Silk is a natural protein secreted by the larvae of several moth species. The larvae use the filaments to construct a cocoon, from which the silk is extracted. Twin filaments of the protein fibroin are secreted and bound together in a single strand with the protein gum sericin. During processing, the sericin is removed, leaving the fibroin protein. Cultivated or cultured silk is produced in very controlled conditions of environment and diet. Tussah or wild silk is harvested from natural sources.