Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin. It is most commonly produced by certain insect larvae to form cocoons. The best-known silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori, which are reared in captivity (sericulture). The shimmery appearance of silk is due to the triangular prism-like structure of the silk fiber, which causes silk cloth to refract incoming light at different angles, thus producing different colors. Harvested silk is produced by numerous insects; generally, only the silk of various moth caterpillars has been used for textile manufacturing. Research into other types of silk, which differ at the molecular level, has been conducted. Silk is produced primarily by the larvae of insects undergoing complete metamorphosis, but some insects, such as webspinners and raspy crickets, produce silk throughout their lives. Silk production also occurs in hymenoptera (bees, wasps, and ants), silverfish, caddisflies, mayflies, thrips, leafhoppers, beetles, lacewings, fleas, flies, and midges. Other types of arthropods also produce silk, most notably various arachnids, such as spiders.
