Natural fibres are obtained from plants and animals. Cotton, jute and flax are all plant fibres. While wool and silk are animal fibres. We get wool from yak, sheep and goat etc. and silk from silk moths or ‘silkworms.’ The rearing of silkworms for obtaining silk is called ‘sericulture’. Sericulture is a very old occupation in India. India is the 2nd largest producer of silk in the world after China and Karnataka is the largest producer of silk in India.
History of silk
The history of production of silk fibre is very interesting. According to one legend, the story of silk began in 2640 BC. Si-Ling-Chi, a Chinese Empress, was walking around her garden while sipping a cup of tea when the cocoon of a silkworm fell into her cup. The cocoon soon began to unravel revealing a long silken fibre. When she looked up, she saw a Mulberry tree with several other cocoons hanging from it and a number of silkworms crawling around. This led her to conclude that the cocoon had come from the silkworm caterpillars.
For 2500 years, the Chinese kept the art of making silk to themselves. They sold silk fabric but refused to reveal the secret of how the fabric was made. In spite of their secrecy, the knowledge of how to make silk reached Korea and India in 200 BC and 140 BC respectively.
Silk production started in Europe when two monks who were visiting China learned the secret of silk-making. They returned to Constantinople and shared this knowledge with Emperor Justinian. The Emperor then sent the monks back to China to source silkworm eggs and mulberry shoots, which they smuggled out of China in their hollowed-out walking sticks.
The life cycle of silkworm
The life cycle of silkworm ranges from 6 to 8 weeks. This is largely dependent on the ambient temperature. The warmer the weather, the sooner the silkworm will complete its life cycle. Other factors that influence the life cycle of the silkworms include exposure to sunlight and humidity. In optimum conditions the eggs can hatch in a week or ten days. The entire lifespan of a silkworm can be broken down into the following stages-
Hatching from Eggs
Female silkworm lays eggs on the leaf of mulberry tree. Eggs hatch about 10 days after they are laid.
As the eggs hatch, they form worm like larvae. This stage of the silkworm’s lifecycle lasts for about 24-33 days. The silkworm larva will moult 4 times before it goes into the pupa stage. Periods between moults are known as silkworm instars. Before the first moult, the silkworm’s head will turn darker than the rest of the body. The silkworm’s growth is fastest during the first instar. During each moult, the larva will shed its skin and grow into a larger, new skin. After the first moult, the silkworm emerges as a white, soft, smooth caterpillar.
Pupa and Cocoon
At an age of between 20 and 33 days, the appearance of the silkworm will change and it will turn yellowish and translucent. This indicates that they are ready to build a net of silk around them which is actually a liquid protein secreted from the head of silk moth or caterpillar. This silk is used as an anchor from which the worm swings back and forth to draw a long continuous fibre and build the cocoon. The fibre can be as long as 1 kilometre. Silkworms can take up to 48 hours to build a complete cocoon. Inside the cocoon, the silkworm moults for the last time and begins the process of becoming a moth.
Sometimes, two silkworms may build a cocoon together. In this case, the cocoon is noticeably larger.
The moth develops within the cocoon for about 10-14 days. The moth emerges as a hairy male or female moth. These moths have a wingspan of about 50mm but cannot fly due to centuries of domestication. Moths have an average lifespan of 5-10 days with females living shorter lives than males. The female moth is bigger than the male and begins searching for a mate almost immediately after emerging from the cocoon. For this, it releases pheromones to attract the males. After mating the female moth lays 300-500 eggs on mulberry leaves. Most female moths die soon after laying the eggs.
Processing of silk
Sericulture refers to the production of silk by raising silkworms. The silkworm must be cared for from the time it hatches from an egg to the completion of the cocoon stage. Silk is harvested by killing the silkworm pupa using heat. The cocoon filament is then wound on a reel, softened and unravelled. The silk produced at this stage is called raw silk. Extracting raw silk from the cocoon of silk moth is known as processing of silk.
Raw silk contains sericin, a gummy substance that protects the strands. This is retained until the yarn is boiled in soap and water. Later, several silk strands may be twisted together to make stronger yarn. It is then treated with chemical substances to add density, increase weight and improve the quality of its draping. Silk is valued for its softness, lustre, resistance to breakage, strength and heat resistance. It may lose its strength and decompose when extensively exposed to sunlight but is usually resistant to mildew.