Life cycle of silkworm

 Silkworm

 Silkworm are a special type of insects which is used to eject fiber and threads from them for the commercial use. The fiber are obtained from a type of moth (an insect ) called silkworm. Silk is extracted during pupa stage in the life cycle of silkworm . In this topic, we will discuss the introduction, life cycle, economic importance and some characteristics of silkworm and silk.

External structure of a silkworm

An adult silkworm is about 2.5 cm long and is creamy white in color. Its body is divided into head, thorax and abdomen. It has a pair of compound eyes and a pair of antennae on its head. The thorax bears 3 pairs of appendages and two pairs of wings . The abdomen is broad and is covered with tiny hairs. It is a unisexual animal. So, male and females are separate.

General characteristics of a silkworm

Introduction

The word silkworm is divided from two words , i.e.  silk and worm. So, a silkworm is a type of worm that produces silk. Silk is  a special type of fiber which is used to make clothes, saris, net, parachutes, etc. Farmers commercially rear silkworms to obtain silk fiber. Silkworms are completely domesticated. So, most of the silkworm are found reared in artificial farms and rarely in their natural habitat. 

Lifespan

A silkworm completes its lifecycle in about 45 days. Different factors such as temperature, humidity and exposure  to sunlight are very important in the lifespan of a silkworm. Generally, the warmer the weather , the quicker the silkworm will complete its life-cycle. Thus, life-cycle of a silkworm generally ranges from 6-8 weeks . The male silkworm dies after mating with the female while female silkworm dies after laying eggs.

food 

An adult silkworm does not eat or drink anything but the larvae of silkworm are voracious eaters. The most  commonly reared silkworm's larvae feed on mulberry leaves.  Another type of silkworm's larvae feed on castor leaves.

Agriculture

There are various types of silkworm that are reared all over the world. The agro-industry of  rearing silkworms to produce silk fibers is called sericulture. Sericulture is the commercial farming of silkworm for silk . It is done extensively in India, China, Nepal, Japan, Brazil, Korea, South Africa, Russia, etc.

Types of Silkworms

Although there are several commercial species of silkworms. Seri silkworm (Bombyx mori) and Eri silkworm (Attacus ricini) are the two types of silkworm that are reared in Nepal . 

Seri Silkworm (Bombyx mori)

Seri silkworm feeds on the leaves of mulberry plant. So, it is also called mulberry silkworm. The seri silkworm produces the highest amount of silk. It produces about 90%  of the world's silk. Therefore, the term 'silk ' in general refers to the silk of the mulberry silkworm. This silkworm is killed inside the silk case during the process of extracting the Seri silk yarns. Thus, the silk produced by Seri silkworm is long, continuous and can be wound easily. Its silk is white or  light yellow.

Eri silkworm (Attacus ricini)

Eri silkworm feeds on the leaves of the castor oil plant (Arandi in Nepali ) .So, it is also called castor silkworm . This silkworm is not killed during the process of extracting the Eri silk yarns. Thus, the silk  produced from Eri is not continuous and long. It cannot be wound . The silk produced by Eri silkworm is white or black red.

Life cycle of a silkworm

Silkworm is a unisexual organism i.e.  male and female sex organs are in different bodies. The female are slightly larger than the males. Reproduction in silkworm moth occurs by internal fertilization in tail to tail mating . For the silk obtaining process, the farmers have to rear the silkworm throughout their lifecycles. 

After the male and the female silkworm mate with each other, the female lays eggs. These eggs develop into adults through a series of different stages: egg, larva, pupa and adult. The transformation of silkworm from egg to adult occurs through a process called metamorphosis. Thus, metamorphosis is a  series of changes through which an organism develops from an early immature stage to an adult. The different stage in the lifecycle of Seri silkworm ( Bombyx mori ) are explained below:

Egg

A female silk moth lays about 300-400 small, white eggs in cluster on mulberry leaves at a time . The eggs are as tiny as the size of a pinhead. An egg is of about 1 to 1.2 mm long and 0.9 to 1 mm wide. the female lays all eggs within 24 hours . After laying eggs, she dies in 3 to 4 days.

