Part 21

In the economy of insects, we constantly observe that the means of defence, not only of the individual creatures, but of their larvæ and pupæ, against the attacks of other insects, and of birds, is proportioned, in the ingenuity of their arrangements, to the weakness of the insect employing them. Those species which multiply the quickest have the greatest number of enemies. Bradley, an English naturalist, has calculated that two sparrows carry, in the course of a week, above three thousand caterpillars to the young in their nests. But though this is, probably, much beyond the truth, it is certain that there is a great and constant destruction of individuals going forward; and yet the species is never destroyed. In this way a balance is kept up, by which one portion of animated nature cannot usurp the means of life and enjoyment which the world offers to another portion. In all matters relating to reproduction, Nature is prodigal in her arrangements. Insects have more stages to pass through before they attain their perfect growth than other creatures. The continuation of the species is, therefore, in many cases, provided for by a much larger number of eggs being deposited than ever become fertile. How many larvæ are produced, in comparison with the number which pass into the pupa state; and how many pupæ perish before they become perfect insects! Every garden is covered with caterpillars; and yet how few moths and butterflies, comparatively, are seen, even in the most sunny season? Insects which lay few eggs are, commonly, most remarkable in their contrivances for their preservation. The dangers to which insect life is exposed are manifold; and therefore are the contrivances for its preservation of the most perfect kind, and invariably adapted to the peculiar habits of each tribe. The same wisdom determines the food of every species of insect; and thus some are found to delight in the rose-tree, and some in the oak. Had it been otherwise, the balance of vegetable life would not have been preserved. It is for this reason that the contrivances which an insect employs for obtaining its food are curious, in proportion to the natural difficulties of its structure. The ant-lion is carnivorous, but he has not the quickness of the spider, nor can he spread a net over a large surface, and issue from his citadel to seize a victim which he has caught in his out-works. He is therefore taught to dig a trap, where he sits like the unwieldy giants of fable, waiting for some feeble one to cross his path. How laborious and patient are his operations--how uncertain the chances of success! Yet he never shrinks from them, because his instinct tells him that by these contrivances alone can he preserve his own existence, and continue that of his species.

CHAPTER XII.

CLOTHES-MOTH AND OTHER TENT-MAKING CATERPILLARS.--LEAF AND BARK MINERS.

There are at least five different species of moths similar in manners and economy, the caterpillars of which feed upon animal substances, such as furs, woollen cloths, silk, leather, and, what to the naturalist is no less vexing, upon the specimens of insects and other animals preserved in his cabinet. The moths in question are of the family named Tinea by entomologists, such as the tapestry-moth (Tinea tapetzella), the fur-moth (Tinea pellionella), the wool-moth (Tinea vestianella), the cabinet-moth (Tinea destructor, Stephens), &c.

The moths themselves are, in the winged state, small and well fitted for making their way through the most minute hole or chink, so that it is scarcely possible to exclude them by the closeness of a wardrobe or a cabinet. If they cannot effect an entrance when a drawer is out, or a door open, they will contrive to glide through the key-hole; and if they once get in, it is no easy matter to dislodge or destroy them, for they are exceedingly agile, and escape out of sight in a moment. Moufet is of opinion that the ancients possessed an effectual method of preserving stuffs from the moth, because the robes of Servius Tullius were preserved up to the death of Sejanus, a period of more than five hundred years. On turning to Pliny to learn this secret, we find him relating that stuff laid upon a coffin will be ever after safe from moths; in the same way as a person once stung by a scorpion will never afterwards be stung by a bee, or a wasp, or a hornet! Rhasis, again, says that cantharides suspended in a house drive away moths; and he adds that they will not touch anything wrapped in a lion's skin!--the poor little insects, says Réaumur, sarcastically, being probably in bodily fear of so terrible an animal. Such are the stories which fill the imagination even of philosophers, till real science entirely expels them.

The effluvium of camphor or turpentine, or fumigation by sulphur or chlorine, may sometimes kill them, when in the winged state, but this will have no effect upon their eggs, and seldom upon the caterpillars; for they wrap themselves up too closely to be easily reached by any agent except heat. This, when it can be conveniently applied, will be certain either to dislodge or to kill them. When the effluvium of turpentine, however, reaches the caterpillar, Bonnet says it falls into convulsions, becomes covered with livid blotches, and dies.

The mother insect takes care to deposit her eggs on or near such substances as she instinctively foreknows will be best adapted for the food of the young, taking care to distribute them so that there may be a plentiful supply and enough of room for each. We have found, for example, some of those caterpillars feeding upon the shreds of cloth used in training wall-fruit trees; but we never saw more than two caterpillars on one shred. This scattering of the eggs in many places renders the effects of the caterpillars more injurious, from their attacking many parts of a garment or a piece of stuff at the same time. (J. R.)

