CHAPTER I.. Manufacture of Silk by Caterpillars of Various Kinds.

MANUFACTURE OF SILK BY CATERPILLARS OF VARIOUS KINDS.

The most important of all the insect manufactures is doubtless that of silk-spinning. It seems almost past belief that the magnificent velvets, satins, and silks which form so elegant a part of female attire, and which are heaped together in such costly profusion in the shops of the metropolis, and in those of all considerable towns, whether in our own, or in foreign countries—that these splendid fabrics, together with all ribbons, gauzes, damasks, or other articles composed of silk, should owe the raw material of which they are formed to the labours of a race of little creeping caterpillars, which in this their early and imperfect state spin for themselves cocoons, or cases of silk, where they may quietly undergo their changes until they become perfect winged insects.

CATERPILLAR SUSPENDED BY ITS SILKEN THREAD. ]

The astonishing task of supplying silk for the whole civilized world is performed almost without exception by the common silkworm, or caterpillar of the mulberry-tree moth. But it must not be supposed that this is the only silk-producing insect. On the contrary, all the caterpillars of butterflies and moths have the power of spinning a certain quantity of silken thread, however small, and however inferior to that of the silkworm, properly so called. It is very common in gardens to see numbers of caterpillars dangling by their silken threads from the young branches of fruit-trees. In this way they let themselves down, or break their fall if blown off by the wind, or otherwise shaken from their favourite tree. And in the case of some caterpillars when the insect has completed the term of its existence and becomes a chrysalis, it suspends itself by silken cords to some fixed point, where it remains in complete repose, without food, perhaps for months before the perfect winged insect bursts forth, as different a creature from the caterpillar as the chrysalis is from either,—and yet these are but three different states of existence of the same insect.

CHRYSALIS SUSPENDED BY SILKEN CORDS. ]

According to the particular species of caterpillar, the silk will vary in strength and fineness, and also in colour, but seldom will it be found strong enough, or in sufficient quantity to be of use to us. The insects themselves employ it in many ways for their own safety and shelter. How common it is to find some of the leaves of a lilac-tree made up into little rolls, or folded together at their edges, where they stick so fast that it requires some little force to pull them asunder. This is the work of a small caterpillar, whose subsistence is found on that tree. A single egg has been laid on each leaf selected by the parent moth, and soon after it is hatched, the caterpillar begins rolling up the lilac leaf, and fastening the edges with silk. This is a work of time, but gradually proceeds as the caterpillar increases in strength, until at last it is shut up in a gallery of its own making, safe from the attacks of birds and larger insects. The leaves of various other trees are rolled in a somewhat similar manner, as those of the oak, the willow, and the rose, and also those of humbler plants, as the plantain, nettle, thistle, &c. The mode in which these leaf-rolling caterpillars set to work to form convenient habitations of the leaves of the plants on which they feed, is well described by Kirby and Spence.

OAK LEAF ROLLED BY A CATERPILLAR. ]

“Some of these merely connect together with a few silken threads several leaves, so as to form an irregular packet, in the centre of which the little hermit lives. Others confine themselves to a single leaf, of which they simply fold one part over the other. A third description form and inhabit a sort of roll, by some species made cylindrical, by others conical, resembling the papers in which grocers put their sugar, and as accurately constructed; only there is an opening left at the smaller extremity for the egress of the insect in case of need. If you were to see one of these rolls, you would immediately ask by what mechanism it could possibly be made—how an insect, without fingers, could contrive to bend a leaf into a roll, and to keep it in that form until fastened with the silk which holds it together? The following is the operation: the little caterpillar first fixes a series of silken cables from one side of the leaf to the other: she next pulls at these cables with her feet; and when she has forced the sides to approach, she fastens them together with stronger threads of silk. If the insect finds that one of the larger nerves of the leaf is so strong as to resist her efforts, she weakens it by gnawing it here and there half through. What engineer could act more sagaciously? To form one of the conical or horn-shaped rolls, which are not composed of a whole leaf, but of a long triangular portion cut out of the edge, some other manœuvres are requisite. Placing herself upon the leaf, the caterpillar cuts out with her jaws the piece which is to compose her roll. She does not, however, entirely detach it; it would then want a base. She detaches that part only which is to form the contour of the horn. This portion is a triangular strap, which she rolls as she cuts. When the body of the horn is finished, as it is intended to be fixed upon the leaf in nearly an upright position, it is necessary to elevate it. To effect this, she proceeds as we should with an inclined obelisk. She attaches threads, or little cables, towards the point of the pyramid, and raises it by the weight of her body.”

Other larvæ form their habitations wholly of silk: one that inhabits the leaves of pear-trees forms a little tent; another a sort of cloak; another, as in the case of the silkworm, a complete ball of silk.

