英文科學讀本 第六冊·Lesson 59 Silk and the Silk-Worm

Lesson 59 Silk and the Silk-Worm

The untutored savage, throughout all time, has clothed himself with the skins of mammals, and adorned his person with the feathers of birds, but it would never dawn upon him to look for his clothing to the insect race. That to him would appear an absurdity, and yet silk, the most beautiful of all fabrics, is the product of an insect—the silk-worm.

The silk-worm is the grub or caterpillar of a peculiar kind of moth, which feeds on the leaves of the mulberry tree. The moth came originally from China. It has a thick-set, hairy body, with short, stout legs and large cream-colored wings marked with dark stripes. The female is bigger than the male, and altogether different in appearance. She dies as soon as she has laid her eggs, which are smaller than grains of mustard-seed, and from 250 to 400 in number; the male does not long survive her.

The favorite food of the silk-worm, as we said just now, is the leaf of the mulberry tree. People, therefore, who rear silk-worms provide a constant supply of fresh mulberry leaves, and the mother-moth lays her eggs on the leaves. The grub, when hatched, emerges from the egg in the form of a little black worm, not more than a quarter of an inch in length, but there is plenty of food at hand, and it feeds and grows quickly. While growing it casts its skin as it becomes too small, and another skin takes its place. During its growth the silk-worm moults, or casts its skin, four times. The first moult takes place about eight days after it leaves the egg; the second, third, and fourth at regular intervals of about five days each.

After the last moult it feeds voraciously on the mulberry leaves and continues to grow for about ten days longer, when it may be said to have reached its full size as a caterpillar. It is then about 2 inches long; its body, which is of a grayish cream color, is supported upon sixteen legs, and consists of twelve ringed segments, from the last of which projects a sort of horn. It now ceases to feed, and after fixing itself to some light object, such as a twig, a bit of straw, or a piece of paper rolled up, it commences to send out, from two small holes under its jaw, a fine, yellow, gum-like substance, which hardens into a silky thread or fiber on exposure to the air. With this the worm completely envelops itself as in a ball.

The first day is usually spent in forming a loose, flossy covering for the outside of the ball. This it afterwards coats with gum, so as to make it into a kind of outer skin. Inside that, during the next three days, the silk-worm spins a firm ball of fine, strong, yellow fiber, which is the silk of commerce. The whole forms an oval ball, about the size of a walnut, and is called a cocoon.

The creature—whatever it is—of course occupies the center of the cocoon. It is not a silk-worm, it is not a moth. It is a lumpy, oval ball, covered with a dark red, shiny skin or shell; but if it were removed from the cocoon, it would be to all appearance lifeless. It is the silk-worm chrysalis. It has taken the first stage of that wonderful change, which is peculiar to the life-history of all insects—from the grub to the pupa or chrysalis, and if left alone it would pass from the pupa to the perfect insect.

Many grubs, when about to enter the pupa or chrysalis stage, envelop themselves in a kind of silky covering, which they spin with material from their own bodies. The destructive rose-maggot may always be met with in the spring, curled up in a leaf of the tree, in a loose, flossy covering of its own spinning. Most caterpillars prepare for the chrysalis stage in the same way, but only the silkworm caterpillar spins a silk which is of use to us.

After from ten to fifteen days the chrysalis would, if left to itself, give ample proof that it is not lifeless. During its imprisonment in the cocoon it undergoes wonderful changes, and at the end of the time emerges from the ball, not as a grub, not as a chrysalis, but as a winged moth able to fly in the air. It is found that the cocoon from which the moth has been allowed to work its way out does not yield such good silk as one that has not been broken through. Hence in silk-producing countries the chrysalis is always killed, by throwing the cocoons into boiling water or steaming them. The hot water, too, softens the gum on the cocoon, so that the loose outer skin is easily pushed aside to allow the inner ball of fine silky fiber to be taken out. Of course some of the cocoons are left for the moths to emerge from them in the natural way, in order that there may be eggs for future hatching.

The process of unwinding the delicate, silky thread of the cocoon without breaking it is one that requires extreme care. As the silk-worm commenced its spinning operations from the outside, the first step in the unwinding is to find the outer end of the thread.This done, the rest is easy with care. The thread is unwound by means of a rotating wheel, each cocoon usually yielding from 300 to 500 yards of silk. It is estimated that 250 cocoons weigh 1 lb., and as it takes 12 lbs. of these cocoons to produce 1 lb. of silk, it therefore follows that a single lb, of this raw silk is the result of the work of 3000 silk-worms.

From the reels the silk is made up into bundles or hanks, and is known as raw silk. A number of these hanks are usually bound together to make what is known in the trade as a "book." In this state it is shipped to the manufacturer to be spun into thread and woven into various fabrics.

The raw silk, however, requires careful treatment before it is fit for the spinning machine. It must first be washed and then thrown. The throwing process consists of twisting several of the delicate threads together to make one sufficiently strong to be spun into yarn for weaving. It is then known as thrown silk. The word thrown comes from the Anglo-Saxon word thrawan, which means to twirl or twist.

The silk-worm will not thrive in cold latitudes. The principal silk-producing countries of the world are China, Japan, India, Persia, Turkey, Italy, and the South of France. China, as we have already observed, was the original home of the silk-worm, and today that country provides by far the greatest supply—more than all the rest put together.

England imports annually from 4 million to 5 million lbs. of raw silk for their home manufactures, but their main supply of silk comes in the form of manufactured goods, mostly from France.

The silk manufacture is not one of England's great industries. Yet they have important centers of activity in the various branches of the trade.

Silks, satins, brocades, and velvets are made in Spitalfields (in the east of London), and in Macclesfield, Glasgow, Paisley, Manchester, and Dublin. Stockings and gloves are made at Nottingham; crape at Norwich and Colchester, and ribbons at Coventry, Macclesfield, and Derby.