When one of the caterpillars of this family issues from the egg, its first care is to provide itself with a domicile, which indeed seems no less indispensable to it than food; for, like all caterpillars that feed under cover, it will not eat while it remains unprotected. Its mode of building is very similar to that which is employed by other caterpillars that make use of extraneous materials. The foundation or framework is made of silk secreted by itself, and into this it interweaves portions of the material upon which it feeds. It is said by Bingley, that "after having spun a fine coating of silk immediately around its body, it cuts the filaments of the wool or fur close by the thread of the cloth, or by the skin, with its teeth, which act in the manner of scissors, into convenient lengths, and applies the bits, one by one, with great dexterity, to the outside of its silken case." This statement, however, is erroneous, and inconsistent with the proceedings not only of the clothes-moth, but of every caterpillar that constructs a covering. None of these build from within outwards, but uniformly commence with the exterior wall, and finish by lining the interior with the finest materials. Réaumur, however, found that the newly-hatched caterpillars lived at first in a case of silk.

We have repeatedly witnessed the proceedings of these insects from the very foundation of their structures; and, at the moment of writing this, we turned out one from the carcase of an "old lady moth" (Mormo maura, Ochsenheim) in our cabinet, and placed it on a desk covered with green cloth, where it might find materials for constructing another dwelling. It wandered about for half a day before it began its operations; but it did not, as is asserted by Bonnet, and Kirby and Spence, "in moving from place to place, seem to be as much incommoded by the long hairs which surround it, as we are by walking amongst high grass," nor, "accordingly, marching scythe in hand," did it, "with its teeth, cut out a smooth road." On contrary, it did not cut a single hair till it selected one for the foundation of its intended structure. This it cut very near the cloth, in order, we suppose, to have it as long as possible; and placed it on a line with its body. It then immediately cut another, and placing it parallel to the first, bound both together with a few threads of its own silk. The same process was repeated with other hairs, till the little creature had made a fabric of some thickness, and this it went on to extend till it was large enough to cover its body; which (as is usual with caterpillars) it employed as a model and measure for regulating its operations. We remarked that it made choice of longer hairs for the outside than for the parts of the interior, which it thought necessary to strengthen by fresh additions; but the chamber was ultimately finished by a fine and closely-woven tapestry of silk. We could see the progress of its work by looking into the opening at either of the ends; for at this stage of the structure the walls are quite opaque, and the insect concealed. It may be thus observed to turn round, by doubling itself and bringing its head where the tail had just been; of course, the interior is left wide enough for this purpose, and the centre, indeed, where it turns, is always wider than the extremities. (J. R.)

When the caterpillar increases in length, it takes care to add to the length of its house, by working in fresh hairs at either end; and if it be shifted to stuffs of different colours, it may be made to construct a party-coloured tissue, like a Scotch plaid. Réaumur cut off with scissors a portion at each end, to compel the insect to make up the deficiency. But the caterpillar increases in thickness as well as in length, so that, its first house becoming too narrow, it must either enlarge it, or build a new one. It prefers the former as less troublesome, and accomplishes its purpose "as dexterously," says Bonnet, "as any tailor, and sets to work precisely as we should do, slitting the case on the two opposite sides, and then adroitly inserting between them two pieces of the requisite size. It does not, however, cut open the case from one end to the other at once; the sides would separate too far asunder, and the insect be left naked. It therefore first cuts each side about half-way down, beginning sometimes at the centre and sometimes at the end (Fig. c), and then, after having filled up the fissure, proceeds to cut the remaining half; so that, in fact, four enlargements are made, and four separate pieces inserted. The colour of the case is always the same as that of the stuff from which it is taken. Thus, if its original colour be blue, and the insect, previously to enlarging it, be put upon red cloth, the circles at the end, and two stripes down the middle, will be red." Réaumur found that they cut these enlargements in no precise order, but sometimes continuously, and sometimes opposite each other, indifferently.

The same naturalist says he never knew one leave its old dwelling in order to build a new; though, when once ejected by force from its house, it would never enter it again, as some other species of caterpillars will do, but always preferred building another. We, on the contrary, have more than once seen them leave an old habitation. The very caterpillar, indeed, whose history we have above given, first took up its abode in a specimen of the ghost-moth (Hepialus humuli), where, finding few suitable materials for building, it had recourse to the cork of the drawer, with the chips of which it made a structure almost as warm as it would have done from wool. Whether it took offence at our disturbing it one day, or whether it did not find sufficient food in the body of the ghost-moth, we know not; but it left its cork house, and travelled about eighteen inches, selected "the old lady," one of the largest insects in the drawer, and built a new apartment, composed partly of cork as before, and partly of bits clipt out of the moth's wings. (J. R.)

