Chapter XVII: Introduction (12)

All the species of Termites are not social; but the solitary ones do not, like their congeners, distinguish themselves in architecture. In other respects, their habits are more similar; for they destroy almost every substance, animal and vegetable. The most common of the solitary species must be familiar to all our readers by the name of wood-louse (_Termes pulsatorium_, Linn.; _Atropos lignarius_, Leach)--one of the insects which produces the ticking superstitiously termed the _death-watch_. It is not so large as the common wood-louse, but whiter and more slender, having a red mouth and yellow eyes. It lives in old books, the paper on walls, collections of insects and dried plants, and is extremely agile in its movements, darting, by jerks, into dark corners for the purpose of concealment. It does not like to run straight forward without resting every half-second, as if to listen or look about for its pursuer, and at such resting times it is easily taken. The ticking noise is made by the insect beating against the wood with its head, and it is supposed by some to be peculiar to the female, and to be connected with the laying of her eggs. M. Latreille, however, thinks that the wood-louse is only the grub of the _Psocus abdominalis_, in which case it could not lay eggs; but this opinion is somewhat questionable. Another death-watch is a small beetle (_Anobium tesselatum_).

_CHAPTER XVII._

STRUCTURES OF SILK SPUN BY CATERPILLARS, INCLUDING THE SILK-WORM.

"Millions of spinning-worms,
That in their green shops weave the smooth-hair'd silk."

Milton's _Comus_.

All the caterpillars of butterflies, moths, and, in general, of insects with four wings, are capable of spinning silk; of various degrees of fineness and strength, and differing in colour, but usually white, yellow, brown, black, or grey. This is not only of advantage in constructing nests for themselves, and particularly for their pupæ, as we have so frequently exemplified in the preceding pages, but it enables them, the instant they are excluded from the egg, to protect themselves from innumerable accidents, as well as from enemies. If a caterpillar, for instance, be exposed to a gust of wind, and blown off from its native tree, it lets itself gently down, and breaks its fall, by immediately spinning a cable of silk, along which, also, it can reascend to its former station when the danger is over. In the same way it frequently disappoints a bird that has marked it out for prey, by dropping hurriedly down from a branch, suspended to its never-failing delicate cord. The leaf-rollers, formerly described, have the advantage of other caterpillars in such cases, by being able to move as quickly backwards as forwards; so that when a bird puts in its bill at one end of the roll, the insect makes a ready exit at the other, and drops along its thread as low as it judges convenient. We have seen caterpillars drop in this way from one to six feet or more; and by means of their cable, which they are careful not to break, they climb back with great expedition to their former place.

The structure of their legs is well adapted for climbing up their singular rope--the six fore-legs being furnished with a curved claw; while the pro-legs (as they have been termed) are no less fitted for holding them firm to the branch when they have regained it, being constructed on the principle of forming a vacuum, like the leather sucker with which boys lift and drag stones. The foot of the common fly has a similar sucker, by which it is enabled to walk on glass, and otherwise support itself against gravity. The different forms of the leg and pro-leg of a spinning caterpillar are represented in the figure.

In order to understand the nature of the apparatus by which a caterpillar spins its silk, it is to be recollected that its whole interior structure differs from that of warm-blooded animals. It has, properly speaking, no heart, though a long tubular _dorsal vessel_, which runs along the back, and pulsates from twenty to one hundred times per minute, has been called so by Malpighi and others, but neither Lyonnet nor Cuvier could detect any vessel issuing from it, and consequently the fluid which is analogous to blood has no circulation. It differs also from the higher orders of animals in having no brain, the nerves running along the body being only united by little knobs, called ganglions. Another circumstance is, that it has no lungs, and does not breathe by the mouth, but by air-holes, or spiracles, eighteen in number, situated along the sides, in the middle of the rings, as may be seen in the following figure from Lyonnet.

These spiracles communicate on each side with tubes, that have been called the wind-pipes (_tracheæ_). The spinning apparatus is placed near the mouth, and is connected with the silk-bags, which are long, slender, floating vessels, containing a liquid gum. The bags are closed at their lower extremity, become wider towards the middle, and more slender towards the head, where they unite to form the spinning-tube, or spinneret. The bags being in most cases longer than the body of the caterpillar, necessarily lie in a convoluted state, like the intestines of quadrupeds. The capacity, or rather the length, of the silk-bags is in proportion to the quantity of silk required for spinning; the _Cossus ligniperda_, for example, from living in the wood of trees spins little, having a bag only one-fourth the length of that of the silk-worm, though the caterpillar is at least twice the dimensions of the latter. The following figure, taken from the admirable treatise of Lyonnet on the anatomy of the _Cossus_, will render these several organs more easily understood than any description.

