We have watched this little architect begin and finish his tent upon a nettle in our study, the whole operation taking more than half an hour. (J. R.) He began by walking over the plant in all directions, examining the leaves severally, as if to ascertain which was best fitted for his purpose by being pliable, and bending with the weight of his body. Having found one to his mind, he placed himself along the mid-rib, to the edge of which he secured himself firmly with the pro-legs of his tail; then stretching his head to the edge of the leaf, he fixed a series of parallel cables between it and the mid-rib, with another series crossing these at an acute angle. The position in which he worked was most remarkable, for he did not, as might have been supposed, spin his cables with his face to the leaf, but throwing himself on his back, which was turned towards the leaf, he hung with his whole weight by his first-made cables. This, by drawing them into the form of a curve, shortened them, and consequently pulled the edge of the leaf down towards the mid-rib. The weight of his body was not, however, the only power which he employed; for, using the terminal pro-legs as a point of support, he exerted the whole muscles of his body to shorten his threads, and pull down the edge of the leaf. When he had drawn the threads as tight as he could, he held them till he spun fresh ones of sufficient strength to retain the leaf in the bent position into which he had pulled it. He then left the first series to hang loose while he shortened the fresh-spun ones as before. This process was continued till he had worked down about an inch and a half of the leaf, as much as he deemed sufficient for his habitation. This was the first part of the architecture.
By the time he had worked to the end of the fold, he had brought the edge of the leaf to touch the mid-rib; but it was only held in this position by a few of the last-spun threads, for all the first-spun ones hung loose within. Apparently aware of this, the insect protruded more than half of its body through the small aperture left at the end, and spun several bundles of threads on the outside precisely similar to those ropes of a tent which extend beyond the canvas, and are pegged into the ground. Unwilling to trust the exposure of his whole body on the outside, lest he should be seized by the first sand-wasp (odynerus) or sparrow which might descry him, he now withdrew to complete the internal portion of his dwelling, where the threads were hanging loose and disorderly. For this purpose he turned his head about, and proceeded precisely as he had done at the beginning of his task, but taking care to spin his new threads so as to leave the loose ones on the outside, and make his apartment smooth and neat. When he again reached the opposite end, he constructed there also a similar series of cables on the outside, and then withdrew to give some final touches to the interior.
It is said by Kirby and Spence,[BJ] that when these leaf-rolling insects find that the larger nervures of the leaves are so strong as to prevent them from bending, they “weaken it by gnawing it here and there half through.” We have never observed the circumstance, though we have witnessed the process in some hundreds of instances; and we doubt the statement, from the careful survey which the insect makes of the capabilities of the leaf before the operation is begun. If she found upon examination that a leaf would not bend, she would reject it, as we have often seen happen, and pass to another. (J. R.)
* * * * *
A species of leaf-roller, of the most diminutive size, merits particular mention, although it is not remarkable in colour or figure. It is without hair, of a greenish-white, and has all the vivacity of the other leaf-rollers. Sorrel is the plant on which it feeds; and the manner in which it rolls a portion of the leaf is very ingenious.
The structure which it contrives is a sort of conical pyramid, composed of five or six folds lapped round each other. From the position of this little cone the caterpillar has other labours to perform, beside that of rolling the leaf. It first cuts across the leaf, its teeth acting as a pair of scissors; but it does not entirely detach this segment. It rolls it up very gradually, by attaching threads of silk to the plane surface of the leaf, as we have before seen; and then, having cut in a different direction, sets the cone upright, by weaving other threads, attached to the centre of the roll and the plane of the leaf, upon which it throws the weight of its body. This, it will be readily seen, is a somewhat complicated effort of mechanical skill. It has been minutely described by M. Réaumur; but the preceding representation will perhaps make the process clearer than a more detailed account.
This caterpillar, like those of which we have already spoken, devours all the interior of the roll. It weaves, also, in the interior, a small and thin cocoon of white silk, the tissue of which is made compact and close. It is then transformed into a chrysalis.
