As the story has it, silk was discovered in
China around 2700 BC during the reign of emperor
Huang Di. At some point either he or his wife,
Hsi Ling-Shi, accidentally dropped a silkworm cocoon into a cup of freshly poured
tea. A single thread unwound from the mass of the cocoon, and intrigued whichever one of them noticed it. In one version of the story
Hsi Ling-Shi discovered all the properties of silk on her own, while in another Huang Di asked her to research the new finding. In either case, Hsi Ling-Shi discovered all the methods of raising
silkworms, then boiling and collecting the silk, and was later deified as
Seine-Than -- goddess of the silkworms.
Whether or not any of this ever happened, Chinese silk production really does go back about 5000 years, which makes it much older than iron-working. In 139 BC the Silk Road was opened between China and the Eastern edge of the Mediterranean sea, and for centuries before that various traders moved the product from East to West. Silk may have been the deciding factor in the meeting of Eastern and Western culture. Even so, the secrets of growing silkworms and making silk (collectively known as sericulture) were guarded well; it wasn't until 300 AD that sericulture even got as far West as India. In 522 AD emperor Justinian was finally able to get silkworm eggs smuggled all the way to Rome, and the prized Chinese secret was secret no more. It caught on quickly in Eastern europe, and when a revolution caused China's foreign trade to be discontinued in 877, Western silk prices were hardly affected.
Western culture made some innovations on silk production, but was never able to reproduce some qualities of Chinese, Korean, and Indian silk. 1801 saw Joseph Jacquard's invention of the Jacquard loom, which could weave patterns from colored silk without (much) human intervention. The loom is still mentioned today because it surprising similarity to a primitive computer, including a counting mechanism and its method of reading patterns from punched cards. Louis Pasteur also contributed to sericulture by stopping the silkworm disease Pebrine; this work may have given him the idea for his germ theory of disease. Despite all the Western interest and innovation, political revolution in China during the late 1970's lead to their again becoming the world's largest silk producer; a process that's still important nearly five millennia after its invention.
The silkworm itself is the only member of its family, and has the name Bombyx Mori. No members of this species have been found in the wild, and if planted there Bombyx Mori will not survive; it is a completely domesticated animal. Silkworms are only able to eat leaves from the mulberry (Morus) genus, though an artificial silkworm food is available. Mulberry leaves and stems exude latex, which is the first step in the chemical process that makes silk, and also makes the mulberry inedible to most other insects.
During its lifetime the silkworm goes through three morphological stages: larva, pupa, and adult moth. Larval stage itself is made up of four molts (technically known as instar stages), and the fifth instar stage is also the pupal stage. As of the fourth instar stage, the silkworm is about three inches long and has eight pairs of legs. At this point the silkworm spends three or four days spinning its cocoon, entering the pupal stage. Were the pupa allowed to continue and become an adult, the moth would have a wingspan of about two inches and be covered with fuzz. Both males and females have fat, un-aerodynamic bodies, but females tend to be a bit larger. Neither sex is able to fly, and both live only a few days as adults.
Silk is made by two glands which produce the protein fibroin. Each fibroin strand is coated within the gland by another protein, sericin, which is sticky and stiffens when exposed to air. A specialized organ named the spinarette (also found in spiders) spins these two fibers together and ejects them from the silkworm's body. As the silkworm makes specialized movements, the silk gradually builds up into a cocoon shape. As has been noted many times, the silk fiber is around five times as strong as a steel fiber of the same diameter, and once protected against predators in the wild.
To harvest the silk, the farmer drops a bunch of cocoons into very hot water (or steams them) to loosen up the sericin. Then a loose end is found, and wound together with the silk from five to fifteen more cocoons. Machines can automate this procedure, but the highest quality silk is hand-harvested. Each cocoon's single thread is somewhere between 800 and 3000 feet long. Because of silk's lightness, it takes 1500 silkworm cocoons to produce one kilogram of silk. Some sericulturists eat the now-boiled pupae, while others crush them for oil, and others simply discard them.