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, flossycovering of its own spinning. Most caterpillars prepare for the chrysalis stage in the same way, but only the silk- worm 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 wonderfulchanges, 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 chrysalisis always killed, by throwing the cocoons into boilingwater 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 isone 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 millionlbs. 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.