Silk was discovered in China at least 4500 years ago. A Chinese legend has it that the emperor Huang Di (the yellow emperor) accidentally dropped a silkworm cocoon in his tea, and observed its shining fiber as it began to unwind.
The silkworm is the caterpillar of the moth Bombyx mori. It is a completely domesticated insect. There are no wild silkworms and silkworms can't live without human care.
The silkworm caterpillar grows to about three inches long on a diet of nothing but white mulberry leaves. Then it spins a cocoon about itself and metamorphoses into an adult moth. The adults mate, lay eggs and die in just a few days.
The silk is a continuous thread, as much as half a mile long, which makes up the cocoon. The silkworm has a pair of glands which produce a liquid form of the protein fibroin. Fibroin emerges from a silk gland as a continuous fiber coated with the other silk protein, a sticky substance called sericin. The two glands form two fibers which are then spun together by a specialized organ, the spinneret.
The collected cocoons are put into very hot water, which, of course kills the silkworm. A single thread is prized loose from the wet cocoon and unrolled. The best quality silk is unrolled by hand. When the whole cocoon has been unrolled, the little cooked silkworm provides a snack for the worker.
Silkworms played a big part in medical history. The Chinese had had a monopoly on silk production for millennia, but in the nineteenth century, French producers got a few cocoons and established a local silk industry. Soon, however, the silkworms began dying of an unknown disease. The silk raisers turned for help to Louis Pasteur, who was able to discover that the disease was caused by bacteria, and prescribe the sanitary procedures that saved the French silk industry. That convinced Pasteur that many diseases were caused by bacteria. It marked the beginning of the modern age of medicine.
It was another scientist studying silkworm diseases, Dr. Ishiwata Shigetane, who first identified the bacterium Bacillus thuringiensis, which is the source of Cry protein, the Bt toxin which is now made by many insect resistant transgenic plants.
Japanese scientists have given silkworms a gene for luminescence. They produce silk that glows in the dark. Although this might be a useful product, the scientists' real goal is to be able to replace the luminescence gene with any useful other gene they choose. If a transgenic silkworm doesn't produce light, the new gene must have successfully replaced the luminescence gene.