Insect Architecture

Emperor-Moth.

The emperor-moth, indeed, is no less worthy of our attention with respect to the ingenuity of its architecture than the beauty of its colours, and has consequently attracted the attention of every Entomologist. The caterpillar feeds on fruit-trees and on the willow, and spins a cocoon, in the form of a Florence flask, of strong silk, so thickly woven that it appears almost like damask or leather. It differs from most other cocoons in not being closed at the upper or smaller end, which terminates in a narrow circular aperture, formed by the convergence of little bundles of silk, gummed together, and almost as elastic as whalebone. In consequence of all these terminating in needle-shaped points, the entrance of depredators is guarded against, upon the principle which prevents the escape of a mouse from a wire trap. The insect, however, not contented with this protection, constructs another in form of a canopy or dome, within the external aperture, so as effectually to shield the chrysalis from danger. We have formerly remarked (page 210) that the caterpillar of the Ægeria asiliformis of Stephens in a similar way did not appear to be contented with a covering of thin wood, without an additional bonnet of brown wax. The cocoon of the emperor-moth, though thus in some measure impenetrable from without, is readily opened from within; and when the moth issues from its pupa case, it easily makes its way out without either the acid or eye-files ascribed to the silk-worm. The elastic silk gives way upon being pushed from within, and when the insect is fairly out, it shuts again of its own accord, like a door with spring hinges,—a circumstance which at first puzzled Roesel not a little when he saw a fine large moth in his box, and the cocoon apparently in the same state as when he had put it there. Another naturalist conjectures that the converging threads are intended to compress the body of the moth as it emerges, in order to force the fluids into the nervures of the wings; for when he took the chrysalis previously out of the cocoon, the wings of the moth never expanded properly.[EC] Had he been much conversant with breeding insects, he would rather, we think, have imputed this to some injury which the chrysalis had received. We have witnessed the shrivelling of the wings which he alludes to, in many instances, and not unfrequently in butterflies which spin no cocoon. The shrivelling, indeed, frequently arises from the want of a sufficient supply of food to the caterpillar in its last stage, occasioning a deficiency in the fluids.

The elasticity of the cocoon is not peculiar to the emperor-moth. A much smaller insect, the green cream-border-moth (Tortrix chlorana) before mentioned (page 190), for its ingenuity in bundling up the expanding leaves of the willow, also spins an elastic shroud for its chrysalis, of the singular shape of a boat with the keel uppermost. Like the caterpillar of Pyralis strigulalis (page 217), whose building, though of different materials, is exactly of the same form, its first spins two approximating walls of whitish silk, of the form required, and when these are completed, it draws them forcibly together with elastic threads, so placed as to retain them closely shut. The passage of the moth out of this cocoon might have struck Roesel as still more marvellous than that of his emperor, in which there was at least a small opening; while in the boat cocoon there is none. We have now before us two of these, which we watched the caterpillars through the process of building, in the summer of 1828, and from one only a moth issued—the other, as often happens, having died in the chrysalis. But what is most remarkable, it is impossible by the naked eye to tell which of these two has been opened by the moth, so neatly has the joining been finished. (J. R.)

Some species of moths spin a very slight silken tissue for their cocoons, being apparently intended more to retain them from falling than to afford protection from other accidents. The gipsy-moth (Hypogymna dispar), rare in most parts of Britain, is one of these. It selects for its retreat a crack in the bark of the tree upon which it feeds, and over this spins only a few straggling threads. We found last summer (1829), in the hole of an elm-tree in the Park at Brussels, a group of half a dozen of these, that did not seem to have spun any covering at all, but trusted to a curtain of moss (Hypna) which margined the entrance. (J. R.) In a species nearly allied to this, the yellow-tussock (Dasychira pudibunda, Stephens), the cocoon, one of which we have now before us, is of a pretty close texture, and interwoven with the long hairs of the caterpillar itself (see figure b, page 17), which it plucks out piecemeal during the process of building,—as is also done by the vapourer (Orgyia antiqua, Hubner), and many others.

These are additional instances of the remarks we formerly made, that caterpillars which spin a slight web are transformed into perfect insects in a much shorter period than those which spin more substantial ones. Thus the cream-spot tiger (Arctia villica, Stephens) lies in chrysalis only three weeks, and therefore does not require a strong web. It is figured above, along with another, which is still slighter, though more ingeniously woven, being regularly meshed like net-work.

A very prettily-netted cocoon is constructed by the grub of a very small grey weevil (Hypera rumicis), which is not uncommon in July, on the seed spikes of docks (Rumices). This cocoon is globular, and not larger than a garden pea, though it appears to be very large in proportion to the pupa of the insect, reminding us not a little of the carved ivory balls from China. The meshes of the net-work are also large, but the materials are strong and of a waxy consistence. Upon remarking that no netting was ever spun over the part of the plant to which the cocoon was attached, we endeavoured to make them spin cocoons perfectly globular by detaching them when nearly finished; but though we tried four or five in this way, we could not make them add a single mesh after removal, all of them making their escape through the opening, and refusing to re-enter in order to complete their structure. (J. R.)

The silk, if it may be so termed, spun by many species of larvæ is of a still stronger texture than the waxy silk of the little weevil just mentioned. We recently met with a remarkable instance of this at Lee, in the cocoons of one of the larger ichneumons (Ophion Vinulæ? Stephens), inclosed in that of a puss-moth (Cerura Vinula)—itself remarkable for being composed of sand as well as wood, the fibres of which had been scooped out of the underground cross-bar of an old paling, to which it was attached. But the most singular portion of this was the junction of the outer wall with the edges of the hollow thus scooped out, which was formed of fibres of wood placed across the fibres of the bar nearly at right angles, and strongly cemented together, as if to form a secure foundation for the building.

In this nest were formed, surreptitiously introduced into the original building, five empty cells of a black colour, about an inch long, and a sixth of an inch in diameter; nearly cylindrical in form, but somewhat flattened; vertical and parallel to one another, though slightly curved on the inner side. The cells are composed of strong and somewhat coarse fibres, more like the carbonized rootlets of a tree than silk, and resembling in texture a piece of coarse milled cloth or felt, such as is used for the bases of plated hats. It is worthy of remark, that all these cells opened towards one end, as if the caterpillars which constructed them had been aware that the wall of the puss-moth, in which the flies would have to make a breach, was very hard, and would require their united efforts to effect an escape. The importance of such a precaution will appear more strikingly, when we compare it with the instance formerly mentioned (page 215), in which only one ichneumon had been able to force its way out. (J. R.)

