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The caterpillar which spins a silken thread

Posted in Industry, Insects, Nature on Monday, 28 November 2011

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This edited article about silk originally appeared in Look and Learn issue number 863 published on  29 July 1978.

Silk, picture, image, illustration

The story of silk in pictures

No-one really knows when man first discovered how to use the threads made by caterpillars of the silkworm moth, bombyx mori, for the spinning of silk.

The origin of this beautiful, much-valued material is lost in legend and fable, but we do know that the silk industry began in China and that silk became an important commodity there under the emperor Huang Ti as long ago as 2,640 BC.

Silkworms are big, smooth, white caterpillars which hatch from tiny eggs laid by the parent moth on mulberry leaves. The eggs are called “silk seed” and are so minute that about 1,500 of them would only weigh about a gram.

When newly hatched, the tiny black larva turns to a creamy colour and grows to 85 millimetres in length. At first, it eats its eggshell, which contains food that is vital to its life and to its spinning. When the eggshell, has been eaten, the caterpillar then begins to eat the food that will provide its only sustenance for the rest of its life – mulberry leaves.

The caterpillar snips off bits of the leaf with its pincer-like jaws at an extremely rapid pace because it does not have to digest its food before swallowing. It eats the edge of the leaf, or makes a small hole in the leaf and then eats round the edge of this hole.

It lifts its head clear and draws it downwards along the edge, the jaws making a snipping sound as they go. A caterpillar’s jaws are not set like ours. To get some idea of how they work, imagine holding a slice of bread to bite on, with one corner of your mouth under your nose and the other on your chin. That’s why the caterpillar eats along the edge of the leaf.

As well as possessing this feeding skill, the caterpillar also has to walk upside down and underneath the leaves. For this, it has a special arrangement of legs. At the front end of the body there are three pairs of legs which are thin and strong, each ending in a claw. Farther back are five pairs of thick, soft legs each ending in a flat “sucker foot”. On this sucker foot is a small oval pad which can be arched when the foot is placed down.

This enables the caterpillar to maintain a strong hold on the leaf even if it is upside-down. Around each pad is a fringe of tiny hooks which gives the caterpillar an extra firm grasp on a rough surface. There are two more sucker feet at the end of the body.

To walk, it puts its tail down first, and then the sucker feet in this order, 5, 4, 3, 2, 1, so that the body is pushed forward.

You can do your own practical experiment if you want to find out exactly how the caterpillar does this. Put your right hand on a table as though ready to play the piano. Now make your fingers walk once across the table from right to left, in order, 5, 4, 3, 2, 1. Leave the thumb on the table, bring the fifth finger up to it, and then put the fingers down again, in order, 4, 3, 2, 1. Do this once or twice. Now you will know how the caterpillar is able to move forward on its sucker feet.

Although you would think that with all these sucker pads and hooks the caterpillar could quite happily cope with the difficult manoeuvre of walking on a leaf and staying there, it has one more safety device. It has what you could correctly call a safety rope. This is really a thread of silk which it spins out from its mouth. The free end fastens to the leaf so that one end is secured and the other is in its mouth. If it does happen to fall off its perch, the caterpillar can easily climb back on again by means of the rope. It is this thread that makes the silkworm such an interesting – and valuable – creature.

As the caterpillar reaches maturity, its glands fill up with a clear, viscous fluid which is used to spin the cocoon. The caterpillar needs to make this cocoon so that it can safely pass through the pupa stage of its life. When it is ready to do this, it spends its time looking for a suitable spot, selecting a corner formed by two leaves to provide adequate shelter.

There, it spins its silk. This is produced by a pair of long tubular spinning glands, coiled within the caterpillar and lying one on each side of the digestive canal. Each gland secretes a fluid called fibroin.

The fibroin from the two glands moves through the common silk duct and enter a muscular thread-press. This thread-press transforms the two fibres into a single flattened thread. Accessory glands, which open into the common silk duct, secrete a substance that causes the two fibres to stick together. The single thread produced solidifies on contact with the air. The caterpillar spins this thread into a hard, compact, oval-shaped cocoon which varies from white to yellow in colour.

The whole process takes roughly a day, and once the cocoon is completed, the silk worm takes a rest for several hours before casting its skin to reveal its pupa form. During the next 10-12 days, the wings and feelers of the growing moth develop, pressing against the side of the pupa case. When the newly-formed moth is ready to emerge, it moistens one end of the cocoon with a special fluid which softens the silk, then pushes its way through the cocoon into the open with damp, crumpled wings.

The wings start to quiver and as they dry they smooth out and expand. The male and female moths mate almost immediately and the female lays her 500 or more eggs within four to six days and soon dies. From the eggs that are laid in the autumn, silkworms hatch in March of the following year and the whole process of larva, pupa, moth starts all over again.

As we saw at the beginning, it was the Chinese who first started to carry out sericulture – the cultivation of silkworm cocoons from which raw silk is reeled. It was a Chinese emperor’s wife who made sericulture fashionable by cultivating mulberry trees, raising worms, and reeling silk.

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