I see already, and I see it with pleasure, that you will not content yourself with being a mere collector of insects. To possess a cabinet well stored, and to know by what name each described individual which it contains should be distinguished, will not satisfy the love that is already grown strong in you for my favourite pursuit; and you now anticipate with a laudable eagerness, the discoveries that you may make respecting the history and economy of this most interesting department of the works of our Creator. I hail with joy this intention to emulate the bright example, and to tread in the hallowed steps of Swammerdam, Leeuwenhoek, Redi, Malpighi, Vallisnieri, Ray, Lister, Reaumur, De Geer, Lyonnet, Bonnet, the Hubers, &c.; and I am confident[Pg 2] that a man of your abilities, discernment, and observation will contribute, in no small degree, to the treasures already poured into the general fund by these your illustrious predecessors.
I feel not a little flattered when you inform me that the details contained in my late letters relative to this subject, have stimulated you to this noble resolution.—Assure yourself, I shall think no labour lost, that has been the means of winning over to the science I love, the exertions of a mind like yours.
But if the facts already related, however extraordinary, have had power to produce such an effect upon you, what will be the momentum, when I lay before you more at large, as I next purpose, the most striking particulars of the proceedings of insects in society, and show the almost incredibly wonderful results of the combined instincts and labours of these minute beings? In comparison with these, all that is the fruit of solitary efforts, though some of them sufficiently marvellous, appear trifling and insignificant: as the works of man himself, when they are the produce of the industry and genius of only one, or a few individuals, though they might be regarded with admiration by a being who had seen nothing similar before, yet when contrasted with those to which the union of these qualities in large bodies has given birth, sink into nothing, and seem unworthy of attention. Who would think a hut extraordinary by the side of a stately palace, or a small village when in the vicinity of a populous and magnificent city?
Insects in society may be viewed under several lights, and their associations are for various purposes and of different durations.
There are societies the object of which is mutual defence; while that of others is the propagation of the species. Some form marauding parties, and associate for prey and plunder;—others meet, as it should seem, under certain circumstances, merely for the sake of company;—again, others are brought together by accidental causes, and disperse when these cease to operate;—and finally, others, which may be said to form proper societies, are associated for the nurture of their young, and, by the union of their labours and instincts, for mutual society, help, and comfort, in erecting or repairing their common habitation, in collecting provisions, and in defending their fortress when attacked.
With respect to the duration of the societies of insects, some last only during their first or larva state; and are occasionally even restricted to its earliest period;—some again only associate in their perfect or imago state; while with others, the proper societies for instance, the association is for life. But if I divide societies of insects into perfect and imperfect, it will, I think, enable me to give you a clearer and better view of the subject. By perfect societies I mean those that are associated in all their states, live in a common habitation, and unite their labours to promote a common object;—and by imperfect societies, those that are either associated during part of their existence only, or else do not dwell in a common habitation, nor unite their labours to promote a common object. In the present letter I shall confine myself to giving you some account of imperfect societies.
Imperfect societies may be considered as of five descriptions:—associations for the sake of company only—associations[Pg 4] of males during the season for pairing—associations formed for the purpose of travelling or emigrating together—associations for feeding together—and associations that undertake some common work.
The first of these associations consists chiefly of insects in their perfect state. The little beetles called whirlwigs (Gyrinus),—which may be seen clustering in groups under warm banks in every river and every pool, and wheeling round and round with great velocity; at your approach dispersing and diving under water, but as soon as you retire resuming their accustomed movements,—seem to be under the influence of the social principle, and to form their assemblies for no other purpose than to enjoy together, in the sunbeam, the mazy dance. Impelled by the same feeling, in the very depth of winter, even when the earth is covered with snow, the tribes of Tipulariæ (usually, but improperly, called gnats) assemble in sheltered situations at midday, when the sun shines, and form themselves into choirs, that alternately rise and fall with rapid evolutions. To see these little aëry beings apparently so full of joy and life, and feeling the entire force of the social principle in that dreary season, when the whole animal creation appears to suffer, and the rest of the insect tribes are torpid, always conveys to my mind the most agreeable sensations. These little creatures may always be seen at all seasons amusing themselves with these choral dances; which Mr. Wordsworth, in one of his poems, has alluded to in the following beautiful lines:
"Nor wanting here to entertain the thought,Creatures that in communities exist,Less, as might seem, for general guardianshipOr through dependance upon mutual aid,Than by participation of delight,And a strict love of fellowship combined.What other spirit can it be that promptsThe gilded summer flies to mix and weaveTheir sports together in the solar beam,Or in the gloom and twilight hum their joy?"
