Silk

Silk is a fiber made up of proteins which are produced by various insects. Although silk fiber can be produced by a number of insect species (bees, wasps, spiders, etc.), silk that is sold as thread is produced exclusively by animals we call “silkworms.” Silk is used to make all sorts of clothing, like ties, scarves, shirts, blouses, suits, pajamas, nightgowns, and underwear.

In Thailand, in addition to bred worms (Bombycidae), wild worms (Saturniidae) are also used to obtain silk. The varieties of worms used for this purpose have been going through a process of selection throughout history in order for their silk thread production to be as high as possible.1 At present, silk producers are trying to lower costs by giving hormones and other chemicals to silkworms.2

Silkworms are also used for other purposes, as feed for other domesticated animals like chickens and fishes.3 In some countries they are also used as food for humans.4

How silk is produced

Silk can be obtained either manually or with machinery. First, the eggs are deposited into containers with special paper. After having been enclosed for 35 days, the worms start to spin cocoons around themselves until they are completely encased. After this happens, most of the worms are put in boiling water or killed with gas.

A small percentage of the worms are not killed, but rather are left to emerge from their cocoons as silk moths in order to lay eggs, so that the cycle of production is continued.

About 6,000 worms are killed to produce one kilogram of silk.5 Additionally, huge quantities of dead chrysalises are reused as feed for fishes in fish farms, for pigs, and sometimes for human consumption.

Silk is often used in ties, handkerchiefs, and accessories. In the last few years, silk has also started being used in the production of hygiene and cosmetic products.

In silk production, only the healthiest moths are used. Their eggs are sorted and tested. The least healthy eggs are burned, while the rest are put in cold storage until they are hatched. Once hatched, they are kept enclosed for another seven days. While normal hatching would occur once a year, in silkworm farms the process is repeated three times a year.

The silkworms are fed mulberry leaves, and they grow until they reach nine centimeters, shedding their skin several times during this process. Their skin changes from gray to a translucent pink.

At a certain time, the worms shake their heads and begin to create their cocoon. To do this, they spin a double chain of fiber in a figure eight, until they have formed a symmetrical wall around themselves. The filament used to create the cocoon is called fibroin. It is held together by sericin, a soluble gum secreted by the worm, which hardens on contact with air.

If there is no interruption to the process, the chrysalis breaks out of the cocoon as a moth. However, silk producers destroy the chrysalis in order to avoid breaking the silk thread. The cocoons are categorized by characteristics such as color and size, with the objective of having the final product be uniform in quality. Then, the cocoons are dipped in hot water in order to remove most of the sericin. That way, the filaments can more easily form a thread.

Ahimsa silk

Another way to produce silk is the ahimsa method, used by Jains. With this method, the cocoon is used after the moth emerges, so as not to kill the moth. The fabric produced by this method is of poor quality, more like wool. Ahimsa silk makes up a relatively small percentage of total sales.

The production of ahimsa silk is not without its problems, because animals are still held captive and harmed in the process.

Further readings

Avizienis, A. (1996) “Silk: How products are made”, Encyclopedia.com [accessed on 8 April 2013].

Cherry, R. (1993) “Sericulture”, Bulletin of the Entomological Society of America, 35, pp. 83-84.

Gleason, C. (2007) The biography of silk, New York: Crabtree, p. 12.

Hyde, N. (1984) “The queen of textiles”, National Geographic, 165, p. 249.

Peigler, R. S. (1993) “Wild silks of the world”, American Entomologist, 39, pp. 151-161.

Tomasik, B. (2016) “Insect suffering from silk, shellac, carmine, and other insect products”, Essays on Reducing Suffering [accessed on 9 December 2016].

Wigglesworth, V. B. (1961) “Physiology of insect metamorphosis”, Journal of Insect Physiology, 7, p. 73.

Notes

1 Nagaraju, J. (2002) “Application of genetic principles for improving silk Production”, Current Science, 83, pp. 409-414.

2 Chang, C. F.; Murakoshi, S.; Tamura, S. (1972) “Giant cocoon formation in the silkworm, Bombyx mori L. topically treated with methylenedioxyphenyl derivates” Agricultural Biology and Chemistry, 36, pp. 629-694. Akai, H.; Kiguchi, K.; Kobari, Y. & Shibukawa, A. (1981) “Practical utilization of juvenoids for increasing silk production”, Scientific Papers of the Institute of Organic Physical Chemistry, 22, pp. 781-792. Sarangi, S. K. (1988) “Effect of juvenile hormone analogue on the silk gland of the silkworm, Bombyx mori L.”, Sericologia, 28, pp. 553-557.

3 Kiuchi, M. & Tamaki, Y. (1990) “Future of edible insects”, Farming Japan, 24, p. 374L.

4 Hoffman, W. E. (1947) “Insects as human food”, Proceedings of the Entomological Society of Washington, 49, pp. 233-237. Defoliart, G. R. (1995) “Edible insects as minilivestock”, Biodiversity and Conservation, 4, pp. 306-321.