Usually refers to silkworm cocoon. The sac-shaped protective layer of silkworm pupa stage, containing the pupal body. The protective layer includes cocoon coat, cocoon layer and pupa lining. The cocoon layer can be reeled into silk, and the cocoon clothing and the waste silk after reeling can be used as raw materials for silk cotton and silk spinning.

The cocoon has different shapes such as oblong, elliptical, girdle, spherical or spindle, or is slightly constricted in the middle. The cocoon comes in white, yellow, light green, flesh red and other colors, and is about 3 to 4 years long. cm, with a diameter of 1.7 to 2.1 cm. The surface is white, with irregular wrinkles, and attached silk, which is fluffy. The silk patterns on its inner wall are very regular. Light and tough, not easy to tear. The unemerged silkworm cocoon contains a yellow-brown silkworm chrysalis and the light brown, shrunken silkworm skin that was shed before pupae.