The caterpillars of two of our largest and handsomest butterflies, the painted lady (Cynthia cardui, Stephens), and the admiral, or Alderman of the London fly-fanciers (Vanessa atalanta), are also leaf-rollers. The first selects the leaves of the great spear-thistle, and sometimes those of the stemless or star-thistle, which might be supposed rather difficult to bend; but the caterpillar is four times as large and strong as those which we have been hitherto describing. In some seasons it is plentiful; in others it is rarely to be met with: but the admiral is seldom scarce in any part of the country; and by examining the leaves of nettles which appear folded edge to edge, in July and August, the caterpillar may be readily found.
Another butterfly (Hesperia malvæ) is met with on dry banks where mallows grow, in May, or even earlier, and also in August, but is not indigenous. The caterpillar, which is grey, with a black head, and four sulphur-coloured spots on the neck, folds around it the leaves of the mallow, upon which it feeds. There is nothing, however, peculiarly different in its proceedings from those above described; but the care with which it selects and rolls up one of the smaller leaves, when it is about to be transformed into a chrysalis, is worthy of remark; it joins it, indeed, so completely round and round, that it has somewhat the resemblance of an egg. Within this green cell it lies secure, till the time arrives when it is ready to burst its cerements, and trust to the quickness of its wings for protection against its enemies.
Among the nests of caterpillars which roll up parcels of leaves, we know none so well contrived as those which are found upon willows and a species of osier. The long and narrow leaves of these plants are naturally adapted to be adjusted parallel to each other; for this is the direction which they have at the end of each stalk, when they are not entirely developed. One kind of small smooth caterpillar (Tortrix chlorana), with sixteen feet, the under part of which is brown, and streaked with white, fastens these leaves together, and makes them up into parcels. There is nothing particularly striking in the mechanical manner in which it constructs them. It does precisely what we should do in a similar case: it winds a thread round those leaves which must be kept together, from a little above their termination to a very short distance from their extreme point; and as it finds the leaves almost constantly lying near each other, it has little difficulty in bringing them together, as is shown in the following cut, a.
The prettiest of these parcels are those which are made upon a kind of osier, the borders of whose leaves sometimes form columnar bundles before they become developed. A section of these leaves has the appearance of filigree-work (see b).
A caterpillar which feeds upon the willow, and whose singular attitudes have obtained for it the trivial name of Ziczac, also constructs for itself an arbour of the leaves, by drawing them together in an ingenious manner. M. Roesel[BK] has given a tolerable representation of this nest, and of the caterpillar. The caterpillar is found in June; and the moth (Notodonta ziczac) from May to July in the following year (see cut, p. 151).
Beside those caterpillars which live solitary in the folds of a leaf, there are others which associate, employing their united powers to draw the leaves of the plants they feed upon into a covering for their common protection. Among these we may mention the caterpillar of a small butterfly, the plantain or Glanville fritillary (Melitea cinxia), which is very scarce in this country.
Although a colony of these caterpillars is not numerous, seldom amounting to a hundred individuals, the place which they have selected is not hard to discover. Their abode may be seen in the meadow in form of a tuft of herbage covered with a white web, which may readily be mistaken, at first view, for that of a spider, but closer inspection soon corrects this notion. It is, in fact, a sort of common tent, in which the whole brood lives, eats, and undergoes the usual transformations. The shape of this tent, for the most part, approaches the pyramidal, though that depends much upon the natural growth of the herbage which composes it. The interior is divided into compartments formed by the union of several small tents, as it were, to which others have been from time to time added according to the necessities of the community.
When they have devoured all the leaves, or at least those which are most tender and succulent, they abandon their first camp, and construct another contiguous to it under a tuft of fresh leaves. Several of these encampments may sometimes be seen within the distance of a foot or two, when they can find plantain (Plantago lanceolata) fit for their purpose; but though they prefer this plant, they content themselves with grass if it is not to be procured.
When they are about to cast their skins, but particularly when they perceive the approach of winter, they construct a more durable apartment in the interior of their principal tent. The ordinary web is thin and semi-transparent, permitting the leaves to be seen through it; but their winter canvas, if we may call it so, is thick, strong, and quite opaque, forming a sort of circular hall without any partition, where the whole community lie coiled up and huddled together.
Early in spring they issue forth in search of fresh food, and again construct tents to protect them from cold and rain, and from the mid-day sun.
