Bombyx mori
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 Domesticated mulberry silkworm
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| Bombyx mori Linnaeus, 1758 |
The silkworm (Bombyx mori, Latin: "silkworm of the mulberry tree") is the larva or caterpillar of a moth in the family Bombycidae, that is very important economically as the producer of silk. It is entirely dependent on humans for its reproduction and no longer occurs in the wild; silk culture has been practiced for at least 5000 years in China (Goldsmith et al., 2004). A silkworm's diet consists solely of mulberry leaves. It is native to northern China. Its nearest wild relative is Bombyx mandarina with which it is able to hybridize (Goldsmith et al., 2004), and which ranges from northern India to northern China, Korea and Japan.
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[edit] Development
Silkworms have a strong appetite, as do all lepidopteran larvae. They eat mulberry leaves day and night. Eggs take about ten days to hatch. When the color of their heads turns darker, it means that it is time for them to moult. After they moult four times, their bodies turn slightly yellow and their skin becomes tighter. The larvae enclose themselves in a cocoon of raw silk produced in the salivary glands that provides protection during the vulnerable, almost motionless pupal state. Many other Lepidoptera produce cocoons, but only a few large Bombycidae and Saturniidae have been exploited for fabric production.
[edit] Silk
The cocoon is made of a single continuous thread of raw silk from 300 to 900 meters (1000 to 3000 feet) long. The fibers are very fine and lustrous, about 10 micrometers (1/2500th of an inch) in diameter. About 2,000 to 3,000 cocoons are required to make a pound of silk. Based on 1 kilometer (2/3 of mile) per cocoon, ten unraveled cocoons could theoretically extend vertically to the height of Mt 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 (Fieldbook of Natural History, 1949), 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.
If the animal is allowed to survive after spinning its cocoon, it will make a hole in the cocoon when it exits as a moth. This would cut short the threads and ruin the silk. Instead, silkworm cocoons are baked at about 100oC for 2 to 2.5 hours, which kills the silkworms and also makes the cocoons easier to unravel. Often, the silkworm itself is eaten.
The adult phase (the moth) cannot fly. The silkworm-moths have a wingspan of 2 inches and a white hairy body. Females are about twice to three times as large as the males, but similarly coloured. Adults in the Bombycidae have reduced mouth parts and do not feed.
[edit] Scientific uses
Due to its large size and ease of culture, the silkworm has long been a model insect in the study Lepidopteran and arthropod biology (Goldsmith et al., 2004). Fundamental findings on pheromones, hormones, brain structures and physiology were made with the silkworm (Grimaldi and Engel 2005). To characterize the first known pheromone, bombykol, extracts were needed from 500,000 individuals because only very small quantities are produced (Scoble 1995).
Currently, research is focusing on genetics of silkworms and genetic engineering. Many hundreds of strains are maintained, and over 400 Mendelian mutations have been described (Goldsmith et al, 2004). One useful mutant for the silk industry confers the ability to feed on food besides mulberry leaves, including an artificial diet (Goldsmith et al. 2004). The genome has been sequenced (Mita et al. 2004), and many projects have worked on genetic engineering of silkworms to produce desirable proteins in the place of silk. Such proteins include human drugs (Grimaldi and Engel, 2005).
[edit] 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 (Chinese: 嫘祖; pinyin: Léi Zǔ). She was drinking tea under a tree when a cocoon fell into her tea. She picked it out and as it started to wrap around her finger, she slowly felt a warm sensation. When the silk ran out, she saw a small cocoon. In an instant, she realized that this cocoon 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 jealously guarded their knowledge of silk. It is said that a Chinese princess smuggled eggs to Japan, hidden in her hair. The Japanese thus began their love affair with silk. A single kimono requires the silk from 2100 silkworm moths to produce.
[edit] Silkworm diseases
Silkworms suffer from infectious diseases caused by protozoans, fungi, viruses, and bacteria. The French microbiologist Louis Pasteur investigated several silkworm diseases, which threatened the European silk production at that time.
[edit] Medical uses
Silkworm is the source of the traditional Chinese medicine "Bombyx batryticatus" or "stiff silkworm" (Simplified Chinese: 僵蚕; Traditional Chinese: 僵蠶; pinyin: jiāngcán). It is the dried body of the 4–5th stage larva which has died of the white muscardine disease caused by the infection of the fungus Beauveria bassiana. Its uses are to dispel wind, dissolve phlegm and relieve spasm.
[edit] Cuisine
Like many insect species (see Entomophagy), silkworm pupae are eaten in some cultures. In Korea they are boiled and seasoned to make a popular snack food known as beondegi. In China street vendors sell roasted silkworm pupae.
[edit] References
- Scoble, MJ, 1995. The Lepidoptera: Form, function and diversity. Princeton Univ. Press.
- Goldsmith, M, Toru Shimada, and Hiroaki Abe. 2004. The genetics and genomics of the silkworm, Bombyx mori. Annual Review of Entomology 50:71-100. PMID 15355234. Online access
- Grimaldi and Engel, 2005. Evolution of the Insects. Cambridge University Press.
- Mita K. et. al., 2004. The genome sequence of the silkworm, Bombyx mori. DNA Research 11:27-35. PMID 15141943. PDF
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