# Popular Science Monthly/Volume 19/October 1881/Intelligence of Ants II

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* INTELLIGENCE OF ANTS.
By GEORGE J. ROMANES.
II.
I HAVE now presented some of the most curious and interesting facts concerning the intelligence of ants in general; I shall next proceed to state some of the more remarkable facts concerning the intelligence of certain species of ants in particular.
Leaf-cutting Ants of the Amazon.—The mode of working practiced by these ants is thus described by Bates:
Other observers have since said that this herbage is regularly felled by the ants in order to make a road. Each ant carries its semicircular piece of leaf upright over its head, so that the home-returning train is rendered very conspicuous. Keener observation shows that this home-returning, or load-carrying, train of workers keeps to one side of the road, while the outgoing, or empty-handed, train keeps to the other side; so that on every road there is a double train of ants going in opposite directions. When the leaves arrive at the nest they are received by a smaller kind of worker, whose duty it is to cut up the pieces into still smaller fragments, whereby the leaves seem to be better fitted for the purpose to which, as we shall presently see, they are put. These smaller workers never take any part in the out-door labor; but they occasionally leave the nest, apparently for the sole purpose of obtaining air and exercise, for when they leave the nest they merely run about doing nothing, and frequently, as in mere sport, mount some of the semicircular pieces of leaf which the carrier-ants are taking to the nest, and so get a ride home.
From his continued observation of these ants Bates concludes—and his opinion has been corroborated by that both of Belt and Müller—that the object of all this labor is a highly remarkable one. The leaves when gathered do not themselves appear to be of any service to the ants as food; but, when cut into small fragments and stored away in the nests, they become suited as a nidus for the growth of a minute kind of fungus on which the ants feed. We may therefore call these insects "gardening ants," inasmuch as all their labor is given to the rearing of nutritious vegetables on artificially prepared soil. They are ​not particular as to the material which they collect and store up for soil, provided that it is a material on which the fungus will grow—orange-peel, certain flowers, etc., being equally acceptable to them. But they are very particular regarding the ventilation of their underground storehouses, on a suitable degree of which the successful growth of the fungus presumably depends. They therefore have numerous holes or ventilating shafts which lead up to the surface from the storehouses or underground gardens, and these they either open or close according to the horticultural requirements as regards temperature and moisture. If the leaves are either too damp or too dry, they will not grow the fungus, and therefore in gathering the leaves the ants are very particular that they should neither be the one nor the other. Thus Bates observed:
Dr. Ellendorf made the experiment of interrupting the advance of a column of these ants, with the interesting result which he thus describes in a letter to Büchner:
The operations here described show clearly that these ants act upon the principle of the division of labor. In this connection I may also quote an observation of Belt, which shows this fact in perhaps even a stronger light. He says:
Ants of this genus are very clever at making tunnels. The Rev. H. Clark says that in one case they have made a tunnel of enormous length under the river Parahylia, where this is as broad as the Thames at London—their object being to reach a storehouse which is on the opposite bank. This statement is not to be considered so incredible as it at first sight unquestionably appears, for Bates has seen the subterranean passages of these ants extending to a distance of seventy yards.
Harvesting Ants.—The harvesting ants belong almost exclusively to a single genus, which, however, comprises a number of species distributed in localized areas over all the four quarters of the globe. Their distinctive habits consist in gathering nutritious seeds of grasses during summer, and storing them in granaries for winter consumption. We owe our present knowledge concerning these insects mainly to Moggridge, who studied them in the south of Europe, Lincecum and McCook, who studied them in Texas; Colonel Sykes and Dr. Jerdon also made some observations upon them in India. They likewise occur in Palestine, where they were clearly known to Solomon and other writers of antiquity, whose claim to accurate observation in this matter has within the last few years been amply vindicated, after having been for many years discredited, on account chiefly of the adverse statements of Huber.
