Which insect makes silk?

The silkworm moth, Bombyx mori, whose caterpillar spins a cocoon from a single continuous thread that can run to a kilometre. To harvest it unbroken the pupa is killed before it emerges, because a moth chewing its way out cuts the thread into useless lengths.

The animal no longer exists in the wild. Five thousand years of domestication have left it flightless, unpigmented and unable to feed itself or escape a predator; it is entirely dependent on people, in the way that a cabbage is. Its wild ancestor still exists, and looks like a different proposition altogether.

China kept the method secret for millennia and enforced it with the death penalty, which is why the overland trade route is named after the cloth rather than the technology. The secret escaped more than once - the tidiest version has two monks arriving in Constantinople around 550 with silkworm eggs concealed in hollow canes, on the orders of the emperor Justinian. Spiders make better silk and always have, but they are territorial and cannibalistic, and nobody has managed to farm them.