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Silk
Silkworm is the common name for the silk-producing larvae of several species of moth. The larva is not a worm at all but a caterpillar. The adult moths are extinct in the wild but are reared in large numbers by the silk industry. Silkworms feed mainly on the leaves of the mulberry tree.Silkworm farming is called sericulture.

Typical Commercial Silkworm Production

The first stage of silk production is the hatching of the silkworm eggs, which takes place in a controlled environment. The female lays 300 to 400 eggs at a time1, each about the size of a pinhead. The female dies almost immediately after depositing the eggs.

The larvae hatch in about 10 days and are about 0.6cm long. Once hatched they are placed under a layer of gauze and fed large quantities of chopped mulberry leaves. The larva will eat 50,000 times its initial weight in plant material.2

After four to six weeks, when the larva has achieved its maximum growth, it stops eating, changes colour and attaches itself to a compartmented frame, twig, tree or shrub in the rearing house ready to become a chrysalis.

A Hard Day’s Night

Over the next four days the silkworm spins a fibrous cocoon around itself. The cocoon is secreted as a continuous silk fibre up to a mile long3 from special glands in the silkworm's head.

If the adult moth were allowed to emerge from the cocoon naturally, it would secrete an alkali, which would eat through the cocoon. Therefore the silkworms are killed by heat to preserve the silk cocoon,4 by immersion in boiling water, steaming or drying in an oven. Only enough adult moths are allowed to emerge to ensure the continuation of the species.

Hundreds Die

The amount of useable silk from each cocoon is small, so it takes hundreds of tiny lives to produce just one silk scarf or tie.

Stud Bank & Breeding Research

A limited number of pupae are allowed to complete their chrysalis stage, the resulting silk moths being the stud bank that produces eggs to breed future generations of silkworm.

Researchers are keen to establish silkworm varieties for low-cost cocoons, disease resistance and polyphagy (ability to utilise more than one type of food) etc. As with other types of animal farming, research and technology are used to increase production.5

Silk Gut

Another silkworm product is silkworm gut. Immediately before the cocoon stage, the silkworm pupae are killed by immersion in an acidic solution. Their bodies are opened and the silk glands extracted and stretched into a strand. These strands or 'silkworm gut' were once favoured by surgeons for stitching and by anglers for lines. Silk gut is no longer used as a surgical suture but silk fibres may be used.6 Silk gut is still found in some angling products.

World Silk Production

World silk production continues to increase. In 2000 world raw-silk production was 84,403 tonnes and it increased to 125,605 tonnes in 2005.7 China and India rank first and second respectively as the world's largest raw-silk producers.

Say No to Silk

Silk may be used for suits, coats, trousers, jackets, shirts, ties, lingerie, hosiery, gloves, lace, curtains, linings and handbags. Synthetic fibres such as nylon and polyester are stronger and cheaper than silk. Rayon, composed of cellulose, is another alternative to silk. Fibres from bamboo can be made into fabrics that look and drape like silk. Bamboo fabric is growing in popularity as a more environmentally friendly fabric since bamboo grows easily without any chemical input and helps to stabilise soils.8

References

1 Schaffer D. Silkworms Minnesota, Mankato: Capstone Press; 1999

2 Morton A. Thailand’s Million Dollar Moth. New Scientist 25 November 1989: 1692

3 Rackesh P. Silk Yarn and Its Production Process. 2007 http://www.articlesbase.com/ask-an-expert-articles/silk-yarn-and-its-production-process-158441.html (accessed 12 November 2009)

4 Franck R. Silk, Mohair, Cashmere and Other Luxury Fibres. Cambridge: Woodhead Publishing Ltd; 2001

5 Aruga H. Principles of Sericulture. London: CRC Press; 1994

6Taylor B. Basic plastic surgery techniques and principles: Choosing the right suture material. Student British Medical Journal 2003; 11:131-174

7 Ministry of Textiles, Government of India. Sericulture Industry. http://ministryoftextiles.gov.in/tex/sector/Silk_Industry_cental_silk_board.pdf (accessed 12 November 2009)

8 Comeau A. The eco-friendly fabric guide. 2007 http://www.sierratradingpost.com/lp2/green-guide.html(accessed 12 November 2009)