M. Réaumur found upon trial, that it was not only the caterpillars hatched from the eggs of the same mother which would unite in constructing the common tent; for different broods, when put together, worked in the same social and harmonious manner. We ourselves ascertained, during the present summer (1829), that this principle of sociality is not confined to the same species, nor even to the same genus. The experiment which we tried was to confine two broods of different species to the same branch, by placing it in a glass of water to prevent their escape. The caterpillars which we experimented on were several broods of the brown-tail moth (Porthesia auriflua) and the lackey (Clisiocampa neustria). These we found to work with as much industry and harmony in constructing the common tent as if they had been at liberty on their native trees; and when the lackeys encountered the brown-tails they manifested no alarm nor uneasiness, but passed over the backs of one another, as if they had made only a portion of the branch. In none of their operations did they seem to be subject to any discipline, each individual appearing to work, in perfecting the structure, from individual instinct, in the same manner as was remarked by M. Huber in the case of the hive-bees. In making such experiments, it is obvious that the species of caterpillars experimented with must feed upon the same sort of plant.[BL] (J. R.)
The design of the caterpillars in rolling up the leaves is not only to conceal themselves from birds and predatory insects, but also to protect themselves from the cuckoo-flies, which lie in wait in every quarter to deposit their eggs in their bodies, that their progeny may devour them. Their mode of concealment, however, though it appear to be cunningly contrived and skilfully executed, is not always successful, their enemies often discovering their hiding-place. We happened to see a remarkable instance of this last summer (1828), in the case of one of the lilac caterpillars which had changed into a chrysalis within the closely-folded leaf. A small ichneumon, aware it should seem of the very spot where the chrysalis lay within this leaf, was seen boring through it with her ovipositor, and introducing her eggs through the punctures thus made into the body of the dormant insect. We allowed her to lay all her eggs, about six in number, and then put the leaf under an inverted glass. In a few days the eggs of the cuckoo-fly were hatched, the grubs devoured the lilac chrysalis, and finally changed into pupæ in a case of yellow silk, and into perfect insects like their parents. (J. R.)
INSECTS FORMING HABITATIONS OF DETACHED LEAVES.
The habitations of the insects which we have just described consist of growing leaves, bent, rolled, or pressed together, and fixed in their positions by silken threads. But there are other habitations of a similar kind, which are constructed by cutting out and detaching a whole leaf, or a portion of a leaf. We have already seen how dexterously the upholsterer-bees cut out small parts of leaves and petals with their mandibles, and fit them into their cells. Some of the caterpillars do not exhibit quite so much neatness and elegance as the leaf-cutting bees, though their structures answer all the purposes intended; but there are others, as we shall presently see, that far excel the bees, at least in the delicate minutiæ of their workmanship. We shall first advert to those structures which are the most simple.
* * * * *
Not far from Longchamps, in a road through the Bois de Boulogne, is a large marsh, which M. Réaumur never observed to be in a dry state even during summer. This marsh is surrounded with very lofty oaks, and abounds with pondweed, the water-plant named by botanists potamogeton. The shining leaves of this plant, which are as large as those of the laurel or orange-tree, but thicker and more fleshy, are spread upon the surface of the water. Having pulled up several of these about the middle of June, M. Réaumur observed, beneath one of the first which he examined, an elevation of an oval shape, which was formed out of a leaf of the same plant. He carefully examined it, and discovered that threads of silk were attached to this elevation. Breaking the threads, he raised up one of the ends, and saw a cavity, in which a caterpillar (Hydrocampa potamogeta) was lodged. An indefatigable observer, such as M. Réaumur, would naturally follow up this discovery; and he has accordingly given us a memoir of the pondweed tent-maker, distinguished by his usual minute accuracy.
In order to make a new habitation, the caterpillar fastens itself on the under side of a leaf of the Potamogeton. With its mandibles it pierces some part of this leaf, and afterwards gradually gnaws a curved line, marking the form of the piece which it wishes to detach. When the caterpillar has cut off, as from a piece of cloth, a patch of leaf of the size and shape suited to its purpose, it is provided with half of the materials requisite for making a tent. It takes hold of this piece by its mandibles, and conveys it to the situation on the under side of its own or another leaf, whichever is found most appropriate. It is there disposed in such a manner that the under part of the patch—the side which was the under part of the entire leaf—is turned towards the under part of the new leaf, so that the inner walls of the cell or tent are always made by the under part of two portions of leaf. The leaves of the potamogeton are a little concave on the under side; and thus the caterpillar produces a hollow cell, though the rims are united.