Another association is that of males during the season of pairing. Of this nature seems to be that of the cockchafer and fernchafer (Melolontha vulgaris andAmphimalla solstitialis), which, at certain periods of the year and hours of the day, hover over the summits of the trees and hedges like swarms of bees, affording, when they alight on the ground, a grateful food to cats, pigs, and poultry. The males of another root-devouring beetle (Hoplia argentea) assemble by myriads before noon in the meadows, when in these infinite hosts you will not find even one female. After noon the congregation is dissolved, and not a single individual is to be seen in the air: while those of M. vulgaris and A. solstitialis are on the wing only in the evening.
At the same time of the day some of the short-lived Ephemeræ assemble in numerous troops, and keep rising and falling alternately in the air, so as to exhibit a very amusing scene. Many of these also are males. They continue this dance from about an hour before sun-set, till the dew becomes too heavy or too cold for them. In the beginning of September, for two successive years,[Pg 6] I was so fortunate as to witness a spectacle of this kind, which afforded me a more sublime gratification than any work or exhibition of art has power to communicate.—The first was in 1811:—taking an evening walk near my house, when the sun declining fast towards the horizon shone forth without a cloud, the whole atmosphere over and near the stream swarmed with infinite myriads of Ephemeræ and little gnats of the genus Chironomus, which in the sun-beam appeared as numerous and more lucid than the drops of rain, as if the heavens were showering down brilliant gems.—Afterwards, in the following year, one Sunday, a little before sun-set, I was enjoying a stroll with a friend at a greater distance from the river, when in a field by the road-side the same pleasing scene was renewed, but in a style of still greater magnificence; for, from some cause in the atmosphere, the insects at a distance looked much larger than they really were. The choral dances consisted principally of Ephemeræ, but there were also some of Chironomi; the former, however, being most conspicuous, attracted our chief attention—alternately rising and falling, in the full beam they appeared so transparent and glorious, that they scarcely resembled any thing material—they reminded us of angels and glorified spirits drinking life and joy in the effulgence of the Divine favour. The bard of Twickenham, from the terms in which his beautiful description of his sylphs is conceived in The Rape of the Lock, seems to have witnessed the pleasing scene here described:
"Some to the sun their insect wings unfold,Waft on the breeze, or sink in clouds of gold;Transparent forms, too fine for mortal sight,Their fluid bodies half dissolv'd in light;Loose to the wind their airy garments flew,Thin glittering textures of the filmy dew,Dipt in the richest tincture of the skies,Where light disports in ever mingling dyes,While every beam new transient colours flings,Colours that change whene'er they wave their wings."
I wish you may have the good fortune next year to be a spectator of this all but celestial dance. In the mean time, in May and June, their season of love, you may often receive much gratification from observing the motions of a countless host of little black flies of the genus Hilara, (H. maura,) which at this period of the year assemble to wheel in aëry circles over stagnant waters, with a rush resembling that of a hasty shower driven by the wind.