It appears indispensable to some grubs to be confined within a certain space in order to construct their cocoons. We saw this well exemplified in the instance of a grub of one of the mason-bees (Osmia bicornis), which we took from its nest, and put into a box with the pollen paste which the mother bee had provided for its subsistence. (See pages 45, 46.) When it had completed its growth, it began to spin, but in a very awkward manner-attaching threads, as if at random, to the bits of pollen which remained undevoured, and afterwards tumbling about to another part of the box, as if dissatisfied with what it had done. It sometimes persevered to spin in one place till it had formed a little vaulted wall; but it abandoned at the least three or four of these in order to begin others, till at length, as if compelled by the extreme urgency of the stimulus of its approaching change, it completed a shell of shining brown silk, woven into a close texture. Had the grub remained within the narrow clay cell built for it by the mother bee, it would, in all probability, not have thus exhausted itself in vain efforts at building, which were likely to prevent it from ever arriving at the perfect state—a circumstance which often happens in the artificial breeding of insects. This bee, however, made its appearance the following spring. (J. R.)

Besides silk, the cocoons of many insects are composed of other animal secretions, intended to strengthen or otherwise perfect their texture. We have already seen that some caterpillars pluck off their own hair to interweave amongst their silk; there are others which produce a peculiar substance for the same purpose. The lackey caterpillar (Clisiocampa neustria, Curtis) in this manner lines its cocoon with pellets of a downy substance, resembling little tufts of the flowers of sulphur. The small egger, again (Eriogaster lanestris, Germar), can scarcely be said to employ silk at all,—the cocoon being of a uniform texture, looking, at first sight, like dingy Paris plaster, or the shell of a pheasant’s egg; but upon being broken, and inspected narrowly, a few threads of silk may be seen interspersed through the whole. In size it is not larger than the egg of the gold-crested wren. It has been considered by Brahm a puzzling circumstance, that this cocoon is usually perforated with one or two little holes, as if made by a pin from without; and Kirby and Spence tell us that their use has not been ascertained.[ED] May they not be left as air-holes for the included chrysalis, as the close texture of the cocoon might, without this provision, prove fatal to the animal? Yet, on comparing one of these with a similar cocoon of the large egger-moth (Lasiocampa quercus), we find no air-holes in the latter, as we might have been led to expect from the closeness of its texture. We found a cocoon of a saw-fly (Trichiosoma), about the same size as that of the egger, attached to a hawthorn-twig, in a hedge at New Cross, Deptford, but of a leathery texture, and, externally, exactly the colour of the bark of the tree. During the summer of 1830 we found a considerable number of the same cocoons. These were all without air-holes. The egger, we may remark, unlike the dock-weevil or the bee-grub just mentioned, can work her cocoon without any point of attachment. We had a colony of these caterpillars in the summer of 1825, brought from Epping Forest, and saw several of them work their cocoons, and we could not but admire the dexterity with which they avoided filling up the little pin-holes. The supply of their building material was evidently measured out to them in the exact quantity required; for when we broke down a portion of their wall, by way of experiment, they did not make it above half the thickness of the previous portion, though they plainly preferred having a thin wall to leaving the breach unclosed. (J. R.)

Several species of caterpillars, that spin only silk, are social, like some of those we formerly mentioned, which unite to form a common tent of leaves (see pages 351, 352, &c.). The most common instance of this is in the caterpillars which feed on the nettle—the small tortoise-shell (Vanessa urticæ), and the peacock’s eye (V. I.). Colonies of these may be seen, after midsummer, on almost every clump of nettles, inhabiting a thin web of an irregular oval shape, from which they issue out to feed on the leaves, always returning when their appetite is satisfied, to assist their companions in extending their premises. Other examples, still more conspicuous from being seen on fruit-trees and in hedges, occur in the caterpillars of the small ermine-moth (Yponomeuta padella), and of the lackey (Clisiocampa neustria), which in some years are but too abundant, though in others they are seldom met with. In the summer of 1826, every hedge and fruit-tree around London swarmed with colonies of the ermine, though it has not since been plentiful; and in the same way, during the summer of 1829, the lackeys were to be seen everywhere. We mention this irregularity of appearance that our readers may not disappoint themselves by looking for what is not always to be found. It is probable that in 1830 the lackeys will be few, for, notwithstanding the myriads of caterpillars last summer, we saw only a single moth of this species, and out of a number of chrysalides which a young friend had in his nurse-boxes, not one moth was bred.

The small ermine does not, besides, feed so indiscriminately as many others, but when the bird-cherry (Prunus padus), its peculiar food, is not to be had, it will put up with blackthorn, plum-tree, hawthorn, and almost any sort of orchard fruit-tree. With respect to such caterpillars as feed on different plants, Réaumur and De Geer make the singular remark, that in most cases they would only eat the sort of plant upon which they were originally hatched.[EE] We verified this, in the case of the caterpillar in question, upon two different nests which we took, in 1806, from the bird-cherry at Crawfordland, in Ayrshire. Upon bringing these to Kilmarnock, we could not readily supply them with the leaves of this tree; and having then only a slight acquaintance with the habits of insects, and imagining they would eat any sort of leaf, we tried them with almost everything green in the vicinity of the town; but they refused to touch any which we offered them. After they had fasted several days, we at length procured some fresh branches of the bird-cherry, with which they gorged themselves so that most of them died. Last summer (1829) we again tried a colony of these caterpillars, found on a seedling plum-tree at Lee, in Kent, with blackthorn, hawthorn, and many other leaves, and even with those of the bird-cherry; but they would touch nothing except the seedling plum, refusing the grafted varieties. (J. R.)