We have seen these caterpillars form their habitations of every sort of insect, from a butterfly to a beetle; and the soft, feathery wings of moths answer their purpose very well: but when they fall in with such hard materials as the musk beetle (Cerambyx moschatus), or the large scolopendra of the West Indies, they find some difficulty in the building.

When the structure is finished, the insect deems itself secure to feed on the materials of the cloth or other animal matter within its reach, provided it is dry and free from fat or grease, which Réaumur found it would not touch. This may probably be the origin of the practice of putting a bit of candle with furs, &c., to preserve them from the moth. For building, it always selects the straightest and loosest pieces of wool, but for food it prefers the shortest and most compact; and to procure these it eats into the body of the stuff, rejecting the pile or nap, which it necessarily cuts across at the origin, and permits to fall, leaving it threadbare, as if it had been much worn. It must have been this circumstance which induced Bonnet to fancy (as we have already mentioned) that it cuts the hairs to make itself a smooth, comfortable path to walk upon. It would be equally correct to say that an ox or a sheep dislikes walking amongst long grass, and therefore eats it down in order to clear the way.

As soon as it can hide itself in an empty cell--an easy matter enough for so tiny a moth, which harmonizes exactly in colour with the bee-combs--it proceeds to lay its eggs, and, having discharged its office, dies. The eggs soon hatch into little grubs and caterpillars with very hard horny heads and soft bodies. As soon as they come into the dark world of the hive, they begin to eat their way through the combs, spinning the while a tunnel of silk, which entirely protects them from the stings of the bees. They can traverse these tunnels with tolerable speed, so that the bees do not know where to find their enemies; and if perchance they should discover one of them at the mouth of its burrow, the hard, horny head is all that is visible, and against its polished surface the sting of the bee is useless. The rapidity with which they drive the silken tubes through the comb is really marvellous; and even if they get among a collection of empty bee-combs, they make as much havoc as if they were bred in the hive from which the combs were taken.

In the accompanying illustration are seen figures of the two species of honey-moths, together with their tunnels. The species may be easily distinguished by the shape of the wings, Galleria alvearia having, as seen at Figs. b, c, the ends of its wings rounded, and Galleria cereana having them squared.

Some moths, also belonging to the vast Family Tineidæ, do much damage to grain, and have also the habit of spinning silken tissues as they eat their way through the grain. One of them is more plentiful on the Continent than in England, but is known in this country by the name of the mottled woollen moth (Tinea granella)].

The caterpillar, which is smooth and white, ties together with silk several grains of wheat, barley, rye, or oats, weaving a gallery between them, from which it projects its head while feeding; the grains, as Réaumur remarks, being prevented from rolling or slipping by the silk which unites them. He justly ridicules the absurd notion of its filing off the outer skin of the wheat by rubbing upon it with its body, the latter being the softer of the two, and he disproved, by experiment, Leeuwenhoeck's assertion that it will also feed on woollen cloth. It is from the end of May till the beginning of July that the moths, which are of a silvery grey, spotted with brown, appear and lay their eggs in granaries.

The caterpillar of another still more singular grain-moth (Tinea Hordei, Kirby and Spence) proves sometimes very destructive of granaries. The mother-moth, in May or June, lays about twenty or more eggs on a grain of barley or wheat; and when the caterpillars are hatched they disperse, each selecting a single grain. M. Réaumur imagines that sanguinary wars must sometimes arise, in cases of preoccupancy, a single grain of barley being a rich heritage for one of these tiny insects; but he confesses he never saw such contests. When the caterpillar has eaten its way into the interior of the grain, it feeds on the farina, taking care not to gnaw the skin nor even to throw out its excrements, so that except the little hole, scarcely discernible, the grain appears quite sound. When it has eaten all the farina, it spins itself a case of silk within the now hollow grain, and changes to a pupa in November.

Tent-making Caterpillars.

The caterpillars of a family of small moths (Tineidæ), which feed on the leaves of various trees, such as the hawthorn, the elm, the oak, and most fruit-trees, particularly the pear, form habitations which are exceedingly ingenious and elegant. They are so very minute that they require close inspection to discover them; and to the cursory observer, unacquainted with their habits, they will appear more like the withered leaf-scales of the tree, thrown off when the buds expand, than artificial structures made by insects. It is only, indeed, by seeing them move about upon the leaves, that we discover they are inhabited by a living tenant, who carries them as the snail does its shell.

These tents are from a quarter of an inch to an inch in length, and usually about the breadth of an oat-straw. That they are of the colour of a withered leaf is not surprising; for they are actually composed of a piece of leaf; not, however, cut out from the whole thickness, but artfully separated from the upper layer, as a person might separate one of the leaves of paper from a sheet of pasteboard.