The caterpillar secures the leaf in its position by threads of white silk. It then weaves in the cavity a cocoon, which is somewhat thin, but of very close tissue. There it shuts itself up, to appear again only in the form of the perfect insect, and is soon transformed into a chrysalis. In this cocoon of silk no point touches the water; whilst the tent of leaves, lined with silk, has been constructed underneath the water. This fact proves that the caterpillar has a particular art by which it repels the water from between the leaves.
When the caterpillar, which has thus conveyed and disposed a patch of leaf against another leaf, is not ready to be transformed into a chrysalis, it applies itself to make a tent or habitation which it may carry everywhere about with it. It begins by slightly fixing the piece against the perfect leaf, leaving intervals all round, between the piece and leaf, at which it may project its head. The piece which it has fixed serves as a model for cutting out a similar piece in the other leaf. The caterpillar puts them accurately together, except at one end of the oval, where an opening is left for the insect to project its head through. When the caterpillar is inclined to change its situation, it draws itself forward by means of its scaly limbs, riveted upon the leaf. The membraneous limbs, which are riveted against the inner sides of the tents, oblige it to follow the anterior part of the body, as it advances. The caterpillar, also, puts its head out of the tent every time it desires to eat.
There is found on the common chickweed (Stellaria media), towards the end of July, a middle-sized smooth green caterpillar, having three brown spots bordered with white on the back, and six legs and ten pro-legs, whose architecture is worthy of observation. When it is about to go into chrysalis, towards the beginning of August, it gnaws off, one by one, a number of the leaves and smaller twigs of the chickweed, and adjusts them into an oval cocoon, somewhat rough and unfinished externally, but smooth, uniform, and finely tapestried with white silk within. Here it undergoes its transformation securely, and, when the period of its pupa trance has expired in the following July, it makes its exit in the form of a yellowish moth, with several brown spots above, and a brown band on each of its four wings below. It is also furnished with a sort of tail.
On the cypress-spurge (Euphorbia cyparissias), a native woodland plant, but not of very common occurrence, may be found, towards the end of October, a caterpillar of a middle size, sparely tufted with hair, and striped with black, white, red, and brown. The leaves of the plant, which are in the form of short narrow blades of grass, are made choice of by the caterpillar to construct its cocoon, which it does with great neatness and regularity, the end of each leaf, after it has been detached from the plant, being fixed to the stem, and the other leaves placed parallel, as they are successively added. The other ends of all these are bent inwards, so as to form a uniformly rounded oblong figure, somewhat larger at one end than at the other.
A caterpillar which builds a very similar cocoon to the last-mentioned may be found upon a more common plant—the yellow snap-dragon or toad-flax (Antirrhinum linaria)—which is to be seen in almost every hedge. It is somewhat shaped like a leech, is of a middle size, and the prevailing colour pearl-grey, but striped with yellow and black. It spins up about the beginning of September, forming the outer coating of pieces of detached leaves of the plant, and sometimes of whole leaves placed longitudinally, the whole disposed with great symmetry and neatness. The moth appears in the following June.
