which grows to tall. It is generally a short-lived tree with a lifespan comparable to that of humans, althoug ...
leaves, though they may eat other mulberry species and even 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 under way for at least 5,000 years in China, whence it spread to India, Korea, Nepal,
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
, and 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 China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
, 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 Age. 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. Domestic silk moths are very different from most members in the genus '' Bombyx''; not only have they lost the ability to fly, but their color pigments have also been lost.


Types

Mulberry silkworms can be categorized into three different but connected groups or types. The major groups of silkworms fall under the univoltine ("uni-"=one, "voltine"=brood frequency) and bivoltine categories. The univoltine type is generally linked with the geographical area within greater Europe. The eggs of this type hibernate during winter due to the cold climate, and cross-fertilize only by spring, generating silk only once annually. The second type is called bivoltine and is normally found in
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
,
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
, and Korea. The breeding process of this type takes place twice annually, a feat made possible by the slightly warmer climates and the resulting two life cycles. The polyvoltine type of mulberry silkworm can only be found in the tropics. The eggs are laid by female moths and hatch within nine to 12 days, so the resulting type can have up to eight separate life cycles throughout the year.


Process

Eggs take about 14 days to hatch into larvae, which eat continuously. They have a preference for
white mulberry ''Morus alba'', known as white mulberry, common mulberry and silkworm mulberry, is a fast-growing, small to medium-sized mulberry tree which grows to tall. It is generally a short-lived tree with a lifespan comparable to that of humans, althoug ...
, 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. 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 lifecycle, 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 vital layer of protection during the vulnerable, almost motionless pupal state. Many other
Lepidoptera Lepidoptera ( ) is an order (biology), order of insects that includes butterfly, butterflies and moths (both are called lepidopterans). About 180,000 species of the Lepidoptera are described, in 126 Family (biology), families and 46 Taxonomic r ...
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. If the animal is allowed to survive after spinning its cocoon and through the pupal phase of its lifecycle, it releases
proteolytic enzyme A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the for ...
s 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 seriously reduces the value of the silk
threads Thread may refer to: Objects * Thread (yarn), a kind of thin yarn used for sewing ** Thread (unit of measurement), a cotton yarn measure * Screw thread, a helical ridge on a cylindrical fastener Arts and entertainment * ''Thread'' (film), 2016 ...
, although these damaged silk cocoons are still used as "stuffing" available in China and elsewhere for doonas, jackets, etc. 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. As the process of harvesting the silk from the cocoon kills the larva, sericulture has been criticized by animal welfare and rights activists. Mahatma Gandhi was critical of silk production based on the
Ahimsa Ahimsa (, IAST: ''ahiṃsā'', ) is the ancient Indian principle of nonviolence which applies to all living beings. It is a key virtue in most Indian religions: Jainism, Buddhism, and Hinduism.Bajpai, Shiva (2011). The History of India ...
philosophy "not to hurt any living thing". This led to Gandhi's promotion of cotton spinning machines, an example of which can be seen at the Gandhi Institute, and an extension of this principle has led to the modern production practice known as Ahimsa silk, which is wild silk (from wild and semiwild silk moths) made from the cocoons of moths that are allowed to emerge before the silk is harvested. The moth – the adult phase of the lifecycle – is not capable of functional flight, in contrast to the wild ''B. mandarina'' and other ''Bombyx'' species, whose males fly to meet females and for evasion from predators. Some may emerge with the ability to lift off and stay airborne, but sustained flight cannot be achieved. This is because their bodies are too big and heavy for their small wings. Silk moths have a wingspan of and a white, hairy body. Females are about two to three times bulkier than males (for they are carrying many eggs), but are similarly colored. Adult Bombycidae have reduced mouthparts and do not feed.


Cocoon

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 very 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.


