Silkworms are caterpillars of (usually) the Bombyx mori moth. During its 3 to 8 day pupating period, the silkworm secretes fibroin, a sticky liquid protein, from its two sericteries (special salivary glands). Pushed through a spinneret (opening on the mouth), the twin pair of continuous threads harden when they come into contact with the air. Next, the silkworm secretes sericin, a bonding agent, from two other glands to hold the two filaments together. While constructing its cocoon, the silkworm will twist in a figure-8 motion about 300,000 times and produce around 1 kilometer of filament.
Since hatching from the cocoon destroys the thread, to harvest the silk, the cocoon is placed in either boiling water, or blasted with steam or hot air, all processes that kill the pupae. Less lethal methods were tried in the past, such as pulling the silk as the worms spun it, but the worms resisted and bit off the filaments (the longest thread harvested in this way was just 6 meters).
Besides killing the pupae, the heat softens the binding agent (sericin), so that the filaments may be unwound. Sometimes, the softened sericin is left on the fibers, and this product is called raw silk. In the end, it takes the deaths of about 2500 caterpillars to make a single pound of raw silk.
From there, raw silk strands are twisted together until a fiber of sufficient strength for knitting or weaving is produced, and different twisting methods produce a different type of thread: crepe, thrown, tram, organzine or singles. Crinkly fabrics are made with crepe, while sheer cloth is made with single thread. Spun silk is comprised of broken filaments that have been processed into a yarn.
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