Moggridge found that from the nest in various directions there proceed outgoing trains, which may be thirty or more yards in length, and each consisting of a double row of ants moving in opposite directions. Like the leaf-cutting ants, those composing the outgoing train are empty-handed, while those composing the incoming train are laden. But here the burdens are grass-seeds. At their terminations in the foraging-ground, or ant-fields, the insects composing these columns disperse by hundreds among the seed-yielding grasses. They then ascend the stems of the grasses, and, seizing the seed or capsule in their jaws, fix their hind-legs firmly as a pivot, round which they turn and turn till the stalk is twisted off. The ant then descends the stem,
​As further evidence that these insects well understand the advantages arising from the division of labor, I may quote one or two other observations. Thus Moggridge once saw a dead grasshopper carried into a nest of harvesting ants by the following means:
Again, Lespis says of the harvesting ant that,
No less, therefore, than the leaf-cutting ants already described, do these harvesting ants appreciate the benefits arising from the division of labor; and, as we shall presently see, there is a kind of ant exhibiting widely different habits, which shows appreciation of this principle in an even higher degree.
When the grain is taken into their nest by the harvesters, it is stored in regular granaries, but not until it has been denuded of its "husks" or "chaff." The denuding process, which corresponds to thrashing, is carried on below-ground, and the chaff is brought up to the surface, where it is laid in heaps to be blown away by the wind. It is not yet understood why the seed, when thus stored in subterranean chambers just far enough below the surface to favor germination, does not germinate. Moggridge proved that the vitality of the seeds is not impaired, for he grew some plants from seeds taken from the granaries; and he also found that the seeds would germinate even in the granaries, if the ants were prevented from obtaining access to them for two or three days. The non-germination of the seeds must, therefore, be due to some influence exerted by the ants. Moggridge thought this influence might be the exhalations from the ants, and so tried inclosing some seeds in a bottled test-tube, containing also earth and ants. The seeds, however, sprouted; and even an atmosphere of formic-acid vapor was found not to prevent germination. Probably, therefore, the ants in their granaries do something to the seeds for the express purpose of preventing germination; and, if so, it would be interesting to botanists to ascertain what this process can be.
But, be this as it may, there is no doubt that the ants are fully aware of the importance in this connection of keeping their garnered seeds as dry as possible; for when the latter prove over-moist after collection, or have been subsequently wetted by soaking rains, the insects bring them up to the surface and spread them out to dry, to be again brought into the nest after a sufficient exposure.
Lastly, Moggridge observed that the process, whatever it is, ​whereby the ants prevent germination, is not invariably successful, but that a small percentage of stored seeds sometimes do begin to germinate. When this was the case, he also observed the highly interesting fact that the ants then knew the most effective method of checking further germination, for he found that in these cases they gnawed off the tip of the sprouting radicle. This fact deserves to be considered as one of the most remarkable among the many remarkable facts of ant psychology.
Passing on now to the harvesting ants of the New World, the insects here remove all the herbage above their nest in the form of a perfect circle, or "disk," fifteen to twenty feet in diameter. Every grass or weed within the disk is carefully felled, and, as the nests are situated in thickly-grown localities, the effect of the bald or shaven disk is highly conspicuous and peculiar, exactly resembling in miniature the "clearings" which are made by settlers in the backwoods. The disk, however, is not merely cleared of herbage, but also carefully leveled—all inequalities of the surface being reduced by pellets of soil being built into the hollows to an extent sufficient to make a uniformly flat surface. In the center of the disk is the gateway of the nest. From the disk in various directions there radiate out-roads or avenues, which are cleared and smoothed like the disk. These roads course through the thick grass, branching and narrowing as they go, till they eventually taper away. They are usually four to seven inches wide at their origin, and may be from sixty to three hundred feet in length. Along these roads there is always passing during the daytime a constant double stream of ants, one being laden and the other not.
In their manner of gathering and garnering grain these harvesters resemble in general the harvesters of Europe; but it is alleged by Dr. Lincecum that in one respect their habits manifest an astonishing, and indeed wellnigh incredible, advance upon those of their European allies. For this observer, who, it must be remembered, was the first to call attention to these ants in the New World, and whose other observations, extending over a number of years, have since been fully confirmed—this observer states in the most positive terms that the ants actually sow the seeds of a certain plant called the ant-rice, for the purpose of subsequently reaping a harvest of grain; hence these ants have been called the "agricultural ants." Now, there is no doubt, from the subsequent observations of McCook and others, that the ant-disks do very frequently support this peculiar kind of grass, and that the ants are particularly fond of its seed. Nevertheless, McCook did not himself witness the process of sowing, although he is not disposed to doubt the statements of his predecessor upon the subject. These statements, as already observed, are most emphatic and precise—Lincecum saying, in italics, that he knows and is certain about the fact; but until corroborated it is safest to regard the fact as not yet fully established.
