Part 18

In the eighteenth century the intendants of the provinces tried, but with very slight success, to give a fresh impetus to sericulture in France. The Abbé Boissier de Sauvages published, about 1760, some works, which prove him to have been a patient observer, an accurate reasoner, and a clever rearer of silkworms. Boissier de Sauvages is the father of modern silk-culture. During the first Revolution, men's minds were occupied with graver subjects than the cultivation of the mulberry tree. But, on the return of peace, they got to work again on all sides. In 1808, the minister Chaptal estimated the weight of the cocoon harvest at between five or six thousand kilogrammes; whilst the invention of the Jacquard loom gave an immense impulse to the weaving of silk stuffs. Amongst those who introduced and benefited the art of sericulture, we must not forget Dandolo. Dandolo, who was born at Venice in 1758, and died in 1819, was the first who, at the beginning of this century, applied himself seriously to the amelioration of the processes employed in the cultivation of silk. He endeavoured to regulate the temperature, to introduce more order into the distribution of the food to the worms, to have more spacious premises, and to have these properly ventilated.

Now we are on this subject, we must mention the names of those who at the present day have rendered important services to sericulture--such as M. Camille Beauvais, who raised silkworm rearing from the inactivity into which it had been plunged; M. Eugène Robert, who founded in the south of France the first successful silkworm nursery; M. Guérin-Méneville, who has devoted his life to the study of the same question, and to whom Europe owes the introduction and the acclimatisation of some species which will render us, perhaps, one day very great services; and lastly, M. Robinet, who has elucidated several practical questions in the art of sericulture. In bringing to a close this rapid historical epitome, we will state that France consumes annually 30,000 kilogrammes of silkworms' eggs, each kilogramme being at the present time worth from 300 to 500 francs, and even more. The value of manufactured silks represents annually about 8,000,000 francs; and we find by official statistics that France exported in 1863 silk stuffs to the value of 384,000,000 francs. This immense trade shows how much silk is now-a-days everywhere appreciated; in those numerous tissues called taffeta, satin, and velvet, each of which seems to have a charm--a peculiar attraction. The consistency of the stuff, the smoothness, the softness of surface, the manner in which silk receives colours, the brightness, fineness, power of reflecting, the rustling, the light or heavy folds,--all these are beauty, elegance, and luxury, in whatever way these words are understood.

The _Bombyx mori_ has, however, nothing alluring in its appearance. Other caterpillars of the genus _Bombyx_ have brilliant liveries; they are adorned with spots, blue as sapphires, green as emeralds, red as rubies, but produce threads without brightness and fineness. The humble silkworm, in a white blouse, like a workman, has nothing brilliant in its dress, and yet it gives to the whole world its most beautiful and gorgeous array. The body of the silkworm is composed of thirteen distinct segments. In front are three pairs of articulated legs, which will become later those of the moth. In the middle and towards the posterior part, are five pairs of membranous legs, furnished with a circle of very fine bristles, which assist the animal to hook itself on to leaves and stalks. On the two sides of its body are eighteen stigmata, or respiratory mouths.

The head of the silkworm is remarkable; it is scaly, horny, and formed of one single piece. The mouth is provided with six small articulated pieces. Below is a simple blade, the upper lip, having in its middle a hollow, into which the animal causes the edge of the leaf it is gnawing to enter, and holds it thus without any exertion. Underneath the lip are inserted two large jaws, which cut the leaf as a pair of scissors. Underneath, some weaker jaws divide the fragments, and a little organ, articulated on to each jaw, that is to say, a palpus, pushes them back towards the mouth, and prevents the smallest particle of the leaf from falling. And lastly, in the space comprised between the two jaws, is an under-lip, which completely closes the mouth below. At the extremity of this piece may be seen a little prolongation, a sort of papilla, pierced with a hole, which is the orifice that gives issue to the silky thread.

The organs which serve for the elaboration and emission of the silk have a peculiar interest for us. If we dissect a silkworm under water, we succeed, sooner or later, after having removed the outer parts, in laying bare a double apparatus, placed along the two sides of the intestinal canal and below it. This is the apparatus which secretes the silk; it is the double silk-bearing gland. Each one of these glands is composed of a tube formed of three distinct parts (Fig. 201). The part which is nearest to the tail of the worm is a bent tube, A B C, of a thirtieth of an inch in diameter, and about nine inches in length, twisted a great many times into irregular zig-zags. This part of the silk-producing organ is continued in an enlarged portion, D E, which is the reservoir of the silky matter. To the extremity, E, of this reservoir, is attached another capillary tube, E F. These two capillary tubes, proceeding from the two glands, unite together like two venous trunks, as the plate shows, in one single, short canal, F, which opens in the mouth of the worm, at its under-lip.

