Popular Science Monthly/Volume 19/August 1881/Intelligence of Ants I
| INTELLIGENCE OF ANTS. |
By GEORGE J. ROMANES.
I HAVE frequently been much struck by the absence of information, even among professed naturalists and professed psychologists, concerning the intelligence of ants. The literature on the subject being scattered and diffused, it is not many persons who have either the leisure or the inclination to search it out for themselves. Most of us, therefore, either rest in a general hazy belief that ants are wonderfully intelligent animals, without knowing exactly in what ways and degrees the intelligent action of these animals is displayed; or else, having read Sir John Lubbock's investigations, we come to the general conclusion that ants are not really such very intelligent animals, after all, but, as was to have been expected from their small size and low position in the zoölogical scale, it only required some such methodical course of scientific investigation to show that previous ideas upon the subject were exaggerated, and that, when properly tested, ants are found to be rather stupid than otherwise. I have therefore thought it well to write a paper for this widely circulated review, in order to diffuse some precise information concerning the facts of this interesting branch of natural history.
Not having any observations of my own to communicate, I have no special right to be heard on this subject; but, as I have recently had occasion to read through the literature connected with it, I am able to render what I may call a filtered abstract of all the facts which have hitherto been observed by others. It is needful, however, to add that the filter has been necessarily a close one; if I had a large volume instead of a short paper as my containing vessel, the filtrate would still require to be a strongly condensed substance.
Powers of Special Sense.—Let us take first the sense of sight. Sir John Lubbock made a number of experiments on the influence of light colored by passing through various tints of stained glass, with the following results: 1. The ants which he observed greatly disliked the presence of light within their nests," hurrying about in search of the darkest corners "when light was admitted. 2. Some colors were much more distasteful to them than others; for while under a slip of red glass there were on one occasion congregated 890 ants, under a green slip there were 544, under a yellow 495, and under a violet only 5. 3. The rays thus act on these ants in a graduated series, which corresponds with the order of their influence on a photographic plate. Experiments were therefore made to test the effect of the rays on either side of the visible spectrum, but with negative results. In considering these experiments, however, it is important to remember that other observers (especially Moggridge in Europe, and McCook in America) have described other species of ants (genus Atta) as fond of light. It would be interesting for any one who has an opportunity to try whether ants of this genus do not show toward the rays of the spectrum a scale of preference the reverse of that which Sir John Lubbock described.
As regards hearing, Sir John found that sounds of various kinds do not produce any effect upon the insects, nor could he obtain any evidence of their emitting sounds, either audible or inaudible to human ears.
It has long been known that the sense of smell in ants is highly developed, and it appears to be the sense on which, like dogs, they mainly rely. Huber proved that they track one another's footsteps in finding their way to food, etc.; for he observed, on drawing his finger across the trail so as to obliterate the scent, that the ants became confused and ran about in various directions, till they again came upon the trail on the other side of the interrupted space. By many ingeniously devised experiments Lubbock has amply confirmed Huber's statements, and concludes that in finding treasure "they are guided in some cases by sight, while in others they track one another by scent"—depending, however, more upon scent than upon sight.
There can be little doubt that ants have a sense of taste, as they are so well able to distinguish sugary substances; and it is unquestionable that in their antennæ they possess highly elaborated organs of touch.
Sense of Direction.—It is certain that ants, in common with many other animals, possess some unaccountable sense of direction, whereby they are able to find their way independently of landmarks, etc. Sir John Lubbock tried a number of experiments in this connection, of which the following is perhaps the most conclusive: Between the nest and the food he placed a hat-box, in each of two opposite sides of which he bored a small hole, so that the ants, in passing from the nest to the food and back again, had to go in at one hole and out at the other. The box was fixed upon a pivot, where it could be easily rotated, and, when the ants had well learned their way to the food through the box, the latter was turned half round as soon as an ant had entered it; "but in every case the ant turned too, thus retaining her direction."