It is worthy of remark, as one of the most striking instances of instinctive foresight, that the caterpillars which build structures of this substantial description are destined to lie much longer in their chrysalis trance than those which spin merely a flimsy web of silk. For the most part, indeed, the latter undergo their final transformation in a few weeks; while the former continue entranced the larger portion of a year, appearing in the perfect state the summer after their architectural labours have been completed. (J. R.) This is a remarkable example of the instinct which leads these little creatures to act as if under the dictates of prudence, and with a perfect knowledge of the time, be it long or short, which will elapse before the last change of the pupa takes place. That the caterpillar, while weaving its cocoon and preparing to assume the pupa state, exercises any reflective faculties, or is aware of what is about to occur relative to its own self, we cannot admit. It enters upon a work of which it has had no previous experience, and which is performed, as far as contingencies allow, in the same manner by every caterpillar of the same species. Its labours, its mode of carrying them on, and the very time in which they are to be commenced, are all pre-appointed; and an instinctive impulse urges and guides; and with this instinct its organic endowments are in precise harmony; nor does instinct ever impel to labours for which an animal is not provided. “The same wisdom,” says Bonnet, “which has constructed and arranged with so much art the various organs of animals, and has made them concur towards one determined end, has also provided that the different operations which are the natural results of the economy of the animal should concur towards the same end. The creature is directed towards his object by an invisible hand; he executes with precision, and by one effort, those works which we so much admire; he appears to act as if he reasoned, to return to his labour at the proper time, to change his scheme in case of need. But in all this he only obeys the secret influence which drives him on. He is but an instrument which cannot judge of each action, but is wound up by that adorable Intelligence, which has traced out for every insect its proper labours, as he has traced the orbit of each planet. When, therefore, I see an insect working at the construction of a nest, or a cocoon, I am impressed with respect, because it seems to me that I am at a spectacle where the Supreme Artist is hid behind the curtain.”[BM]
There is a small sort of caterpillar which may be found on old walls, feeding upon minute mosses and lichens, the proceedings of which are well worthy of attention. They are similar, in appearance and size, to the caterpillar of the small cabbage-butterfly (Pontia rapæ), and are smooth and bluish. The material which they use in building their cocoons is composed of the leaves and branchlets of green moss, which they cut into suitable pieces, detaching at the same time along with them a portion of the earth in which they grow. They arrange these upon the walls of their building, with the moss on the outside, and the earth on the inside, making a sort of vault of the tiny bits of green moss turf, dug from the surface of the wall. So neatly, also, are the several pieces joined, that the whole might well be supposed to be a patch of moss which had grown in form of an oval tuft, a little more elevated than the rest growing on the wall. When these caterpillars are shut up in a box with some moss, without earth, they construct with it cells in form of a hollow ball, very prettily plaited and interwoven.
In May last (1829), we found on the walls of Greenwich Park a great number of caterpillars, whose manners bore some resemblance to those of the grub described by M. Réaumur. (J. R.) They were of middle size, with a dull-orange stripe along the back; the head and sides of the body black, and the belly greenish. Their abodes were constructed with ingenuity and care. A caterpillar of this sort appears to choose either a part where the mortar contains a cavity, or it digs one suited to its design. Over the opening of the hollow in the mortar it builds an arched wall, so as to form a chamber considerably larger than is usual with other architect caterpillars. It selects grains of mortar, brick, or lichen, fixing them, by means of silk, firmly into the structure. As some of these vaulted walls were from an inch to an inch and a half long, and about a third of an inch wide and deep, it may be well imagined that it would require no little industry and labour to complete the work; yet it does not demand more than a few hours for the insect to raise it from the foundation. Like all other insect architects, this caterpillar uses its own body for a measuring-rule, and partly for a mould, or rather a block or centre to shape the walls by, curving itself round and round concentrically with the arch which it is building.
We afterwards found one of these caterpillars, which had dug a cell in one of the softest of the bricks, covering itself on the outside with an arched wall of brick-dust, cemented with silk. As this brick was of a bright-red colour, we were thereby able to ascertain that there was not a particle of lichen employed in the structure.
The neatness mentioned by Réaumur, as remarkable in his moss-building caterpillars, is equally observable in that which we have just described; for, on looking at the surface of the wall, it would be impossible for a person unacquainted with those structures to detect where they were placed, as they are usually, on the outside, level with the adjoining brick-work; and it is only when they are opened by the entomologist, that the little architect is perceived lying snug in his chamber. If a portion of the wall be thus broken down, the caterpillar immediately commences repairing the breach, by piecing in bits of mortar and fragments of lichen, till we can scarcely distinguish the new portion from the old.
CADDIS-WORMS AND CARPENTER-CATERPILLARS.
