CHAPTER VII.

DESCRIPTION OF THE SILK-WORM, &c.

Preliminary observations--The silk-worm--Various changes of the silk-worm--Its superiority above other worms--Beautiful verses on the May-fly, illustrative of the shortness of human life--Transformations of the silk-worm--Its small desire of locomotion--First sickness of the worm--Manner of casting its Exuviæ--Sometimes cannot be fully accomplished--Consequent death of the insect--Second, third, and fourth sickness of the worm--Its disgust for food--Material of which silk is formed--Mode of its secretion--Manner of unwinding the filaments--Floss-silk--Cocoon--Its imperviousness to moisture--Effect of the filaments breaking during the formation of the cocoon--Mr. Robinet’s curious calculation on the movements made by a silk-worm in the formation of a cocoon--Cowper’s beautiful lines on the silk-worm--Periods in which its various progressions are effected in different climates--Effects of sudden transitions from heat to cold--The worm’s appetite sharpened by increased temperature--Shortens its existence--Various experiments in artificial heating--Modes of artificial heating--Singular estimate of Count Dandolo--Astonishing increase of the worm--Its brief existence in the moth state--Formation of silk--The silken filament formed in the worm before its expulsion--Erroneous opinions entertained by writers on this subject--The silk-worm’s Will.

It can never be too strongly impressed upon a mind anxious for the acquisition of knowledge, that the commonest things by which we are surrounded are deserving of minute and careful attention. The most profound investigations of Philosophy are necessarily connected with the ordinary circumstances of our being, and of the world in which our every-day life is spent. With regard to our own existence, the pulsation of the heart, the act of respiration, the voluntary movement of our limbs, the condition of sleep, are among the most ordinary operations of our nature; and yet how long were the wisest of men struggling with dark and bewildering speculations before they could offer anything like a satisfactory solution of these phenomena, and how far are we still from an accurate and complete knowledge of them! The science of Meteorology, which attempts to explain to us the philosophy of matters constantly before our eyes, as dew, mist, and rain, is dependent for its illustrations upon a knowledge of the most complicated facts, such as the influence of heat and electricity upon the air; and this knowledge is at present so imperfect, that even these common occurrences of the weather, which men have been observing and reasoning upon for ages, are by no means satisfactorily explained, or reduced to the precision that every science should aspire to. Yet, however difficult it may be entirely to comprehend the phenomena we daily witness, everything in nature is full of instruction. Thus the humblest flower of the field, although, to one whose curiosity has not been excited, and whose understanding has, therefore, remained uninformed, it may appear worthless and contemptible, is valuable to the botanist, not only with regard to its place in the arrangement of this portion of the Creator’s works, but as it leads his mind forward to the consideration of those beautiful provisions for the support of vegetable life, which it is the part of the physiologist to study and admire[128].

[128] “Insect Architecture,” vol. i. p. 9. London: Charles Knight & Co., Ludgate St. 1845.

This train of reasoning is peculiarly applicable to the economy of insects. They constitute a very large and interesting part of the animal kingdom. They are everywhere about us. The _spider_ weaves his curious web in our houses; the _caterpillar_ constructs his silken cell in our gardens; the _wasp_ that hovers over our food has a nest not far removed from us, which she has assisted to build with the nicest art; the _beetle_ that crawls across our path is also an ingenious and laborious mechanic, and has some curious instincts to exhibit to those who will feel an interest in watching his movements; and the _moth_ that eats into our clothes has something to plead for our pity, for he came, like us, naked into the world, and he has destroyed our garments, not in malice or wantonness, but that he may clothe himself with the same wool which we have stripped from the sheep. An observation of the habits of these little creatures is full of valuable lessons, which the abundance of the examples has no tendency to diminish. The more such observations are multiplied, the more we are led forward to the freshest and the most delightful parts of knowledge; the more do we learn to estimate rightly the extraordinary provisions and most abundant resources of a creative Providence; and the better do we appreciate our own relations with all the infinite varieties of Nature, and our dependence, in common with the _ephemeron_ that flutters its little hour in the summer sun, upon that Being in whose scheme of existence the humblest as well as the highest creature has its destined purposes. “If you speak of a _stone_,” says St. Basil, “if you speak of a _fly_, a _gnat_, or a _bee_, your conversation will be a sort of demonstration of his power whose hand formed them, for the wisdom of the workman is commonly perceived in that which is of little size. He who has stretched out the Heavens, and dug up the bottom of the sea, is also He who has pierced a passage through the sting of the bee for the ejection of its poison.”

