Five thousand years of silk start by killing a pupa
Bombyx mori, the domestic silk moth, exists because humans bred it from a wild relative, and it can no longer truly fly. Chinese sericulture, at least five thousand years old, usually kills the pupa inside its cocoon so that a single unbroken thread, hundreds of metres long, can be unwound.
The domestic silk moth (family Bombycidae) is closest kin to wild Bombyx mandarina. Its caterpillars are the economic engine of raw silk. Sericulture arose in China at least five thousand years ago and spread through India, Korea, Nepal, Japan, and eventually the West. Conventional reeling kills the pupa; only then can a continuous filament be unwound. Domestication likely postdates Neolithic toolkits for bulk thread; captive and wild forms can still hybridize. Archaeological traces of domestication reach back more than 5,000 years to the Yangshao culture of northern China, and carp were often raised in ponds on silk farms.
Breeders sort stocks by how many broods they raise yearly: univoltine (one brood, often European, eggs overwinter), bivoltine, and multivoltine lines. Eggs hatch in about fourteen days. Larvae prefer white mulberry, drawn to the odorant cis-jasmone, and pass five instars. They are essentially monophagous on Morus leaves; artificial diets have revealed which amino acids they require. Adults have a wingspan of 3 to 5 centimetres, white hairy bodies, and reduced mouthparts, so they never eat, and unlike their wild relatives they cannot sustain flight.
After four molts the body yellows and the skin tightens; salivary glands then spin a cocoon of raw silk as the insect pupates. A single cocoon yields roughly 300–900 meters of filament about ten micrometers thick. Roughly four to seven thousand cocoons make a kilogram of silk, and at least thirty-two kilograms of mulberry leaves feed the worms behind one kilogram of silk. A pupa left alive dissolves an exit hole with enzymes that break the filament into short random lengths, which is why reeling kills it; Gandhi, following the ahimsa principle of harming no living thing, criticized silk and promoted cotton spinning instead. China makes about 70 percent of the world's silk, a fibre under 0.2 percent of global output, and the moth's full genome, around 432 million base pairs, was published in 2008.
Source: Bombyx mori
