Bombyx mori, commonly known as the domestic silk moth, is a domesticated moth species belonging to the family Bombycidae. It is the closest relative of Bombyx mandarina, the wild silk moth. Silkworms are the larvae of silk moths. The silkworm is of particular economic value, being a primary producer of silk. The silkworm's preferred food are the leaves of white mulberry, though they may eat other species of mulberry, and even leaves of other plants. Domestic silk moths are entirely dependent on humans for reproduction, as a result of millennia of selective breeding. Wild silk moths, which are other species of Bombyx, are not as commercially viable in the production of silk.
Sericulture, the practice of breeding silk moths for the production of raw silk, has existed for at least 5,000 years in China, whence it spread to India, Korea, Nepal, Japan, and then the West. The conventional process of sericulture kills the silkworm in the pupal stage.Sh. S.D. Pateriya. "Introduction to Sericulture". https://www.ignfa.gov.in/document/biodiversity-cell-ntfp-related-issues4.pdf The domestic silk moth was domesticated from the wild silk moth Bombyx mandarina, which has a range from northern India to northern China, 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 period. Before then, the tools to manufacture quantities of silk thread had not been developed. The domesticated Bombyx mori and the wild Bombyx mandarina can still breed and sometimes produce hybrids. It is unknown if B. mori can hybridize with other Bombyx species. Compared to most members in the genus Bombyx, domestic silk moths have lost their coloration as well as their ability to fly.
Types
Mulberry silk moths can be divided into three major categories based on seasonal brood frequency. Univoltine silk moths 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 warmer climates. In addition, there are polyvoltine silk moths 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. Mulberry silkworms are highly sensitive to environmental variables, and diapause - a physiological arrest in development - can be significantly influenced by photoperiod, temperature and humidity.Rahmathulla, V. K. (2012). Influence of environmental factors on the biology of the silkworm, Bombyx mori L.: A review. International Journal of Industrial Entomology, 25(1), 1–13. https://doi.org/10.7852/ijie.2012.25.1.001Nair, K. S., Radhika, K., & Premalatha, V. (2010). Photoperiodic regulation of diapause in the silkworm Bombyx mori L. Journal of Applied Entomology, 134(1), 44–52. https://doi.org/10.1111/j.1439-0418.2009.01447.x Recent studies have reported the occurrence of non-diapause eggs in bivoltine races, indicating a more flexible diapause expression than previously understood.Rudramuni, K., Neelaboina, B. K., Shivkumar, S., Ahmad, M. N., & Chowdhury, S. R. (2021). Variation of embryonic diapause induction in bivoltine silkworm Bombyx mori L (Lepidoptera: Bombycidae) under controlled conditions. International Journal of Industrial Entomology, 43(2), 37–44. https://doi.org/10.7852/IJIE.2021.43.2.37
Description and life cycle
Larvae (silkworms)
Eggs take about 14 days to hatch into larvae. Larvae have a preference for white mulberry, having an attraction to the mulberry odorant cis-jasmone. There are five instars before pupation.
The silkworm 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.
Pupae (cocoon)
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 long. The fibers are fine and lustrous, about 10 μm in diameter. The number of cocoons required to produce a final yield of silk is approximately ; at least of raw silk are produced each year, requiring nearly 10 billion cocoons.
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, 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.
Moth
The moth is the adult phase of the silk worm's life cycle. Silk moths have a wingspan of 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. 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.
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 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.
In 2018, Illumina's short reads for 137 strain genomes were published. In 2022, Nanopore's long reads for 545 strain genomes were published.
Interaction with humans
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.
The earliest archaeological evidence for silk moth domestication is associated with the Yangshao culture of northern China, dating to more than 5,000 years ago. The Chinese also domesticated the common carp over 5,000 years ago. Carp were often raised in ponds on silk farms and were fed silk moth larvae and faeces. This allowed sericulture and aquaculture to be practiced in conjunction.
Economic value
Silk is a valuable product, representing under 0.2% of the world's production of fibre but with a market value predicted to be $18.1 billion in 2027. Production is labour-intensive as it takes over 100 kg of mulberry leaves to produce 1 kg of raw silk. 70% of the world's silk is produced in China.
Breeding
Silk moth 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, 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 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. Other than the U.S., this domestic silk moth is utilized in educational settings in several countries, including China, South Africa, Zimbabwe, Iran, and Taiwan.
Research