If it be granted that making discoveries is one of the most satisfactory of human pleasures, then we may without hesitation affirm, that the study of insects is one of the most delightful branches of natural history, for it affords peculiar facilities for its pursuit. These facilities are found in the almost inexhaustible variety which insects present to the curious observer.

There is, perhaps, no situation in which the lover of nature and the observer of animal life may not find opportunities for increasing his store of facts. It is told of a state prisoner under a cruel and rigorous despotism, that when he was excluded from all commerce with mankind, and was shut out from books, he took an interest and found consolation in the visits of a _spider_; and there is no improbability in the story. The operations of that persecuted creature are among the most extraordinary exhibitions of mechanical ingenuity; and a daily watching of the workings of its instinct would beget admiration in a rightly constituted mind. The poor prisoner had abundant leisure for the speculations in which the spider’s web would enchain his understanding. We have all of us, at one period or other of our lives, been struck with some singular evidence of contrivance in the economy of insects, which we have seen with our own eyes. Want of leisure, and probably want of knowledge, have prevented us from following up the curiosity which for a moment was excited. And yet some such accident has made men Naturalists, in the highest meaning of the term. Bonnet, evidently speaking of himself, says, “I knew a naturalist, who, when he was seventeen years of age, having heard of the operations of the ant-lion, began by doubting them. He had no rest till he had examined into them; and he verified them, he admired them, he discovered new facts, and soon became the disciple and the friend of the Pliny of France[129]” (Réaumur). It is not the happy fortune of many to be able to devote themselves exclusively to the study of nature, unquestionably the most fascinating of human employments; but almost every one may acquire sufficient knowledge to be able to derive a high gratification from beholding the more common operations of animal life. His materials for contemplation are always before him.

[129] Contemplation de la Nature, part ii. ch. 42.

The silk-worm is a species of caterpillar which, like all other insects of the same class, undergoes a variety of changes during the short period of its life; assuming, in each of three successive transformations, _a form wholly dissimilar to that with which it was previously invested_.

Among the great variety of caterpillars, the descriptions of which are to be found in the records of natural history, the silk-worm occupies a place far above the rest. Not only is our attention called to the examination of its various transformations, by the desire of satisfying our curiosity as entomologists, but our artificial wants incite us likewise to the study of its nature and habits, that we may best and most profitably apply its instinctive industry to our own advantage.

It has been well observed by Pullein, a writer on this subject, that “there is scarcely anything among the various wonders which the animal creation affords, more admirable than the variety of changes which the silk-worm undergoes;” but the curious texture of that silken covering with which it surrounds itself when it arrives at the perfection of its animal life, vastly surpasses what is made by other animals of this class. All the caterpillar kind do, indeed, pass through changes like those of the silk-worm, and the beauty of many in their butterfly state greatly exceeds it; but the covering which they put on before this mutation is poor and mean, when compared to that golden tissue in which the silk-worm wraps itself. They, indeed, come forth in a variety of colors, their wings bedropped with gold and scarlet, yet are they but the beings of a summer’s day; both their life and beauty quickly vanish, and they leave no remembrance after them; but the silk-worm leaves behind it such beautiful, such beneficial monuments, as at once to record both the wisdom of their Creator and his bounty to man.”

We may without impropriety, here introduce the following truly beautiful comparison of the shortness of human life, as well as in illustration of this part of our subject, as evidenced in the May-fly.

“The angler’s May-fly, the most short-lived in its perfect state of any of the insect race, emerges from the water, where it passes its _aurelia_ state, about six in the evening, and dies about eleven at night.”--WHITE’S _Selborne_.

The sun of the eve was warm and bright When the May-fly burst his shell, And he wanton’d awhile in that fair light O’er the river’s gentle swell; And the deepening tints of the crimson sky Still gleam’d on the wing of the glad May-fly.

The colors of sunset pass’d away, The crimson and yellow green, And the evening-star’s first twinkling ray In the waveless stream was seen; Till the deep repose of the stillest night Was hushing about his giddy flight.

The noon of the night is nearly come-- There’s a crescent in the sky;-- The silence still hears the myriad hum Of the insect revelry. The hum has ceas’d--the quiet wave Is now the sportive May-fly’s grave.

Oh! thine was a blessed lot--to spring In thy lustihood to air, And sail about, on untiring wing, Through a world most rich and fair, To drop at once in thy watery bed, Like a leaf that the willow branch has shed.

And who shall say that his thread of years Is a life more blest than thine! Has his feverish dream of doubts and fears Such joys as those which shine In the constant pleasures of thy way, Most happy child of the happy May?

For thou wert born when the earth was clad With her robe of buds and flowers, And didst float about with a soul as glad As a bird in the sunny showers; And the hour of thy death had a sweet repose, Like a melody, sweetest at its close.

