Sometimes a still more wonderful arch or tunnel is made by the ants, for it is a living one composed of the bodies of some of their number. These, apparently, stand in two rows upon their hinder legs, and by interlocking their jaws and intertwining their anterior legs and antennæ make a covered way for the workers to pass along. From this, it would appear that certain of the ants feel the heat less than the ordinary workers. Apparently, however, the ants only act in this way when the sky is clouded, and when, as a consequence, one would not have expected any covering to be necessary. Dr. Savage, who gives this interesting account of ant body-building, as one may call it, has not been sufficiently explicit in regard to the details and circumstances attending it.
More extraordinary even than their habit of making a living arch or gallery, is the method which these ants employ of passing rivers. To do this they climb a tree upon one or other of its banks, and running out along a branch overhanging the water, let themselves down by clinging one to another, until a rope is formed of their united bodies. This soon reaches the water, and becoming constantly longer as fresh ants run down and affix themselves, is swept out from the shore by the force of the current, until at length its free end is washed against the opposite bank. There is, now, a thin bridge of ants, like a ribbon and of immense length, stretched slanting-wise from shore to shore, and over it the main body of the ants ceaselessly pass, till there are no more to come. Only the bridge itself now remains, but the ants helping to form this, on the nearer side of the stream, detach themselves now from the tree, when the bridge changes to a rope in the water, and this, being carried at once down the stream, is soon washed against the further bank, to which its corresponding end is attached.[6] As soon as this has been accomplished, the living ants composing this organic work of engineering skill, crawl on shore and continue their march, bringing up the rear of the column. It has been asserted, I know—for I have read it somewhere, and well remember the accompanying illustration—that the monkeys inhabiting the Brazilian forests are accustomed to cross the smaller rivers that flow through them, in the same way. As the ants do so, there seems nothing absolutely impossible in the thing, but as years have gone by and I have met with no reference to so interesting a fact in any work of standing, I have got to distrust the only authority I can remember for it—a boy’s book, namely, by Mayne Reid.
Du Chaillu, whose account of the driver-ants, or bashikonays, I have already quoted, describes their manner of bridging streams in a slightly different way, which, if correct, makes it still more remarkable. He says: “When, on their line of march, they require to cross a narrow stream, they throw themselves across, and form a tunnel—a living tunnel—connecting two trees or high bushes on opposite sides of the little stream, whenever they can find such, to facilitate the operation. This is done with great speed, and is effected by a great number of ants, each of which clings with its fore claws to its next neighbour’s body or hind claws. Thus they form a high, safe tubular bridge, through which the whole vast regiment marches in regular order. If disturbed, or if the arch is broken by the violence of some animal, they instantly attack the offender with the greatest animosity.” This presents the matter in a still more interesting light, and as it is the account of a man who professes to have seen what he describes, it should rank, perhaps, before the other, which, though I have taken it from a trustworthy source, was not there given as a first-hand account. Both versions, however, may be correct.
If streams are not sufficient to daunt the driver-ant, neither are floods. When these occur, numbers of them rush together and cling to one another, forming a ball-shaped mass, that, being lighter than the water, floats upon it, till such time as the flood has retired. The size of these balls is, for the most part, that of an orange, but they may be either larger or smaller—tangerine orange-balls in the latter case. The natives say that the larger and stronger ants form the outer circumference of the globe, whilst the weakly ones—or, as they express it, the women and children—are contained and guarded in the centre.
I have never seen the real driver-ants, not having been in any really tropical country. In South Africa, however, I have often seen the armies, or, as the Kaffirs call them, impis, of a black, stinging ant, that seems to take their place. When these insects are disturbed in their march, the whole column makes a hissing noise, which can be very distinctly heard. How the sound is produced I do not know, but it is more like a hiss than anything else, and is accompanied, if I remember rightly, with a strong smell of formic acid. Though these black ants are fierce and bold, so that the Kaffirs admire them, call them warriors, and compare them with themselves, their marches are not attended with the striking sights which belong to those of the drivers, nor have they the wonderful habits or instincts of the latter. They are less than half their size, moreover, and their chief weapon being a sting, the mandibles are not extraordinarily developed. I never myself happened to be stung by one, but have heard others complain bitterly.
