Part 22

When these little architects prepare for their change into chrysalides before becoming moths, they attach their tents securely to the stone over which they have hitherto rambled, by spinning a strong mooring of silk, so as not only to fill up every interstice between the main entrance of the tent and the stone, but also weaving a close, thick curtain of the same material, to shut up the entire aperture.

It is usual for insects which form similar structures to issue, when they assume the winged state, from the broader end of their habitation; but our little stone-mason proceeds in a different manner. It leaves open the apex of the cone from the first, for the purpose of ejecting its excrements, and latterly it enlarges this opening a little, to allow of a free exit when it acquires wings; taking care, however, to spin over it a canopy of silk, as a temporary protection, which it can afterwards burst through without difficulty. The moth itself is very much like the common clothes-moth in form, but is of a gilded-bronze colour, and considerably smaller.

In the same locality, M. de Maupertuis found a numerous brood of small caterpillars, which employed grains of stone, not, like the preceding, for building feeding-tents, but for their cocoons. This caterpillar was of a brownish-grey colour, with a white line along the back, on each side of which were tufts of hair. The cocoons which it built were oval, and less in size than a hazel-nut, the grains of the stone being skilfully woven into irregular meshes of silk.

In June, 1829, we found a numerous encampment of the tent-building caterpillars described by MM. de la Voye and Réaumur, on the brick wall of a garden at Blackheath, Kent. (J. R.) They were so very small, however, and so like the lichen on the wall, that had not our attention been previously directed to their habits, we should have considered them as portions of the wall; for not one of them was in motion, and it was only by the neat, turbinated, conical form in which they had constructed their habitations that we detected them. We tried the experiment above mentioned, of ejecting one of the caterpillars from its tent, in order to watch its proceedings when constructing another; but probably its haste to procure shelter, or the artificial circumstances into which it was thrown, influenced its operations, for it did not form so good a tent as the first, the texture of the walls being much slighter, while it was more rounded at the apex, and of course not so elegant. Réaumur found, in all his similar experiments, that the new structure equalled the old; but most of the trials of this kind which we have made correspond with the inferiority which we have here recorded. The process indeed is the same, but it seems to be done with more hurry and less care. It may be, indeed, in some cases, that the supply of silk necessary to unite the bits of stone, earth, or lichen employed, is too scanty for perfecting a second structure.

We remarked a very singular circumstance in the operations of our little architect, which seems to have escaped the minute and accurate attention of Réaumur. When it commenced its structure, it was indispensable to lay a foundation for the walls about to be reared; but as the tent was to be moveable like the shell of a snail, and not stationary, it would not have answered its end to cement the foundation to the wall. We had foreseen this difficulty, and felt not a little interested in discovering how it would be got over. Accordingly, upon watching its movements with some attention, we were soon gratified to perceive that it used its own body as the primary support of the building. It fixed a thread of silk upon one of its right feet, warped it over to the corresponding left foot, and upon the thread thus stretched between the two feet it glued grains of stone and chips of lichen, till the wall was of the required thickness. Upon this, as a foundation, it continued to work till it had formed a small portion in form of a parallelogram; and proceeding in a similar way, it was not long in making a ring a very little wider than sufficient to admit its body. It extended this ring in breadth, by working on the inside only, narrowing the diameter by degrees, till it began to take the form of a cone. The apex of this cone was not closed up, but left as an aperture through which to eject its excrements.

It is worthy of remark, that one of the caterpillars which we deprived of its tent attempted to save itself the trouble of building a new one, by endeavouring to unhouse one of its neighbours. For this purpose, it got upon the outside of the inhabited tent, and, sliding its head down to the entrance, tried to make its way into the interior. But the rightful owner did not choose to give up his premises so easily, and fixed his tent down so firmly upon the table where we had placed it, that the intruder was forced to abandon his attempt. The instant, however, that the other unmoored his tent and began to move about, the invader renewed his efforts to eject him, persevering in the struggle for several hours, but without a chance of success. At one time we imagined that he would have accomplished his felonious intentions; for he bound down the apex of the tent to the table with cables of silk. But he attempted his entrance at the wrong end. He ought to have tried the aperture in the apex, by enlarging which a little he would undoubtedly have made good his entrance; and as the inhabitant could not have turned upon him for want of room, the castle must have been surrendered. This experiment, however, was not tried, and there was no hope for him at the main entrance.

Muff-shaped Tents.

