Caterpillar of the Goat Moth (Cossus ligniperda).
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, with the coil of bile vessels.
These spiracles communicate on each side with tubes, that have been called the wind-pipes (tracheæ). The spinning apparatus is placed near the mouth, and is connected with the silk-bags, which are long, slender, floating vessels, containing a liquid gum. The bags are closed at their lower extremity, become wider towards the middle, and more slender towards the head, where they unite to form the spinning-tube, or spinneret. The bags being in most cases longer than the body of the caterpillar, necessarily lie in a convoluted state, like the intestines of quadrupeds. The capacity, or rather the length, of the silk-bags is in proportion to the quantity of silk required for spinning; the Cossus ligniperda, for example, from living in the wood of trees spins little, having a bag only one-fourth the length of that of the silk-worm, though the caterpillar is at least twice the dimensions of the latter. The following figure, taken from the 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 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[DQ],] through which she draws two drops of the gum that fills her two bags. These instruments 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.
Labium, or lower lip of Cossus.—a, Silk-tube.
“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.”[DR] 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.