Nor too brief the date of thy cheerful race-- ’Tis its use that measures time-- And the mighty Spirit that fills all space With His life and His will sublime, May see that the May-fly and the Man Each flutter out the same small span;

And the fly that is born with the sinking sun, To die ere the midnight hour, May have deeper joy, ere his course be run, Than man in his pride and power; And the insect’s minutes be spared the fears And the anxious doubts of our threescore years.

The years and the minutes are as one-- The fly drops in his twilight mirth, And the man, when his long day’s work is done, Crawls to the self-same earth. Great Father of each! may _our_ mortal day Be the prelude to an endless May[130]!

[130] “See,” exclaims Linnæus, “the large, elegant painted wings of the butterfly, four in number, covered with delicate feathery scales! With these it sustains itself in the air a whole day, rivalling the flight of birds and the brilliancy of the peacock. Consider this insect through the wonderful progress of its life,--how different is the first period of its being from the second, and both from the parent insect! Its changes are an inexplicable enigma to us: we see a green caterpillar, furnished with sixteen feet, feeding upon the leaves of a plant; this is changed into a chrysalis, smooth, of golden lustre, hanging suspended to a fixed point, without feet, and subsisting without food; this insect again undergoes another transformation, acquires wings, and six feet, and becomes a gay butterfly, sporting in the air, and living by suction upon the honey of plants. What has Nature produced more worthy of our admiration than such an animal coming upon the stage of the world, and playing its part there under so many different masks?” The ancients were so struck with the transformations of the butterfly, and its revival from a seeming temporary death, as to have considered it an emblem of the soul, the Greek word _psyche_ signifying both the soul and a butterfly; and it is for this reason that we find the butterfly introduced into their allegorical sculptures as an emblem of immortality. Trifling, therefore, and perhaps contemptible, as to the unthinking may seem the study of a butterfly, yet when we consider the art and mechanism displayed in so minute a structure,--the fluids circulating in vessels so small as almost to escape the sight--the beauty of the wings and covering--and the manner in which each part is adapted for its peculiar functions,--we cannot but be struck with wonder and admiration, and allow, with Paley, that “the production of beauty was as much in the Creator’s mind in painting a butterfly as in giving symmetry to the human form.”

Silk-worms proceed from eggs which are deposited during the summer by a grayish kind of moth, of the genus palæna. These eggs are about equal in size to a grain of mustard seed: their color when first laid is yellow; but in three or four days after, they acquire a bluish cast. In temperate climates, and by using proper precautions, these eggs may be preserved during the winter and spring, without risk of premature hatching. The period of their animation may be accelerated or retarded by artificial means, so as to agree with the time when the natural food of the insect shall appear in ample abundance for its support.

All the curious changes and labors which accompany and characterize the life of the silk-worm are performed within the space of a very few weeks. This period varies, indeed, according to the climate or temperature in which its life is passed; all its vital functions being quickened, and their duration proportionally abridged, by warmth. With this sole variance, its progressions are alike in all climates, and the same mutations accompany its course.

The three successive states of being put on by this insect are, that of the worm or caterpillar, of the chrysalis or aurelia, and moth. In addition to these more decided transformations, the progress of the silk-worm in its _caterpillar state_ is marked by _five distinct stages of being_.

When first hatched, it appears as a small black worm about a quarter of an inch in length. Its first indication of animation is the desire which it evinces for obtaining food, in search of which, if not immediately supplied, it will exhibit more power of locomotion than characterizes it at any other period. So small is the desire of change on the part of these insects, that of the generality it may be said, their own spontaneous will seldom leads them to travel over a greater space than three feet throughout the whole duration of their lives. Even when hungry, the worm still clings to the skeleton of the leaf from which its nourishment was last derived. If, by the continued cravings of its appetite, it should be at length incited to the effort necessary for changing its position, it will sometimes wander as far as the edge of the tray wherein it is confined, and some few have been found sufficiently adventurous to cling to its rim; but the smell of fresh leaves will instantly allure them back. It would add incalculably to the labors and cares of their attendants, if silk-worms were endowed with a more rambling disposition. So useful is this peculiarity of their nature, that one is irresistibly tempted to consider it the result of design, and a part of that beautiful system of the fitness of things, which the student of natural history has so many opportunities of contemplating with delight and admiration.