PENDULOUS NESTS OF CATERPILLARS. ]

Among leaf-rolling caterpillars, one of the most curious is described by Bonnet, in which the nest hangs suspended from the branch of a fruit-tree by a strong silken thread. It is formed of one or two leaves neatly folded and fastened together with silk, and in this small enclosure several caterpillars live harmoniously together.

LEAF CUTTING CATERPILLAR. ]

There is also a very curious and beautiful nest formed by a rare insect, which has only yet been observed in its caterpillar state; but its proceedings therein are interesting and highly remarkable. The length of the body in this caterpillar varies from six to eight inches, the thickness is half a line. The general colour is bluish green, tending to yellow about the head; the feet are black, and six in number. At the extremity of the body are two points projecting sideways. The eyes are visible and prominent in this caterpillar, and besides the antennæ there are two or three pairs of palpi or feelers, which perform the office of hands. This insect inhabits the nut-tree, and cuts out from its leaves a most ingenious case, which serves for its dwelling-place. This case is of a singular form, being an elongated cone, very narrow at the extremity, and tolerably wide at the orifice. It is formed of a strip of nut-leaf, wound up in a spiral manner, but so cleverly contrived that the strip is very narrow at the extremity, and gradually widens as the work proceeds. This happens from the gradual progress of the work, which is begun in the early stage of the caterpillar’s existence, when it does not require a large dwelling-place, and becomes more and more extended to suit the growth of its inmate. When it is completed, it is rather more than an inch long, and two lines in diameter at the orifice. The outside of the case presents the upper surface of the leaf in which the indentations are preserved. The dwelling is a spacious one for the caterpillar, so that it can turn round in it with perfect ease. The band of which the cone is composed is taken from the edge of the leaf, which the caterpillar cuts away in proportion as it winds it round its own body. When one portion has taken the required form, a little more is cut out, but always in a direction parallel with the edge of the leaf. While the insect is thus gradually rolling itself up in a case, it also feeds vigorously on the leaf, taking care, however, always to spare the band which forms its habitation. Another part of the cleverness of this insect consists in maintaining this strip of leaf in its spiral form, which would otherwise naturally unwind by its own weight, and flutter in the wind as a torn fragment. Here it is that silk is again skilfully employed. The caterpillar commences much as other leaf-rolling insects do, by fastening a thread to the edge of the leaf and drawing it towards the desired point. But as the work proceeds the insect generally has three skeins or bands of silk extended from the opening of its case to the leaf. The first is the shortest and the most direct, issuing from the interior of the roll; the second is fastened to the middle of the last spiral, and extends to the leaf; the third is more extensive still, and all three are admirably disposed for keeping the leaf bent to the proper form. These skeins are composed of a great number of threads parallel with each other, which not only serve to retain the leaf in its spiral form, but also serve the caterpillar as a kind of ladder, by which it mounts, and on which it rests while cutting out fresh portions of leaf.

LEAF-NEST UNWOUND. ]

It sometimes happens accidentally to this caterpillar that the case becomes detached from the leaf, or naturalists have purposely severed a portion to see what would be the result; but this does not greatly disturb the insect, for as long as a single thread of silk remains to hold the pieces together, the caterpillar is able to repair the evil. It will unite the fractured portion almost imperceptibly by means of silken threads, and proceed with its case as before; but if the leaf dry up and wither, the insect is then compelled to desert it, and to select a younger and fresher leaf for its operations.

What is still more curious, on the completion of the dwelling, the caterpillar sometimes wishes to make an excursion to another part of the leaf, and to take its house with it, and this it accomplishes in the following manner. Coming more than three parts out of the cone, the creature makes new skeins in advance of the old, and attempts to pull the cone forward by their means; but finding that this does not succeed, it bends its body back, and quickly severs the old threads and the portion of leaf which keeps the case immovable. It is now able to draw the case onwards a certain distance, after which it has only to repeat the operation, and make other new skeins in advance of these, to continue its onward journey. Thus it arrives at the opposite edge of the leaf, where it carefully adjusts its case to the under side, fastening it with great ingenuity, and drawing the threads tighter where the cone is not properly balanced. In all these cases, silk is the useful material by which the caterpillar secures its nest, and provides for its own safety: it is also a constant resource in case of danger, or of accident, as the following anecdote will show. A caterpillar of the goat-moth, being confined in a smooth glass sugar-basin, managed to crawl up the slippery sides and escape. This excited great surprise in the naturalist (Rösel) who had imprisoned the creature, and he therefore took occasion closely to watch its proceedings when again placed in a similar vessel. It was with surprise and admiration that he now saw the caterpillar constructing a silken ladder on the side of the glass; the natural gum of the silk being sufficient to secure it even to that perfectly smooth surface. Up this ladder the creature crawled, and thus easily and expeditiously made its escape.