The driver-ants of Africa are represented in tropical America by the Ecitons—a family containing numerous species—of which we have some interesting accounts by travellers who were, at the same time, naturalists. Speaking of the Eciton drepanophora, Mr. Bates, in his well-known Naturalist on the River Amazon, says: “When the pedestrian falls in with a train of these ants, the first signal given him is a twittering and restless movement of small flocks of plain-coloured birds (ant-thrushes) in the jungle. If this be disregarded until he advances a few steps farther, he is sure to fall into trouble, and find himself suddenly attacked by numbers of the ferocious little creatures. They swarm up his legs with incredible rapidity, each one driving his pincer-like jaws into his skin, and, with the purchase thus obtained, doubling in its tail and stinging with all its might. There is no course left but to run for it.” However, it is almost as easy to “fly from oneself” (a hard thing, Horace tells us) as from ants that have once crawled up beneath one’s garments and embedded their jaws in one’s flesh. Only after a halt, and special attention paid to each individual, are these to be got rid of, and then only by degrees, since these determined little warriors—all undecorated, and without even a thought of crosses or promotions—are content to let their bodies be torn from their heads, as long as they can leave the latter, with the jaws attached, sticking in the wounds they have made.
“The errand,” continues Mr. Bates, “of the vast ant armies is plunder, and wherever they move the whole animal world is set in commotion, and every creature tries to get out of their way. But it is, especially, the various tribes of wingless insects that have cause for fear, such as heavy-bodied spiders, ants of other species, maggots, caterpillars, larvæ of cockroaches, and so forth, all of which live under fallen leaves or in decaying wood.” Unlike the bashikonay ants that we have been considering, these Ecitons do not ascend trees to any great height, so that young birds in their nests for the most part escape. Both species consist, like other ant communities, of males, females, and workers, but the differentiation of the latter into two castes, differing both in size and shape from one another, is most marked amongst the Ecitons. The members composing these two classes are known as the worker-majors and worker-minors respectively, and whilst the latter make up the majority of the host, and thus present the standard size and appearance, the former are much larger, with heads disproportionately big, and greatly lengthened jaws.
Both the African and American kinds hunt with method and system, and each species has its own particular way of setting to work. Of that employed by the one under consideration, Mr. Bates gives us the following account. “The main column, from four to six deep, moves forward in a given direction, clearing the ground of all animal matter, dead or alive, and throwing off, here and there, a thinner column, to forage for a short time on the flanks of the main army, and re-enter it again after their task is accomplished. If some very rich place be encountered anywhere near the line of march—for example, a mass of rotten wood abounding in insect larvæ—a delay takes place, and a very strong force of insects is concentrated upon it. The excited creatures search every cranny, and tear in pieces all the large grubs they drag to light. It is curious to see them attack wasps’ nests, which are sometimes built on low shrubs. They gnaw away the papery covering, to get at the larvæ, pupæ, and newly hatched wasps, and cut everything to tatters, regardless of the infuriated owners which are flying about them. In bearing off their spoil in fragments, the pieces are apportioned to the carriers with some degree of regard to fairness of load: the dwarfs taking the smallest pieces, and the strongest fellows, with small heads, the heaviest portions. Sometimes two ants join together in carrying one piece, but the worker-majors, with their unwieldy and distorted jaws, are incapacitated from taking any part in the labour.”
The precise part in the life of the community which is played by these great worker-majors, with the relation which it no doubt bears to their superior size and modified shape, has long been a puzzle to naturalists. The first idea was that they formed a soldier caste—a natural supposition in view of their great armour-plated heads, and elongated twisted jaws. Observation, however, does not bear out this theory. The jaws, in spite of their size, are not so well adapted for seizing on a plane surface—the skin, for instance, of an animal—as are those of the smaller workers; and, moreover, these large ants seemed to Mr. Bates to be less pugnacious than the others. “The position,” he tells us, “of the large-headed individuals in the marching column was rather curious. There was one of these extraordinary fellows to about a score of the smaller class; none of them carried anything in their mouths, but all trotted along empty-handed and outside the column, at pretty regular intervals from each other, like subaltern officers in a marching regiment of soldiers. I did not see them change their position, or take any notice of their small-headed comrades marching in the column, and when I disturbed the line, they did not prance forth or show fight so eagerly as the others.” Mr. Bates then hazards a conjecture that these big ants may serve indirectly to preserve the community, by being indigestible to birds, and that their great, twisted mandibles may be effective, whilst lying in the crops or stomachs of the latter. This seems possible, since a certain number of unpalatable individuals in a community of ants might make birds disinclined to eat any of them. I think, myself, however, that it is premature to speculate on the part in life which these curiously modified worker ants may be designed to play, until we know something more of their home economy, and particularly of their architecture. This, it is true, is of a very rude kind, nor do these marauding ants appear to have any permanent place of abode. Still, they may do something in the shape of building, and the peculiar jaws of the worker-major class suggest that they are formed for seizing some special object, or performing some special kind of labour.