​Honey-making Ants.—These ants are found in Texas and New Mexico. Their remarkable habits have been observed by Captain Fleeson, who communicated his observations to Mr. Darwin.
The community consists of three distinct kinds of ants, which appear to belong to two distinct genera. These are:
I. Yellow workers; nursers and feeders of II.
II. Yellow honey-makers; sole function to secrete a kind of honey in their large globose abdomens, on which the other ants are supposed to feed. They never quit the nest, and are fed and tended by I.
III. Black workers; guards and purveyors, which surround the nest as sentinels, and also forage for the food required for I. They are much larger and stronger than either I or II, and are provided with very formidable mandibles.
The nest is in the form of an absolutely perfect square, of which each side measures from four to five feet, and the surface of which is kept quite unbroken save at two points, at each of which there is a very minute hole or entrance. One of these minute holes occurs near the west side of the square, and the other near the southeast corner; for it must be remarked that the square is always built with precise reference to the points of the compass, in such a way that one side faces due north, and consequently the others due south, east, and west. These boundaries are rendered very conspicuous by the guard of black workers or soldiers (III), which continuously parade round three of the sides in a close double line of defense, moving in opposite directions. This sentry-path occupies the north, east, and west boundaries, the south side of the square being left open; but, if an enemy approaches on this or any other side, a number of the guards leave their stations and sally forth to face the foe, raising themselves on their hind legs on meeting the enemy, and moving their large mandibles in defiance. After tearing the enemy to pieces the guards return to their places in the line of defense, their object in destroying any insect or other small intruders being defense of the encampment, and not the obtaining of food.
The southern side of the square encampment, or rather fortress, is left open as just described in order to admit of a free entry of supplies. While some of the black workers are on duty as gurad, another and larger division are engaged on duty as purveyors. These enter and leave the quadrangle by its southwest corner in a double line (one laden and the other not), which follows exactly the diagonal of the square to its central point, where all the booty, consisting of flowers and aromatic leaves, is deposited in a heap. Passing from this central heap to the entrance at the southeast corner of the quadrangle, and therefore occupying the other semi-diagonal of the square, there is another double line of workers constantly engaged in carrying the booty from the central deposit into the storehouses below-ground. These workers are exclusively composed of Class II, whose whole life ​is therefore spent in running backward and forward upon this semi-diagonal of the square, carrying in food and feeding Class I. Ko black ant is ever seen on the eastern diagonal, and no yellow ant is ever seen on the western; but each keeps to his own separate station, and here works with a steadfastness and apparent adherence to discipline which are not less remarkable than those exhibited by the sentries. The western hole before mentioned seems to be intended only as a ventilating shaft; it is never used as a gateway.
Section of the nest reveals, besides passages and galleries, a small chamber, across which is spread, like a spider's web, a network of squares spun by the insects. In each of these squares, supported by the web, sits one of the honey-secreting ants (II). Here the honey makers live in perpetual confinement, and receive a constant supply of flowers, pollen, etc., which is continually being brought them by I, and which, by a process of digestion and secretion, they convert into honey. It is particularly noteworthy that in this truly wonderful exhibition of social coöperation the black and yellow workers appear to belong to two distinct genera; for hitherto this is the only case known of two distinct species of animals coöperating for a common end.
