THE SILKWORM-DISEASE.
The life of the population of certain departments in the South of France hangs on the existence of silkworms. In each house there is nothing to be seen but hurdles, over which the worms crawl. They are placed even in the kitchens, and often in well-to-do families they occupy the best rooms. In the largest cultivations, regular stages of these hurdles are raised one above the other in immense sheds, under roofs of disjointed tiles, where thousands and thousands of silkworms crawl upon the litters which they have the instinct never to leave. Great or small, the silkworm-rearing establishments exist everywhere. When people accost each other, instead of saying 'How are you?' they say 'How are the silkworms?' In the night they get up to feed them or to keep up around them a suitable temperature. And then what anxiety is felt at the least change of weather! Will not the mulberry leaves be wet? Will the worms digest well? Digestion is a matter of great importance to the health of the worms, which do nothing all their lives but eat! Their appetites become especially insatiable during the last days of rearing. All the world is then astir, day and night. Sacks of leaves are incessantly brought in and spread out on the litters. Sometimes the noise of the worms munching these leaves resembles that of rain falling upon thick bushes. With what impatience is the moment waited for when the worms arrive at the last moulting! Their bodies swollen with silk, they mount upon the brambles prepared for them, there they shut themselves up in their golden prisons and become chrysalides. What days of rejoicing are those in which the cocoons are gathered; when, to use the words of Olivier de Serres, the silk harvest is garnered in!
Just as in all agricultural harvests, this ingathering of the silk is exposed to many risks. Nearly always, however, it pays the cultivator for his trouble, and sometimes pays him largely. But in 1849, after an exceptionally good year, and without any atmospheric conditions to account for the fact, a number of cultivations entirely broke down. A disease which little by little took the proportions of an epidemic fell upon the silkworm nurseries. Worms hardly hatched, and worms arrived at the last moulting, were equally stricken in large numbers. It mattered little in what phase the silkworm happened to be: in all it was assailable by the evil.
There is hardly a schoolboy who has not reared in the recesses of his desk some five or six silkworms, feeding them, in default of mulberry leaves, with leaves of lettuce or salsify. Therefore it is hardly necessary to remind my readers how the silkworm is born, grows, and is transformed. Coming out of its egg, which is called a grain, because of its resemblance to a small vegetable seed, the silkworm appears in the first fine days of spring. It does not then weigh more than one or two milligrammes. Little by little its size and its activity augment. The seventh day after its birth it rests on a leaf and appears to sleep. It remains thus for nearly thirty hours. Presently, its head moves, as if it did not belong to the rest of the body, and under the skin of this head appears a second quite new head. Just as if it came out of a case, the silkworm disengages itself from its old withered skin. Here are its front feet, there the false feet (fausses pattes), which it carries behind. At length the worm is quite complete. It rests a while and then begins to eat. At the end of a few days new sleep, new skin, new shedding of the skin, then a third, and then a fourth metamorphosis. About eight days after the fourth shedding of its skin, the worm ceases to eat, its body becomes more slender, more transparent; it seeks to leave its litter, it raises its head and appears uneasy. Some twigs of dried heather are then arranged for it to fasten upon; these it climbs, never to descend again. It spins its cocoon and becomes a chrysalis. When the worms of a cultivation have all spun their cocoons, they are smothered in a steam stove, and, after being dried in the sun, they are handed over to the spinners. If it is desired to reserve some of the cocoons for seed, instead of being smothered, they are strung together in chaplets. After about three weeks, the moth comes out of its chrysalis. It pierces the cocoon by means of a liquid which issues from its mouth, and which has the property of so softening the silk that the moth is able to pass through the cocoon. It has hardly dried itself and developed its wings when the males and females pair for several hours. Then the females lay their eggs, of which they can produce from four to six hundred. These are all the phases through which silkworms pass in the space of two months.
In the epidemic which ravaged the silkworm nurseries in 1849 the symptoms were numerous and changeable. Sometimes the disease exhibited itself immediately. Many of the eggs were sterile, or the worms died during the first days of their existence. Often the hatching was excellent, and the worms arrived at their first moulting, but that moulting was a failure. A great number of the worms, taking little nourishment at each repast, remained smaller than the others, having a rather shining appearance and a blackish tint. Instead of all the worms going through the phases of this first moulting together, as is usually the case in a batch of silkworms, they began to present a marked inequality, which displayed itself more and more at each successive moulting. Instead of the worms swarming on the tables, as if their number was uniformly augmenting, empty spaces were everywhere seen; every morning corpses were collected on the litters.
Sometimes the disease manifested itself under still more painful circumstances. The batch would progress favourably to the third, and even to the fourth moulting, the uniform size and the health of the worms leaving nothing to be desired; but after the fourth moulting the alarm of the husbandman began. The worms did not turn white, they retained a rusty tint, their appetite diminished, they even turned away from the leaves which were offered to them. Spots appeared on their bodies, black bruises irregularly scattered over the head, the rings, the false feet, and the spur. Here and there dead worms were to be seen. On lifting the litter, numbers of corpses would be found. Every batch attacked was a lost batch. In 1850 and 1851 there were renewed failures. Some cultivators, discouraged, attributed these accidents to bad eggs, and got their supplies from abroad.
At first everything went as well as could be wished. The year 1853, in which many of these eggs were reared in France, was one of the most productive of the century. As many as twenty-six millions of kilogrammes of cocoons were collected, which produced a revenue of 130,000,000 francs. But the year following, when the eggs produced by the moths of these fine crops of foreign origin were tried, a singular degeneracy was immediately recognised. The eggs were of no more value than the French eggs. It was in fact a struggle with an epidemic. How was it to be arrested? Would it be always necessary to have recourse to foreign seed? and what if the epidemic spread into Italy, Spain, and the other silk cultivating countries?
The thing dreaded came to pass. The plague spread; Spain and Italy were smitten. It became necessary to seek for eggs in the Islands of the Archipelago, in Greece, or in Turkey. These eggs, at first very good, became infected in their turn in their native country; the epidemic had spread even to that distance. The eggs were then procured from Syria and the provinces of the Caucasus. The plague followed the trade in the eggs. In 1864 all the cultivations, from whatever corner of Europe they came, were either diseased or suspected of being so. In the extreme East, Japan alone still remained healthy.