The next description of insect associations is of those that congregate for the purpose of travelling or emigrating together. De Geer has given an account of the larvæ of certain gnats (Tipulariæ) which assemble in considerable numbers for this purpose, so as to form a band of a finger's breadth, and of from one to two yards in length. And, what is remarkable, while upon their march, which is very slow, they adhere to each other by a kind of glutinous secretion; but when disturbed they separate without difficulty. Kuhn mentions another of the same tribe (from the antennæ in his figure, which is very indifferent, it should seem a species of agaric-gnat (Mycetophila)), the larvæ of which live in society and[Pg 8] emigrate in files, like the caterpillar of the procession-moth. First goes one, next follow two, then three, &c., so as to exhibit a serpentine appearance, probably from their simultaneous undulating motion and the continuity of the files; whence the common people in Germany call them (or rather the file when on march) heerwurm, and view them with great dread, regarding them as ominous of war. These larvæ are apodes, white, subtransparent, with black heads.—But of insect emigrants none are more celebrated than the locusts, which, when arrived at their perfect state, assemble as before related, in such numbers, as in their flight to intercept the sunbeams, and to darken whole countries; passing from one region to another, and laying waste kingdom after kingdom:—but upon these I have already said much, and shall have occasion again to enlarge.—The same tendency to shift their quarters has been observed in our little indigenous devourers, the Aphides. Mr. White tells us, that about three o'clock in the afternoon of the first of August 1785, the people of the village of Selborne were surprised by a shower of Aphides or smother-flies, which fell in those parts. Those that walked in the street at that juncture found themselves covered with these insects, which settled also upon the hedges and in the gardens, blackening all the vegetables where they alighted. His annuals were discoloured by them, and the stalks of a bed of onions quite coated over for six days after. These armies, he observes, were then, no doubt, in a state of emigration, and shifting their quarters; and might have come from the great hop-plantations of Kent or Sussex, the wind being all that day in[Pg 9] the east. They were observed at the same time in great clouds about Farnham, and all along the vale from Farnham to Alton. A similar emigration of these flies I once witnessed, to my great annoyance, when travelling later in the year, in the Isle of Ely. The air was so full of them, that they were incessantly flying into my eyes, nostrils, &c.; and my clothes were covered by them. And in 1814, in the autumn, the Aphides were so abundant for a few days in the vicinity of Ipswich, as to be noticed with surprise by the most incurious observers.
As the locust-eating thrush (Turdus gryllivorus) accompanies the locusts, so the lady-birds (Coccinellæ) seem to pursue the Aphides; for I know no other reason to assign for the vast number that are sometimes, especially in the autumn, to be met with on the sea-coast or the banks of large rivers. Many years ago, those of the Humber were so thickly strewed with the common Lady-bird (C. septempunctata), that it was difficult to avoid treading upon them. Some years afterwards I noticed a mixture of species, collected in vast numbers, on the sand-hills on the sea-shore, at the north-west extremity of Norfolk. My friend the Rev. Peter Lathbury made long since a similar observation at Orford, on the Suffolk coast; and about five or six years ago they covered the cliffs, as I have before remarked, of all the watering-places on the Kentish and Sussex coasts, to the no small alarm of the superstitious, who thought them forerunners of some direful evil. These last probably emigrated with the Aphides from the hop-grounds. Whether the latter and their devourers cross the sea has not been ascertained;[Pg 10] that the Coccinellæ attempt it, is evident from their alighting upon ships at sea, as I have witnessed myself.—This appears clearly to have been the case with another emigrating insect, the saw-fly (Athalia?) of the turnip (which, though so mischievous, appears never to have been described; it is nearly related to A. Centifoliæ). It is the general opinion in Norfolk, Mr. Marshall informs us, that these insects come from over sea. A farmer declared he saw them arrive in clouds so as to darken the air; the fishermen asserted that they had repeatedly seen flights of them pass over their heads when they were at a distance from land; and on the beach and cliffs they were in such quantities, that they might have been taken up by shovels-full. Three miles in-land they were described as resembling swarms of bees. This was in August 1782. Unentomological observers, such as farmers and fishermen, might easily mistake one kind of insect for another; but supposing them correct, the swarms in question might perhaps have passed from Lincolnshire to Norfolk.—Meinecken tells us, that he once saw in a village in Anhalt, on a clear day, about four in the afternoon, such a cloud of dragon-flies (Libellulina) as almost concealed the sun, and not a little alarmed the villagers, under the idea that they were locusts: several instances are given by Rösel of similar clouds of these insects having been seen in Silesia and other districts; and Mr. Woolnough of Hollesley in Suffolk, a most attentive observer of nature, once witnessed such an army of the smaller dragon-flies (Agrion) flying in-land from the sea, as to cast a slight shadow[Pg 11] over a field of four acres as they passed.