In about eight days from its being hatched, its head becomes perceptibly larger, and the worm is attacked by its first sickness. This lasts for three days; during which time it refuses food, and remains motionless as in a kind of lethargy. Some have thought this to be sleep, but the fatal termination which so frequently attends these sicknesses seems to afford a denial to this hypothesis. The silk-worm increases its size so considerably, and in so short a space of time,--its weight being multiplied many thousand fold in the course of one month,--that if only one skin had been assigned to it, which should serve for its whole caterpillar state, it would with difficulty have distended itself sufficiently to keep pace with the insect’s growth. The economy of nature has therefore admirably provided the embryos of other skins, destined to be successively called into use; and this sickness of the worm, and its disinclination for food, may very probably be occasioned by the pressure of the skin, now become too small for the body which it encases.

At the end of the third day from its first refusal of food, the animal appears, on that account, much wasted in its bodily frame; a circumstance which materially assists in the painful operation of casting its skin: this it now proceeds to accomplish. To facilitate this moulting, a sort of humor is thrown off by the worm, which, spreading between its body and the skin about to be abandoned, lubricates their surfaces, and causes them to separate the more readily. The insect also emits from its body silken traces, which, adhering to the spot where it rests, serves to confine the skin to its then existing position. These preliminary steps seem to call for some considerable exertion, as after them the worm remains quiet for a short space of time, to recover from its fatigue. It then proceeds, by rubbing its head among the leafy fibres surrounding it, to disencumber itself of the scaly covering. Its next effort is to break through the skin nearest to the head, which, as it is there the smallest, calls for the greatest exertion; and no sooner is this accomplished and the two front legs are disengaged, than the remainder of the body is quickly drawn forth, the skin being still fastened to the spot in the manner already described.

This moulting is so complete, _that not only is the whole covering of the body cast off, but that of the feet, the entire skull, and even the jaws, including the teeth_. These several parts may be discerned by the unassisted eye; but become very apparent when viewed through a magnifying lens of moderate power.

In two or three minutes from the beginning of its efforts the worm is wholly freed, and again puts on the appearance of health and vigor; feeding with recruited appetite upon its leafy banquet. It sometimes happens that the outer skin refuses to detach itself wholly, but breaks and leaves an annular portion adhering to the extremity of its body, from which all the struggles of the insect cannot wholly disengage it. The pressure thus occasioned induces swelling and inflammation in other parts of the body; and, after efforts of greater or less duration, death generally terminates its sufferings.

Worms newly freed from their exuviæ are easily distinguished from others by the pale color and wrinkled appearance of their new skin. This latter quality, however, soon disappears, through the repletion and growth of the insect, which continues to feed during five days. At this time its length will be increased to half an inch; when it is attacked by a second sickness, followed by a second moulting, the manner of performing which is exactly similar to the former. Its appetite then again returns, and is indulged during other five days, in the course of which time its length increases to three quarters of an inch: it then undergoes its third sickness and moulting. These being past in all respects like the former, and five more days of feeding having followed, it is seized by its fourth sickness, and casts its skin for the last time in the caterpillar state. The worm is now about one and a half or two inches long. This last change being finished, the worm devours its food most voraciously, and increases rapidly in size during ten days.

The silk-worm has now attained to its full growth, and is a slender caterpillar from two and a half to three inches in length (See Figure 1. Plate III.). The peculiarities of its structure may be better examined now than in its earlier stages. It can readily be seen that the worm has twelve membranous rings round its body, parallel to each other; and which, answering to the movements of the animal, mutually contract and elongate. It has sixteen legs, in pairs: six in front, which are covered with a sort of shell or scale, and are placed under the three first rings, and cannot be either sensibly lengthened, or their position altered. The other ten legs are called holders: these are membranous, flexible, and attached to the body under the rings, being furnished with little hooks, which assist the insect in climbing. The skull is inclosed in a scaly substance, similar to the covering of the first six legs. The jaws are indented or serrated like the teeth of a saw, and their strength is great considering the size of the insect. Its mouth is peculiar, having a vertical instead of an horizontal aperture; and the worm is furnished with eighteen breathing holes, placed at equal distances down the body, nine on each side. Each of these holes is supposed to be the termination of a particular organ of respiration. On either side of the head, near to the mouth, seven small eyes may be discerned. The two broad appearances higher upon the head, which are frequently mistaken for eyes, are bones of the skull. The two apertures through which the worm draws its silken filament are placed just beneath the jaw, and close to each other; these being exceedingly minute.

At the period above-mentioned the desire of the worm for food begins to abate: the first symptom of this is the appearance of the leaves nibbled into small portions and wasted. It soon after entirely ceases even to touch the leaves; appears restless and uneasy; erects it head; and moves about from side to side, with a circular motion, in quest of a place wherein it can commence its labor of spinning. Its color is now light green, with some mixture of a darker hue. In twenty-four hours from the time of its abstaining from food, the material for forming its silk will be digested in its reservoirs; its green color will disappear; its body will have acquired a degree of glossiness, and have become partially transparent towards its neck. Before the worm is quite prepared to spin, its body will have acquired greater firmness, and be in a trifling measure lessened in size.