The spinneret itself was supposed by Réaumur to have two outlets for the silk; but Lyonnet, upon minute dissection, found that the two tubes united into one before their termination; and he also assured himself that it was composed of alternate slips of horny and membranaceous substance,--the one for pressing the thread into a small diameter, and the other for enlarging it at the insect's pleasure. It is cut at the end somewhat like a writing-pen, though with less of a slope, and is admirably fitted for being applied to objects to which it may be required to attach silk. The following are magnified figures of the spinneret of the _Cossus_, from Lyonnet.

"You may sometimes have seen," says the Abbé de la Pluche, "in the work-rooms of goldsmiths or gold wire-drawers, certain iron plates, pierced with holes of different calibres, through which they draw gold and silver wire, in order to render it finer. The silk-worm has under her mouth such a kind of instrument, perforated with a pair of holes [united into one on the outside[DQ],] through which she draws two drops of the gum that fills her two bags. These instruments are like a pair of distaffs for spinning the gum into a silken thread. She fixes the first drop of gum that issues where she pleases, and then draws back her head, or lets herself fall, while the gum, continuing to flow, is drawn out and lengthened into a double stream. Upon being exposed to the air, it immediately loses its fluidity, becomes dry, and acquires consistence and strength. She is never deceived in adjusting the dimensions of the [united] apertures, or in calculating the proper thickness of the thread, but invariably makes the strength of it proportionable to the weight of her body.

"It would be a very curious thing to know how the gum which composes the silk is separated and drawn off from the other juices that nourish the animal. It must be accomplished like the secretions formed by glands in the human body. I am therefore persuaded that the gum-bags of the silk-worm are furnished with a set of minute glands, which being impregnated with gum, afford a free passage to all the juices of the mulberry-leaf corresponding with this glutinous matter, while they exclude every fluid of a different quality."[DR] When confined in an open glass vessel, the goat-moth caterpillar will effect its escape by constructing a curious silken ladder, as represented by Roesel.

Caterpillars, as they increase in size, cast their skins as lobsters do their shells, and emerge into renewed activity under an enlarged covering. Previous to this change, when the skin begins to gird and pinch them, they may be observed to become languid, and indifferent to their food, and at length they cease to eat, and await the sloughing of their skin. It is now that the faculty of spinning silk seems to be of great advantage to them; for, being rendered inactive and helpless by the tightening of the old skin around their expanding body, they might be swept away by the first puff of wind, and made prey of by ground beetles or other carnivorous prowlers. To guard against such accidents, as soon as they feel that they can swallow no more food, from being half choked by the old skin, they take care to secure themselves from danger by moorings of silk spun upon the leaf or the branch where they may be reposing. The caterpillar of the white satin-moth (_Leucoma salicis_, Stephens) in this way draws together with silk one or two leaves, similar to the leaf-rollers (_Tortricidæ_), though it always feeds openly without any covering. The caterpillar of the puss-moth again, which, in its third skin, is large and heavy, spins a thick web on the upper surface of a leaf, to which it adheres till the change is effected.

The most important operation, however, of silk-spinning is performed before the caterpillar is transformed into a chrysalis, and is most remarkable in the caterpillars of moths and other four-winged flies, with the exception of those of butterflies; for though these exhibit, perhaps, greater ingenuity, they seldom spin more than a few threads to secure the chrysalis from falling, whereas the others spin for it a complete envelope or shroud. We have already seen, in the preceding pages, several striking instances of this operation, when, probably for the purpose of husbanding a scanty supply of silk, extraneous substances are worked into the texture. In the case of other caterpillars, silk is the only material employed.

Of this the cocoon of the silk-worm is the most prominent example, in consequence of its importance in our manufactures and commerce, and on that account will demand from us somewhat minute details, though it would require volumes to incorporate all the information which has been published on the subject.

Silk-Worm.

The silk-worm, like most other caterpillars, changes its skin four times during its growth. The intervals at which the four moultings follow each other depend much on climate or temperature, as well as on the quality and quantity of food. It is thence found, that if they are exposed to a high temperature, say from 81° to 100° Fahrenheit, the moultings will be hastened; and only five days will be consumed in moulting the third or fourth time, whilst those worms that have not been hastened take seven or eight days.[DS]

The period of the moultings is also influenced by the temperature in which the eggs have been kept during the winter. When the heat of the apartment has been regulated, the first moulting takes place on the fourth or fifth day after hatching, the second begins on the eighth day, the third takes up the thirteenth and fourteenth days, and the last occurs on the twenty-second and twenty-third days. The fifth age, in such cases, lasts ten days, at the end of which, or thirty-two days after hatching, the caterpillars attain their full growth, and ought to be three inches in length; but if they have not been properly fed, they will not be so long.