A circumstance not a little remarkable in so very nice a feeder is, that in some cases the mother moth will deposit her eggs upon trees not of indigenous growth, and not even of the same genus with her usual favourites. Thus, in 1825, the cherry-apple, or Siberian crab (Pyrus prunifolia, Willdenow), so commonly grown in the suburbs of London, swarmed with them. On a single tree at Islington we counted above twenty nests, each of which would contain from fifty to a hundred caterpillars; and though these do not grow thicker than a crow-quill, so many of them scarcely left a leaf undevoured, and, of course, the fruit, which showed abundantly in spring, never came to maturity. The summer following they were still more abundant on the hawthorn hedges, particularly near the Thames, by Battersea and Richmond. Since then we have only seen them sparingly; and last summer we could only find the single nest upon which we tried the preceding experiment. (J. R.)

The caterpillars of other moths, which are in some years very common—such as the brown-tail (Porthesia auriflua), and the golden-tail (P. chrysorrhœa), are also social; and, as the eggs are hatched late in the summer, the brood passes the winter in a very closely-woven nest of warm silk. This is usually represented as composed of leaves which have had their pulpy parts eaten as food by the colonists; but from minute observation of at least twenty of these nests in the winter of 1828-9, we are quite satisfied that leaves are only an accidental, and not a necessary, part of the structure. When a leaf happens to be in the line of the walls of the nest, it is included; but there is no apparent design in pressing it into the service, nor is a branch selected because it is leafy. On the contrary, by far the greater number of these nests do not contain a single leaf, but are composed entirely of grey silk. In external form, no two of these nests are alike; as it depends entirely upon the form of the branch. When, therefore, there is only one twig, it is somewhat egg-shaped; but when there are several twigs, it commonly joins each, assuming an angular shape, as may be seen in the left-hand figure.

This irregularity arises from the circumstance of each individual acting on its own account, without the direction or superintendence of the others. The interior of the structure is, for the same reason, more regular, being divided into compartments, each of which forms a chamber for one or more individuals. Previous to the cold weather, these chambers have but slight partitions; but before the frosts set in the whole is made thick and warm.

None of the preceding details, however, appear so striking as what is recorded of the brown-tail moth (Porthesia auriflua) by Mr. W. Curtis,[EF] whose multitudinous colonies spread great alarm over the country in the summer of 1782. This alarm was much increased by the exaggeration and ignorant details which found their way into the newspapers. The actual numbers of these caterpillars must have been immense, since Curtis says, “in many of the parishes near London subscriptions have been opened, and the poor people employed to cut off the webs at one shilling per bushel, which have been burnt under the inspection of the church-wardens, overseers, or beadle of the parish: at the first onset of this business fourscore bushels, as I was most credibly informed, were collected in one day in the parish of Clapham.”

It is not, therefore, very much to be wondered at, that the ignorant, who are so prone to become the victims of groundless fears, should have taken serious alarm on having so unusual a phenomenon forced upon their attention. Some alarmists accordingly asserted that the caterpillars “were the usual presage of the plague;” and others that they not only presaged it, but would actually cause it, for “their numbers were great enough to render the air pestilential;” while, to add to the mischief, “they would destroy every kind of vegetation, and starve the cattle in the fields.” “Almost every one,” adds Curtis, “ignorant of their history, was under the greatest apprehensions concerning them; so that even prayers were offered up in some churches to deliver the country from the apprehended approaching calamity.”

It seems to have been either the same caterpillar, or one very nearly allied to it, probably that of the golden-tail (Porthesia chrysorrhœa), which in 1731-2 produced a similar alarm in France. Réaumur, on going from Paris to Tours, in September, 1730, found every oak, great and small, literally swarming with them, and their leaves parched and brown as if some burning wind had passed over them; for when newly hatched, like the young buff-tips, they only eat one of the membranes of the leaf, and of course the other withers away. These infant legions, under the shelter of their warm nests, survived the winter in such numbers, that they threatened the destruction not only of the fruit-trees, but of the forests,—every tree, as Réaumur says, being overrun with them. The Parliament of Paris thought that ravages so widely extended loudly called for their interference, and they accordingly issued an edict, to compel the people to uncaterpillar (décheniller) the trees; which Réaumur ridiculed as impracticable, at least in the forests. About the middle of May, however, a succession of cold rains produced so much mortality among the caterpillars, that the people were happily released from the edict; for it soon became difficult to find a single individual of the species.[EG] In the same way the cold rains, during the summer of 1829, seem to have nearly annihilated the lackeys, which in the early part of the summer swarmed on every hedge around London. The ignorance displayed in France, at the time in question, was not inferior to that recorded by Curtis; for the French journalists gravely asserted that part of the caterpillars were produced by spiders; and that these spiders, and not the caterpillars, constructed the webs of the slime of snails, which they were said to have been seen collecting for the purpose! “Verily,” exclaims Réaumur, “there is more ignorance in our age than one might believe.”

It is justly remarked by Curtis, that the caterpillar of the brown-tail moth is not so limited a feeder as some, nor so indiscriminate as others; but that it always confines itself to trees or shrubs, and is never found on herbaceous plants, whose low growth would seldom supply a suitable foundation for its web. Hence the absurdity of supposing it would attack the herbage of the field, and produce a famine among cattle. Curtis says, it is found on the “hawthorn most plentifully, oak the same, elm very plentifully, most fruit-trees the same, blackthorn plentifully, rose-trees the same, bramble the same, on the willow and poplar scarce. None have been noticed on the elder, walnut, ash, fir, or herbaceous plants. With respect to fruit-trees the injuries they sustain are most serious, as, in destroying the blossoms as yet in the bud, they also destroy the fruit in embryo; the owners of orchards, therefore, have great reason to be alarmed.”

The sudden appearance of great numbers of these caterpillars in particular years, and their scarcity in others, is in some degree explained by a fact stated by Mr. Salisbury. “A gentleman of Chelsea,” he says, “has informed me that he once took a nest of moths and bred them; that some of the eggs came the first year, some the second, and others of the same nest did not hatch till the third season.”[EH] We reared, during 1829, several nests both of the brown-tails and of the golden-tails, and a number of the females deposited their eggs in our nurse-cages; but, contrary to the experiment just quoted, all of these were hatched during the same autumn. (J. R.) The difference of temperature and moisture in particular seasons may produce this diversity.

A no less remarkable winter nest, of a small species of social caterpillar, is described by M. Bonnet, which we omitted to introduce when treating of the Glanville fritillary. The nest in question is literally pendulous, being hung from the branch of a fruit tree by a strong silken thread. It consists of one or two leaves neatly folded, and held together with silk, in which the caterpillars live harmoniously together.