The ingenuity of man has pressed into his service not only the wool, the hair, and even the skins of animals, but has most extensively searched the vegetable kingdom for the materials of his clothing. In all this, however, he is rivalled by the tiny inhabitants of the insect world, as we have already seen; and we are about now to give an additional instance of the art of a species of caterpillars which select a warmer material for their tents than even the caterpillar of the clothes-moth. It may have been remarked by many who are not botanists, that the seed-catkins of the willow become, as they ripen, covered with a species of down or cotton, which, however, is too short in the fibre to be advantageously employed in our manufactures. But the caterpillars, to which we have alluded, find it well adapted for their habitations.

The muff-looking tent in which we find these insects does not require much trouble to construct; for the caterpillar does not, like the clothes-moth caterpillar, join the willow-cotton together, fibre by fibre--it is contented with the state in which it finds it on the seed. Into this it burrows, lines the interior with a tapestry of silk, and then detaches the whole from the branch where it was growing, and carries it about with it as a protection while it is feeding.

An inquiring friend of Réaumur having found one of these insects floating about in its muff-tent upon water, concluded that they feed upon aquatic plants; but he was soon convinced that it had only been blown down by an accident, which must frequently happen, as willows so often hang over water. May it not be, that the buoyant materials of the tent were intended to furnish the little inhabitant with a life-boat, in which, when it chanced to be blown into the water, it might sail safely ashore and regain its native tree?

Leaf-mining Caterpillars.

The process of mining between the two membranes of a leaf is carried on to a farther extent by minute caterpillars allied to the tent-makers above described. The tent-maker never deserts his house, except when compelled, and therefore can only mine to about half the length of his own body; but the miners now to be considered make the mine itself their dwelling-place, and as they eat their way, they lengthen and enlarge their galleries. A few of these mining caterpillars are the progeny of small weevils (Curculionidæ), some of two-winged flies (Diptera), but the greater number are produced from a genus of minute moth (Oecophora, Latr.), which, when magnified, appear to be amongst the most splendid and brilliant of Nature's productions, vying even with the humming-birds and diamond-beetles of the tropics in the rich metallic colours which bespangle their wings. Well may Bonnet call them "tiny miracles of Nature," and regret that they are not en grand.

There are few plants or trees whose leaves may not, at some season of the year, be found mined by these caterpillars, the track of whose progress appears on the upper surface in winding lines. Let us take one of the most common of these for an example,--that of the rose-leaf, produced by the caterpillar of Ray's golden-silver spot (Argyromiges Rayella? Curtis), of which we have just gathered above a dozen specimens from one rose-tree. (J. R.)

It may be remarked that the winding line is black, closely resembling the tortuous course of a river on a map,--beginning like a small brook, and gradually increasing in breadth as it proceeds. This representation of a river exhibits, besides, a narrow white valley on each side of it, increasing as it goes, till it terminates in a broad delta. The valley is the portion of the inner leaf from which the caterpillar has eaten the pulp (parenchyma), while the river itself has been formed by the liquid ejectamenta of the insect, the watery part becoming evaporated. In other species of miners, however, the dung is hard and dry, and consequently these only exhibit the valley without the river (see p. 255).

On looking at the back of the leaf, where the winding line begins, we uniformly find the shell of the very minute egg from which the caterpillar has been hatched, and hence perceive that it digs into the leaf the moment it escapes from the egg, without wandering a hair's-breadth from the spot; as if afraid lest the air should visit it too roughly. The egg is, for the most part, placed upon the mid-rib of the rose-leaf, but sometimes on one of the larger nervures. When once it has got within the leaf, it seems to pursue no certain direction, sometimes working to the centre, sometimes to the circumference, sometimes to the point, and sometimes to the base, and even, occasionally, crossing or keeping parallel to its own previous track.

The most marvellous circumstance, however, is the minuteness of its workmanship; for though a rose-leaf is thinner than this paper, the insect finds room to mine a tunnel to live in, and plenty of food, without touching the two external membranes. Let any one try with the nicest dissecting instruments to separate the two plates of a rose-leaf, and he will find it impossible to proceed far without tearing one or other. The caterpillar goes still further in minute nicety; for it may be remarked, that its track can only be seen on the upper, and not on the under surface of the leaf, proving that it eats as it proceeds only half the thickness of the pulp, or that portion of it which belongs to the upper membrane of the leaf.

We have found this little miner on almost every sort of rose-tree, both wild and cultivated, including the sweet-briar, in which, the leaf being very small, it requires nearly the whole parenchyma to feed one caterpillar. They seem, however, to prefer the foreign monthly rose to any of our native species, and there are few trees of this where they may not be discovered.

Tunnels very analogous to the preceding may be found upon the common bramble (Rubus fruticosus); and on the holly, early in spring, one which is in form of an irregular whitish blotch. But in the former case, the little miner seems to proceed more regularly, always, when newly hatched, making directly for the circumference, upon or near which also the mother-moth deposits her egg, and winding along for half the extent of the leaf close upon the edge, following, in some cases, the very indentations formed by the terminating nervures.

