How Silk is Made by Silk-worms?

Though various species of insects produce silk, which is a secretion of their salivary glands, silk of commercial quality is produced only by a very limited number of species. The silk produced by the mulberry silk worm of the species Bombyx mori is the most valued of these. This species belongs to the family Bombycidae. This silk is one of the most valuable and widely used products. It is the secretion of the paired salivary glands of the full grown last larval stage of the silk moth Bombyx mori (Fig. 17.5). The worm feeds on mulberry leaves (Morus aha).

The adult moth is creamy white about 5 cm across with fully spread wings. From head to the tip of the abdomen, it measures about 3 cm. The wings are however feeble and the insect scarcely flies. As adult it takes no food and lives for only 2-3 days during which the female lays 300-500 eggs. The eggs hatch into tiny larvae by 8-12 days. These feed on mulberry leaves and grow in size. They molt four times to reach full grown size of about 8 cm in length. It is grayish and has a hump behind the head and a spine-like horn at the hind end dorsally. Full growth is reached by 28-30 days. When full grown it becomes restless and spins a cocoon.

Spinning of cocoon takes about 3 days. During this operation the head is constantly moved from side to side swiftly at the rate of about 65 times per minute. As this larva does so, the secretion of the salivary gland is continuously poured out through the common opening at the tip of a median cylindrical spinneret on the lower lip or labium.

This is a clear viscous fluid, but as it is exposed to the air it hardens into the fine silk fiber. This fiber forming the cocoon is continuous and ranges in length from 700 to 1100 meters. Silk consists of two proteins: fibroin and sericin. The silk thread is elastic, resistant and non-conductor of heat and electricity. It has also good tensile strength, comparable to steel. The cocoons are oval. The color of the silk varies from white to a beautiful golden yellow.

The larva pupates within the cocoon. The cocoon is made up of a single reel-able thread. The pupa is the inactive stage undergoing transformation to become adult moth. The adult moth emerges from the cocoon after 10-12 days. While coming out it softens one end of the cocoon by an alkaline secretion which enables it to break through the strands of silk. Such cocoons through which the moths emerge are called pierced cocoons. These are of low value because they cannot be reeled.

For reeling (winding the thread on a wheel), the cocoons are gathered about 8 days after spinning begins, and the pupae are killed usually by heat from steam or hot air or even by fumigation and then thoroughly dried. Next, the cocoons are immersed in warm water. This loosens the fiber. The fibers from four or five cocoons are caught up together and twisted into a thread that is wound on the reel.

Usefulness of silk was first discovered by Empress Lotzu in China about 2700 B.C. Since then silkworms were reared and silk was produced in that country as a monopoly for about next 2000 years. In the year 550 A.D. two European monks smuggled out eggs of silk moth and thus introduced silk culture in Europe.

Today, sericulture, or the commercial production of silk, is an important industry in several countries including China, Japan, India, France, Spain and Italy. In India the mulberry silk is produced extensively in Karnataka, Tamil Nadu, Assam, West-Bengal, Jammu-Kashmir and Punjab.