In a recently-published volume of ‘Travels in Mexico,’ we find a very remarkable account of some pendulous nests of caterpillars, which appear to be almost as curious as the nests of the pasteboard-making wasps, described at p. 177. The author of these Travels does not define the species of caterpillar whose constructions attracted his observation. He says, "After having ascended for about an hour, we came to the region of oaks and other majestically tall trees, the names of which I could not learn. Suspended from their stately branches were innumerable nests, enclosed, apparently, in white paper bags, in the manner of bunches of grapes in England, to preserve them from birds and flies. I had the curiosity to examine one of them, which I found to contain numberless caterpillars. The texture is so strong that it is not easily torn; and the interior contained a quantity of green leaves, to support the numerous progeny within."[EI]

[We will now give a brief account of several foreign insects that are remarkable for the pendulous nests which they make.

The first of these is built by a hymenopterous insect belonging to the genus Larrada. It is fastened to the under side of a leaf, and is made of vegetable fibres, cut up very short, and masticated by the insect, much like the materials used by the Fungus Ant, described on page 311. The insect which forms this nest is black in colour and has very thick legs. The wings are clouded. It is but a small insect, being only three-eighths of an inch in length. Both nest and insect are in the collection of Mr. F. Smith, of the British Museum.

In the next illustration are seen two nests built by hymenopterous insects belonging to the hymenopterous genus Polybia. The left-hand figure represents a nest made by Polybia sedula, a Brazilian insect. It is fixed to a large leaf, and, as may be seen, has the entrance at the end of a long neck. The exterior of the nest is a very thin sheet of pale-brown substance, almost identical with the paper with which our British wasps make their nests. It is, however, very much stronger and very much thinner, and is very close in texture, so that it effectually excludes rain.

The right-hand figure shows a nest also brought from Brazil. It has no neck, the opening being a mere hole beneath. The name Polybia is derived from two Greek words signifying that many insects live together, and has been given to the genus on account of the social habits of its members.

Our next illustration contains some very remarkable nests.

The large central nest is the cocoon of a moth belonging to the genus Oiketicus, or Housebuilder. There are many species of Housebuilder moths, and all are remarkable for the fact that the larva never exhibits itself, but builds a dwelling in which it conceals itself, just as does the well-known caddis-worm. Indeed the nests of several Oiketici look exactly as if the dwelling of a caddis-worm had been greatly enlarged and hung up in a tree.

The nest of this species, however, differs from that of the common Oiketicus by being covered with a coating of greyish silk. If we cut open the silk, we find a great number of little sticks and leaf-stems crossed on each other, and showing their ends through the silken cover. Within these defences there is a layer of leaves cut into small pieces, and lastly comes the cell inhabited by the caterpillar. It is lined with a silken web similar in character to that on the outside, but finer, stronger, and whiter The caterpillar is therefore defended by four distinct barriers. First comes the strong silken web which lines the cell, and next is the layer of leaf fragments. Outside them comes the chevaux de frise of crossed sticks, and lastly we have the grey silken web. This outer wrapper has no connection with the interior of the cell, and is only lightly attached to the ends of the cross sticks.

Within this curious dwelling the caterpillar conceals the whole of its body, clinging to the branch or leaf by its feet, and if alarmed drawing itself up so that the mouth of the cocoon is pressed tightly against the branch, and effectually conceals even the feet which hold it.

The other figure on the right hand represents the dwelling of another Housebuilder caterpillar. It looks very much as if it were made of drab cloth. The most remarkable point about it is the lower end. When the insect is within the dwelling the extremity has a spiral twist, but when the moth has escaped the spiral form is destroyed, and it appears as represented in the illustration. The female Oiketicus never attains the winged state nor leaves her house, but lives and dies in it, almost unchanged in shape. In fact, the adult female is even more undeveloped in appearance than the caterpillar, and looks like a large, fat, unwieldy grub, covered with down. The male, on the contrary, is a tolerably active moth, with sharply-pointed wings and beautiful feathered antennæ.

Another kind of Housebuilder’s residence is shown in the lower left-hand figure, enveloped and almost concealed by leaves.

The remaining figures represent the dwellings of two unknown insects, both from Australia. The upper left-hand nest is made wonderfully like that of the weaver-bird, being composed of fibres like cow-hairs woven loosely together. It is brown outside and white in the interior.

The last nest is made of some substance which is smooth, and hard as horn, brown within, and dark grey on the outside. The circular lid by which the enclosed insect escapes is shown open.

In the accompanying illustration, we have five remarkable pensile nests of insects, some British, and others exotic.

Fig. 1 represents the nest of a Pelopæus from Natal. It is made of dried cow-dung, and is fixed to straws. The length is from three to five inches, and there are sometimes found three or more in a row upon a single straw. The insect is about an inch in length, black-blue in colour, and with clouded wings. The abdomen is small, sharply pointed, and placed on a long footstalk.

At Fig. 2 is seen the nest of Pelopæus Flavipes, a North American insect, which is also fixed along its whole length to the supporting object, which is sometimes a wall, and sometimes, as in the illustration, a branch. It is made of mud, and the insects seem to have a sort of gregarious instinct, loving to fix their nests in rows, one above the other. There is only one larva in each cell. The Pelopæi are, by the way, allied to the English genus Ammophila.

Fig. 3 shows the nest of Anthidium cordatum, one of the solitary bees of Natal. It is made of vegetable fibres. The insect as well as the nest is represented of the natural size. It is black and shining, with the under part and sides and legs yellowish.

At Fig. 4 are seen three of the nests of Trypoxylon aurifrons, a Brazilian insect. They are built of mud, and are remarkable for their elegant shape, which looks as if it had been formed by the hand of the potter, and for the manner in which the mouth is turned over so as to form a distinct neck. The larvæ is fed with a store of spiders. The insect is represented of the natural size; its colour is black, and the face is covered with short golden hairs, a fact which has gained for it the name of aurifrons, or golden-fronted.

Our last example, Fig. 5, is the nest of an English insect, Eumenes coarctata. The insect is represented of its natural size. It is very pretty in colour as well as elegant in shape, being black, diversified with yellow bands and spots. The nest is made of clay, and is found upon the heath twigs. The larvæ of the Eumenes are fed with those of a species of Crambus. The insect is tolerably common in Surrey and Hampshire, and appears in July and August.

