Silkworms- part 1
Two things are needed to produce silk, silkworms and a supply of food for them, which is exclusively mulberry leaves. They seem to prefer the white mulberry, but the black mulberry variety has been used as well. According to Dr. John Feltwell in his book “The Story of Silk”, there are about 1,200 species of caterpillars that produce silk cocoons. The one species that accounts for the vast majority of silk production in the world is Bombyx mori. This species has been domesticated for over 5,000 years. It originated in China, the only place where the silkworms and mulberry trees lived together. It is one thing to think about a domesticated mammal, such as a dog or cat, but it is not often one thinks about domesticated insects. That is in fact the case for Bombyx mori, however. They no longer exist in the wild, and they are only kept alive for silk production. They do not wander far from their pens, where groups of caterpillars are often kept on flat beds or trays, stacked to the ceiling of a room, perhaps in about 10 layers, to maximize the space available for raising them and to give them enough space and air to grow. The moths have wings, but they are useless and the moths cannot fly. Only enough moths are allowed to hatch to ensure enough eggs and silkworms for the next generation of silk production.
With natural production as practiced before modern times there was one generation of silkworms per year. The female moth would emerge from its cocoon and then would immediately mate with a nearby male which fertilized her eggs. She would then lay the eggs on her cocoon or on leaves and branches nearby. Typically about 300-500 eggs would be laid by each female. These would remain there and hatch just when the new mulberry leaves were ready to sprout in the springtime. The springtime warmth would start the incubation process, which takes about 10 days, at the proper time to coincide with the emergence of new leaves on the mulberry trees. Eventually processes were developed with artificial heating and lighting to produce multiple generations of silkworms each year, even as many as 6 to 8 generations. Artificial food was developed which could be made available at any time of the year. This consists of ground mulberry leaves that are dried and then given to the caterpillars when mixed with water. It has been proven that silkworms take to this substitute for natural leaves quite well.
There are 4 stages in the life of silkworms. These are commonly referred to as egg, caterpillar, chrysalis and adult. Scientifically the stages are called ova, larva, pupa and imago. In the caterpillar stage the silkworms are essentially mulberry leaf eating machines. In the month of feeding that is required for them to mature they need to be fed about every four hours, day and night. Their body weight will increase by a factor of 10, and they will grow in size from about 1mm in length to about 70-80 mm in that time. During the total period of their development they will shed their skin a total of five times, and the fifth shedding will actually occur in the finished cocoon. When growth reaches a certain point and the skin gets to tight, it is shed and the process continues with another layer of skin that allows the caterpillar to grow larger. The fully developed silkworms are generally oyster-white in color and sometimes have black bands. They have eight pairs of legs and breathe through nine pairs of air tubes on the sides of the body. After the fourth layer of skin is shed, the caterpillar starts to extrude silk from glands below its mouth to produce a cocoon. The purpose of the cocoon is to provide protection for the pupa, which will undergo metamorphosis and turn into a moth. Silkworms work for 2-3 days extruding a single strand of silk that is typically 400-600 meters in length. The head moves back and forth continuously and secretes a liquid substance which dries into silk when it is exposed to air. After the cocoon is finished following those 2-3 days, the silkworm rests. It then sheds its skin one last time and enters into the pupa stage.
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