There is a very interesting class of grubs which live under water, where they construct for themselves moveable tents of various materials as their habits direct them, or as the substances they require can be conveniently procured. Among the materials used by these singular grubs, well-known to fishermen by the name of caddis-worms, and to naturalists as the larvæ of the four-winged flies in the order Trichoptera of Kirby and Spence, we may mention sand, stones, shells, wood, and leaves, which are skilfully joined and strongly cemented. One of these grubs forms a pretty case of leaves glued together longitudinally, but leaving an aperture sufficiently large for the inhabitant to put out its head and shoulders when it wishes to look about for food. Another employs pieces of reed cut into convenient lengths, or of grass, straw, wood, &c., carefully joining and cementing each piece to its fellow as the work proceeds; and he frequently finishes the whole by adding a broad piece longer than the rest to shade his door-way overhead, so that he may not be seen from above. A more laborious structure is reared by the grub of a beautiful caddis-fly (Phryganea), which weaves together a group of the leaves of aquatic plants into a roundish ball, and in the interior of this forms a cell for its abode. The fallowing figure from Roesel will give a more precise notion of this structure than a lengthened description.
Another of these aquatic architects makes choice of the tiny shells of young fresh-water mussels and snails (Planorbis), to form a moveable grotto; and as these little shells are for the most part inhabited, he keeps the poor animals close prisoners, and drags them without mercy along with him. These grotto-building grubs are by no means uncommon in ponds; and in chalk districts, such as the country about Woolwich and Gravesend, they are very abundant.
One of the most surprising instances of their skill occurs in the structures of which small stones are the principal material. The problem is to make a tube about the width of the hollow of a wheat-straw or a crow-quill, and equally smooth and uniform. Now the materials being small stones full of angles and irregularities, the difficulty of performing this problem will appear to be considerable, if not insurmountable: yet the little architects, by patiently examining their stones and turning them round on every side, never fail to accomplish their plans. This, however, is only part of the problem, which is complicated with another condition, and which we have not found recorded by former observers, namely, that the under-surface shall be flat and smooth, without any projecting angles which might impede its progress when dragged along the bottom of the rivulet where it resides. The selection of the stones, indeed, may be accounted for, from this species living in streams where, but for the weight of its house, it would to a certainty be swept away. For this purpose, it is probable that the grub makes choice of larger stones than it might otherwise want; and therefore also it is that we frequently find a case composed of very small stones and sand, to which, when nearly finished, a large stone is added by way of ballast. In other instances, when the materials are found to possess too great specific gravity, a bit of light wood, or a hollow straw, is added to buoy up the case.
| Stone Nest of Caddis-Worm. | Sand Nest balanced with a Stone. |
Nest of Caddis-Worm balanced with Straws. |
It is worthy of remark, that the cement, used in all these cases, is superior to pozzolana[BN] in standing water, in which it is indissoluble. The grubs themselves are also admirably adapted for their mode of life, the portion of their bodies which is always enclosed in the case being soft like a meal-worm, or garden-caterpillar, while the head and shoulders, which are for the most part projected beyond the door-way in search of food, are firm, hard, and consequently less liable to injury than the protected portion, should it chance to be exposed.
We have repeatedly tried experiments with the inhabitants of those aquatic tents, to ascertain their mode of building. We have deprived them of their little houses, and furnished them with materials for constructing new ones, watching their proceedings from their laying the first stone or shell of the structure. They work at the commencement in a very clumsy manner, attaching a great number of chips to whatever materials may be within their reach with loose threads of silk, and many of these they never use at all in their perfect building. They act, indeed, much like an unskilful workman trying his hand before committing himself upon an intended work of difficult execution. Their main intention is, however, to have abundance of materials within reach: for after their dwelling is fairly begun, they shut themselves up in it, and do not again protrude more than half of their body to procure materials; and even when they have dragged a stone, a shell, or a chip of reed within building reach, they have often to reject it as unfit. (J. R.)
[We have here some examples of the latter kind of nest, i. e., those habitations which are made of stones and shells. Beginning at the upper left-hand figure, we find one that is made of moderately-sized stones cemented together in a way that reminds the observer of the manner in which a builder forms irregular stones into a wall. Next to it is another, in which the stones are larger and narrower, and are arranged much as some of the caddis-worms arrange pieces of stick and straw.
In the second, and on the left-hand side, is a very long and simple tube, made of a grass stem, and balanced by three little sticks attached to its centre. The next figure represents a number of sand-tubes attached to each other. These are built up laboriously of single particles of sand, and are remarkable for their peculiar horn-like shape, the tube having the same regular curve as the horn of an ox or antelope, and tapering gradually from the base to the top. A somewhat similar tube, but of larger size, is shown in the right-hand figure.