It is in the narrow hinder tubes that the silky matter is formed. It collects in the swollen part, D E, which is, properly speaking, the reservoir; and remains there in the glutinous state. Having reached the capillary tubes, it begins to assume consistency, and forms two threads, which are united together at the point of junction of the tubes, and come out through the orifice, with the appearance of a single thread, to be conducted and directed by the animal to those points it has selected.

It was hoped that by taking from the body of the worm the viscous matter contained in the glands, silk could be formed. But this hope was disappointed. It was found possible, it is true, to take the silk out; to draw it out into threads more or less fine; but up to this time it has only been possible in this way to obtain a matter which, when dried, more or less resembles catgut, and is easily enough spoilt by water.

The viscous substance contained in the glands must then be elaborated by the insect itself. When it arrives in the conduit common to the capillary tubes, under the form of a thread, it is impregnated with a sort of varnish, which is poured into them from two neighbouring glands. The varnish unites the two threads into one single thread, and imparts to it the brilliancy of silk, and the property of resisting the action of water. It is during the last phases of the worm's development that the silky matter becomes abundant in the glands. At this period the animal eats much; and it is certain that the substance to be converted is furnished by the leaf of the tree on which the insect feeds.

In consequence of this having been remarked, some manufacturers have attempted to obtain their silk directly from the mulberry leaf; but they only get a bad floss or refuse silk. This is because the silk is not formed in the mulberry leaf. The organs of insects are laboratories, in which manipulations unknown to man are carried on, manipulations which he has not been able to imitate.

After this rapid glance at the fundamental parts of the organism of the silkworm, we will occupy ourselves with the natural history, properly so called, of this insect, and with its rearing, carried on with a view to the production of silk.

As belonging to the first part of this programme, we have to speak of the _moult_, of the _ages_ of the silkworm, of its maturity, of its _mounting_ or _ascending season_, of the formation of the cocoon, of the chrysalis, of the moth, and the eggs.

The name moult has been given to a sort of crisis during which the renewing of the skin of larvæ takes place. When it approaches, the silkworm changes its colour. Its robe, which was white or grey, and opaque, becomes yellow and somewhat transparent. The head swells considerably, especially above, and the skin becomes wrinkled (Fig. 202). The worm then fasts, and prepares to cast its skin. It places here and there some silk threads on the surrounding objects. It then slips under these threads, so that during its movements the old skin which it will abandon is, so to speak, gathered up. It then assumes a peculiar position, that represented in Fig. 203, and remains in it in a state of immobility which has been called sleep (_sommeil_).

During this sleep the new skin is formed under the old. A liquid oozes forth between the two membranes which separates them, and allows the silkworm to leave its old skin. To effect this, the worm begins by raising its head, and by making contortions. The old skin splits round the muzzle, or snout, on the head and back; then by different movements the animal emerges from its skin, which remains held up by the silken threads. The duration of the time occupied in moulting varies with the degree of the heat or humidity of the atmosphere; but in general the state of _sleep_ lasts from twelve to twenty-four hours. One hour after the crisis the worm begins again to eat.

The _ages_ of the silkworm are the periods of time which elapse between one moult and another. If we observe some silkworms when the temperature is favourable, we shall find that there are four moults, and consequently five ages. At the first age (Fig. 204), the silkworm is black and hairy; then of a nut colour at the moment when the first moult is going to take place. "The appearance presented by these worms collected together on a leaf," says Dandolo, "is that of a downy surface of a dark chestnut colour, in the midst of which one sees nothing but a movement of little animals having their heads raised, working them about, and presenting black, shiny muzzles. Their bodies are completely covered with hairs arranged in straight lines, between which one perceives along the whole length of the body other longer hairs."[60]

[60] "L'Art d'élever les Vers à Soie," par le Comte Dandolo. In 8vo. 2e édition. Lyon, 1825.

The first age lasts for five days. At the second (Fig. 205), the worm is grey, almost without down, then of a yellowish white, and one sees the crescents making their appearance on the second and fifth segment. At the third age (Fig. 206), there is not a single hair remaining, and the worm becomes whitish, and is always becoming lighter. The third age lasts six days, as does also the fourth (Fig. 207). At the fifth (Fig. 208), the worm has very nearly reached the end of its career in the caterpillar state, and now is the time of its greatest voracity. This age is the longest; it lasts nine days.