Ecitons.—We have lastly to consider the most astonishing insects, if not the most astonishing animals, in the world. These are the so called "foraging," or, as they might more appropriately be called, the military ants of the Amazon. They belong to several species of the same genus, and have been carefully watched by Bates, Belt, and other naturalists. The following facts must therefore be regarded as fully established:
Eciton legionis moves in enormous armies, and everything that these insects do is done with the most perfect instinct of military organization. The army marches in the form of a rather broad and regular column, hundreds of yards in length. The object of the march is to capture and plunder other insects, etc., for food, and, as the well-organized host advances, its devastating legions set all other terrestrial life at defiance. From the main column there are sent out smaller lateral columns, the composing individuals of which play the part of scouts, branching off in various directions, and searching about with the utmost activity for insects, grubs, etc., over every log and under every fallen leaf. If prey is found in sufficiently small quantities for them to manage alone, it is immediately seized and carried to the main column, but, if the amount is too large for the scouts themselves to deal with, messengers are sent back to the main column, whence there is immediately dispatched a detachment large enough to cope with the requirements. Insects or other prey which, when killed, are too large for single ants to carry, are torn in pieces, and the pieces conveyed back to the main army by different individuals. Many insects in trying to escape run up bushes and shrubs, where they are ​pursued from branch to branch and twig to twig by their remorseless enemies, till, on arriving at some terminal ramification, they must either submit to immediate capture by their pursuers or drop down amid the murderous hosts beneath. As already stated, all the spoils which are taken by the scouts, or by the detachments sent out in answer to their demands for assistance, are immediately taken back to the main army or column. When they arrive there, they are conveyed to the rear of that column by two smaller columns of carriers, which are constantly running in two double rows (one of each being laden and the other not) on either side of the main column. On either side of the main column there are also constantly running up and down a few individuals of smaller size, lighter color, and having larger heads than the other ants. These appear to perform the duty of officers, for they never leave their stations, and, while actively running up and down the outsides of the column, they seem intent only on maintaining order in the march—stopping every now and then to touch some member of the rank and file with their antennæ, as if giving directions.
When the scouts discover a wasp's nest in a tree, a strong force is sent out from the main army, the nest is pulled to pieces, and all the larvæ in the nest are carried by the carrier-columns to the rear of the army, while the wasps fly around defenseless against the invading multitudes. Or, if the nest of any other species of ant is found, a similarly strong force is sent out, or even the whole army may be deflected toward it, when with the utmost energy the innumerable insects set to work to sink shafts and dig mines till the whole nest is rifled of its contents. In these mining operations the Ecitons work with an extraordinary display of organized coöperation; for those low down in the shafts do not lose time by carrying up the earth which they excavate, but pass on the pellets to those above, and the ants on the surface, when they receive the pellets, carry them only just far enough to insure that they shall not roll back again into the shaft, and, after having deposited them at a safe distance, immediately hurry back for more.
The Ecitons have no fixed nest themselves, but live, as it were, on a perpetual campaign. At night, however, they call a halt and pitch a camp. For this purpose they usually select a piece of broken ground, in the interstices of which they temporarily store their plunder. In the morning the army is again on the march, and before an hour or' two has passed not a single ant is to be seen where thousands and millions had previously covered the ground.
The habits of E. humana and E. drepanophora are in general similar to those of the species just described. The latter, however, march in a narrower column (only four to six deep), which is therefore proportionally longer—sometimes extending to over half a mile. Bates tried the effect of interfering with a column of this species by ​abstracting an individual from it. "News of the disturbance was quickly communicated to a distance of several yards to the rear, and the column at that point commenced retreating." It was also this species that the same naturalist describes as enjoying periods of leisure and recreation when they call a halt in "the sunny nooks of the forest."
On such occasions,
E. prædator differs from the others of its genus in not hunting in columns, but "in dense phalanxes consisting of myriads of individuals."
A phalanx occupies from four to six square yards of ground, and the ants composing it do not move "altogether in one straightforward direction, but in variously spreading contiguous columns, now separating a little from the general mass, now reuniting with it. The margins of the phalanx spread out at times like a cloud of skirmishers from the flanks of the main army."