“The substance,” says Mr. Porter, “of which the silk is composed, _is secreted in the form of a fine yellow transparent gum in two separate vessels of slender dimensions, wound, as it were, on two spindles in the stomach; and if unfolded, these vessels would be about ten inches in length_[131].” This statement is proved to be erroneous, as the reader will perceive, at the conclusion of this chapter.

[131] Porter’s “Treatise on the Silk Manufacture,” p. 111.

When the worm has fixed upon some angle, or hollow place, whose dimensions agree with the size of its intended silken ball or cocoon, it begins its labor by throwing forth thin and irregular threads, see Figure 2. Plate III., which are intended to support its future dwelling.

During the first day, the insect forms upon these a loose structure of an oval shape, which is called floss silk, and within which covering, in the three following days, it forms the firm and consistent yellow ball; the laborer, of course, always remaining on the inside of the sphere which it is forming[132].

[132] If at this time any of the threads intended for the support of the cocoon should be broken, the worm will find, in the progress of its work, that the ball, not being properly poised, becomes unsteady, so that the insect is unable properly to go forward with its labors. Under these circumstances the worm pierces and altogether quits the unfinished cocoon, and throws out its remaining threads at random wherever it passes; by which means the silk is wholly lost, and the worm, finding no place wherein to prepare for its change, dies without having effected it. It may sometimes happen, but such a thing is of unfrequent occurrence, that the preparatory threads before mentioned are broken by another worm working in the neighborhood, when the same unsatisfactory result will be experienced.--_Obs. on the Culture of Silk_, _by_ A. STEPHENSON.

The silken filament, which when drawn out appears to be one thread, is composed of two fibres, unwound through the two orifices before described; and these fibres are brought together by means of two hooks, placed within the silk-worm’s mouth for the purpose. The worm rests on its lower extremity throughout the unwinding operation, and employs its mouth and front legs in the task of directing and uniting the two filaments. The filament is not wound in regular concentric circles round the interior surface of the ball, but in spots, going backwards and forwards with a sort of wavy motion. This apparently irregular manner of proceeding is plainly perceptible when the silk is being reeled off the ball; which does not make more than one or two entire revolutions while ten or twelve yards of silk are being transferred to the reel[133].

[133] Mr. Robinet, of Paris, made the following curious calculation on the movements a silk-worm must make in forming a cocoon supposed to contain a thread of 1500 metres. It is known, says Mr. Robinet, that the silk-worm, in forming his cocoon, does not _spin_ the silken filament in concentric circles round the interior surface of the ball, but in a zigzag manner. This it effects by the motions of its head. Now if each one of these motions gives half a centimetre of the silken filament; it follows that the worm must make 300,000 motions of its head to form it; and if the labor requires 72 hours in the performance, the creature makes 100,000 motions every 24 hours, 4,166 per hour, 69 per minute, and a little more than one in a second!

At the end of the third or fourth day, the worm will have completed its task; and we have then a _silk cocoon_ (See Figure 3. Plate III.), with the worm imprisoned in its centre; the cocoon being from an inch to an inch and a half long, and of a yellow or orange color.

When the insect has finished its labor of unwinding, it smears the entire internal surface of the cocoon with a peculiar kind of gum, very similar in its nature to the matter which forms the silk itself; and this is no doubt designed as a shield against rain or the humidity of the atmosphere, for the chrysalis in its natural state; when of course it would be subject to all varieties of weather. The silken filament of which the ball is made up, is likewise accompanied, throughout its entire length, by a portion of gum, which serves to give firmness and consistency to its texture; and assists in rendering the dwelling of the chrysalis impervious to moisture. This office it performs so well, that when, for the purpose of reeling the silk with greater facility, the balls are thrown into basins of hot water, they swim on the top with all the buoyancy of bladders; nor, unless the ball be imperfectly formed, does the water penetrate within until the silk is nearly all unwound. In figure 4, plate III., the cocoons are drawn two-thirds of the usual size, and are shown with part of the outward floss silk removed.

The continual emission of the silken material during the formation of its envelope, together with its natural evaporation, uncompensated by food, causes the worm gradually to contract in bulk; it becomes wrinkled, and the rings of its body approach nearer to each other and appear more decidedly marked. When the ball is finished, the insect rests awhile from its toil, and then throws off its caterpillar garb. If the cocoon be now opened, its inhabitant will appear in the form of a chrysalis or aurelia, in shape somewhat resembling a kidney-bean (See Figure 5. plate III.), but pointed at one end, having a smooth brown skin. Its former covering, so dissimilar to the one now assumed, will be found lying beside it.