Sir John then placed in the stead of a hat-box a disk of white paper. When an ant was on the disk making toward the food, he gently drew the paper to the other side of the food, so that the ant was conveyed by the moving surface in the same direction as that in which she was going, but beyond the point to which she intended to go. Under these circumstances the ant did not turn round, but went on to the farther edge of the disk, "when she seemed a good deal surprised at finding where she was."
These results seem to indicate that the sense of direction is due to a process of registering all the changes of direction which may be made during the out-going journey, and that this power of registration has reference only to lateral movements; it has no reference to variations in the velocity of advance along the line in which the animal is progressing.
Powers of Communication.—Huber, Forel, Kirby and Spence, Dujardin, Burmeister, Franklin, and other observers have all expressed themselves as holding the opinion that ants are able to communicate information to one another by some system of language or signs. The facts, however, on which the opinion of these earlier observers rested, have not been stated with that degree of caution and detail which the acceptance of their opinion would require. But the more recent observations of Bates, Belt, Moggridge, Hague, Lincecum, McCook, and Lubbock, leave no doubt upon the subject. Two or three instances will be enough to select in order to prove the general fact. Hague, the geologist, kept upon his mantel-shelf a vase of flowers, and he noticed a file of small red ants on the wall above the shelf passing upward and downward between the latter and a small hole near the ceiling. The ants, whose object was to get at the flowers, were at first few; but they increased in number during several successive days, until an unbroken succession was formed all the way down the wall. To get rid of the ants, Hague then tried frequently brushing them off the wall upon the floor in great numbers; but the only result was that another train was formed to the flowers ascending from the floor. He, therefore, took more severe measures, and struck the end of his finger lightly upon the descending train near the flower-vase, so killing some and disabling others. "The effect of this was immediate and unexpected. As soon as those ants which were approaching arrived near to where their fellows lay dead and suffering, they turned and fled with all possible haste, and in half an hour the wall above the mantel-shelf was cleared of ants." The stream from below continued to ascend for an hour or two, the ants advancing "hesitatingly just to the edge of the shelf, when, extending their antennæ and stretching their necks, they seemed to peep cautiously over the edge until beholding their suffering companions, when they too turned, expressing by their behavior great excitement and terror." Both columns of ants thus entirely disappeared. For several days there was a complete absence of ants: then a few began to reappear; "but, instead of visiting the vase which had been the scene of the disaster, they avoided it altogether," and made for another vessel containing flowers at the other end of the shelf. Hague here repeated the same experiment, with exactly the same result. After this for several days no ants reappeared; and during the next three months it was only when fresh and particularly fragrant flowers were put into the vases that a few of the more daring ants ventured to straggle toward them. Hague concludes his letter to Mr. Darwin, in which these observations are contained, by saying:
Many other cases might be quoted to show that ants are able to communicate information to one another; but, to save space, I shall pass on to Sir John Lubbock's direct experiments upon this subject. Three similar and parallel tapes were stretched from an ant's nest to three similar glass vessels. In one of the latter Sir John placed several hundred larvæ, in another only two or three larvæ, and the third he left empty. The object of the empty glass was to see whether any ants might not run along the tapes without any special reference to the obtaining of larvæ; and this was found not to be the case. Sir John then put an ant to each of the other two glasses; they each took a larva, carried it to the nest, returned for another, and so on. Each time a larva was taken out of the glass containing only two or three, Sir John replaced it with another, so that the supply should not become exhausted. Lastly, every ant (except the two which had first been put to the larvæ), before reaching home with her burden, was caught and imprisoned till the observation terminated.