With the age of the caterpillar, its appetite increases, and is at its maximum after the fourth moulting, when it also attains its greatest size. The silk gum is then elaborated in the reservoirs, while the caterpillar ceases to eat, and soon diminishes again in size and weight. This usually requires a period of nine or ten days, commencing from the fourth moulting, after which it begins to spin its shroud of silk. In this operation it proceeds with the greatest caution, looking carefully for a spot in which it may be most secure from interruption.

"We usually," says the Abbé de la Pluche, "give it some little stalks of broom, heath, or a piece of paper rolled up, into which it retires, and begins to move its head to different places, in order to fasten its thread on every side. All this work, though it looks to a bystander like confusion, is not without design. The caterpillar neither arranges its threads nor disposes one over another, but contents itself with distending a sort of cotton or floss to keep off the rain; for Nature having ordained silk-worms to work under trees, they never change their method even when they are reared in our houses.

"When my curiosity led me to know how they spun and placed their beautiful silk, I took one of them, and frequently removed the floss with which it first attempted to make itself a covering; and as by this means I weakened it exceedingly, when it at last became tired of beginning anew, it fastened its threads on the first thing it encountered, and began to spin very regularly in my presence, bending its head up and down, and crossing to every side. It soon confined its movements to a very contracted space, and, by degrees, entirely surrounded itself with silk; and the remainder of its operations became invisible, though these may be understood from examining the work after it is finished. In order to complete the structure, it must draw out of the gum-bag a more delicate silk, and then with a stronger gum bind all the inner threads over one another.

"Here, then, are three coverings entirely different, which afford a succession of shelter. The outer loose silk, or floss, is for keeping off the rain; the fine silk in the middle prevents the wind from causing injury; and the glued silk, which composes the tapestry of the chamber where the insect lodges, repels both air and water, and prevents the intrusion of cold.

"After building her cocoon, she divests herself of her fourth skin, and is transformed into a chrysalis, and subsequently into a moth (_Bombyx mori_), when, without saw or centre-bit, she makes her way through the shell, the silk, and the floss; for the Being who teaches her how to build herself a place of rest, where the delicate limbs of the moth may be formed without interruption, instructs her likewise how to open a passage for escape.

"The cocoon is like a pigeon's egg, and more pointed at one end than the other; and it is remarkable that the caterpillar does not interweave its silk towards the pointed end, nor apply its glue there as it does in every other part,[DT] by bending itself all around with great pliantness and agility: what is more, she never fails, when her labour is finished, to fix her head opposite to the pointed extremity. The reason of her taking this position is, that she has purposely left this part less strongly cemented, and less exactly closed. She is instinctively conscious that this is to be the passage for the perfect insect which she carries in her bowels, and has therefore the additional precaution never to place this pointed extremity against any substance that might obstruct the moth at the period of its egress.

"When the caterpillar has exhausted herself to furnish the labour and materials of the three coverings, she loses the form of a worm; her spoils drop all around the chrysalis; first throwing off the skin, with the head and jaws attached to it, and the new skin hardening into a sort of leathery consistence. Its nourishment is already in its stomach, and consists of a yellowish mucus, but gradually the rudiments of the moth unfold themselves,--the wings, the antennæ, and the legs becoming solid. In about a fortnight or three weeks, a slight swelling in the chrysalis may be remarked, which at length produces a rupture in the membrane that covers it, and by repeated efforts the moth bursts through the leathery envelope into the chamber of the cocoon.

"The moth then extends her antennæ, together with her head and feet, towards the point of the cone, which not being thickly closed up in that part gradually yields to her efforts; she enlarges the opening, and at last comes forth, leaving at the bottom of the cone the ruins of its former state--namely, the head and entire skin of the caterpillar, which bear some resemblance to a heap of foul linen."[DU]

Réaumur was of opinion that the moth makes use of its eyes as a file, in order to effect its passage through the silk; while Malpighi, Peck, and others, believe that it is assisted by an acid which it discharges in order to dissolve the gum that holds the fibres of the silk together (see p. 338). Mr. Swayne denies that the threads are broken at all, either by filing or solution; for he succeeded in unwinding a whole cocoon from which the moth had escaped. The soiling of the cocoon by a fluid, however, we may remark, is no proof of the acid; for all moths and butterflies discharge a fluid when they assume wings, whether they be inclosed in a cocoon or not; but it gives no little plausibility to the opinion, that "the end of the cocoon is observed to be wetted for an hour, and sometimes several hours, before the moth makes its way out."[DV] Other insects employ different contrivances for escape, as we have already seen, and shall still further exemplify.

