The fully grown caterpillar larva of Bombyx mori stops feeding and develops salivary glands, then it undergoes pupation. In this, the larva secretes a sticky fluid through a narrow pore situated on the hypopharynx. This secreted fluid when comes in contact with air, takes the form of long thread of silk and is wrapped around the body of caterpillar in the form of a covering called as cocoon. The silk threads are then removed from cocoon after killing them.

Hence, silk is secreted by caterpillar larva of silkworm but is obtained from the cocoon.