Farmers collect eggs and keep them in cold places. In cold, the eggs become inert and they don't hatch for a long time . It helps farmers to preserve the eggs. But as soon as the temperature is increased , the eggs hatch into larvae in a week . In sericulture farm, the eggs are collected and stored in a cold place in times of scarcity of mulberry leaves to feed the larvae, But , as soon as enough leaves become available  , the temperature is slowly increased up to 18 degree C to 25 degree C to hatch eggs. After this, the eggs hatch into larvae in 10 to 12 days. 

Larvae

The larvae hatched from the eggs are creamy white in color. Their size is about 6 mm. The body of larvae is divided into  three parts, viz. head, thorax and abdomen. They feed heavily feeders . They continue to feed on the mulberry leaves for 25-32 days. During this time , they grow very fast and shed their skin. The process of shedding their skin is called moulting or ecdysis . A single larvae moults up to four times . During moulting, larvae completely become inactive. They neither move nor eat for about 20-24 hours. 

The lateral sides of the head of larvae bear a pair of long sac- like structure known as silk glands. These silk glands are actually salivary glands. Larvae produce sticky saliva like fluid with the help of these glands. This saliva turns into threads as soon as it comes in correct with air. These threads are extracted  as silk threads. When the larva stops eating and turns yellow , it becomes ready to make silk. The larva continuously makes threads and wraps around its body making a capsule like structure called a cocoon. Thus, the white or yellow, thick oval capsule -shaped protective silk case spun by the larvae of a silkworm is called a cocoon. The cocoon helps to protect the larva, pupa and early adult stage of a silkworm from unfavourable conditions. The color of cocoon depends upon the type of mulberry leaves and the type of silkworm.

The larval stage is completed in five small stages known as instars. That is, instar is one of the five stages of  larvae period during which the silkworm moults and changes its color. In between two instars, the larva sleeps. The silkworm makes silk in the fifth instar. The larval stage finally gets completed in about 30 days.

Pupa or Chrysalis

Pupa is the third stage in the life cycle of a silkworm. This stage is called an inactive or a resting stage. It is because the pupa does not eat anything, and remains inactive inside the cocoon. But, it keeps on developing. The pupa stage is about 12-14 days. 

The cocoon is kept in warm water or air to extract the silk thread. In this process, the pupa dies due to high temperature. Farmers kill the pupa because if they turn into adults, they produce a special liquid that dissolves the part of the cocoon and can change the whole color and texture of the silk. But, some farmers have developed a nature friendly method of silk extraction without killing the pupa. To produce a silk for a single sari, thousands of pupae were killed. 

Adult 

Farmers use about 80% of the cocoon in extraction of the silk. The remaining 20% of the cocoons are developed into adults. But, this number may vary according to farmers and places. These adults are preserved for producing eggs by further mating in the future generations. 

The pupa inside a cocoon undergoes an active metamorphosis and finally grows 3 pairs of legs and 12 pairs of wings. It is transformed into an adult now. The adult silk moth comes out of the cocoon by dissolving a part of cocoon using a special fluid secreted from its mouth. After emerging out, it dries up its wings completely and tries to fly. But a domesticated silkworm cannot fly. Adults survive for 5-7 days during which, male searches female and they mate in a tail to tail position. The male dies after mating and the female also dies after laying eggs. 

Characteristics of silk

1. The silk produced from silkworm is long, strong, elastic, shiny and durable. 
2. Natural silk fiber can be easily colored and ironed. 
3. It is highly elastic and has long threads. 
4. Natural silk can absorb water readily and can be dried easily. 

Economic importance of silkworm 

• Silk is used to make textiles, clothes, curtains, parachutes, etc. 
• Silk is used in fishing lines, sieves of flower mills, and tires of racing cars. 
• The clothes made up of silk are suitable to be worn in all seasons. So, silk clothes have business throughout the year. 
• Silk clothes generally do not decay easily and are warm, light and elastic. So, most of the people like to wear these clothes. 
• Sericulture can be done in small land area. So, common farmers can also practice it. 
• All the byproducts of sericulture can be sold. The pupae are fed to animals. Similarly, spoilt cocoons can be used as food for fish in fish farms. 
• Sericulture provides employment opportunities and generate national income.