Silkworm Cocoon

the casing formed by the caterpillars of silkworms before metamorphosis into a pupa; the raw material for obtaining natural silk.

The cocoons of the mulberry silkworm have the greatest commercial value, but cocoons of the Chinese oak silkworm, the ailanthus silkworm, and other silkworms are also used. The caterpillars of these silkworms secrete a double silk thread consisting of the protein fibroin, the sticky substance sericin, pigment, and other substances. In winding its cocoon on a cocoon support the caterpillar lays the thread around its body in the form of the figure eight, and these loops constitute the basic, reelable part of the casing. A continuous silk thread can reach a length of 1,200 m and a thickness of 20–30 micrometers. The cocoons of the mulberry silkworm are oval, spindle-shaped, or cylindrical; cocoons of the oak silkworm are less regular in shape. The cocoons selected for reeling are treated with hot air or steam to kill the pupa and prevent its metamorphosis into a moth, which damages the cocoon as it emerges. The cocoons are then dried so that they will not spoil during storage. The weight of a live cocoon ranges from 1.5 to 3.5 g, and the weight of the reelable casing is 350–550 mg. The cocoon of females is on the average 20 percent heavier than that of males.