Two species of Eciton are totally blind, and the habits of these differ from those above described in that they march exclusively under covered roads or tunnels. The van of the column is constantly engaged in rapidly constructing the tunnels through which the army or regiment advances as quickly as they are made. Under the protection of these covered ways the ants travel at a surprising rate, and, when they reach a rotten log or other promising hunting-ground, they pour into all the crevices, etc., iii search of prey. Bates says:
These two blind species of Eciton are particularly interesting from the fact that in a part of the world so remote from them as Western Africa there is another genus of military ant, also blind, which in all its habits closely resembles the blind Ecitons of Brazil. For, like the latter, Annornia arcens march in long, close columns through tunnels, have no fixed nest, but make temporary halts in shaded places, and are no less organized, remorseless, and irresistible than their American congeners. In one curious particular, however, they differ: the relative position of the marchers and the carriers is reversed, for here the carrier-columns occupy the middle place, while the marching columns with their officers occupy the flanks. When overtaken by a sudden African rain-storm, these ants congregate in a close mass, with the younger ants in the center; they thus form a floating island.
It is remarkable that ants of different hemispheres should manifest so close a similarity with respect to all these wonderful habits. The Chasseur ants of Trinidad, and, according to Madame Merian, the ants of Visitation of Cayenne, also display habits of the same kind.
Special Instances of the Display of High Intelligence.—I shall conclude this brief résumé of the more important facts at present known concerning the psychology of ants with a few selected observations of the display of high intelligence. It is always difficult to draw the line between instinct and reason, between adjustive action due to hereditary or purposeless habit and adjustive action due to individual and purposive adaptation. But we may be least diffident in accepting as evidence of the latter cases where animals exhibit a power of adapting their actions to meet the requirements of novel circumstances or circumstances which can not be supposed to have been of sufficiently frequent occurrence in the life-history of the species to have developed instincts of mechanical response in the individual. It is in view of this consideration that the following instances are selected.
Ebrard records in his "Études de Mœurs" an observation of his own on F. fusca. The ants were engaged in building walls, and when the work was nearly completed there still remained an interspace of twelve or fifteen millimetres to be covered in. For a moment the ants were thrown out, and
This observation naturally leads to two others by two different observers. Thus, Moggridge says: "I was able to watch the operation of removing roots which had pierced through their galleries, belonging to seedling plants growing on the surface, and which was performed by two ants, one pulling at the free end of the root, and the other gnawing at its fibers where the strain was greatest, until at length it gave way." Again, as previously quoted in another connection, he says that two ants sometimes combine their efforts, one stationing itself near the base of a footstalk and gnawing at the point of greatest tension, while the other hauls upon and twists it.
The other observer to whom I have referred is McCook, who says of the harvesting ants of America that he has seen "the workers, in several cases, leave the point at which they had begun a cutting, ascend the blade, and pass as far toward the point as possible. The blade was thus borne downward, and, as the ant swayed up and down, it really seemed that she was taking advantage of the leverage thus gained, and was bringing the augmented force to bear upon the fracture. In two or three cases there appeared to be a division of labor; that is to say, while the cutter at the roots kept on with his work, another ant climbed the grass-blade and applied the power at the opposite end of the lever. This position may have been quite accidental, but it certainly had the appearance of voluntary coöperation."
These observations serve to render less improbable the following quotation taken from Bingley's account of Captain Cook's expedition in New South Wales, and vouched for by Sir J. Banks. Green ants were seen forming their nests in trees by "bending down several of the leaves, each of which is as broad as a man's hand, and gluing the points of them together so as to form a purse. . . . We saw thousands uniting all their strength to hold them in this position, while other busy multitudes within were employed in applying the gluten that was to prevent their returning back."
Moggridge says that he has seen the harvesting ants of Europe clustering round the larva of a certain beetle, and directing it toward some small opening in the soil, "which it would quickly enlarge and disappear down"; and he believes that "these attentions were purely selfish," the ants availing "themselves of the tunnel thus made down into the soil."
McCook says of the harvesters of America that they dislike shade, so that if a tree grows up in their vicinity and casts a shadow over their nest they forthwith migrate. He gives in this connection a statement which I regard as bordering on the incredible, and therefore I desire it to be specially observed that it is not very evident from ​McCook's account whether he himself witnessed the facts. The facts, however, which he narrates, are that a peach-tree having grown up so as to overshadow a nest of harvesting ants, the latter climbed the tree to strip off the leaves. "I am convinced," says McCook, "that the reason for this onslaught was the desire to be rid of the obnoxious shade." If this statement had been met with in any ordinary book on animal intelligence, of course I should not have quoted it; but as McCook went to Texas for the express purpose of studying these ants in a scientific manner, and as the numerous other observations which he made, both on these and on the mound-building species, entitle him to respect, I have not felt justified in suppressing this statement.