Any one who wishes to see one of these creatures rebuild its house can do so by carefully removing it from its tube, and supplying it with fresh material. Very great care must be taken in the removal, as the grub is easily damaged, and it holds so tightly to the tube with a pair of pincers at the end of its body, that it must rather be coaxed than driven out.
If desirable, they can be made to build their new houses of most singular materials. A lady, Miss Smee, was very successful in a series of experiments which she made with these insects, forcing them to make tubes of different colours and patterns, by supplying them with coloured sand, pieces of stained glass of various hues, gold dust, and similar materials. Although there was scarcely any material which they would not use, they seemed to consider a certain amount of angularity as essential, and rejected any object, such as a bead, of which the surface was perfectly rounded, while they would accept the same, if it were broken or indented.
When the caddis-grub has ceased from feeding, and is about to pass into the perfect stage, it spins over the mouth of the tube a strong silken web. This web is made in quite a pretty pattern, and being woven with rather wide meshes, it allows the water to flow through the tube while it prevents any aquatic foes from penetrating and destroying the pupa.
The remaining figures of the illustration represent tubes, around which are built a quantity of small shells. Generally, stones are mixed with the shells; but in some cases, shells seem to be almost the only material.]
Carpenter-Caterpillars.
Insects, though sometimes actuated by an instinct apparently blind, unintelligent, or unknown to themselves, manifest in other instances a remarkable adaptation of means to ends. We have it in our power to exemplify this in a striking manner by the proceedings of the caterpillar of a goat-moth (Cossus ligniperda) which we kept till it underwent its final change.
This caterpillar, which abounds in Kent and many other parts of the island, feeds on the wood of willows, oaks, poplars, and other trees, in which it eats extensive galleries; but it is not contented with the protection afforded by these galleries during the colder months of winter, before the arrival of which it scoops out a hollow in the tree, if it does not find one ready prepared, sufficiently large to contain its body in a bent or somewhat coiled-up position. On sawing off a portion of an old poplar in the winter of 1827, we found such a cell with a caterpillar coiled up in it.
It had not, however, been contented with the bare walls of the retreat which it had hewn out of the tree, for it had lined it with a fabric as thick as coarse broadcloth, and equally warm, composed of the raspings of the wood scooped out of the cell, united with the strong silk which every species of caterpillar can spin. In this snug retreat our caterpillar, if it had not been disturbed, would have spent the winter without eating; but upon being removed into a warm room and placed under a glass along with some pieces of wood, which it might eat if so inclined, it was roused for a time from its dormant state, and began to move about. It was not long, however, in constructing a new cell for itself, no less ingenious than the former. It either could not gnaw into the fir plank, where it was now placed with a glass above it, or it did not choose to do so; for it left it untouched, and made it the basis of the edifice it began to construct. It formed, in fact, a covering for itself precisely like the one from which we had dislodged it,—composed of raspings of wood detached for the purpose from what had been given it as food, the largest piece of which was employed as a substantial covering and protection for the whole. It remained in this retreat, motionless, and without food, till revived by the warmth of the ensuing spring, when it gnawed its way out, and began to eat voraciously, to make up for its long fast.
These caterpillars are three years in arriving at their final change into the winged state; but as the one just mentioned was nearly full grown, it began, in the month of May, to prepare a cell, in which it might undergo its metamorphosis. Whether it had actually improved its skill in architecture by its previous experience we will not undertake to say, but its second cell was greatly superior to the first. In the first there was only one large piece of wood employed; in the second, two pieces were placed in such a manner as to support each other, and beneath the angle thus formed an oblong structure was made, composed, as before, of wood-raspings and silk, but much stronger in texture than the winter cell. In a few weeks (four, if we recollect aright) the moth came forth. (J. R.)
[I have now before me a series of three cocoons, made by one caterpillar of the goat-moth, showing its increase in size during the three years that it remained in the larval state. They were found in an old willow tree, and occupied different parts of the same burrow. The ravages which a goat-moth caterpillar can make in a tree are almost incredible to those who have not seen the long and tortuous burrows which the insect will construct, burrows which at first are small and insignificant, but which afterwards become large enough to admit a man’s finger.
