The silk covering over caterpillar is known as cocoon.
The life history of a silk moth starts when a female silk moth lays eggs.
The larvae or caterpillars hatched from the eggs of the silk moth. These silkworms feed on fresh mulberry leaves the silkworm grows in size and then becomes a pupa.
In the pupa stage, it weaves a net to hold itself. It then swings its head from side to side, secreting a fibre that hardens on contact with air. This fibre is made of a protein and becomes the silk fibre.
The caterpillar covers itself completely with silk fibre and turns into a pupa, this covering is known as the cocoon. The moth continues to develop within the cocoon. The silk thread or yarn is obtained from the silk moth’s cocoon.