The three figures in the next illustration represent the cocoons of three species of the Bombycidæ, and are given in order to show the different modes by which they are fastened. The upper nest is hung by a slight cord, which spreads into a broad silken band wrapped round the branch for some distance. The right-hand figure shows a very remarkable cocoon suspended by a long footstalk affixed to a ring. The remarkable point in the construction of this ring is that it is very hard and horny, and is not fastened to the branch, but passes loosely round it, so that the cocoon swings backwards and forwards in the breeze. The cocoon is about two inches in length, and is covered with thick black veinings. The lowermost cocoon is most curiously fixed to the branch by bending the leaves round the exterior of the dwelling, and fixing them to it with silk. All these specimens were brought from Northern India.]

In all the nests of social caterpillars, care is taken to leave apertures for passing out and in. It is remarkable, also, that however far they may ramble from their nest, they never fail to find their way back when a shower of rain or nightfall renders shelter necessary. It requires no great shrewdness to discover how they effect this: for by looking closely at their track it will be found that it is carpeted with silk—no individual moving an inch without constructing such a pathway, both for the use of his companions and to facilitate his own return. All these social caterpillars, therefore, move more or less in processional order, each following the road which the first chance traveller has marked out with his strip of silk carpeting.

There are some species, however, which are more remarkable than others in the regularity of their processional marchings, particularly two which are found in the south of Europe, but are not indigenous in Britain. The one named by Réaumur the Processionary (Cnethocampa processionea, Stephens) feeds upon the oak; a brood dividing, when newly hatched, into one or more parties of several hundred individuals, which afterwards unite in constructing a common nest nearly two feet long, and from four to six inches in diameter. As it is not divided like that of the brown-tails into chambers, but consists of one large hall, it is not necessary that there should be more openings than one; and accordingly, when an individual goes out and carpets a path, the whole colony instinctively follow in the same track, though from the immense population they are often compelled to march in parallel files from two to six deep. The procession is always headed by a single caterpillar; sometimes the leader is immediately followed by one or two in single file, and sometimes by two abreast, as represented in the cut. A similar procedure is followed by a species of social caterpillars which feed on the pine in Savoy and Languedoc; and though their nests are not half the size of the preceding, they are more worthy of notice, from the strong and excellent quality of their silk, which Réaumur was of opinion might be advantageously manufactured. Their nest consists of more chambers than one, but is furnished with a main entrance, through which the colonists conduct their foraging processions.

[In the accompanying illustration is shown a nest of the Processionary caterpillar, part of which has been torn away to show the interior. Inside may be seen the larva of a certain beetle (Calosoma sycophanta), which feeds on these caterpillars, and one of the beetles is seen below in the act of ascending the tree. This beetle, although exceedingly scarce in England, is very common on the Continent, and trees have been cleared of Processionary caterpillars by the simple process of putting a few female beetles upon the branches.]

CHAPTER XVIII.

STRUCTURES OF SPIDERS.

Modern naturalists do not rank spiders among insects, because they have no antennæ, and no division between the head and the shoulders. They breathe by leaf-shaped gills, situated under the belly, instead of spiracles in the sides; have a heart connected with these; have eight legs instead of six; and eight fixed eyes. But as spiders are popularly considered insects, it will sufficiently suit our purpose to introduce them here as such.

The apparatus by which spiders construct their ingenious fabrics is much more complicated than that which we have described as common to the various species of caterpillars. Caterpillars have only two reservoirs for the materials of their silk; but spiders, according to the dissections of M. Treviranus, have four principal vessels, two larger and two smaller, with a number of minute ones at their base. Several small tubes branch towards the reservoirs, for carrying to them, no doubt, a supply of the secreted material. Swammerdam describes them as twisted into many coils of an agate colour.[EJ] We do not find them coiled, but nearly straight, and of a deep-yellow colour. From these, when broken, threads can be drawn out like those spun by the spider, though we cannot draw them so fine by many degrees.

From these little flasks or bags of gum, situated near the apex of the abdomen, and not at the mouth, as in caterpillars, a tube originates, and terminates in the external spinnerets, which may be seen by the naked eye in the larger spiders, in the form of five little teats surrounded by a circle, as represented in the following figure.

We have seen that the silken thread of a caterpillar is composed of two united within the tube of the spinneret, but the spider’s thread would appear, from the first view of its five spinnerets, to be quintuple, and in some species which have six teats, so many times more. It is not safe, however, in our interpretations of nature to proceed upon conjecture, however plausible, nor to take anything for granted which we have not actually seen; since our inferences in such cases are almost certain to be erroneous. If Aristotle, for example, had ever looked narrowly at a spider when spinning, he could not have fancied, as he does, that the materials which it uses are nothing but wool stripped from its body. On looking, then, with a strong magnifying glass, at the teat-shaped spinnerets of a spider, we perceive them studded with regular rows of minute bristle-like points, about a thousand to each teat, making in all from five to six thousand. These are minute tubes which we may appropriately term spinnerules, as each is connected with the internal reservoirs, and emits a thread of inconceivable fineness. In the following figure, this wonderful apparatus is represented as it appears in the microscope.

We do not recollect that naturalists have ventured to assign any cause for this very remarkable multiplicity of the spinnerules of spiders, so different from the simple spinneret of caterpillars. To us it appears to be an admirable provision for their mode of life. Caterpillars neither require such strong materials, nor that their thread should dry as quickly. It is well known in our manufactures, particularly in rope-spinning, that in cords of equal thickness, those which are composed of many smaller ones united are greatly stronger than those which are spun at once. In the instance of the spider’s thread, this principle must hold still more strikingly, inasmuch as it is composed of fluid materials that require to be dried rapidly, and this drying must be greatly facilitated by exposing so many to the air separately before their union, which is effected at the distance of about a tenth of an inch from the spinnerets. In the following figure each of the threads represented is reckoned to contain one hundred minute threads, the whole forming only one of the spider’s common threads.

Leeuwenhoeck, in one of his extraordinary microscopical observations on a young spider not bigger than a grain of sand, upon enumerating the threadlets in one of its threads, calculated that it would require four millions of them to be as thick as a hair of his beard.