At each of these periods in the life of the silkworm may be remarked a physiological fact to which has been given the name of _frèze_. When the silkworm has just moulted it eats little, but the time very soon arrives when it does so with extraordinary avidity. It is indeed insatiable. The _frèze_ of the last age is called the _grande frèze_. It takes place about the seventh day. During this day worms, the produce of thirty grammes[61] of eggs, consume in weight as much as four horses, and the noise which their little jaws make resembles that of a very heavy shower of rain. It is at the end of this stage that the insect prepares the shelter in which is to be brought about its metamorphosis into a chrysalis.

[61] One gramme = 15·4325 gr. troy.

A little while before this it ceases to eat, turns yellow, and becomes as transparent as a grape. It is now said to have reached its _maturity_. Up to this moment the worm had never tried to leave its litter. It lived a sedentary life, and never thought of wandering away from its food. Now it is seized with an imperious desire for changing its quarters. It gets up, it roams about, and moves its head in all directions to find some place to cling on to. It walks over everything within its reach, particularly over those obstacles which are placed vertically. It aspires, not to descend, like the heroes of classic tragedy, but to rise. It is for this reason that this period of the silkworm's life has received the name of the _mounting_ or _ascending season_. It now looks for a convenient place in which to establish its cocoon. Every one has remarked how the animal sets to work to accomplish its task. It begins by throwing from different sides threads destined for fixing the cocoon; this is what we call _refuse silk_. The proper space having been circumscribed by this means, the worm begins to unwind its thread--a continuous thread of about a thousand mètres long.

It has been calculated, let us say by the way, that forty thousand cocoons would suffice to surround the earth at the equator with one thread of silk. Folded on itself almost like a horse-shoe, its back within, its legs without, the worm arranges its thread all round its body, describing ovals with its head. It approximates gradually the points of attachment of the thread. As long as the cocoon is not very thick one can watch it through the meshes of the web applying and fixing its thread, still to a certain degree soft, in such a manner as to make it adhere closely to the parts already formed.

"We can state," says M. Robinet, "that the silkworm makes every second a movement extending over about five millimètres. The length of the threads being known, it follows that the worm moves its head three hundred thousand times in making its cocoon. If it employs seventy-two hours at its work, it is a hundred thousand movements every twenty-four hours, four thousand one hundred and sixty-six an hour, and sixty-nine a minute, that is to say, a little more than one a second."

About the fourth day, after having expended all its silk,[62] the worm shut up in the cocoon becomes of a waxy white colour, and swollen in the middle of its body. The abdominal legs wither away; the six fore legs approach each other and become black. The parts of the mouth tend downwards; the skin wrinkles. Very soon it is detached and pushed down towards the hinder part, and the chrysalis appears under the rents in the skin. It is at first white, but speedily becomes of a brown red.

[62] "Manuel de l'Educateur du Ver à Soie," p. 37.

The silkworm remains in general from fifteen to seventeen days in the pupa state. At the moment of hatching, the moth begins by breaking the skin in which it is shut up, and which is pretty thin. But how can it come out of the silky prison which it has itself built? To effect this it makes use of a peculiar liquid contained in a little bladder with which its head is provided, and which was discovered by M. Guérin-Méneville. It moistens the cocoon with this liquid; which soaks through and penetrates the whole thickness of the silken wall which confines it. The threads of silk of which it is composed are moistened and disunited, but not broken. The moth opens a passage for itself through the threads thus separated, and makes its appearance in the light of day. Its wings are folded back on themselves, and it is still quite wet, but it seeks immediately for a good place in which to dry itself, and in a little time assumes its final appearance (Figs. 209, 210). The female (Fig. 210) has whitish wings, the antennæ only slightly developed and pale, the abdomen voluminous, cylindrical, and well filled. It is quiet, heavy, and stationary. The male is smaller; its wings are tinged with grey, its antennæ blackish; it moves about, beats its wings together, and is lively and petulant.

Before laying her eggs, the female looks out for a place suitable for this purpose. When she has found this place, she ejects an egg covered with a viscous liquid, which causes it to adhere to the body upon which it falls. Very soon she lays a second egg by the side of the first, then a third by the side of the second, and so on. She very rarely piles them up on each other. The laying lasts about three days; the number of eggs is from 300 to 700 for each female. These eggs are generally tentacular and flattened towards the centre. At the moment at which they are laid they are of a bright yellow. In a week they become brown. The colour changes then to a reddish grey; lastly it becomes of a slaty grey, remaining this colour during the autumn, winter, and a great part of the spring. Then as the temperature rises, the colour of the eggs passes successively through bluish, violet, ashy, and yellowish shades. And, lastly, they become more and more whitish every day as the hatching time approaches.