—Professor Walch states, that one night about eleven o'clock, sitting in his study, his attention was attracted by what seemed the pelting of hail against his window, which surprising him by its long continuance, he opened the window, and found the noise was occasioned by a flight of the froth frog-hopper (Cercopis spumaria), which entered the room in such numbers as to cover the table. From this circumstance and the continuance of the pelting, which lasted at least half an hour, an idea may be formed of the vast host of this insect passing over. It passed from east to west; and as his window faced the south, they only glanced against it obliquely. He afterwards witnessed, in August, a similar emigration of myriads of a kind of ground-beetle (Amara vulgaris,).—Another writer in the same work, H. Kapp, observed on a calm sunny day a prodigious flight of the noxious cabbage-butterfly (Pontia Brassicæ), which passed from north-east to south-west, and lasted two hours. Kalm saw these last insects midway in the British Channel. Lindley, a writer in the Royal Military Chronicle, tells us, that in Brazil, in the beginning of March 1803, for many days successively there was an immense flight of white and yellow butterflies, probably of the same tribe as the cabbage-butterfly. They were observed never to settle, but proceeded in a direction from north-west to south-east. No buildings seemed to stop them from steadily pursuing their course; which being to the ocean, at only a small distance, they must consequently perish. It is remarked that at this time no other kind of butterfly is[Pg 12] to be seen, though the country usually abounds in such a variety.—Major Moor, while stationed at Bombay, as he was playing at chess one evening with a friend in Old Woman's Island, near that place, witnessed an immense flight of bugs (Geocorisæ), which were going westward. They were so numerous as to cover every thing in the apartment in which he was sitting.—When staying at Aldeburgh, on the eastern coast, I have, at certain times, seen innumerable insects upon the beach close to the waves, and apparently washed up by them. Though wetted, they were quite alive. It is remarkable, that of the emigrating insects here enumerated, the majority—for instance the lady-birds, saw-flies, dragon-flies, ground-beetles, frog-hoppers, &c.—are not usually social insects, but seem to congregate, like swallows, merely for the purpose of emigration. What incites them to this is one of those mysteries of nature, which at present we cannot penetrate. A scarcity of food urges the locusts to shift their quarters; and too confined a space to accommodate their numbers occasions the bees to swarm: but neither of these motives can operate in causing unsocial insects to congregate. It is still more difficult to account for the impulse that urges these creatures, with their filmy wings and fragile form, to attempt to cross the ocean, and expose themselves, one would think, to inevitable destruction. Yet, though we are unable to assign the cause of this singular instinct, some of the reasons which induced the Creator to endow them with it may be conjectured. This is clearly one of the modes by which their numbers are kept within due limits, as, doubtless, the great majority of these adventurers perish in the[Pg 13] waters. Thus, also, a great supply of food is furnished to those fish in the sea itself, which at other seasons ascend the rivers in search of them; and this probably is one of the means, if not the only one, to which the numerous islands of this globe are indebted for their insect population. Whether the insects I observed upon the beach wetted by the waves, had flown from our own shores, and falling into the water had been brought back by the tide; or whether they had succeeded in the attempt to pass from the continent to us, by flying as far as they could, and then falling had been brought by the waves, cannot certainly be ascertained; but Kalm's observation inclines me to the latter opinion.
The next order of imperfect associations is that of those insects which feed together:—these are of two descriptions—those that associate in their first or last state only, and those that associate in all their states. The first of these associations is often very short-lived: a patch of eggs is glued to a leaf; when hatched, the little larvæ feed side by side very amicably, and a pleasant sight it is to see the regularity with which this work is often done, as if by word of command; but when the leaf that served for their cradle is consumed, their society is dissolved, and each goes where he can to seek his own fortune, regardless of the fate or lot of his brethren. Of this kind are the larvæ of the saw-fly of the gooseberry, whose ravages I have recorded before, and that of the cabbage-butterfly; the latter, however, keep longer together, and seldom wholly separate. In their final state, I have noticed that the individuals of Thrips Physapus, the fly that causes us in hot weather such intolerable titillation, are[Pg 14] very fond of each other's company when they feed. Towards the latter end of last July, walking through a wheat-field, I observed that all the blossoms of Convolvulus arvensis, though very numerous, were interiorly turned quite black by the infinite number of these insects, which were coursing about within them.
But the most interesting insects of this order are those which associate in all their states.—Two populous tribes, the great devastators of the vegetable world, the one in warm and the other in cold climates, to which I have already alluded under the head of emigrations—you perceive I am speaking of Aphides and Locusts—are the best examples of this order: although, concerning the societies of the first, at present we can only say that they are merely the result of a common origin and station: but those of the latter, the locusts, wear more the appearance of design, and of being produced by the social principle.
