Interior structure of the Cossus.-A, silk bags; B, silk tube, through which the viscid matter, of which the silk threads are formed, is forced by a peristaltic motion; C, stomach; D, D, intestines.

admirable treatise of Lyonnet on the anatomy of the Cossus, will render these several organs more easily understood than any description.

The spinneret itself was supposed by Réaumur to have two outlets for the silk; but Lyonnet, upon minute dissection, found that the two tubes united into one before their termination; and he also almost assured himself that it was composed of alternate slips of horny and membranaceous substance,-the one for pressing the thread into a small diameter, and the other for enlarging it at the insect's pleasure. It is cut at the end somewhat like a writing-pen, though with less of a slope, and is admirably fitted for being applied to objects to which it may be required to attach silk. The following are magnified figures of the spinneret of the Cossus from Lyonnet.

Side view of the Silk-tube. Section of the Silk-tube, magnified 22,000 times.

"You may sometimes have seen," says the Abbé de la Pluche, "in the work-rooms of goldsmiths or gold-wire drawers, certain iron plates, pierced with holes of different calibres, through which they draw gold and silver-wire, in order to render it finer. The silk-worm has under her mouth such a kind of instrument, perforated with a pair of holes [united into one on the outside*], through which she draws two drops of the gum that fills her two bags. These instruments

artists biostrina, silk-tube. Labium, or lower lip of Cossus.-a,

are like a pair of distaffs for spinning the gum into a silken thread. She fixes the first drop of gum that issues where she pleases, and then draws back her head, or lets herself fall, while the gum, continuing to flow, is drawn out and lengthened into a double stream. Upon being exposed to the air, it immediately loses its fluidity, becomes dry, and acquires consistence and strength. She is never deceived in adjusting the dimensions of the [united] apertures, or in calculating the proper thickness of the thread, but invariably makes the strength of it proportionable to the weight of her body.

"It would be a very curious thing to know how the gum which composes the silk is separated and drawn off from the other juices that nourish the animal. It must be accomplished like the secretions formed by glands in the human body. I am therefore persuaded that the gum-bags of the silk-worm are furnished with a set of minute glands, which, being impregnated with gum, afford a free passage to all the juices of the mulberry-leaf corresponding

with this glutinous matter, while they exclude every fluid of a different quality."* When confined in an open glass vessel, the goat-moth caterpillar will effect its escape, by constructing a curious silken ladder, as represented by Roesel.

Caterpillars, as they increase in size, cast their skins as lobsters do their shells, and emerge into renewed activity under an enlarged covering. Previous to this change, when the skin begins to gird and pinch them, they may be observed to become languid, and indifferent to their food, and at length they cease to eat, and await the sloughing of their skin. It is now that the faculty of spinning silk seems to be of great advantage to them; for being rendered inactive and helpless by the tightening of the old skin around their expanding body, they might be swept away by the first puff of wind, and made prey of by ground-beetles or other carnivorous prowlers. To guard against such accidents, as soon as they feel that they can swallow no more food from being half choked by the old skin, they take care to secure themselves from danger by moorings of silk spun upon the leaf or the branch where they may be reposing. The caterpillar of the white satin-moth (Leucoma salicis, STEPHENS) in this way draws together with silk one or two leaves, similar to the leaf-rollers (Tortricidae), though it always feeds openly without any covering. The caterpillar of the puss-moth again, which, in its third skin, is large and heavy, spins a thick web on the upper surface of a leaf, to which it adheres till the change is effected.

The most important operation, however, of silkspinning is performed before the caterpillar is transformed into a chrysalis, and is most remarkable in the caterpillars of moths and other four-winged flies, * Spectacle de la Nature, vol. i, + See Insect Transformations, chap, vii.

with the exception of those of butterflies; for though these exhibit, perhaps, greater ingenuity, they seldom spin more than a few threads to secure the chrysalis from falling, whereas the others spin for it a complete envelope or shroud. We have already seen, in the preceding pages, several striking instances of this operation, when, probably for the purpose of husbanding a scanty supply of silk, extraneous substances are worked into the texture. In the case of other caterpillars, silk is the only material employed. Of this the cocoon of the silk-worm is the most prominent example, in consequence of its importance in our manufactures and commerce, and on that account. will demand from us somewhat minute details, though it would require volumes to incorporate all the information which has been published on the subject.

SILK-WORM.

The silk-worm, like most other caterpillars, changes its skin four times during its growth. The intervals at which the four moultings follow each other depend much on climate or temperature, as well as on the quality and quantity of food. It is thence found, that if they are exposed to a high temperature, say from 81° to 100° Fahrenheit, the moultings will be hastened; and only five days will be consumed in moulting the third or fourth time, whilst those worms that have not been hastened take seven or eight days.*

The period of the moultings is also influenced by the temperature in which the eggs have been kept during the winter. When the heat of the apartment has been regulated, the first moulting takes place on the fourth or fifth day after hatching, the second begins on the eighth day, the third takes up the thirteenth and fourteenth days, and the last occurs

* Cours d'Agriculture, par M. Rozier. Paris, 1801.

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