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.