Another important advantage derived by the spider from the multiplicity of its threadlets is, that the thread affords a much more secure attachment to a wall, a branch of a tree, or any other object, than if it were simple; for, upon pressing the spinneret against the object, as spiders always do when they fix a thread, the spinnerules are extended over an area of some diameter, from every hair’s-breadth of which a strand, as rope-makers term it, is extended to compound the main cord. The preceding figure exhibits this ingenious contrivance.

Those who may be curious to examine this contrivance will see it best when the line is attached to any black object, for the threads, being whitish, are, in other cases, not so easily perceived.

Shooting of the Lines.

It has long been considered a curious though a difficult investigation, to determine in what manner spiders, seeing that they are destitute of wings, transport themselves from tree to tree, across brooks, and frequently through the air itself, without any apparent starting point. On looking into the authors who have treated upon this subject, it is surprising how little there is to be met with that is new, even in the most recent. Their conclusions, or rather their conjectural opinions, are, however, worthy of notice; for by unlearning error, we the more firmly establish truth.

1. One of the earliest notions upon this subject is that of Blancanus, the commentator on Aristotle, which is partly adopted by Redi, by Henricus Regius of Utrecht, by Swammerdam, [EK] by Lehmann, and by Kirby and Spence.[EL] “The spider’s thread,” says Swammerdam, “is generally made up of two or more parts, and after descending by such a thread, it ascends by one only, and is thus enabled to waft itself from one height or tree to another, even across running waters; the thread it leaves loose behind it being driven about by the wind, and so fixed to some other body.” “I placed,” says Kirby, "the large garden spider (Epeira diadema) upon a stick about a foot long, set upright in a vessel containing water.… It let itself drop, not by a single thread, but by two, each distant from the other about the twelfth of an inch, guided, as usual, by one of its hind feet, and one apparently smaller than the other. When it had suffered itself to descend nearly to the surface of the water, it stopped short, and by some means, which I could not distinctly see, broke off, close to the spinners, the smallest thread, which still adhering by the other end to the top of the stick, floated in the air, and was so light as to be carried about by the slightest breath. On approaching a pencil to the loose end of this line, it did not adhere from mere contact. I therefore twisted it once or twice round the pencil, and then drew it tight. The spider, which had previously climbed to the top of the stick, immediately pulled at it with one of its feet, and finding it sufficiently tense, crept along it, strengthening it as it proceeded by another thread, and thus reached the pencil."

We have repeatedly witnessed this occurrence, both in the fields and when spiders were placed for experiment, as Kirby has described; but we very much doubt that the thread broken is ever intended as a bridge cable, or that it would have been so used in that instance, had it not been artificially fixed and accidentally found again by the spider. According to our observations, a spider never abandons, for an instant, the thread which she despatches in quest of an attachment, but uniformly keeps trying it with her feet, in order to ascertain its success. We are, therefore, persuaded that when a thread is broken in the manner above described, it is because it has been spun too weak; and spiders may often be seen breaking such threads in the process of netting their webs. (J. R.)

The plan, besides, as explained by these distinguished writers, would more frequently prove abortive than successful, from the cut thread not being sufficiently long. They admit, indeed, that spiders’ lines are often found “a yard or two long, fastened to twigs of grass not a foot in height.… Here, therefore, some other process must have been used.”[EM]

2. Our celebrated English naturalist, Dr. Lister, whose treatise upon our native spiders has been the basis of every subsequent work on the subject, maintains that "some spiders shoot out their threads in the same manner that porcupines do their quills;[EN] that whereas the quills of the latter are entirely separated from their bodies, when thus shot out, the threads of the former remain fixed to their anus, as the sun’s rays to its body."[EO] A French periodical writer goes a little farther, and says, that spiders have the power of shooting out threads, and directing them at pleasure towards a determined point, judging of the distance and position of the object by some sense of which we are ignorant.[EP] Kirby also says, that he once observed a small garden spider (Aranea reticulata) “standing midway on a long perpendicular fixed thread, and an appearance caught” his “eye, of what seemed to be the emission of threads.” “I, therefore,” he adds, “moved my arm in the direction in which they apparently proceeded, and, as I had suspected, a floating thread attached itself to my coat, along which the spider crept. As this was connected with the spinners of the spider, it could not have been formed” by breaking a “secondary thread.”[EQ] Again, in speaking of the gossamer-spider, he says, “it first extends its thigh, shank, and foot, into a right line, and then, elevating its abdomen till it becomes vertical, shoots its thread into the air, and flies off from its station.”[ER]

Another distinguished naturalist, Mr. White of Selborne, in speaking of the gossamer-spider, says, “Every day in fine weather in autumn do I see these spiders shooting out their webs, and mounting aloft: they will go off from the finger, if you will take them into your hand. Last summer, one alighted on my book as I was reading in the parlour; and running to the top of the page, and shooting out a web, took its departure from thence. But what I most wondered at was, that it went off with considerable velocity in a place where no air was stirring; and I am sure I did not assist it with my breath.”[ES]

Having so often witnessed the thread set afloat in the air by spiders, we can readily conceive the way in which those eminent naturalists were led to suppose it to be ejected by some animal force acting like a syringe; but as the statement can be completely disproved by experiment, we shall only at present ask, in the words of Swammerdam—“how can it be possible that a thread so fine and slender should be shot out with force enough to divide and pass through the air?—is it not rather probable that the air would stop its progress, and so entangle it and fit it to perplex the spider’s operations?”[ET] The opinion, indeed, is equally improbable with another, suggested by Dr. Lister, that the spider can retract her thread within the abdomen, after it has been emitted.[EU] De Geer[EV] very justly joins Swammerdam in rejecting both of these fancies, which, in our own earlier observations upon spiders, certainly struck us as plausible and true. There can be no doubt, indeed, that the animal has a voluntary power of permitting the material to escape, or stopping it at pleasure, but this power is not projectile.

