8

Chitinous Membranes and Analogous Material

115

Fig. 8.4 Life history of silk moth, Bombyx mori (L) (Metamorphosis)

genetics and genetic engineering. The present article reviews the information on silk

from [17] and [18].

It is a natural fibre secreted by the silkworm. The larvae of silkworm secrete the

true silk. Bombyx mori is the most common domesticated silkworm. It belongs to

the small family known as Bombycidae. The adult moths are robust. The caterpillar

larvae are rough, wrinkled, naked, white and with a projection on the opposite side.

The larvae feed on large quantities of mulberry leaves and spin silk cocoons. Silk

production and trade are more common in China, India, Japan and some parts of

Europe.

The life cycle (metamorphosis) involves four stages: Egg, Larva (silkworm

caterpillars), Pupa and Adult Moth (Fig. 8.4).

Structure and Properties of the Silk Fibres

The silk is secreted by silk glands consisting of two long tubes of variable thickness

present in the body of the caterpillar. Each gland has three parts and they are (i)

anterior (ii) middle and (iii) posterior parts.

Silk fibre consists of 75–83% of fibroin and cementing material sericin ranges

from 17 to 25% depending on various factors. Fibroin and sericin are the two major

proteins involved in the formation of silk thread (Table 8.1).

Fibroin is made up of 16 amino acids, and raw silk is composed of two filaments

that are produced by fibroin. These two filaments are generally cemented together

by a gum-like substance which is known as sericin, which involves different types

of mineral salts, fat and wax.