Research

Due to its small size and ease of culture, the silkworm has become a
model organism A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...
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 silkworm. Many research works have focused on the 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 1,000 inbred domesticated strains are kept worldwide. One useful development for the silk industry is 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. ''Bombyx mori'' females are also one of the few organisms with homologous chromosomes held together only by the
synaptonemal complex The synaptonemal complex (SC) is a protein structure that forms between homologous chromosomes (two pairs of sister chromatids) during meiosis and is thought to mediate synapsis and recombination during meiosis I in eukaryotes. It is currentl ...
(and not crossovers) during meiosis.
Kraig Biocraft Laboratories Kraig Biocraft Laboratories, Inc. is an American biotechnology company headquartered in Ann Arbor, Michigan. It develops and manufactures recombinant spider silks and other high-performance polymers using spider silk gene sequences. Their most ...
has used research from the Universities of Wyoming and
Notre Dame Notre Dame, French for "Our Lady", a title of Mary, mother of Jesus, most commonly refers to: * Notre-Dame de Paris, a cathedral in Paris, France * University of Notre Dame, a university in Indiana, United States ** Notre Dame Fighting Irish, th ...
in a collaborative effort to create a silkworm that is genetically altered to produce spider silk. In September 2010, the effort was announced as successful. Researchers at Tufts developed scaffolds made of spongy silk that feel and look similar to human tissue. They are implanted during reconstructive surgery to support or restructure damaged ligaments, tendons, and other tissue. They also created implants made of silk and drug compounds which can be implanted under the skin for steady and gradual time release of medications. Researchers at the MIT Media Lab experimented with silkworms to see what they would weave when left on surfaces with different curvatures. They found that on particularly straight webs of lines, the silkworms would connect neighboring lines with silk, weaving directly onto the given shape. Using this knowledge they built a silk pavilion with 6,500 silkworms over a number of days. Silkworms have been used in antibiotics discovery, as they have several advantageous traits compared to other invertebrate models. Antibiotics such as lysocin E, a non-ribosomal peptide synthesized by Lysobacter sp. RH2180-5 and GPI0363 are among the notable antibiotics discovered using silkworms. In addition, antibiotics with appropriate pharmacokinetic parameters were selected that correlated with therapeutic activity in the silkworm infection model. Silkworms have also been used for the identification of novel virulence factors of pathogenic microorganisms. A first large-scale screening using transposon mutant library of Staphylococcus aureus USA300 strain was performed which identified 8 new genes with roles in full virulence of S. aureus. Another study by the same team of researchers revealed, for the first time, the role of YjbH in virulence and oxidative stress tolerance in vivo.


Domestication

The domestic species ''B. mori'', compared to the wild species (e.g., ''B. mandarina''), has increased cocoon size, body size, growth rate, and efficiency of its digestion. It has gained tolerance to human presence and handling, and also to living in crowded conditions. The domestic silk moths cannot fly, so the males need human assistance in finding a mate, and it lacks fear of potential predators. The native color pigments have also been lost, so the domestic silk moths are leucistic, since camouflage is not useful when they only live in captivity. These changes have made ''B. mori'' entirely dependent upon humans for survival, and it does not exist in the wild. The eggs are kept in incubators to aid in their hatching.


Silkworm breeding

Silkworms were first domesticated in China more than 5,000 years ago. Since then, the silk production capacity of the species has increased nearly tenfold. The silkworm is one of the few organisms wherein the principles of genetics and breeding were applied to harvest maximum output . It is second only to maize in exploiting the principles of heterosis and crossbreeding. Silkworm breeding is aimed at the overall improvement of silkworms from a commercial point of view. The major objectives are improving fecundity (the egg-laying capacity of a breed), the health of larvae, quantity of cocoon and silk production, and disease resistance. Healthy larvae lead to a healthy cocoon crop. Health is dependent on factors such as better pupation rate, fewer dead larvae in the mountage, shorter larval duration (this lessens the chance of infection) and bluish-tinged fifth-instar larvae (which are healthier than the reddish-brown ones). Quantity of cocoon and silk produced are 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.


Hobby raising and school projects

In the U.S., teachers may sometimes introduce the insect life cycle to their students by raising domestic silk moths in the classroom as a science project. Students have a chance to observe complete life cycles of insects from eggs to larvae to pupae to moths. The domestic silk moth has been raised as a hobby in countries such as China, South Africa, Zimbabwe, and Iran. Children often pass on the eggs, creating a non-commercial population. The experience provides children with the opportunity to witness the life cycle of silk moths. The practice of raising silk moths by children as pets has, in non-silk farming South Africa, led to the development of extremely hardy landraces of silk moths, because they are invariably subjected to hardships not encountered by commercially farmed members of the species. However, these worms, not being selectively bred as such, are possibly inferior in silk production and may exhibit other undesirable traits.


Genome

The full genome of the domestic silk moth was published in 2008 by the International Silkworm Genome Consortium. Draft sequences were published in 2004. The genome of the domestic silk moth is mid-range with a genome size around 432 megabase pairs. High genetic variability has been found in domestic lines of silk moths, though this is less than that among wild silk moths (about 83 percent of wild genetic variation). This suggests a single event of domestication, and that it happened over a short period of time, with a large number of wild silkworms having been collected for domestication. Major questions, however, remain unanswered, according to Jun Wang, co-author of a related study published in 2008, who stated: "Whether this event was in a single location or in a short period of time in several locations cannot be deciphered from the data", and research also has yet to identify the area in China where domestication arose.


