Laurent de l'Arbousset showed in i go5 that i oz. of seed of 3o grams produc ing 30,00o to 35,000 silkworms (30,000 may be depended upon to reach the co coon stage) will give a harvest of 13o to 14o1b. fresh cocoons and ultimate yield of about I21b. raw silk properly reeled. The amount of nourishment required for this rearing is as follows:—hatching to first moult, about glb. of leaves of tender growth, equal to 4o to 451b. ripe leaves; first to second moult, 241b., representing oolb. ripe leaves ; second to third moult, 8olb., representing 24o1b. ripe leaves; third to fourth moult, 2361b., representing 4721b. ripe leaves; fourth moult to mounting, I,43o1b., representing 1,54o1b. ripe leaves, totalling to about one ton of ripe leaves for a complete rearing. The growth of the worms during their larval stage is stated by Count Dandolo to be as follows : the small, black, newly hatched worm weighs about one-hundredth of a grain, and is about one-twelfth of an inch long; when it has reached its full growth, the large white worm weighs about ten grains, and is more than three inches long. Or, in tabular form: The Cocoons.—The worms show that they are ready to spin their cocoons by raising the forepart of the body and waving it slowly from side to side. Provision is now made for the struc ture on which they can spin. This consists of "bushes" of scrub oak or other many-branched shrub which are placed vertically in the centre of the shelves on which the worms have hitherto spent their lives. Obedient to the dictates of nature, the worms, now replete with glutinous fibre, mount up into the bushes and proceed to spin the cocoons around themselves. If they are over crowded there is the danger of two worms combining to spin one cocoon; this latter is practically useless for making silk as the combined threads are inextricably mixed up and the cocoon is incapable of being reeled into fine silk. After eight days, the bushes are removed from the shelves and the cocoons are picked off them. The importance of an even incubation is again demon strated here, for if the hatch-out were irregular some worms would be ready to spin before others and the rearer would run the risk of taking down the bushes before all the worms had completed their spinning, resulting in some cocoons of inferior quality. On the other hand, the rearer dare not wait more than eight days, otherwise the chrysalis would complete its transforma tion and change into a moth which, in emerging from the cocoon, would cut through the silk fibres and destroy it for reeling pur poses and thus render it practically valueless.
With the exception of those selected for reproduction of eggs, the cocoons are now treated so as to preserve them intact for reeling. The chrysalis must be killed without damage to the cocoon. The worm spins the cocoon with one continuous thread in a manner forming the figure 8, therefore the cutting of the co coon at one end to allow the moth to escape means the cutting of the one continuous thread into many thousands of short ones and, naturally makes it impossible to unwind ("reeling silk" is only another way of saying "unwinding cocoons"). The method adopted for killing the chrysalides is that of suffocation.
Leaving cocoons exposed to the hot sun will suffocate the chrys alides, but it also hardens the gum in the thread, making un winding difficult and wasteful, and withdraws to a certain extent the colour from yellow cocoons. Another method is suffocation by steam. The cocoons are placed in shallow drawers in a cup
board which is constructed over a common washing copper. The bottoms of the drawers are constructed so as to allow of steam percolating through them. A fire is lit below the cauldron. which is filled with water, and steam is generated. The cocoons remain in the steam from eight to ten minutes and the chrysalis is suffocated. The cocoons are then spread lightly on canvas beds, sheltered from the sun, but where air can circulate freely. They remain on the beds from six weeks to two months, during which time they require to be turned over twice daily to prevent heating and the dead chrysalis gradually dries up without be coming putrid. In this method there is the risk of either keeping the cocoons in the steam too long and damaging the fibre, ren dering the thread brittle, or not keeping them long enough so that the chrysalis, which has marvellous powers of recovery, will complete its metamorphosis and the moth will cut through after all. The safest and most practical method is suffocation by hot air. Sechoirs, or dryers, are constructed to take a large quantity of cocoons at one charge, and air is fanned through a steam-coiled chamber rendering it about 200° F. The hot air circulates by means of channels through the chambers containing the cocoons, and the chrysalis is suffocated and all moisture in it dried up in one process of twelve hours' duration. The cocoons are put into sacks and stored without fear of deterioration. Exposure to air and wind, which also means exposure to dust and dirt, is unnecessary, and the colour of the silk when the cocoons are reeled is richer, while the water used in the reeling keeps clean.
Selection of Eggs for Reproduction.
—The promiscuous production of eggs for the following crop by the rearers is strongly to be discouraged and in many countries it is absolutely prohibited, be cause of the great risk of propagating dis eases to which the silkworm is prone. Re production, if a virile and healthy race of worm is to be preserved, must be under the supervision of experts. These experts choose rearers who are known to be ex tremely adept at cocoon-raising and give them specially selected eggs, each separate laying of which has been microscopically examined for the presence of disease. These eggs are known as "cellular seed." The experts pay periodical visits to the magnanerie during the rearing to examine the condition of the worms and take away any of doubtful appearance for microscopic examination. If the rearing is entirely successful and no trace of disease is found, the cocoons are taken to the "seed station," where they are examined. Ill-formed or imperfect cocoons are sent to be stifled and the selected ones are threaded together into long ropes and suspended from the ceiling to within a foot of the floor. In due course the moths emerge from the cocoons, mating takes place and the female is placed in a small linen bag, about tin. square, which has been washed and disinfected; the mouth is tied up and these bags are strung together and sus pended from the ceiling of another room. The female lays its eggs in the bag and dies. The males, after two or three mat ings, are destroyed. This part of the process is carried on until the cocoon crop is at an end. The ratio of reproduction is about 25o to I, so the seed stations require about i,000lb. of cocoons for seed for a crop of 25,000lb. of cocoons the next season.