Bombyx mori

Mulberry silkworms can be divided into three major categories based on seasonal brood frequency. Univoltine silkworms produce only one brood a season, and they are generally found in and around Europe. Univoltine eggs must hibernate through the winter, ultimately cross-fertilizing in spring. Bivoltine varieties are normally found in East Asia, and their accelerated breeding process is made possible by slightly warmer climates. In addition, there are polyvoltine silkworms found only in the tropics. Their eggs typically hatch within 9 to 12 days, meaning there can be up to eight generations of larvae throughout the year.^[7]

Eggs take about 14 days to hatch into larvae, which eat continuously. They have a preference for white mulberry, 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 life cycle, 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 layer of protection during the vulnerable, almost motionless pupal state. Many other Lepidoptera 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.

The cocoon is made of a thread of raw silk from 300 to about 900 metres (980 to about 3,000 ft) long. The fibers are fine and lustrous, about 10 μm (0.0004 in) in diameter. About 2,000 to 3,000 cocoons are required to make one pound (0.45 kg). At least 70 million lb (32 million kg) of raw silk are produced each year, requiring nearly 10 billion cocoons.^{[8][better source needed]}

If the animal survives through the pupal phase of its life cycle, it releases proteolytic enzymes 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 reduces the value of the silk threads, although these damaged silk cocoons are still used as "stuffing" available in China and elsewhere in the production of duvets, jackets, and other purposes. To prevent this, silkworm cocoons are boiled in water. The heat kills the silkworms, and the water makes the cocoons easier to unravel. Often, the silkworm is eaten.

As the process of harvesting the silk from the cocoon kills the pupa, sericulture has been criticized by animal welfare and rights activists. Mahatma Gandhi was critical of silk production based on the ahimsa 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,^[9] 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 is the adult phase of the silk worm's life cycle. Silk moths have a wingspan of 3–5 cm (1.2–2.0 in) and a white, hairy body. Females are about two to three times bulkier than males (due to carrying many eggs). All adult Bombycidae moths have reduced mouthparts and do not feed.

The wings of the silk moth develop from larval imaginal disks.^[10] The moth is not capable of functional flight, in contrast to the wild B. mandarina and other Bombyx species, whose males fly to meet females. Some may emerge with the ability to lift off and stay airborne, but sustained flight cannot be achieved as their bodies are too big and heavy for their small wings.

 

 

The legs of the silk moth develop from the silkworm's larval (thoracic) legs. Developmental genes like Distalless and extradenticle have been used to mark leg development. In addition, removing specific segments of the thoracic legs at different ages of the larva resulted in the adult silk moth not developing the corresponding adult leg segments.^[10]

 

 

 

Due to its small size and ease of culture, the silkworm has become a model organism in the study of lepidopteran and general arthropod biology. Fundamental findings on genetics, pheromones, hormones, brain structures, and physiology have been made with the silkworm.^{[11][citation needed]} One example of this was the molecular identification of the first known pheromone, bombykol, which required extracts from 500,000 individuals, due to the small quantities of pheromone produced by any individual silkworm.^{[citation needed]}

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.^[12] Another source suggests 1,000 inbred domesticated strains are kept worldwide.^[13] One useful development for the silk industry is silkworms that can feed on food other than mulberry leaves, including an artificial diet.^[12] 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 (and not crossovers) during meiosis.^[14] In the oocytes of B. mori, meiosis is completely achiasmate (lacking crossovers).^[15] Even though synaptonemal complexes are formed during the pachytene stage of meiosis in B. mori, crossing-over homologous recombination does not occur between the paired chromosomes.^[16]

Kraig Biocraft Laboratories^[17] has used research from the Universities of Wyoming and Notre Dame 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.^[18]

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.^[19]

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 antibiotic discovery, as they have several advantageous traits compared to other invertebrate models.^[20] Antibiotics such as lysocin E,^[21] a non-ribosomal peptide synthesized by Lysobacter sp. RH2180-5^[22] and GPI0363^[23] 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.^[24]

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.^[25] 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.^[26]

 

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.^[27] The eggs are kept in incubators to aid in their hatching.

 

Silkworms were first domesticated in China more than 5,000 years ago.^[28][29]

 

 

Silkworm breeding is aimed at the overall improvement of silkworms from a commercial point of view. The major objectives are improving fecundity, 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,^[30] 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.

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 to the next generation, creating a non-commercial population. The experience provides children with the opportunity to witness the life cycle of silk moths.

The full genome of the domestic silk moth was published in 2008 by the International Silkworm Genome Consortium.^[13] Draft sequences were published in 2004.^[31][32]

The genome of the domestic silk moth is mid-range with a genome size around 432 million base pairs. A notable feature is that 43.6% of the genome are repetitive sequences, most of which are transposable elements. At least 3,000 silkworm genes are unique, and have no homologous equivalents in other genomes. The silkworm's ability to produce large amounts of silk correlates with the presence of specific tRNA clusters, as well as some clustered sericin genes. Additionally, the silkworm's ability to consume toxic mulberry leaves is linked to specialized sucrase genes, which appear to have been acquired from bacterial genes.^[13]

In 2018, Illumina's short reads for 137 strain genomes were published.^[33] In 2022, Nanopore's long reads for 545 strain genomes were published.^[34]

 

Silk moth pupae are edible insects and are eaten in some cultures:

• In Assam, India, 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. Live pupae may be eaten raw, boiled or fried.^[35]
• In Korea, they are boiled and seasoned to make a popular snack food known as beondegi (번데기).^[36]
• In China, street vendors sell roasted silk moth pupae.
• In Japan, silkworms are usually served as a tsukudani (佃煮), i.e., boiled in a sweet-sour sauce made with soy sauce and sugar.
• In Vietnam, this is known as nhộng tằm, 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.^[37]

In China, a legend indicates the discovery of the silkworm's silk was by an ancient empress named Leizu, 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.^[38] About AD 550, Christian monks are said to have smuggled silkworms hidden in a hollow stick out of China, selling the secret to the eastern Romans.

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 was 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.^{[citation needed]}

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.^[39] 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

• Beauveria bassiana, 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 is a disease caused by a parasitic microsporidian, Nosema bombycis. 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 die in their worm stage. To prevent this disease, eggs from infected moths are ruled out 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.

• Cocoonase
• History of silk
• Silk Road
• List of animals that produce silk
• Samia cynthia
• Thai silk
• Lao silk
• Japanese silk
• List of domesticated animals

• Kelly, Henrietta Aiken (1903). The culture of the mulberry silkworm. Washington DC: U.S. Department of Agriculture, Government Printing Office. Retrieved 17 January 2012.
• Grimaldi, David A.; Engel, Michael S. (2005). Evolution of the Insects. Cambridge University Press. ISBN 978-0-521-82149-0.
• Johnson, Sylvia (1989). Silkworms. Lerner Publications. ISBN 978-0-8225-9557-1.
• Scoble, M. J. (1995). The Lepidoptera: Form, Function and Diversity. Princeton University Press. ISBN 978-0-19-854952-9.
• 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.
• Trevisan, Adrian. "Cocoon Silk: A Natural Silk Architecture". Sense of Nature. Archived from the original on 7 May 2012.

 

Wikimedia Commons has media related to Bombyx mori.

• Student page on silkworm
• WormSpit, a site about silkworms, silk moths, and silk
• Information about silkworms for classroom teachers with many photos
• 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