The result was, that during forty-seven and a half hours the ants which had access to the glass containing numerous larvæ brought 257 friends to their assistance; while during an interval of five and a half hours longer those which visited the glass with only two or three larvae brought only 82 friends. This result appears very conclusive as proving some power of definite communication, not only as to where food is to be found, but also as to the road which leads to the largest store. Further experiments, however, proved that these ants are not able to describe the precise locality where treasure is to be found. For, having exposed larvæ as before and placed an ant upon them, he watched every time that she came out of the nest with friends to assist her; but, instead of allowing her to pilot the way, he took her up and carried her to the larvæ, allowing her to return with a larva upon her own feet. Under these circumstances the friends, although evidently coming out with the intention of finding some treasure, were never able to find it, but wandered about in various directions for a while, and then returned to the nest. Thus, during two hours, she brought out altogether in her successive journeys no less than 120 ants, of which number only five in their unguided wanderings happened by chance to find the sought-for treasure.
Memory.—The general fact that, whenever an ant finds her way to a store of food or larvæ, she will return to it again and again in a more or less direct line from her nest, constitutes ample proof that the ant remembers her way to the store of food. It is of interest to note that the nature of this insect-memory appears to be identical with that of memory in general. Thus, a new fact becomes impressed upon ant-memory by repetition, and the impression is liable to become effaced by lapse of time. Sir John Lubbock found it necessary to teach the insects, by a repetition of several lessons, their way to treasure, if that way were long or unusual. With regard to the duration of memory in ants, it does not appear that any direct experiments have been made; but the following observation by Mr. Belt on its apparent duration in the leaf-cutting ant may be here stated: In June, 1859, he found his garden invaded by these ants, and on following up their paths he found their nest about a hundred yards distant. He poured down their burrows a pint of diluted carbolic acid. The marauding parties were at once drawn off from the garden to meet the danger at home, while in the burrows themselves the greatest confusion prevailed. Next day he found the ants busily engaged in bringing up the ant-food from the old burrows and carrying it to newly formed ones a few yards distant. These, however, turned out to be intended only as temporary repositories; for in a few days both old and new burrows were entirely deserted, so that he supposed all the ants to have died. Subsequently, however, he found that they had migrated to a new site, about two hundred yards from the old one, and there established themselves in a new nest. Twelve months later the ants again invaded his garden, and again he treated them to a strong dose of carbolic acid. The ants, as on the previous occasion, were at once withdrawn from his garden, and two days afterward he found "all the survivors at work on one track that led directly to the old nest of the year before, where they were busily employed in making new excavations. . . . It was a wholesale and entire migration." Mr. Belt adds, "I do not doubt that some of the leading minds in this formicarium recollected the nest of the year before, and directed the migration to it." Of course, it is possible that the leaders of the migration may have simply stumbled on the old burrows by accident, and, finding them already prepared as a nest, forthwith proceeded to transfer the food and larvæ; but, as the old and the new burrows were separated from one another by so considerable a distance, this supposition does not seem probable, and the only other one open is that the ants remembered their former home for a period of twelve months. This supposition is rendered the more probable from a somewhat analogous case recorded by Karl Vogt in his "Lectures on Useful and Harmless Animals." For several successive years ants from a certain nest used to go through certain inhabited streets to a chemist's shop six hundred metres distant, in order to obtain access to a vessel filled with sirup. As it can not be supposed that this vessel was found in successive working seasons by as many successive accidents, it can only be concluded that the ants remembered the sirup-store from season to season.
Recognition.—I shall now pass on to consider a class of highly remarkable facts. It has been known since the observations of Huber that all the ants of the same community recognize one another as friends, while an ant introduced from another nest, even though it be an ant of the same species, is known at once to be a foreigner, and is usually maltreated or put to death. Huber found that, when he removed an ant from a nest and kept it away from its companions for a period of four months, it was still recognized as a friend, and caressed by its previous fellow-citizens after the manner in which ants show friendship, viz., by stroking antennæ. Sir John Lubbock, after repeating and fully confirming these observations, extended them as follows:
He first tried prolonging the period of separation beyond four months, and found that it might be made more than three times as long without the ants forgetting their absent friend. Thinking that this fact could only be explained, either by all the ants knowing each other's personal appearance, or by their all having a distinctive smell peculiar to each nest, or by their all having a sign, like a pass-word, differing in different nests. Sir John tried separating some ants from a nest while still in the condition of larvæ, and, when they emerged as perfect insects, transferring them back to the nest from which they had been taken as larvæ. Of course, in this case the ants in the nest could never have seen those which had been removed, for a larval ant is as unlike the mature insect as a caterpillar is unlike a butterfly; neither can it be supposed that the larvæ, thus kept away from the nest, should retain, when hatched out as perfect insects, any smell belonging to their parent nest; nor, lastly, is it reasonable to imagine that the animals, while still in the condition of larval grubs, can have been taught any gesture or sign used as a pass-word by the matured animals. Yet, although all these possible hypotheses seem to be thus fully excluded by the conditions of the experiment, the result showed unequivocally that the ants all recognized their transformed larvæ as native-born members of their community.