It is the middle portion of the cocoon, after removing the floss or loose silk on the exterior, which is used in our manufactures; and the first preparation is to throw the cocoons into warm water, and to stir them about with twigs, to dissolve any slight gummy adhesions which may have occurred when the caterpillar was spinning. The threads of several cones, according to the strength of the silk wanted, are then taken and wound off upon a reel. The refuse, consisting of what we may call the tops and bottoms of the cones, are not wound, but carded, like wool or cotton, in order to form coarser fabrics. We learn from the fact of the cocoons being generally unwound without breaking the thread, that the insect spins the whole without interruption. It is popularly supposed, however, that if it be disturbed during the operation by any sort of noise, it will take alarm, and break its thread; but Latreille says this is a vulgar error.[DW]

The length of the unbroken thread in a cocoon varies from six hundred to a thousand feet; and as it is all spun double by the insect, it will amount to nearly two thousand feet of silk, the whole of which does not weigh above three grains and a half; five pounds of silk from ten thousand cocoons is considerably above the usual average. When we consider, therefore, the enormous quantity of silk which is used at present, the number of worms employed in producing it will almost exceed our comprehension. The manufacture of the silk, indeed, gives employment, and furnishes subsistence, to several millions of human beings; and we may venture to say, that there is scarcely an individual in the civilized world who has not some article made of silk in his possession.

In ancient times, the manufacture of silk was confined to the East Indies and China, where the insects that produce it are indigenous. It was thence brought to Europe in small quantities, and in early times sold at so extravagant a price, that it was deemed too expensive even for royalty. The Emperor Aurelian assigned the expense as a reason for refusing his empress a robe of silk; and our own James I., before his accession to the crown of England, had to borrow of the Earl of Mar a pair of silk stockings to appear in before the English ambassador, a circumstance which probably led him to promote the cultivation of silk in England.[DX] The Roman authors were altogether ignorant of its origin,--some supposing it to be grown on trees as hair grows on animals,--others that it was produced by a shell-fish similar to the mussel, which is known to throw out threads for the purpose of attaching itself to rocks,--others that it was the entrails of a sort of spider, which was fed for four years with paste, and then with the leaves of the green willow, till it burst with fat,--and others that it was the produce of a worm which built nests of clay and collected wax. The insect was at length spread into Persia; and eggs were afterwards, at the instance of the Emperor Justinian, concealed in hollow canes by two monks, and conveyed to the Isle of Cos. This emperor, in the sixth century, caused them to be introduced into Constantinople, and made an object of public utility. They were thence successively cultivated in Greece, in Arabia, in Spain, in Italy, in France, and in all places where any hope could be indulged of their succeeding. In America the culture of the silk-worm was introduced into Virginia in the time of James I., who himself composed a book of instructions on the subject, and caused mulberry-trees and silk-worms' eggs to be sent to the colony. In Georgia, also, lands were granted on condition of planting one hundred white mulberry-trees on every ten acres of cleared land.[DY]

The growth of the silk-worm has also been tried, but with no great success, in this country. Evelyn computed that one mulberry-tree would feed as many silk-worms annually as would produce seven pounds of silk. "According to that estimate," says Barham,[DZ] "the two thousand trees already planted in Chelsea Park (which take up one-third of it) will make 14,000 lbs. weight of silk; to be commonly worth but twenty shillings a pound, those trees must make 14,000_l._ per annum." During the last century, some French refugees in the south of Ireland made considerable plantations of the mulberry, and had begun the cultivation of silk with every appearance of success; but since their removal the trees have been cut down.[EA] In the vicinity of London, also, a considerable plantation of mulberry-trees was purchased by the British, Irish, and Colonial Silk Company in 1825; but we have not learned whether this Company have any active measures now in operation.

The manufacture of silk was introduced into this country in 1718, at Derby, by Mr. John Lombe, who travelled into Italy to obtain the requisite information; but so jealous were the Italians of this, that according to some statements which have obtained belief, he fell a victim to their revenge, having been poisoned at the early age of twenty-nine.[EB]

There are not only several varieties of the common silk-worm (_Bombyx mori_), but other species of caterpillars, which spin silk capable of being manufactured, though not of so good qualities as the common silk. None of our European insects, however, seem to be well fitted for the purpose, though it has been proposed by Fabricius and others to try the crimson under-wing (_Catocala sponsa_, Schrank), &c. M. Latreille quotes from the 'Recreations of Natural History,' by Wilhelm, the statement that the cocoons of the emperor-moth (_Saturnia pavonia_) had been successfully tried in Germany, by M. Wentzel Hegeer de Berchtoldsdorf, under an imperial patent.

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. chrysorrhoea_), 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 chrysorrhoea_), 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.