The account which has been given of the progressions of the silk-worm shows, that, in its various modifications, _the animal organization of the insect has been always tending towards its simplification_. Count Dandolo, writing upon this subject, observes, “Thus the caterpillar is in the first instance composed of animal, silky, and excremental particles; this forms the state of the _growing caterpillar_: in the next stage it is composed of animal and silky particles; it is then the _mature caterpillar_: and lastly, it is reduced to the animal particles alone; and is termed in this state the _chrysalis_. The poet Cowper, in the following lines, beautifully illustrates this subject:

The beams of April, ere it goes, A worm, scarce visible, disclose; All winter long content to dwell The tenant of his native shell. The same prolific season gives The sustenance by which he lives, The mulberry leaf, a simple store, That serves him--till he needs no more! For, his dimensions once complete, Thenceforth none ever sees him eat; Though till his growing time be past Scarce ever is he seen to fast. That hour arrived, his work begins. He spins and weaves, and weaves and spins; Till circle upon circle, wound Careless around him and around, Conceals him with a veil though slight, Impervious to the keenest sight. Thus self-inclosed, as in a cask, At length he finishes his task: And, though a worm when he was lost, Or caterpillar at the most, When next we see him, wings he wears, And in papilio pomp appears; Becomes oviparous; supplies With future worms and future flies The next ensuing year--and dies! Well were it for the world if all Who creep about this earthly ball, Though shorter-lived than most he be, Were useful in their kind as he.

It has been already noticed that the progressions of the insects are accelerated by an increase of temperature; and some variation will equally be experienced where different modes of treatment are followed; and, in particular, where different periods of the year are chosen in which to produce and rear the worm. Malpighius, in his “Anatomy of the Silk-worm,” says, that worms which he hatched in May were eleven days old ere they were attacked by their first sickness; others hatched in July were ten days, and those brought forth in August nine days, before they refused their food, preparatory to their first moulting. Eight days appear to be the most usual term for their first attack; and by his judicious treatment count Dandolo shortened even this term by two days. In Europe, except where recourse is had to artificial aid, the term of the caterpillar state is usually that which has been already mentioned.

Sudden transitions from cold to heat, or vice versa, are highly injurious to the silk-worm; but it can bear a very high degree of heat, if uniformly maintained, without sustaining injury. Count Dandolo observed, that “the greater the degree of heat in which it is reared, the more acute are its wants, the more rapid its pleasures, and the shorter its existence.” Monsieur Boissier de Sauvagues made many experiments on this point. One year, when by the early appearance of the mulberry leaves, which were developed by the end of April, he was forced to hurry forward the operations of his filature, he raised the heat of the apartment in which the newly-hatched worms were placed to 100°; gradually diminishing this during their first and second ages to 95°. In consequence of the animal excitement thus induced, there elapsed only nine days between the hatching and the second moulting inclusively. It was the general opinion of those cultivators who witnessed the experiment, that the insects would not be able to exist in so intensely heated an atmosphere. The walls of the apartment, and the wicker hurdles on which the worms were placed, could scarcely be touched from the great heat, and yet all the changes and progressions went forward perfectly well, and a most abundant crop of silk was the result.

The same gentleman, on a subsequent occasion, exposed his brood to the temperature of 93° to 95° during their first age; of 89° to 91° in the second age; and remarked that the attendant circumstances were the same as in his former experiment, the changes of the worm being performed in the same space of time; whence he came to the conclusion, that it is not practicable to accelerate their progress beyond a certain point by any superadditions of heat. In both of these experiments the quantity of food consumed, was as great as is usually given during the longer period employed in the common manner of rearing. After the second moulting had taken place in the last experiment, the temperature was lowered to 82°; and it is remarkable that the worms occupied only five days in completing their third and fourth changes, although others which had been accustomed to this lower degree from their birth occupied seven or eight days for each of these moultings. It would therefore seem that the constitution of the insects can be affected, and an impetus given to their functions at the period of their first animation, which accompanies them through their after stages. So far from this forcing system proving injurious to the health of silk-worms, M. de Sauvagues found that his broods were unusually healthy; and that while the labors of cultivation were abridged in their duration, much of the attendant anxiety was removed.