The observation made by Colonel Sykes on certain ants in India has gained a wide notoriety from its having been published by Spence in his popular work on instinct. Colonel Sykes was a good observer, so that his account ought not to be questioned. He says that in order to guard his provisions from the ants he put them on a table, the four legs of which he placed in as many basins filled with water. Some ants still succeeded in scrambling across the water, and so the legs of the table were likewise painted with turpentine. The ants then ran up a wall near which the table stood, and when about a foot above its level, they sprang from the wall to the table.
Somewhat analogous to this is the observation of Professor Leuckhart, who placed round the trunk of a tree, which had been visited by ants as a pasture for aphides, a broad cloth soaked in tobacco-water. When the ants, returning home down the trunk of the tree, arrived at the soaked cloth, they turned round, went up the tree again to some of the overhanging branches, and allowed themselves to drop clear of the obnoxious barrier. On the other hand, the ants which desired to mount the tree first examined the nature of the obstruction, then turned back and procured some pellets of earth, which they carried in their jaws and deposited, one after another, upon the cloth till a harmless road of earth was made across it.
This observation of Professor Leuckhart is in turn a corroboration of an almost identical one made more than a century ago by Cardinal Fleury, and communicated by him to Réaumur, who published it in his "Natural History of Insects" (1734). The Cardinal smeared the trunk of a tree with bird-lime, in order to prevent the ants from ascending it; but the insects overcame the obstacle by making a road of earth, small stones, etc., as in the case just mentioned. On another occasion the Cardinal saw a number of ants make a bridge across a vessel of water surrounding the bottom of an orange-tree tub. They did so by conveying a number of little pieces of wood, the choice of that material, instead of earth or stones, as in the previous case, apparently betokening no small knowledge of practical engineering—a knowledge which, as we shall presently see, is also shared by the Ecitons.
​Büchner, in his recently published and translated work on "Mind in Animals," gives a singular observation analogous to the above, which was communicated to him by Herr G. Theuerkauf. A maple tree standing in the grounds of Herr Vollbaum, of Elbing, swarmed with ants and aphides. In order to check the mischief, the proprietor smeared about a foot width of the ground around the tree with tar. The first ants that arrived stuck fast; but the next, seeing the predicament of their companions, turned back and fetched a number of aphides from the tree, which they stuck down on the tar one after another till they had made a bridge over which they could cross without danger.
It will be observed that all these cases, being so analogous although recorded independently by different observers, serve to corroborate one another. As such corroboration in matters of this kind is of value, I shall here add two or three cases which go to confirm the observation of Cardinal Fleury regarding the construction of a floating bridge. Dr. Ellendorf writes to Professor Büchner that he protected a cupboard of his provisions from the invasion of ants by standing the legs of the cupboard in saucers filled with water. He adds:
The military ants, both in America and Africa, exhibit still more extraordinary resources in the way of bridge-making. Thus Belt says of the Ecitons:
​It is remarkable that the military or driving ants of Africa exhibit precisely similar devices for the bridging of streams as the Ecitons of America, namely, by forming a chain of individuals over which the others pass. By means of similar chains they also let themselves down from trees.
But of the Ecitons another and more recent observer gives an account of a yet more remarkable device, although no doubt a development of the one just described. This observer is Herr H. Kreplin, who lived for nearly twenty years in South America as an engineer, and often had the opportunity of watching the Ecitons. He writes to Büchner under date 1876 as follows:
I shall now bring these numerous instances to a close with a quotation from Belt, which reveals in a most unequivocal manner astonishing powers of observation and reason in the leaf-cutting ants of South America, the general habits of which we have already considered:
Such, then, are some of the more well-established facts regarding the intelligence of ants, and taken altogether they certainly seem to justify the remark of the most illustrious of naturalists, "The brain of an ant is one of the most marvelous atoms of matter in the world, perhaps more so than the brain of a man."—Nineteenth Century.