3. “There are many people,” says the Abbé de la Pluche, “who believe that the spider flies when they see her pass from branch to branch, and even from one high tree to another; but she transports herself in this manner: she places herself upon the end of a branch, or some projecting body, and there fastens her thread; after which, with her two hind feet, she squeezes her dugs (spinnerets), and presses out one or more threads of two or three ells in length, which she leaves to float in the air till it be fixed to some particular place.”[EW] Without pretending to have observed this, Swammerdam says, “I can easily comprehend how spiders, without giving themselves any motion, may, by only compressing their spinnerets, force out a thread, which being driven by the wind, may serve to waft them from one place to another.”[EX] Others, proceeding upon a similar notion, give a rather different account of the matter. “The spider,” says Bingley, "fixes one end of a thread to the place where she stands, and then with her hind paws draws out several other threads from the nipples, which, being lengthened out and driven by the wind to some neighbouring tree or other object, are by their natural clamminess fixed to it."[EY]

Observation gives some plausibility to the latter opinion, as the spider always actively uses her legs, though not to draw out the thread, but to ascertain whether it has caught upon any object. The notion of her pressing the spinneret with her feet must be a mere fancy; at least it is not countenanced by anything which we have observed.

4. An opinion much more recondite is mentioned, if it was not started, by M. D’Isjonval, that the floating of the spider’s thread is electrical. “Frogs, cats, and other animals,” he says, “are affected by natural electricity, and feel the change of weather; but no other animal more than myself and my spiders.” During wet and windy weather he accordingly found that they spun very short lines, “but when a spider spins a long thread, there is a certainty of fine weather for at least ten or twelve days afterwards.”[EZ] A periodical writer, who signs himself Carolan,[FA] fancies that in darting out her thread the spider emits a stream of air, or some subtle electric fluid, by which she guides it as if by magic.

A living writer (Mr. John Murray), whose learning and skill in conducting experiments give no little weight to his opinions, has carried these views considerably farther. “The aëronautic spider,” he says, "can propel its thread both horizontally and vertically, and at all relative angles, in motionless air, and in an atmosphere agitated by winds; nay more, the aërial traveller can even dart its thread, to use a nautical phrase, in the ‘wind’s eye.’ My opinion and observations are based on many hundred experiments.… The entire phenomena are electrical. When a thread is propelled in a vertical plane, it remains perpendicular to the horizontal plane, always upright, and when others are projected at angles more or less inclined, their direction is invariably preserved; the threads never intermingle, and when a pencil of threads is propelled, it ever presents the appearance of a divergent brush. These are electrical phenomena, and cannot be explained but on electrical principles."

“In clear, fine weather, the air is invariably positive; and it is precisely in such weather that the aëronautic spider makes its ascent most easily and rapidly, whether it be in summer or in winter.” “When the air is weakly positive, the ascent of the spider will be difficult, and its altitude extremely limited, and the threads propelled will be but little elevated above the horizontal plane. When negative electricity prevails, as in cloudy weather, or on the approach of rain, and the index of De Saussure’s hygrometer rapidly advancing towards humidity, the spider is unable to ascend.”[FB]

Mr. Murray had previously told us, that “when a stick of excited sealing-wax is brought near the thread of suspension, it is evidently repelled; consequently, the electricity of the thread is of a negative character,” while “an excited glass tube brought near, seemed to attract the thread, and with it the aëronautic spider.”[FC] His friend, Mr. Bowman, further describes the aërial spider as “shooting out four or five, often six or eight, extremely fine webs several yards long, which waved in the breeze, diverging from each other like a pencil of rays.” One of them “had two distinct and widely-diverging fasciculi of webs,” and “a line uniting them would have been at right angles to the direction of the breeze.”[FD]

Such is the chief evidence in support of the electrical theory; but though we have tried these experiments, we have not succeeded in verifying any one of them. The following statements of Mr. Blackwall come nearer our own observations.

5. “Having procured a small branched twig,” says Mr. Blackwall, "I fixed it upright in an earthen vessel containing water, its base being immersed in the liquid, and upon it I placed several of the spiders which produce gossamer. Whenever the insects thus circumstanced were exposed to a current of air, either naturally or artificially produced, they directly turned the thorax towards the quarter whence it came, even when it was so slight as scarcely to be perceptible, and elevating the abdomen, they emitted from their spinners a small portion of glutinous matter, which was instantly carried out in a line, consisting of four finer ones, with a velocity equal, or nearly so, to that with which the air moved, as was apparent from observations made on the motion of detached lines similarly exposed. The spiders, in the next place, carefully ascertained whether their lines had become firmly attached to any object or not, by pulling at them with the first pair of legs; and if the result was satisfactory, after tightening them sufficiently, they made them pass to the twig; then discharging from their spinners, which they applied to the spot where they stood, a little more of their liquid gum, and committing themselves to these bridges of their own constructing, they passed over them in safety, drawing a second line after them, as a security in case the first gave way, and so effected their escape.

"Such was invariably the result when spiders were placed where the air was liable to be sensibly agitated: I resolved, therefore, to put a bell-glass over them; and in this situation they remained seventeen days, evidently unable to produce a single line by which they could quit the branch they occupied, without encountering the water at its base; though, on the removal of the glass, they regained their liberty with as much celerity as in the instances already recorded.

“This experiment, which, from want of due precaution, has misled so many distinguished naturalists, I have tried with several geometric spiders, and always with the same success.”[FE]

Mr. Blackwall, from subsequent experiments, says he is “confident in affirming, that in motionless air spiders have not the power of darting their threads even through the space of half an inch.”[FF] The following details are given in confirmation of this opinion. Mr. Blackwall observed, the 1st Oct., 1826, a little before noon, with the sun shining brightly, no wind stirring, and the thermometer in the shade ranging from 55°·5 to 64°, a profusion of shining lines crossing each other at every angle, forming a confused net-work, covering the fields and hedges, and thickly coating his feet and ankles, as he walked across a pasture. He was more struck with the phenomenon, because on the previous day a strong gale of wind had blown from the south, and as gossamer is only seen in calm weather, it must have been all produced within a very short time.

“What more particularly arrested my attention,” says Mr. Blackwall, "was the ascent of an amazing quantity of webs, of an irregular, complicated structure, resembling ravelled silk of the finest quality and clearest white; they were of various shapes and dimensions, some of the largest measuring upwards of a yard in length, and several inches in breadth in the widest part; while others were almost as broad as long, presenting an area of a few square inches only.