Next, therefore. Sir John Lubbock tried dividing a nest into two parts before the queen ants had become pregnant. Seven months after the division the queens laid their eggs, and five months later these eggs had developed into perfect insects. He then transferred some of these young ants from the division of the nest in which they had been born to the division in which they had never been, even in the state of the egg. Yet these ants also were received as friends, in marked contrast to the reception accorded to ants from any other nest. It therefore seems to be blood-relationship that ants are able, in some way that is as yet wholly inexplicable, to recognize. It ought, however, to be remembered in this connection that, in an experiment made by Forel on slave-making ants, it was proved that they almost instantaneously recognized their own slaves from other slaves of the same species—and this after their slaves had been kept away from the nest for a period of four months.
Under this heading I may also allude to the unquestionable evidence concerning enormous multitudes, or, as we might say, a whole nation of ants, all recognizing one another as belonging to the same nationality. No doubt the principle (whatever it may be) on which the power of recognition depends is the same here as it is in the case of a single nest; but, in the cases which I am about to quote, the operation of this principle is indefinitely and incalculably extended. The cases to which I allude are those in which new ants' nests spring up as off-shoots from the older ones, so that a nation of towns, as it were, gradually spreads to an immense circumference round an original center. Forel describes such a nation of Formica exsecta which comprised more than two hundred nests, and covered a space of nearly two hundred square metres. Individual ants must here have been numbered by the million, and yet they all knew each other as friends—even those taken from farthermost nests—while they would admit no foreigners within their territory.
A still more remarkable case is recorded by McCook of what he calls an "ant town." The one he has described occurs in the Alleghany Mountains of North America, and consists of sixteen or seventeen hundred nests, which rise in cones to a height of from two to five feet. The ground below is riddled in every direction with subterranean passages of communication. The inhabitants are all on the most friendly terms, so that if any one nest is injured it is repaired by help from the other nests. Here, also, foreign ants of the same species were not tolerated; so that we should have an analogous case if all the inhabitants of Europe should be directly known to one another as friends, while an American or an Australian, on setting foot upon European ground, should be immediately set upon as an enemy.
Emotions.—The pugnacity, valor, and rapacity of ants are too well and generally known to require the narration of special instances of their display. With regard to the tenderer emotions, however, there is among observers a difference of opinion. Sir John Lubbock found that the species of ants on which he experimented are apparently deficient in feelings both of affection and of sympathy. He tried burying some specimens of Lasius niger beneath an ant-road; but none of the ants traversing the road made any attempt to release their imprisoned companions. He repeated the same experiment, with the same result, on various other species. Even when the friends in difficulty were actually in sight, it by no means followed that their companions would assist them. On imprisoning some friends in one bottle, the mouth of which was covered with muslin, and some strangers of the same species (F. fusca) in another bottle similarly protected, and placing both bottles in the nests, "the ants which were at liberty took no notice of the bottle containing their imprisoned friends. The strangers in the other bottle, on the other hand, excited them considerably." For days they crowded round this bottle, endeavoring to gnaw through the muslin by which its mouth was closed. This on the seventh day they succeeded in doing, when they killed the imprisoned strangers. "The friends throughout were quite neglected," so that this experiment, as Sir John observes, seems to show that "in these curious insects hatred is a stronger passion than affection." This experiment always gave the same result in the case of this species; but, when tried with Formica rufescens, the ants took no notice of either bottle, and showed no signs either of affection or hatred; so that, as Sir John again observes, "one is almost tempted to surmise that the spirit of these ants is broken by slavery"—i. e., by the habit of keeping slaves.