Like other caterpillars, the silk-worm is not a warm-blooded animal, and its temperature is therefore always equal to that of the atmosphere in which it is placed. In the silk-producing countries, where modes of artificial heating have not been studied practically and scientifically, the difficulty and expense that must attend the prosecution of this heating system, form abundant reasons why it cannot be generally adopted. The great susceptibility of the insect to atmospheric influences would also in a great degree render unsuitable the more common arrangements for the purpose. The plan of warming apartments by means of stoves, in its passage through which the air becomes highly heated before it mixes with and raises the general temperature of the air in the chamber, is liable to this inconvenience,--that the portion so introduced, having its vital property impaired by the burning heat through which it has passed, injures, proportionably, the respirable quality of the whole atmosphere; an effect which is easily perceptible by those who inhale it. A better plan of heating has lately been suggested, and is rapidly coming into practice, viz., of warming buildings by a current of hot water (an American invention), which is, by a very simple process, kept constantly flowing in close channels through the apartment, where it continually gives off its heat by radiation; and the degree of this being far below the point which is injurious to the vital quality of air, the evil before alluded to is avoided. If the expense of fuel be not too great, as compared with that of the labor which would be saved by this invention, the adoption in silk countries of such a mode of raising and regulating the temperature might, probably, prove advantageous.

The silk-worm remains in the form of a chrysalis, for periods which, according to the climate or the temperature wherein it may be placed, vary from fifteen to thirty days. In India, the time is much shorter (See Chapter VIII.); in Spain and Italy, eighteen to twenty days. In France three weeks; and in the climate of England, when unaccelerated by artificial means, thirty days will elapse from the time the insect begins to spin until it emerges in its last and perfect form. It then throws off the shroud which had confined it in _seeming lifelessness_, and appears as a large moth of a grayish-white color, furnished with four wings, two eyes, and two black horns or antlers which present a feathery appearance (See Figure 6. plate III.).

If left until this period within the cocoon, the moth takes immediate measures for its extrication: ejecting from its mouth a liquor with which it moistens and lessens the adhesiveness of the gum wherewith it had lined the interior surface of its dwelling, and the insect is enabled, by frequent motions of its head, to loosen, without breaking, the texture of the ball; then using its hooked feet, it pushes aside the filaments and makes a passage for itself into light and freedom. It is erroneously said that the moth recovers its liberty by gnawing the silken threads; but it is found, on the contrary, that if carefully unwound, their continuity is by this means rarely broken.

One of the most remarkable circumstances connected with the natural history of silk-worms, is the degree in which their bulk and weight is increased, and the limited time wherein that increase is attained. Count Dandolo, who appears to have neglected nothing that could tend to the right understanding of the subject, and to the consequent improvement of the processes employed, had patience enough to count and weigh many hundred thousand eggs, as well as follow out to the ultimate result his inquiries respecting their produce. He found that on an average sixty-eight sound silk-worm’s eggs weighed one grain. One ounce[134], therefore, comprised, 39,168 eggs. But one twelfth part of this weight evaporates previous to hatching, and the shells are equal to one fifth more. If, therefore, from one ounce, composed of 576 grains, 48 grains be deducted for evaporation, and 115 for the shells, 413 grains will remain equal to the weight of 39,168 young worms; and, at this rate, 54,526 of the insects when newly hatched, are required to make up the ounce. After the first casting of the skin, 3840 worms are found to have this weight, so that the bulk and weight of the insects have in a few days been multiplied more than _fourteen times_. After the second change 610 worms weigh an ounce, their weight being increased in the intermediate time six fold. In the week passed between the second and third ages, the number of insects required to make up the same weight, decreases from 610 to 144, their weight being therefore more than quadrupled. During the fourth age, a similar rate of increase is maintained: thirty-five worms now weigh an ounce. The fifth age of the caterpillar comprises nearly a third part of its brief existence, and has been described, by an enthusiastic writer on the subject, as the happiest period of its life, during which it rapidly increases in size, preparing and secreting the material it is about to spin. When the silk-worms are fully grown, and have arrived at their period of finally rejecting food, six of them make up the weight of an ounce. They have, therefore, since their last change, again added to their weight _six fold_.

[134] This ounce contains 576 grains; 8.5325 of these grains equal seven grains troy. One ounce avoirdupoise is therefore equal to about 533 grains, and between 11-12 and 11-13 ounce avoirdupoise equals one of the above ounces.

It is thus seen that, in a few short weeks, the insect has multiplied its weight more than _nine thousand fold_! From this period, and during the whole of its two succeeding states of being, the worm imbibes no nourishment, and gradually diminishes in weight; being supported by its own substance, and appearing to find sufficient occupation in forming its silken web, and providing successors for our service, without indulging that grosser appetite which forms the beginning and the end of their desires during their caterpillar existence.

