Sericulture and life cycle of silkworm

Silkworm is beneficial to us as we extract silk from its cocoon.
In Nepal, two types of silkworm are reared for the commercial production of raw silk.
- Eri silkworm (Attacus ricini) which feeds on castor leaves.

External morphology of silk moth:

An adult silk moth is strongly built, about 2.5 cm long and is creamy white in color.
The body is divided into head, thorax and abdomen.
The head bears a pair of compound eyes and a pair of feathery antennae.
The proboscis is reduced, so they don’t feed.
The thorax bears three pairs of legs and two pairs of wings.
Minute powdery scales cover the entire body.
Male and female are distinct where the female is bigger than the male with distended body.

Life cycle:

Sexes are separate. The adult moths seldom feed and are concerned with reproduction only.
Male and female copulate in tail to tail position and the fertilization is internal.
It completes its life cycle in about 45 days. Male dies soon after mating and female dies after laying eggs.
Like any other insect, its life cycle completes in four stages, i.e. egg, larva, pupa and adult.

Egg:

Female lays about 300 small, white, round (pin head shaped) eggs in cluster on mulberry leaves and she dies.

In cold, the eggs don’t hatch for a long time and the increase in temperature causes them to hatch faster.
In sericulture, the eggs are collected and stored in a cold place when the food (mulberry leaves) are not abundant to feed the larvae.
In the early spring, with the enough supply of food, the eggs are hatched inside the incubator at the temperature of 18-25⁰
Eggs hatch into larvae within 10-12 days.

Larva:

The newly hatched caterpillar is 6mm long, brownish in color and moves in a characteristic looping manner.
The body is divided into head, thorax and abdomen.
They are actively voracious and feed on mulberry leaves for 25-32 days.
They grow very fast and undergo four moltings (changing the skin) to change into different instars.

* Instar is the larval stage between two successive moltings in the life cycle of any insect. In silkworm’s life cycle, there are 5 instars of larva.

After each molting, the larva neither feeds nor moves for the next 20-24 hours.
The fifth instar larva is fully grown and consists of three pairs of jointed legs and five pairs of unjointed stumpy prolegs.
It has spiracles on either lateral side of the abdomen meant for gaseous exchange.
It also develops a pair of salivary glands and starts secreting the sticky fluid which turns into fine, long and solid thread of silk after coming in contact with air.
The thread becomes wrapped around its body forming a pupal case known as the cocoon.
The larval stage lasts for 25-32 days.

Pupa:

The larva changes into a pupa within a cocoon, which is the third stage in the life cycle.
Pupa is completely inactive. It neither feeds nor moves and if permitted to live, it undergoes active metamorphosis and becomes an adult in about 10-12 days.
The adult secretes an alkaline fluid to moisten one end of the cocoon and emerges through it and the long silk thread is broken into many short fragments.
To save the silk, the cocoons are kept in hot water or hot air to kill the pupae inside.

Adult:

Few pupae with cocoon are selected for further development into adults. These moths are kept to lay next batch of eggs.
The adult coming out of the cocoon has wet wings and hence cannot fly. It flies after its wings are completely dry.
Adults survive for 5-7 days during which they mate, produce eggs and continue the generation.

Properties of silk:

One cocoon yields about 1000 feet of silk thread. It is unwound from the cocoons and reeled into threads of commercial silk.
It is a natural fiber which is soft, shiny, strong and durable.
It is highly elastic and has long threads.
It easily absorbs water.
It is extremely light in weight and warmer than cotton and can be easily dyed and ironed.

Hence, silk is also called the “Queen of fibers”.

Uses of silk:

It is used in the manufacture of woven materials and knitted fabrics (men’s and women’s fashionable clothing).
It is used in fishing lines and sieves of flour mills.
It is used in the insulation coil for telephone, wireless receivers and tyres of racing cars.

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