Silk production selected for traits that suit a rearing tray and nothing else. Modern Bombyx mori adults are pale, heavy-bodied and effectively flightless, and larvae have lost most of the pigmentation, alarm response and dispersal behaviour of their presumed wild ancestor, the mulberry-feeding Bombyx mandarina. If a tray of larvae is left unattended they do not reliably move to fresh leaves.
The silk itself is a single continuous filament, produced as two fibroin strands cemented by sericin, which the larva lays down in a figure-of-eight motion to build a cocoon. A single cocoon can yield a filament of several hundred metres to over a kilometre, though the reeled usable length is shorter. Conventional reeling requires killing the pupa with heat or steam so the emerging moth does not cut the filament.
How long ago domestication began is debated. Archaeological silk finds in China date back several millennia, and genetic work supports a single domestication from Bombyx mandarina, but the precise timing and number of founding events remain under discussion.