During pupation, silkworms release silk to create cocoons. Silk cocoons are harvested before the pupae develop into moths.
During the pupal stage (3-8 days), silkworms produce a liquid and insoluble protein called friboin, which is secreted from their mouths. This liquid protein hardens when exposed to air.
A silkworm can release nearly 1000 meters of silk in just 2 days.
Then, silkworms continue to secrete sericin – the adhesive, which helps to stick the small silk fibers together. They continue to spin silk until the cocoon is complete. With each cocoon, the silk is braided about 800,000 times in the figure 8. A silkworm can release nearly 1,000 meters of silk in just 2 days.
When the pupae develop into moths, they destroy the cocoon to get out. Therefore, silk needs to be harvested before the pupae develop into moths.
Silkworm cocoons will be dipped in boiling water to kill the pupae. The boiling water also helps soften the sericin binder that separates the silk. It takes about 2500 silkworms to make 1 pound (0.45 grams) of raw silk.
Silk is mostly used to make clothes. Scientists are also studying how to mutate silkworms so that they can create silk for medical applications, such as to replace knee ligaments.