Silkworm is a domesticated caterpillar used for silk production. The silkworm is the larva (young) of the domestic silkmoth. The silkmoth is the only fully domesticated insect. It was domesticated around 2700 B.C. in China. Its closest relative in nature is the wild silkmoth. Domestic silkworms and silkmoths cannot survive in the wild.
The silkworm has a head and 13 body segments. The caterpillar has three pairs of true legs. It also usually has five pairs of leglike prolegs farther back on its body.
The female silkmoth lays her eggs in summer or fall. The silk farmer puts the eggs in cold storage. The following spring, the eggs are placed in an incubator, a device that keeps them at a suitable temperature for hatching. About 20 days later, the eggs hatch into tiny silkworms that are only about 1/12 inch (0.2 centimeter) long. The silk farmer feeds the silkworms leaves of the mulberry tree. The caterpillars eat almost continually, night and day, shedding their skins four times. Over the course of about 35 days, the larvae grow to about 70 times their original size, about 3 inches (7.5 centimeters) long and 1/2 inch (1.0 centimeter) wide.
When fully grown, the silkworm stops eating. It is ready to spin its cocoon. The caterpillar creeps into a tiny wooden compartment containing twigs or stems of straw that the farmer has prepared. The silkworm spins a net or web to hold itself to a twig or stem. It then forms a cocoon out of one continuous strand of silk. To do this, it swings its head from side to side in a series of figure-eight movements. Two glands near the silkworm’s lower jaw give off a fluid that hardens into fine silk threads as it comes into contact with the air. At the same time, the silkworm gives off a gum called sericin. The sericin cements the two threads of silk together. Once the caterpillar has created its cocoon, it metamorphoses (changes) first into a relatively inactive stage called a pupa and then into a moth.
When a silkmoth bursts out of its cocoon, it breaks the long silk thread into many short ones. For this reason, silk farmers allow only a small number of pupae to develop into imagos (adults). These moths are kept to lay the next batch of eggs. Silk farmers kill the other pupae by heating the cocoons in an oven, preserving the single long strand of silk.