As food

Silk moth pupae are
edible insects Insects as food or edible insects are insect species used for human consumption. More than 2,000 insects species worldwide are considered edible. However, a much smaller number is discussed for industrialized mass production and partly regiona ...
and are eaten in some cultures: * In
Assam, India Assam (; ) is a state in northeastern India, south of the eastern Himalayas along the Brahmaputra and Barak River valleys. Assam covers an area of . The state is bordered by Bhutan and Arunachal Pradesh to the north; Nagaland and Manipur to ...
, they are boiled for extracting silk and the boiled pupae are eaten directly with salt or fried with chili pepper or herbs as a snack or dish. * In Korea, they are boiled and seasoned to make a popular snack food known as ''
beondegi ''Beondegi'' (), literally "pupa", is a Korean insect-based street food made with silkworm pupae. The boiled or steamed snack food is served in paper cups with toothpick skewers. Its aroma has been described as "nutty, shrimp-like, and a bit ...
'' (번데기). * In
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
, street vendors sell roasted silk moth pupae. * In
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
, silkworms are usually served as a '' tsukudani'' (佃煮), i.e., boiled in a sweet-sour sauce made with
soy sauce Soy sauce (also called simply soy in American English and soya sauce in British English) is a liquid condiment of Chinese origin, traditionally made from a fermented paste of soybeans, roasted grain, brine, and '' Aspergillus oryzae'' or ''Asp ...
and sugar. * In Vietnam, this is known as ', usually boiled, seasoned with fish sauce, then stir-fried and eaten as main dish with rice. * In Thailand, roasted silkworm is often sold at open markets. They are also sold as packaged snacks. Silkworms have also been proposed for cultivation by astronauts as space food on long-term missions.


Silkworm legends


China

In China, a legend indicates the discovery of the silkworm's silk was by an ancient empress named
Leizu Leizu (), also known as Xi Ling-shi (, Wade–Giles Hsi Ling-shih), was a legendary Chinese empress and wife of the Yellow Emperor. According to tradition, she discovered sericulture, and invented the silk loom, in the 27th century BC. Myths A ...
, the wife of the Yellow Emperor, also known as Xi Lingshi. She was drinking tea under a tree when a silk cocoon fell into her tea. As 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 this caterpillar larva was the source of the silk. She taught this to the people and it became widespread. Many more legends about the silkworm are told. The Chinese guarded their knowledge of silk, but, according to one story, a Chinese princess given in marriage to a Khotan prince brought to the oasis the secret of silk manufacture, "hiding silkworms in her hair as part of her dowry", probably in the first half of the first century AD. About AD 550, Christian monks are said to have smuggled silkworms, in a hollow stick, out of China and sold the secret to the Byzantine Empire.


Vietnam

According to a Vietnamese folk tale, silkworms were originally a beautiful housemaid running away from her gruesome masters and living in the mountain, where she was protected by the mountain god. One day, a lecherous god from the heaven came down to Earth to seduce women. When he saw her, he tried to rape her but she was able to escape and hidden by the mountain god. The lecherous god then tried to find and capture her by setting a net trap around the mountain. With the blessing of Guanyin, the girl was able to safely swallow that net into her stomach. Finally, the evil god summons his fellow thunder and rain gods to attack and burn away her clothes, forcing her to hide in a cave. Naked and cold, she spit out the net and used it as a blanket to sleep. The girl died in her sleep, and as she wished to continue to help other people, her soul turned into silkworms.


Silkworm nutrition

''Bombyx mori'' is essentially monophagous, exclusively eating mulberry leaves (''Morus'' spp.). By developing techniques for using artificial diets, the amino acids needed for development are known.Hamed Kioumarsi, Nazanin Amani ''Silkworm/''Bombyx mori'': An Overview of What You Need to Know ''. AREEO, 2021. . Page 27. The various amino acids can be classified into five categories: *Those which, when removed, cause larval development to stop entirely: Lysine, Leucine, Isoleucine, Histidine, Arginine, Valine, Tryptophan, Threonine, Phenylalanine, Methionine *Those which, when removed, impede later stages of larval development: Glutamate and Aspartate *Semi-essential amino acids, with negative effects that can be eliminated by supplementing with other amino acids: Proline (Ornithine can be substituted) *Non-essential amino acids that can by replaced through biosynthesis by the larvae: Alanine, Glycine, Serine *Non-essential amino acids that can be removed with no effect at all: Tyrosine