If looked at closely, one remarks a black spot and a brownish crescent extending along the circumference. The black spot is the head of the worm, which closely touches the shell; the crescent is the body, which is already covered with little hairs. When it leaves the egg, the silkworm gnaws through the shell on its side, never on its flat surface. When the opening is large enough, it breaks out through it, head foremost, and immediately fixes a thread of silk to any object it can reach, no doubt in order to prevent itself from falling. Sometimes the opening is too small to allow of the head passing out, and the larva is forced to come out backwards, that is to say, tail foremost. At times, not being able to get its head free, the poor animal very soon dies of fatigue and hunger.

We will now give a summary of the rearing of the silkworm, that is to say, of the attention which must be paid to this insect that it may construct its cocoon advantageously. We will call to our aid in this very rapid summary the works or notices of MM. Robinet, Guérin-Méneville, Eugène Robert, and Louis Leclerc, and we must not forget the excellent and classical Dandolo.[63]

[63] "L'Art d'élever des Vers à Soie, par le Comte de Dandolo, traduit par Philibert Fontaneilles." In 8vo. Lyons, 1825. Robinet, "Manuel de l'Education des Vers à Soie." In 8vo. Paris. Guérin-Méneville et Eugène Robert, "Manuel de l'Education des Vers à Soie." In 18mo. Paris. Louis Leclerc, "Petite Magnanerie." In 18mo. Paris.

When it is desired to rear silkworms--_magnans_, as they were called in old French, and as they are still called in the patois of Languedoc--the first thing to do is to obtain good eggs, good _grain_, to use the technical word, and then to choose suitable premises. The essential, the fundamental point, in the rearing, is to possess premises in which the air is easily renewed. The worms should have as much air as possible given to them without ever being allowed to be chilled. There is no better means of attaining this end than by keeping a constant open fire in a room, and by letting air into the room from another chamber which separates it from the open air. One has, in this way, the best workroom for a small rearing.

In the workshop are arranged racks, by the aid of which are placed, at the distance of 50 centimètres from each other, frames made of reeds. These frames, or _canisses_, as they are called in the Cévennes, may be from 1 mètre to 1-3/4 mètres in breadth. They should be placed in such a manner that one can easily pass round them to place and remove the worms, and to distribute their leaves to them uniformly. They should be protected by a small border of a few centimètres in height, to prevent the worms from falling. And lastly, they should be covered at the bottom with large sheets of paper. (PLATE VI.) A provident silkworm-rearer has always at his disposal a cellar or cool room, so as to be able to stow away his leaves as soon as they are brought in from the country.

What we have just said applies especially to a small rearing. In large establishments, or even those of second-rate importance, everything is in advance of this, and mathematically regulated: aspect and arrangement of rooms, furniture of these rooms, warming, ventilation, &c. So, for a rearing house for 300 grammes of eggs, the building should be constructed in such a manner that its front and back look east and west, to avoid any inequality in the heat derived from the sun. It ought to consist of a ground-floor, a very lofty first-floor, and of a rather low roof. The ground-floor comprises the chamber of incubation, the store-room for leaves, and the air-chamber with the grate intended for warmth and ventilation. The first-floor constitutes the rearing-room properly so called.

But let us leave these grand industrial establishments, to return to our rearing houses on a small scale, such as are found among the peasants of the Cévennes. They generally receive the silkworms' eggs before the end of the winter. In order to preserve them till the hatching season, they are placed in thin layers, in a piece of folded woollen stuff, which must be hung up in a cool, but not a damp place, exposed to the north. As soon as the buds of the mulberry tree begin to be partially open, they proceed to the incubation of the eggs. They are spread out on sheets of paper, in very thin layers, placed on a table in a room having a southern aspect, and left thus during three or four days, taking care to prevent the rays of the sun from touching them. It is necessary also, from time to time, to open the windows. After three or four days, the fire is lighted, taking care not to have more heat than 13° Centigrade round about the table which supports the eggs, and which should be placed as far as possible from the fire. Each day the room is warmed a little more, in such a way that the temperature is raised 1° to 2° a day, until 25° Centigrade of heat have been attained, at which temperature it is to be maintained when the eggs have reached the last stage, and till the hatching is terminated. On the first day few worms are hatched; but the hatching of the second day is very abundant, as also that of the third. Of these newly-born worms two divisions are made, separated by an interval of twenty-four hours. The worms which are born afterwards are thrown away, unless they are so abundant that they can be made a third batch of, which is to be mixed up with the second at the period of the moult.