Sometimes the tunnel runs just under the bark, and sometimes it goes straight towards the centre of the tree; and no small labour is required before it can be fully traced. Still, the result is worth the labour, for it is most interesting to trace the creature through its whole existence, from the tiny hole which it made soon after its exit from the egg, to the large aperture through which it emerged as a moth. The whole of the tunnel is strongly imbued with the peculiar and unpleasant odour which has given to the goat-moth its popular name; and the scent is so persistent, that it adheres to the fingers which have touched the sides of the tunnel, and can scarcely be removed even by repeated washings.
The moth itself is a well-known insect, though rarely seen except by night. It is large, brown, round bodied; the wings are covered with a soft and downy clothing, which strongly reminds the observer of the plumage of an owl.]
A wood-boring caterpillar, of a species of moth much rarer than the preceding (Ægeria asiliformis, Stephens), exhibits great ingenuity in constructing a cell for its metamorphosis. We observed above a dozen of them during this summer (1829) in the trunk of a poplar, one side of which had been stripped of its bark. It was this portion of the trunk which all the caterpillars selected for their final retreat, not one having been observed where the tree was covered with bark. The ingenuity of the little architect consisted in scooping its cell almost to the very surface of the wood, leaving only an exterior covering of unbroken wood, as thin as writing-paper. Previous, therefore, to the chrysalis making its way through this feeble barrier, it could not have been suspected that an insect was lodged under the smooth wood. We observed more than one of these in the act of breaking through this covering, within which there is, besides, a round moveable lid of a sort of brown wax. (J. R.)
Another architect caterpillar, frequently to be met with in July on the leaves of the willow and the poplar, is, in the fly-state, called the puss-moth (Cerura vinula). The caterpillar is produced from brown-coloured shining eggs, about the size of a pin’s head, which are deposited—one, two, or more together—on the upper surface of a leaf. In the course of six or eight weeks (during which time it casts its skin thrice) it arrives at its full growth, when it is about as thick, and nearly as long, as a man’s thumb, and begins to prepare a structure in which the pupa may sleep securely during the winter. As we have, oftener than once, seen this little architect at work, from the foundation till the completion of its edifice, we are thereby enabled to give the details of the process.
| Eggs of the Puss-Moth. | Rudiments of the Cell of the Puss-Moth. |
The puss, it may be remarked, does not depend for protection on the hole of a tree, or the shelter of an overhanging branch, but upon the solidity and strength of the fabric which it rears. The material it commonly uses is the bark of the tree upon which the cell is constructed; but when this cannot be procured, it is contented to employ whatever analogous materials may be within reach. One which we had shut up in a box substituted the marble paper it was lined with for bark, which it could not procure.[BO] With silk it first wove a thin web round the edges of the place which it marked out for its edifice, then it ran several threads in a spare manner from side to side, and from end to end, but very irregularly in point of arrangement; these were intended for the skeleton or framework of the building. When this outline was finished, the next step was to strengthen each thread of silk by adding several (sometimes six or eight) parallel ones, all of which were then glued together into a single thread, by the insect running its mandibles, charged with gluten, along the line. The meshes, or spaces, which were thus widened by the compression of the parallel threads, were immediately filled up with fresh threads, till at length only very small spaces were left. It was in this stage of the operation that the paper came into requisition, small portions of it being gnawed off the box and glued into the meshes. It was not, however, into the meshes only that the bits of paper were inserted; for the whole fabric was in the end thickly studded over with them. In about half a day from the first thread of the framework being spun the building was completed. It was at first, however, rather soft, and yielded to slight pressure with the finger; but as soon as it became thoroughly dry, it was so hard that it could with difficulty be penetrated with the point of a penknife. (J. R.)
[One puss-moth larva, which I reared, made its nest in a rather curious manner. After it had ceased feeding it had been placed on a marble mantelpiece under a glass tumbler, as a temporary residence until a more appropriate dwelling could be found for it. But its instincts urged it to make its nest without delay, and it accordingly set to work, and spun itself up in a cocoon composed entirely of its own silk, neither the glass tumbler or the mantelpiece affording it any material with which to harden the walls of its dwelling.