The moth enjoys its liberty for only a very brief space. Its first employment is to seek its mate; after which the female deposits her eggs; and both in the course of two or three days after, end their being.

Formation of Silk. By M. H. Straus, of Durckheim.--“It is generally admitted by naturalists that the thread of the caterpillar is produced by a simple emission of liquid matter through the orifice of the spinner, and that it acquires solidity at once from the drying influence of the air. It was easy to entertain such an hypothesis, for nothing is more simple than the formation of a very fine thread by such a process. But a little reflection will soon show us, even _à priori_, that it is not possible; for how can we comprehend that so fine a fibre, liquid at the instant of its issue from the aperture, should _instantly_ acquire such a consistence as to bear the weight of the animal suspended by it, and at the same time that it is rapidly produced? Though the fluid, holding the silk in solution, should be quickly volatilised, it must still be a matter of conjecture, how the animal suspended by this thread could be able to arrest its issue, holding on only by the thread itself, for it cannot pinch the thread, seeing that it is only in a liquid state inside, and the thread cannot be glued to the edge of the opening, as its rapid adhesion would prevent its issue while the animal is spinning. A little examination would satisfy us that silk cannot be produced in this manner, but that it is secreted in the _form of silk_ in the silk vessels, and that the spinning apparatus _only winds it_. The thread is produced in the slender posterior part of the vessel, the inflated portion of which consists of the reservoir of ready formed silk, where it is found in the form of a skein; each thread being rolled up so as to occupy in the silk-worm (_Bombex mori_) a space of only about a sixth part of the real length of the skein. The fact is shown by the following experiment I made for the purpose of ascertaining whether the silk is formed in the body of the caterpillars.

‘_Take one of the animals when about to form its cocoon, clean it in common vinegar, in which it may remain from four to six hours, open it on the back and extract the silk vessels, there being one on each side of the alimentary canal. Take them up by the hinder end, just where they begin to swell (further back the silk is not solid enough), and draw them out. The membrane forming the vessel is easily torn open, and the contents expand to six or seven times its original length. The skein having attained its full length by the letting out of its gathers, we obtain a cord perfectly equal in size throughout, except at the end, where it is attenuated._ This cord resembles a large horse-hair, and constitutes what fishermen call “_Florence hair_.” I ought to add that in simply drawing out the silk vessel, the Florence hair is found enveloped in a golden yellow gummy matter, forming the glutinous portion by which the worm fastens its thread. This must be got rid of by drawing the cord through the fold formed on the inside of the joint of the left fore finger, converted into a canal by applying to it the end of the thumb. The glutinous substance and the membranes being thus separated, we have the _naked hair_. In this state, before the silk becomes dry and hard, not only will it be indefinitely divided longitudinally, which proves its fibrous structure, but in trying to split it by drawing it transversely, _the little filaments of silk which form it are perfectly separated_, making _a bundle of extremely fine fibrils_.’

We cannot better conclude this interesting portion of our subject, than by quoting the following beautiful lines by Miss H. F. Gould:--

THE SILK-WORM’S WILL.

On a plain rush hurdle a silk-worm lay, When a proud young princess came that way: The haughty child of a human king, Threw a sidelong glance at the humble thing, That took, with a silent gratitude, From the mulberry leaf, her simple food; And shrunk, half scorn and half disgust, Away from her sister child of dust-- Declaring she never yet could see Why a reptile form like this should be, And that she was not made with nerves so firm, As calmly to stand by a “crawling worm!”

With mute forbearance the silk-worm took The taunting words, and the spurning look: Alike a stranger to self and pride, She’d no disquiet from aught beside-- And lived of a meekness and peace possessed, Which these debar from the human breast. She only wished, for the harsh abuse, To find some way to become of use To the haughty daughter of lordly man; And thus did she lay a noble plan, To teach her wisdom, and make it plain, That the humble worm was not made in vain; A plan so generous, deep and high, That, to carry it out, she must even die!

“No more,” said she, “will I drink or eat! I’ll spin and weave me a winding-sheet, To wrap me up from the sun’s clear light, And hide my form from her wounded sight. In secret then, till my end draws nigh, I’ll toil for her; and when I die, I’ll leave behind, as a farewell boon, To the proud young princess, my whole cocoon, To be reeled and wove to a shining lace, And hung in a veil o’er her scornful face! And when she can calmly draw her breath Through the very threads that have caused my death;

When she finds, at length, she has nerves so firm As to wear the shroud of a crawling worm, May she bear in mind, that she walks with pride In the winding-sheet where the silk-worm died!”