Silkworm diseases

*''
Beauveria bassiana ''Beauveria bassiana'' is a fungus that grows naturally in soils throughout the world and acts as a parasite on various arthropod species, causing white muscardine disease; it thus belongs to the entomopathogenic fungi. It is used as a biological ...
'', a fungus, destroys the entire silkworm body. This fungus usually appears when silkworms are raised under cold conditions with high humidity. This disease is not passed on to the eggs from moths, as the infected silkworms cannot survive to the moth stage. This fungus, however, can spread to other insects. * Grasserie, also known as nuclear polyhedrosis, milky disease, or hanging disease, is caused by infection with the ''Bombyx mori nucleopolyhedrovirus'' (aka ''Bombyx mori nuclear polyhedrosis virus'', genus '' Alphabaculovirus''). If grasserie is observed in the chawkie stage, then the chawkie larvae must have been infected while hatching or during chawkie rearing. Infected eggs can be disinfected by cleaning their surfaces prior to hatching. Infections can occur as a result of improper hygiene in the chawkie rearing house. This disease develops faster in early instar rearing. *
Pébrine Pébrine, or "pepper disease," is a disease of silkworms, which is caused by protozoan microsporidian parasites, mainly ''Nosema bombycis'' and, to a lesser extent, ''Vairimorpha'', '' Pleistophora'' and '' Thelohania'' species. The parasites infe ...
is a disease caused by a parasitic microsporidian, ''
Nosema bombycis Nosema bombycis is a species of Microsporidia of the genus '' Nosema'' infecting silkworms, responsible for pébrine. This species was the first microsporidium described, when pebrine decimated silkworms in farms in the mid-19th century. This des ...
''. Diseased larvae show slow growth, undersized, pale and flaccid bodies, and poor appetite. Tiny black spots appear on larval integument. Additionally, dead larvae remain rubbery and do not undergo putrefaction after death. ''N. bombycis'' kills 100% of silkworms hatched from infected eggs. This disease can be carried over from worms to moths, then to eggs and worms again. This microsporidium comes from the food that the silkworms eat. Female moths pass the disease to the eggs, and 100% of silkworms hatching from the diseased eggs will die in their worm stage. To prevent this disease, it is extremely important to rule out all eggs from infected moths by checking the moth's body fluid under a microscope. * Flacherie infected silkworms look weak and are colored dark brown before they die. The disease destroys the larva's gut and is caused by viruses or poisonous food. *Several diseases caused by a variety of funguses are collectively named
Muscardine Muscardine is a disease of insects. It is caused by many species of entomopathogenic fungus. Many muscardines are known for affecting silkworms.Singh, T. ''Principles And Techniques Of Silkworm Seed Production''. Discovery Publishing House. 2004. ...
.


See also

*
Cocoonase Cocoonase is a trypsin-like proteolytic enzyme produced by silkworms (of both ''Bombyx'' and ''Antheraea'' species) as they near the final stages of their metamorphosis. It is produced by cells in the proboscis and exuded onto the galeae. Its fun ...
* History of silk *
Silk Road The Silk Road () was a network of Eurasian trade routes active from the second century BCE until the mid-15th century. Spanning over 6,400 kilometers (4,000 miles), it played a central role in facilitating economic, cultural, political, and reli ...
*
List of animals that produce silk Silk is produced by a variety of animals, for different purposes, with various types being produced. Insects *Silkworms produce silk when undergoing larval to adult metamorphosis. This includes not only the domesticated ''Bombyx mori'', but a wide ...
* '' Samia cynthia'' * Thai silk *
Lao silk Lao silk is produced in Laos with ancient weaving techniques that produce high quality silk Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and is pr ...
* Japanese silk *
List of domesticated animals This page gives a list of domesticated animals, also including a list of animals which are or may be currently undergoing the process of domestication and animals that have an extensive relationship with humans beyond simple predation. This includ ...


References


Further reading

* * * * * * *


External links


Student page on silkworm




* ttps://web.archive.org/web/20070626074209/http://morus.ab.a.u-tokyo.ac.jp/ SilkBase Silkworm full length cDNA Database
Silk worm Life cycle photos

Silkworm School Science Project Instruction

Life Cycle Of A Silkworm
1943 article with first photographic study of subject {{DEFAULTSORT:Bombyx Mori Moths described in 1758 Bombycidae Domesticated animals Edible insects Moths of Asia Moths of Japan Moths of Africa Moths of North America Silk Sericulture Traditional Chinese medicine Moths of New Zealand Insects of Turkey Taxa named by Carl Linnaeus Insects in culture Animal models