Worms Turn Mulberry Leaves Into Silk
According to Webster's dictionary, silk is a fine, strong, lustrous fiber produced by various insect larvae. To be sure, spiders also produce a type of silk, but the silk that is used to make fabric comes from caterpillars of the insect known appropriately as the silkworm.
Insect silk consists primarily of two proteins, a structural fibrous one called "fibroin" and a sticky one called "sericin." Silk also contains some peptides that apparently function to inhibit the breakdown of silk fibers by bacteria and fungi. Silk is produced as a single strand from salivary glands located in the head of an insect.
There are many kinds of insects other than the well-known silkworm that produce silk. Most insects that are silk-producers use the fiber to construct cocoons. Cocoons serve as shelter for the insect when it is in the pupal stage. It is the pupa where the insect changes from an immature form, like a caterpillar, to an adult, like a moth.
Many silk-producing insects are moths -- but not all. For example, bees and wasps also construct pupal cases from silk. Some gardeners will see white egg-like sacs attached to tomato hornworm caterpillars. These sacs are silken cocoons spun by larval parasites that have fed inside the hornworm. Tiny wasps will emerge from the cocoons.
Insects also put silk to use in ways other than as a winding sheet for pupae. Some caterpillars spin a single line of silk that serves as a rope, which they attach to a branch or leaf and use to descend or ascend as meets their fancy. Such a use of silk is the insect equivalent of a helmet line, which functions to raise and lower a person in a deep-sea diving suit.
Even a type of insect that lives in water makes use of silk. Larvae of caddisflies are aquatic, and most species construct cases in which they live. The cases are made of leaves, twigs or pebbles. The materials are fastened together with silk. In some instances, the cases are made entirely of silk.
Even though many species of insects produce silk, only one group is known as the silkworm. The most common of the silkworm moths is known scientifically as Bombyx mori (L.). Humans have pampered these insects for so long that they depend on us for their very survival. The silkworm is a totally domesticated insect and probably no longer exists in the wild.
The discovery of silk production by B. mori occurred about 2700 B. C. in China. Other discoveries soon followed, including how to unwind the strands of silk from the cocoon. Commercial production of silk, known as sericulture, has flourished in China for centuries. Silk became the most important commodity that passed along the famous "Silk Road" stretching from Eastern China to the Mediterranean Sea.
Like all animal production systems, raising silkworms on a large scale can present problems. Food for the animals is a case in point. Like many insect larvae, silkworm caterpillars are big eaters. They are also picky eaters. They feed only on mulberry leaves. So massive production of silk requires acres of mulberry trees to produce the tons of food needed to feed the caterpillars.
Another problem in sericulture is disease outbreaks among the animals, in this case, the insects. Louis Pasteur solved a disease problem that was killing silkworms being raised in France. The disease turned out to be caused by bacteria, and Pasteur described the sanitary conditions necessary for control. Another scientist, Ishiwata Shigetae, was studying silkworm disease and discovered the bacterium Bacillus thuringiensis that today is used in insect control.
The best quality silk is produced using primarily hand labor to rear the insects and unwind the cocoons. Consequently, a lot of work goes into producing silk clothing. It takes about 500 cocoons to make a necktie or 8,000 cocoons to produce an evening dress. All of this gives meaning to the old proverb, "Patience and the mulberry leaf become a silk gown."