The tents of this class of caterpillars, which are found on the elm, the alder, and other trees with serrated leaves, are much in the shape of a minute goldfish. They are convex on the back, where the indentations of the leaf out of which they have been cut add to the resemblance, by appearing like the dorsal fins of the fish. By depriving one of those caterpillars common on the hawthorn of its tents, for the sake of experiment, we put it under the necessity of making another; for, as Pliny remarks of the clothes-moth, they will rather die of hunger than feed unprotected. When we placed it on a fresh hawthorn leaf, it repeatedly examined every part of it, as if seeking for its lost tent, though, when this was put in its way, it would not again enter it; but, after some delay, commenced a new one. (J. R.)

For this purpose, it began to eat through one of the two outer membranes which compose the leaf and enclose the pulp (parenchyma), some of which, also, it devoured, and then thrust the hinder part of its body into the perforation. The cavity, however, which it had formed, being yet too small for its reception, it immediately resumed the task of making it larger. By continuing to gnaw into the pulp between the membranes of the leaf (for it took the greatest care not to puncture or injure the membranes themselves), it soon succeeded in mining out a gallery rather larger than was sufficient to contain its body. We perceived that it did not throw out as rubbish the pulp it dug into, but devoured it as food--a circumstance not the least remarkable in its proceedings.

As the two membranes of leaf thus deprived of the enclosed pulp appeared white and transparent, every movement of the insect within could be distinctly seen; and it was not a little interesting to watch its ingenious operations while it was making its tent from the membranes prepared as we have just described. These, as Réaumur has remarked, are in fact to the insect like a piece of cloth in the hands of a tailor; and no tailor could cut out a shape with more neatness and dexterity than this little workman does. As the caterpillar is furnished in its mandibles with an excellent pair of scissors, this may not appear to be a difficult task; yet, when we examine the matter more minutely, we find that the peculiar shape of the two extremities requires different curvatures, and this, of course, renders the operation no less complex, as Réaumur subjoins, than the shaping of the pieces of cloth for a coat. The insect, in fact, shapes the membranes slightly convex on one side and concave on the other, and at one end twice as large as the other. In the instance which we observed, beginning at the larger end, it bent them gently on each side by pressing them with its body thrown into a curve. We have not said it cuts, but shapes its materials; for it must be obvious that if the insect had cut both the membranes at this stage of its operations, the pieces would have fallen and carried it along with them.

To obviate such an accident it proceeded to join the two edges, and secure them firmly with silk, before it made a single incision to detach them. When it had in this manner joined the two edges along one of the sides, it inserted its head on the outside of the joining, first at one end and then at the other, gnawing the fibres till that whole side was separated. It proceeded in the same manner with the other side, joining the edges before it cut them: and when it arrived at the last fibre, the only remaining support of its now finished tent, it took the precaution, before snipping it, to moor the whole to the uncut part of the leaf by a cable of its own silk. Consequently, when it does cut the last nervure, it is secure from falling, and can then travel along the leaf, carrying its tent on its back, as a snail does its shell. (J. R.)

We have just discovered (Nov. 4th, 1829) upon the nettle a tent of a very singular appearance, in consequence of the materials of which it is made. The caterpillar seems, indeed, to have proceeded exactly in the same manner as those which we have described, mining first between the two membranes of the leaf, and then uniting these and cutting out his tent. But the tent itself looks singular from being all over studded with the stinging bristles of the nettle, and forming a no less formidable coat of mail to the little inhabitant than the spiny hide of the hedgehog. In feeding it does not seem to have mined into the leaf, but to have eaten the whole of the lower membrane, along with the entire pulp, leaving nothing but the upper membrane untouched. (J. R.) During the summer of 1830 we discovered a very large tent which had been formed out of a blade of grass; and another stuck all over with chips of leaves upon the common maple.

Tents of Stone-Mason Caterpillars.

The caterpillar of a small moth (Tinea) which feeds upon the lichens growing on walls, builds for itself a moveable tent of a very singular kind. M. de la Voye was the first who described these insects; but though they are frequently overlooked, from being very small, they are by no means uncommon on old walls. Réaumur observed them regularly for twenty years together on the terrace-wall of the Tuileries at Paris; and they may be found in abundance in similar situations in this country. This accurate observer refuted by experiment the notion of M. de la Voye that the caterpillars fed upon the stones of the wall; but he satisfied himself that they detached particles of the stone for the purpose of building their tents or sheaths (fourreaux), as he calls their dwellings. In order to watch their mode of building, Réaumur gently ejected half-a-dozen of them from their homes, and observed them detach grain after grain from a piece of stone, binding each into the wall of their building with silk till the cell acquired the requisite magnitude, the whole operation taking about twenty-four hours of continued labour. M. de la Voye mentions small granular bodies of a greenish colour, placed irregularly on the exterior of the structure, which he calls eggs; but we agree with Réaumur in thinking it more probable that they are small fragments of moss or lichen intermixed with the stone: in fact, we have ascertained that they are so. (J. R.)