The bramble-leaf miner seems also to differ from that of the rose-leaf, by eating the pulp both from the upper and under surface, at least the track is equally distinct above and below; yet this may arise from the different consistence of the leaf pulp, that in the rose being firm, while that of the bramble is soft and puffy.

On the leaves of the common primrose (Primula veris), as well as on the garden variety of it, the polyanthus, one of those mining caterpillars may very frequently be found. It is, however, considerably different from the preceding, for there is no black trace--no river to the valley which it excavates: its ejectamenta, being small and solid, are seen, when the leaf is dried, in little black points like grains of sand. This miner also seems more partial than the preceding to the mid-rib and its vicinity, in consequence of which its path is seldom so tortuous, and often appears at its extremity to terminate in an area comparatively extensive, arising from its recrossing its previous tracks. (J. R.)

Swammerdam describes a mining caterpillar which he found on the leaves of the alder, though it did not, like those we have just described, excavate a winding gallery; it kept upon the same spot, and formed only an irregular area. A moth was produced from this, whose upper wings, he says, "shone and glittered most gloriously with crescents of gold, silver, and brown, surrounded by borders of delicate black." Another area miner which he found on the leaves of willows, as many as seventeen on one leaf, producing what appeared to be rusty spots, was metamorphosed into a very minute weevil (Curculio Rhionoc.). He says he has been informed that, in warm climates, worms an inch long are found in leaves, and adds, with great simplicity, "on these many fine experiments might have been made, if the inhabitants had not laboured under the cursed thirst of gold."

The vine-leaf miner, when about to construct its cocoon, cuts, from the termination of its gallery, two pieces of the membrane of the leaf, deprived of their pulp, in a similar manner to the tent-makers described above, uniting them and lining them with silk. This she carries to some distance before she lays herself up to undergo her change. Her mode of walking under her burthen is peculiar, for, not contented with the security of a single thread of silk, she forms, as Bonnet says, "little mountains (monticules) of silk, from distance to distance, and seizing one of these with her teeth, drags herself forward, and makes it a scaffolding from which she can build another." Some of the miners, however, do not leave their galleries, but undergo their transformations there, taking the precaution to mine a cell, not in the upper, but in the under surface; others only shift to another portion of the leaf.

Social Leaf-Miners.

The preceding descriptions apply to caterpillars who construct their mines in solitude, there being seldom more than one on a leaf or leaflet, unless when two mother-flies happen to lay their eggs on the same leaf; but there are others, such as the miners of the leaves of the henbane (Hyoscyamus niger), which excavate a common area in concert--from four to eight forming a colony. These are very like flesh-maggots, being larger than the common miners; the leaves of this plant, from being thick and juicy, giving them space to work and plenty to eat.

Most of the solitary leaf-miners either cannot or will not construct a new mine, if ejected by an experimenter from the old, as we have frequently proved; but this is not the case with the social miners of the henbane-leaf. Bonnet ejected one of these, and watched it with his glass till it commenced a new tunnel, which it also enlarged with great expedition; and in order to verify the assertion of Réaumur, that they neither endeavour nor fear to meet one another, he introduced a second. Neither of them manifested any knowledge of the other's contiguity, but both worked hard at the gallery, as did a third and a fourth which he afterwards introduced; for though they seemed uneasy, they never attacked one another, as the solitary ones often do when they meet.

Bark-mining Caterpillars.

A very different order of mining caterpillars are the progeny of various beetles, which excavate their galleries in the soft inner bark of trees, or between it and the young wood (alburnum). Some of these, though small, commit extensive ravages, as may readily be conceived when we are told that as many as eighty thousand are occasionally found on one tree. In 1783 the trees thus destroyed by the printer-beetle (Tomicus typographus, Latr.), so called from its tracks resembling letters, amounted to above a million and a half in the Hartz forest. It appears there periodically, and confines its ravages to the fir. This insect is said to have been found in the neighbourhood of London.

On taking off the bark of decaying poplars and willows, we have frequently met with the tracks of a miner of this order, extending in tortuous pathways, about a quarter of an inch broad, for several feet and even yards in length. The excavation is not circular, but a compressed oval, and crammed throughout with a dark-coloured substance like sawdust--the excrement no doubt of the little miner, who is thereby protected from the attacks of Staphylinidæ, and other predaceous insects from behind. But though we have found a great number of these subcortical tracks, we have never discovered one of the miners, though they are very probably the grubs of the pretty musk-beetle (Cerambyx moschatus), which are so abundant in the neighbourhood of the trees in question, that the very air in summer is perfumed with their odour. (J. R.)