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Most commercial silk is Mulberry silk that is produced from cocoons of the Mulberry silkworm moth, Bombyx mori, and comes mainly from China and India.
Silkworm moths go through four stages of development – egg, larva, pupa and adult – as do the majority of insects. Butterflies and moths, two-winged flies, bees and wasps, and beetles all have this 4-stage life cycle. Mulberry Silkworms are the caterpillars or larvae of silkworm moths and feed on the leaves of mulberry trees (Morus).
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new laid silkworm eggs
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fertile silkworm eggs
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The female moths lay 200-300 pale-yellow eggs over a couple of days and usually die within 2 weeks. Fertile eggs then turn to brown or purple in a week or so.
The eggs hatch in 10-14 days to produce a very small blackish first instar larva, less than 2mm or ⅛ inch in length. At ideal temperatures (25° to 30°C), the larva grows to 3 inches (75mm) in length in 25-30 days and Rennie and Westwood calculate that it increases 9,000 times in weight during that time. To accommodate this enormous increase in size, it sheds its skin four times and therefore goes through 5 larval stages or instars before it is ready to pupate.
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3rd instar silkworm larva
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5th instar feeding on mulberry
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4th instar silkworm larva
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silkworm larva spinning hammock
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When the silkworm larva is fully grown and ready to pupate, it stops feeding, looks for suitable location and starts to spin a hammock of silk in which to form the cocoon. The hammock is a loose, irregular protective network of silk in which the cocoon will be suspended.
Once the hammock is complete, the larva settles to produce an even, regular, lozenge-shaped and closely-knit cocoon of silk.
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cocoon suspended in hammock
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The cocoon is much denser and is formed from one length of a fine, strong, lustrous silk thread that is the source of commercial silk and that may be up to 1 kilometre in length.
When the cocoon is complete, the larva starts to shrink in length, develops a hard skin and turns into a pupa, inside which the adult moth develops and metamorphoses (changes from larva to adult). Metamorphosis takes about two to three weeks and the silkworm moths then emerge from their cocoons. |