So much as the world has suffered from these animals, it is extraordinary that so few observations have been made upon their history, economy, and mode of proceeding. One of the best accounts seems to be that of Professor Pallas, in his Travels into the Southern Provinces of the Russian Empire. The species to which his principal attention was paid appears to have been the Locusta italica, in its larva and pupa state. "In serene warm weather," says he, "the locusts are in full motion in the morning immediately after the evaporation of the dew; and if no dew has fallen, they appear as soon as the sun imparts his genial warmth. At first some are seen running about like messengers among the reposing swarms,[Pg 15] which are lying partly compressed upon the ground, at the side of small eminences, and partly attached to tall plants and shrubs. Shortly after the whole body begins to move forward in one direction and with little deviation. They resemble a swarm of ants, all taking the same course, at small distances, but without touching each other: they uniformly travel towards a certain region as fast as a fly can run, and without leaping, unless pursued; in which case, indeed, they disperse, but soon collect again and follow their former route. In this manner they advance from morning to evening without halting, frequently at the rate of a hundred fathoms and upwards in the course of a day. Although they prefer marching along high roads, footpaths, or open tracts; yet when their progress is opposed by bushes, hedges, and ditches, they penetrate through them: their way can only be impeded by the waters of brooks or canals, as they are apparently terrified at every kind of moisture. Often, however, they endeavour to gain the opposite bank with the aid of overhanging boughs; and if the stalks of plants or shrubs be laid across the water, they pass in close columns over these temporary bridges; on which they even seem to rest and enjoy the refreshing coolness. Towards sunset the whole swarm gradually collect in parties, and creep up the plants, or encamp on slight eminences. On cold, cloudy, or rainy days they do not travel.—As soon as they acquire wings they progressively disperse, but still fly about in large swarms."
"In the month of May, when the ovaries of these insects were ripe and turgid," says Dr. Shaw, "each of these swarms began gradually to disappear, and retired[Pg 16] into the Mettijiah, and other adjacent plains, where they deposited their eggs. These were no sooner hatched in June, than each of the broods collected itself into a compact body, of a furlong or more in square; and marching afterwards directly forwards toward the sea, they let nothing escape them——they kept their ranks, like men of war; climbing over, as they advanced, every tree or wall that was in their way; nay, they entered into our very houses and bed-chambers, like so many thieves.——A day or two after one of these hordes was in motion, others were already hatched to march and glean after them.——Having lived near a month in this manner——they arrived at their full growth, and threw off their nympha-state by casting their outward skin. To prepare themselves for this change, they clung by their hinder feet to some bush, twig, or corner of a stone; and immediately, by using an undulating motion, their heads would first break out, and then the rest of their bodies. The whole transformation was performed in seven or eight minutes; after which they lay for a small time in a torpid and seemingly in a languishing condition; but as soon as the sun and the air had hardened their wings, by drying up the moisture that remained upon them after casting their sloughs, they reassumed their former voracity, with an addition of strength and agility. Yet they continued not long in this state before they were entirely dispersed." The species Dr. Shaw here speaks of is probably not the Locusta migratoria.
The old Arabian fable, that they are directed in their flights by a leader or king, has been adopted: but I[Pg 17] think without sufficient reason, by several travellers. Thus Benjamin Bullivant, in his observations on the Natural History of New England, says that "the locusts have a kind of regimental discipline, and as it were some commanders, which show greater and more splendid wings than the common ones, and arise first when pursued by the fowls or the feet of the traveller, as I have often seriously remarked." And in like terms Jackson observes, that "they have a government amongst themselves similar to that of the bees and ants; and when the (Sultan Jerraad) king of the locusts rises, the whole body follow him, not one solitary straggler being left behind." But that locusts have leaders, like the bees or ants, distinguished from the rest by the size and splendour of their wings, is a circumstance that has not yet been established by any satisfactory evidence; indeed, very strong reasons may be urged against it. The nations of bees and ants, it must be observed, are housed together in one nest or hive, the whole population of which is originally derived from one common mother, and the leaders of the swarms in each are the females. But the armies of locusts, though they herd together, travel together, and feed together, consist of an infinity of separate families, all derived from different mothers, who have laid their eggs in separate cells or houses in the earth; so that there is little or no analogy between the societies of locusts and those of bees and ants; and this pretended sultan is something quite different from the queen-bee or the female ants. It follows, therefore, that as the locusts have no common mother, like the bees, to lead their swarms, there is no one that nature,[Pg 18] by a different organization and ampler dimensions, and a more august form, has destined to this high office. The only question remaining is, whether one be elected from the rest by common consent as their leader, or whether their instinct impels them to follow the first that takes flight or alights. This last is the learned Bochart's opinion, and seems much the most reasonable. The absurdity of the other supposition, that an election is made, will appear from such queries as these, at which you may smile.—Who are the electors? Are the myriads of millions all consulted, or is the elective franchise confined to a few? Who holds the courts and takes the votes? Who casts them up and declares the result? When is the election made?