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)