SILKEN LADDER SPUN BY THE GOAT-MOTH CATERPILLAR. ]

Silkworms abound in our own country, as well as in others; but the silk they spin is too fragile and scanty to be employed for other purposes than their own. It is different with some of the caterpillars of India. Although the great supply of silk is there obtained, as with us, from the silkworm of the mulberry-tree, yet the caterpillars of various moths also furnish a considerable quantity. The most important of these are the Tusseh and Arindy silkworms, both natives of Bengal. The first feeds on the leaves of the jujube tree and of the asseen, and is found in such abundance as from time immemorial to have afforded a constant supply of coarse, dark-coloured silk, which is woven into a cheap but very durable cloth.

THE TUSSEH SILKWORM. ]

When the caterpillars approach their full size they are too heavy to crawl in search of their food with the back upwards, as is usual with most caterpillars, but traverse the small branches suspended by the feet, as is shown in the figure.

COCOON OF THE TUSSEH SILKWORM. ]

When the caterpillar is ready to spin its cocoon, it connects by means of the silk, (which is always glutinous when newly spun,) two or three new leaves into an outer envelope, which serves as a basis to spin the complete cocoon in, besides the cocoon being suspended from a branch of the tree in a wonderful manner, by a thick, strong, consolidated cord, spun of the same material by this persevering creature.

The cocoon is of an exact oval shape, and very firm in texture: in it the animal remains dormant, and perfectly protected, for about nine months, namely from October until July; so that it makes its appearance in time for the caterpillars to come into existence, when Providence has furnished them with the greatest plenty of proper food. When the insect is prepared to make its escape and be changed into its perfect state, it discharges from its mouth a large quantity of liquid, with which the upper end of the case is so perfectly softened as to enable the moth to work its way out in a very short time; an operation which is always performed during the night. In their perfect state these insects do not exist many days: the female deposits her eggs in the branches of the tree she may be resting on, to which they adhere firmly by means of the gluten they are covered with when newly laid.

The eggs are white, round, and compressed, with a depression in the centre on each side. They hatch in from two to four weeks, according to the state of the weather. The larvæ or caterpillars acquire their full size, which is about four inches in length, and three in circumference, in about six weeks: they are nearly the colour of the leaves they feed on, with a light yellowish stripe on each side; under these stripes the middle segments are marked with an oblong gold-coloured speck. The back is also marked with a few round darker coloured spots, from which issue a few long, coarse, distinct hairs, while others of smaller size are scattered over the insect.

The Tusseh silkworm is found in such abundance over many parts of Bengal and the adjoining provinces, as to afford to the natives a large supply of the durable, coarse, dark-coloured silk, already mentioned, commonly called Tusseh silk, which is woven into a kind of cloth called Tusseh doot’hies, much worn by Brahmins and other sects of Hindoos. This substance would no doubt be highly useful to the inhabitants of many parts of America and the south of Europe, where a cheap, light, cool, durable dress, such as this silk makes, is much wanted. Millions of cocoons of the Tusseh silkworm are annually collected in the jungle, and brought to the silk factories near Calcutta. In other parts the people gather and transplant them to the trees near their own dwellings, that they may watch over the safety of the caterpillars, which are very liable to be devoured by crows during the day time, and by bats at night.

The Arindy silkworm feeds only on the leaves of the Palmi Christi; it produces so delicate and flossy a silk that it cannot be wound from the cocoons; it is therefore spun like cotton, and the thread woven into a coarse kind of white cloth, of a loose texture, but so durable that a person can scarcely in his lifetime wear out a garment made of it.

Eleven different species of silkworm have been enumerated as natives of India, which has thus the internal means of providing the whole of Europe with a material which would rival cotton and woollen cloth, and would often be preferred to both, could it be obtained at a low price. The produce of the Arindy silkworm, when sent to this country was much admired, and some manufacturers to whom it was shown seemed to think that they had been hitherto deceived in the account of the shawls of India being made from the wool of a goat, and that this silk, if sent home, could be made into shawls equal to any manufactured in India.

Many of the larvæ of the European moths afford a very strong silk, and it is said, that a manufacture of silk from the cocoons of the emperor moth was at one time established in Germany. There is no doubt, however, that silk might be collected in abundance from many native silkworms in America. Cocoons have been described eight inches long, made of grey silk, which the inhabitants of Chilpancingo, Tixtala, and other places in South America, manufacture into stockings and handkerchiefs. Humboldt also observed similar nests in the provinces of Mechoacan, and the mountains of Santa Rosa; they were of dense tissue, resembling Chinese paper, of a brilliant whiteness, and formed of distinct and separate layers. The interior layers, which are the thinnest, and of extraordinary transparency, were used by the ancient Mexicans as writing tablets.