—The larvæ appear to be as much under government as the perfect insect.—Is the monarch then chosen by his peers when they first leave the egg and emerge from their subterranean caverns? or have larva, pupa, and imago each their separate king? The account given us in Scripture is certainly much the most probable, that the locusts have no king, though they observe as much order and regularity in their movements as if they were under military discipline, and had a ruler over them. Some species of ants, as we learn from the admirable history of them by M. P. Huber, though they go forth by common consent upon their military expeditions, yet the order of their columns keeps perpetually changing; so that those who lead the van at the first setting out, soon fall into the rear, and others take their place: their successors do the same; and such is the constant order of their march. It seems probable, as these columns are extended[Pg 19] to a considerable length, that the object of this successive change of leaders is to convey constant intelligence to those in the rear, of what is going forward in the van. Whether any thing like this takes place for the regulation of their motions in the innumerable locust-armies, which are sometimes co-extensive with vast kingdoms; or whether their instinct simply directs them to follow the first that moves or flies, and to keep their measured distance, so that, as the prophet speaks, "one does not thrust another, and they walk every one in his path," must be left to future naturalists to ascertain. And I think that you will join with me in the wish that travellers, who have a taste for Natural History, and some knowledge of insects, would devote a share of attention to the proceedings of these celebrated animals, so that we might have facts instead of fables.
The last order of imperfect associations approaches nearer to perfect societies, and is that of those insects which the social principle urges to unite in some common work for the benefit of the community.
Amongst the Coleoptera, Ateuchus pilularius, a beetle before mentioned, acts under the influence of this principle. "I have attentively admired their industry and mutual assisting of each other," says Catesby, "in rolling those globular balls from the place where they made them, to that of their interment, which is usually the distance of some yards, more or less. This they perform breech foremost, by raising their hind parts, forcing along the ball with their hind feet. Two or three of them are sometimes engaged in trundling one ball, which, from meeting with impediments from the unevenness[Pg 20] of the ground, is sometimes deserted by them: it is however attempted by others with success, unless it happens to roll into some deep hollow chink, where they are constrained to leave it; but they continue their work by rolling off the next ball that comes in their way. None of them seem to know their own balls, but an equal care for the whole appears to affect all the community."
Many larvæ also of Lepidoptera associate with this view, some of which are social only during part of their existence, and others during the whole of it. The first of these continue together while their united labours are beneficial to them; but when they reach a certain period of their life, they disperse and become solitary. Of this kind are the caterpillars of a little butterfly (Melitæa Cinxia) which devour the narrow-leaved plantain. The families of these, usually amounting to about a hundred, unite to form a pyramidal silken tent, containing several apartments, which is pitched over some of the plants that constitute their food, and shelters them both from the sun and the rain. When they have consumed the provision which it covers, they construct a new one over other roots of this plant; and sometimes four or five of these encampments may be seen within a foot or two of each other. Against winter they weave and erect a stronger habitation of a rounder form, not divided by any partitions, in which they lie heaped one upon another, each being rolled up. About April they separate, and continue solitary till they assume the pupa.
Reaumur, to whom I am indebted for this account, has also given us an interesting history of another insect, the[Pg 21] gold-tail-moth (Arctia chrysorhœa) before mentioned, whose caterpillars are of this description. They belong to that family of Bombycidæ, which envelop their eggs in hair plucked from their own body. As soon as one of these young caterpillars is disclosed from the egg, it begins to feed; another quickly joins it, placing itself by its side; thus they proceed in succession till a file is formed across the leaf:—a second is then begun; and after this is completed, a third—and so they proceed till the whole upper surface of the leaf is covered:—but as a single leaf will not contain the whole family, the remainder take their station upon the adjoining ones. No sooner have they satisfied the cravings of hunger, than they begin to think of erecting a common habitation, which at first is only a vaulted web, that covers the leaf they inhabit, but by their united labours in due time grows into a magnificent tent of silk, containing various apartments sufficient to defend and shelter them all from the attack of enemies and the inclemency of the seasons. As our caterpillars, like eastern monarchs, are too delicate to adventure their feet upon the rough bark of the tree upon which they feed, they lay a silken carpet over every road and pathway leading to their palace, which extends as far as they have occasion to go for food. To the habitation just described they retreat during heavy rains, and when the sun is too hot:—they likewise pass part of the night in them;—and, indeed, at all times some may usually be found at home. Upon any sudden alarm they retreat to them for safety, and also when they cast their skins:—in the winter they are wholly confined to them, emerging again in the spring: but in May and June they entirely desert them; and, losing all their[Pg 22] love for society, live in solitude till they become pupæ, which takes place in about a month. When they desert their nests, the spiders take possession of them; which has given rise to a prevalent though most absurd opinion, that they are the parents of these caterpillars.
