The secret to silk production is the tiny creature known as the silkworm, which is the caterpillar of the silk moth Bombyx mori. It feeds solely on the leaves of mulberry trees. Only one other species of moth, the Antheraea mylitta, also produces silk fiber. This is a wild creature, and its silk filament is about three times heavier than that of the cultivated silkworm. Its coarser fiber is called tussah.