So much as the world has suffered from these animals[20], 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, 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[21]."
"In the month of May, when the ovaries of these insects were ripe and turgid," says Dr. Shaw[22], "each of these swarms began gradually to disappear, and retired 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[23], has been adopted: but I think without sufficient reason, by several travellers. Thus Benjamin Bullivant, in his observations on the Natural History of New England[24], 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[25]." 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, 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[26]. 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[27]. 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 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[28]," 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 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[29]."
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 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 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[30].
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[31]. I have observed similar nidi in this country; the insects that form them belong to the Fabrician genus Lyda.
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, 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[32].
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[33]. 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 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[34].