"These webs, it was quickly perceived, were not formed in the air, as is generally believed, but at the earth’s surface. The lines of which they were composed, being brought into contact by the mechanical action of gentle airs, adhered together, till, by continual additions, they were accumulated into flakes or masses of considerable magnitude, on which the ascending current, occasioned by the rarefaction of the air contiguous to the heated ground, acted with so much force as to separate them from the objects to which they were attached, raising them in the atmosphere to a perpendicular height of at least several hundred feet. I collected a number of these webs about mid-day, as they rose; and again in the afternoon, when the upward current had ceased, and they were falling; but scarcely one in twenty contained a spider: though, on minute inspection, I found small winged insects, chiefly aphides, entangled in most of them.

"From contemplating this unusual display of gossamer, my thoughts were naturally directed to the animals which produced it, and the countless myriads in which they swarmed almost created as much surprise as the singular occupation that engrossed them. Apparently actuated by the same impulse, all were intent upon traversing the regions of air: accordingly, after gaining the summits of various objects, as blades of grass, stubble, rails, gates, &c., by the slow and laborious process of climbing, they raised themselves still higher by straightening their limbs; and elevating the abdomen, by bringing it from the usual horizontal position into one almost perpendicular, they emitted from their spinning apparatus a small quantity of the glutinous secretion with which they construct their webs. This viscous substance being drawn out by the ascending current of rarefied air into fine lines several feet in length, was carried upward, until the spiders, feeling themselves acted upon with sufficient force in that direction, quitted their hold of the objects on which they stood, and commenced their journey by mounting aloft.

“Whenever the lines became inadequate to the purpose for which they were intended, by adhering to any fixed body, they were immediately detached from the spinners, and so converted into terrestrial gossamer, by means of the last pair of legs, and the proceedings just described were repeated; which plainly proves that these operations result from a strong desire felt by the insects to effect an ascent.”[FG] Mr. Blackwall has recently read a paper (still unpublished) in the Linnean Society, confirmatory of his opinions.

6. Without going into the particulars of what agrees or disagrees in the above experiments with our own observations, we shall give a brief account of what we have actually seen in our researches. (J. R.) So far as we have determined, then, all the various species of spiders, how different soever the form of their webs may be, proceed in the circumstance of shooting their lines precisely alike; but those which we have found the most manageable in experimenting, are the small gossamer spider (Aranea obtextrix, Bechstein), known by its shining blackish-brown body and reddish-brown semi-transparent legs; but particularly the long-bodied spider (Tetragnatha extensa, Latr.), which varies in colour from green to brownish or grey—but has always a black line along the belly, with a silvery white or yellowish one on each side. The latter is chiefly recommended by being a very industrious and persevering spinner, while its movements are easily seen, from the long cylindrical form of its body and the length of its legs.

We placed the above two species with five or six others, including the garden, the domestic and the labyrinthic spiders, in empty wine-glasses, set in tea-saucers filled with water to prevent their escape. When they discovered, by repeated descents from the brims of the glasses, that they were thus surrounded by a wet ditch, they all set themselves to the task of throwing their silken bridges across. For this purpose they first endeavoured to ascertain in what direction the wind blew, or rather (as the experiment was made in our study) which way any current of air set,—by elevating their arms as we have seen sailors do in a dead calm. But, as it may prove more interesting to keep to one individual, we shall first watch the proceedings of the gossamer spider.

Finding no current of air on any quarter of the brim of the glass, it seemed to give up all hopes of constructing its bridge of escape, and placed itself in the attitude of repose; but no sooner did we produce a stream of air, by blowing gently towards its position, than, fixing a thread to the glass, and laying hold of it with one of its feet, by way of security, it placed its body in a vertical position, with its spinnerets extended outwards; and immediately we had the pleasure of seeing a thread streaming out from them several feet in length, on which the little aëronaut sprung up into the air. We were convinced, from what we thus observed, that it was the double or bend of the thread which was blown into the air; and we assigned as a reason for her previously attaching and drawing out a thread from the glass, the wish to give the wind a point d’appui—something upon which it might have a purchase, as a mechanic would say of a lever. The bend of the thread, then, on this view of the matter, would be carried out by the wind,—would form the point of impulsion,—and, of course, the escape bridge would be an ordinary line doubled.

Such was our conclusion, which was strongly corroborated by what we subsequently found said by M. Latreille—than whom no higher authority could be given. “When the animal,” says he, “desires to cross a brook, she fixes to a tree or some other object one of the ends of her first threads, in order that the wind or a current of air may carry the other end beyond the obstacle;”[FH] and as one end is always attached to the spinnerets, he must mean that the double of the thread flies off. In his previous publications, however, Latreille had contented himself with copying the statement of Dr. Lister.

In order to ascertain the fact, and put an end to all doubts, we watched, with great care and minuteness, the proceedings of the long-bodied spider above mentioned, by producing a stream of air in the same manner, as it perambulated the brim of the glass. It immediately, as the other had done, attached a thread, and raised its body perpendicularly, like a tumbler, standing on his hands with his head downwards; but we looked in vain for this thread bending, as we had at first supposed, and going off double. Instead of this it remained tight, while another thread, or what appeared to be so, streamed off from the spinners, similar to smoke issuing through a pin-hole, sometimes in a line, and sometimes at a considerable angle, with the first, according to the current of the air,—the first thread, extended from the glass to the spinnerets, remaining all the while tight drawn in a right line. It further appeared to us, that the first thread proceeded from the pair of spinnerets nearest the head, while the floating thread came from the outer pair,—though it is possible in such minute objects we may have been deceived. That the first was continuous with the second, without any perceptible joining, we ascertained in numerous instances, by catching the floating line and pulling it tight, in which case the spider glides along without attaching another line to the glass; but if she has to coil up the floating line to tighten it, as usually happens, she gathers it into a packet and glues the two ends tight together. Her body, while the floating line streamed out, remained quite motionless, but we distinctly saw the spinnerets not only projected, as is always done when a spider spins, but moved in the same way as an infant moves its lips when sucking. We cannot doubt, therefore, that this motion is intended to emit (if eject or project be deemed too strong words) the liquid material of the thread; at the same time, we are quite certain that it cannot throw out a single inch of thread without the aid of a current of air. A long-bodied spider will thus throw out in succession as many threads as we please, by simply blowing towards it; but not one where there is no current, as under a bell-glass, where it may be kept till it die, without being able to construct a bridge over water of an inch long. We never observed more than one floating thread produced at the same time; though other observers mention several.