The larva extrudes fibroin from paired salivary glands through a spinneret, coating it with the gum sericin, and lays it down in a figure-of-eight motion for two to three days. Because the flow is uninterrupted, the cocoon is a single strand. Reported lengths vary widely with breed and rearing conditions; figures from several hundred to well over a thousand metres are cited, and a portion of any cocoon is too fine or tangled to reel.
The adult moth escapes by secreting a proteolytic enzyme that softens one end of the cocoon, pushing through and cutting the filament into short lengths. Commercial reeling therefore stifles the pupa with heat or steam first. Silk produced from cocoons the moth has left, sometimes sold as peace silk or ahimsa silk, is spun from short fibres and has a different character.
Domestication has gone unusually far. Adults are flightless and unpigmented, larvae have lost the ability to disperse or to avoid predators, and no feral population of Bombyx mori exists anywhere. The species survives only in cultivation, which makes it among the most completely domesticated animals on Earth.