With other caterpillars the association continues during the whole of the larva state. De Geer mentions one of the saw-flies (Serrifera) of this description which form a common nidus by connecting leaves together with silken threads, each larva moreover spinning a tube of the same material for its own private apartment, in which it glides backwards and forwards upon its back. I have observed similar nidi in this country; the insects that form them belong to the Fabrician genusLyda.
The most remarkable insects, however, that arrange under this class of imperfect associates, are those that observe a particular order of march. Though they move without beat of drum, they maintain as much regularity in their step as a file of soldiers. It is a most agreeable sight, says one of Nature's most favoured admirers, Bonnet, to see several hundreds of the larvæ of Trichoda Neustria marching after each other, some in straight lines, others in curves of various inflection, resembling, from their fiery colour, a moving cord of gold stretched upon a silken ribband of the purest white; this ribband is the carpeted causeway that leads to their leafy pasture from their nest. Equally amusing is the progress of another moth, the Pityocampa, before noticed; they march together from their common citadel,[Pg 23] consisting of pine leaves united and inwoven with the silk which they spin, in a single line: in following each other they describe a multitude of graceful curves of varying figure, thus forming a series of living wreaths, which change their shape every moment:—all move with a uniform pace, no one pressing too forward or loitering behind; when the first stops, all stop, each defiling in exact military order.
A still more singular and pleasing spectacle, when their regiments march out to forage, is exhibited by the caterpillars of the Processionary moth Lasiocampa processionea. This moth, which is a native of France, and has not yet been found in this country, inhabits the oak. Each family consists of from 600 to 800 individuals. When young, they have no fixed habitation, but encamp sometimes in one place and sometimes in another, under the shelter of their web: but when they have attained two-thirds of their growth, they weave for themselves a common tent, before described. About sun-set the regiment leaves its quarters; or, to make the metaphor harmonize with the trivial name of the animal, the monks their cœnobium. At their head is a chief, by whose movements their procession is regulated. When he stops, all stop, and proceed when he proceeds; three or four of his immediate followers succeed in the same line, the head of the second touching the tail of the first: then comes an equal series of pairs, next of threes, and so on as far as fifteen or twenty. The whole procession moves regularly on with an even pace, each file treading[Pg 24] upon the steps of those that precede it. If the leader, arriving at a particular point, pursues a different direction, all march to that point before they turn. Probably in this they are guided by some scent imparted to the tracks by those that pass over them. Sometimes the order of procession is different; the leader, who moves singly, is followed by two, these are succeeded by three, then come four, and so on. When the leader,—who in nothing differs from the rest, and is probably the caterpillar nearest the entrance to the nest, followed, as I have described,—has proceeded to the distance of about two feet, more or less, he makes a halt; during which those which remain come forth, take their places, the company forms into files, the march is resumed, and all follow as regularly as if they kept time to music. These larvæ may be occasionally found at mid-day out of their nests, packed close one to another without making any movement; so that, although they occupy a space sufficiently ample, it is not easy to discover them. At other times, instead of being simply laid side by side, they are formed into singular masses, in which they are heaped one upon another, and as it were interwoven together. Thus also they are disposed in their nests. Sometimes their families divide into two bands, which never afterwards unite.
I have nothing further of importance to communicate to you on imperfect societies: in my next I shall begin the most interesting subject that Entomology offers; a subject, to say the least, including as great a portion both of instruction and amusement as any branch of Natural[Pg 25] History affords;—I mean those perfectassociations which have for their great object the multiplication of the species, and the education, if such a term may be here employed, of the young. This is too fertile a theme to be confined to a single letter, but must occupy several.
I am, &c.
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