But there is no lack of evidence to show, per contra, that the tenderer emotions have a place in ant-psychology. Even the hardhearted species which Sir John Lubbock observed grew sympathetic toward sick or injured friends. Thus he observed that a specimen of F. fusca, which was congenitally destitute of antennæ, and which had been attacked by an ant of another species, excited the sympathy of a friend on being placed near her own nest. This friend "examined the poor sufferer carefully, then picked her up tenderly, and carried her away into the nest. It would have been difficult for any one who witnessed this scene to have denied to this ant the possession of humane feelings." Again, Moggridge has seen one ant carry another sick and apparently dead ant "down the twig which formed their path to the surface of the water, and, after dipping it in for a minute, carry it laboriously up again, and lay it in the sun to dry and recover."
But some species of ants seem habitually to show affection and sympathy even toward healthy companions in distress. Thus Belt writes of the Eciton humata, that "one day watching a small column of these ants, I placed a little stone on one of them to secure it. The next that approached, as soon as it discovered its situation, ran backward in an agitated manner, and soon communicated the intelligence to the others. They rushed to the rescue," and by their concerted action effected the release of their companion. Similarly ants of this species which Belt buried were always dug out by their friends. To quote one such instance, the ant which first found the buried one
Ford and McCook have also observed displays of sympathy and affection by other species.
Nursing.—This may appropriately be considered in connection with the emotions, as it seems to imply something akin to maternal affection. The eggs will not develop into larvæ unless nursed, and the nursing is effected by licking the surface of the eggs, which under the influence of this process increase in size, or grow. In about a fortnight—during which time the workers carry the eggs from higher to lower levels of the nest, and vice versa, according to the circumstances of heat, moisture, etc.—the larvæ are hatched out, and require no less careful nursing than the eggs. The workers feed them by placing mouths together—the larvæ stretching out their heads to receive the nourishment after the manner of young birds. When fully grown the larvæ spin cocoons, and are then pupæ, or the "ants' eggs" of the pheasant-rearers. These require no food, but still need incessant attention with reference to warmth, moisture, and cleanliness. When the time arrives for their emergence as perfect insects, the workers assist them to get out of their larval cases by biting through the walls of the latter. When it emerges, the newly-born ant is inclosed in a thin membrane like a shirt, which has to be pulled off. "When we see," says Büchner, "how neatly and gently this is done, and how the young creature is then washed, brushed, and fed, we are involuntarily reminded of the nursing of human babies." The young ants are then educated. They are led about the nest and taught their various domestic duties. Later on they learn to distinguish between friends and foes; and when an ant's nest is attacked by foreign ants the young ones never join in the fight, but confine themselves to removing the pupæ. That the knowledge of hereditary enemies is not wholly instinctive is proved by the experiment of Forel, who put young uneducated ants of three different species into a glass case with pupæ of six other species—all the nine species being naturally hostile to one another. Yet the young ants did not quarrel, but worked together to tend the pupæ. When the latter hatched out, an artificial colony was formed of a number of naturally hostile species, all living together like the "happy families" of the showmen.
Keeping Aphides.—It is well and generally known that various species of ants keep aphides, as men keep milch-cows, to supply a nutritious secretion. Huber first observed this fact, and noticed that the ants collected the eggs of the aphides, and treated them with as much apparent care as they treated their own. When these eggs hatch out, the aphides are usually kept and fed by the ants. Sometimes the stems and branches on which they live are incased by the ants in clay walls, in which doors are left large enough to admit the ants, but too small to allow the aphides to escape. The latter are therefore imprisoned in regular stables. The sweet secretion is yielded to the ants by a process of "milking," which consists in the ants stroking the aphides with their antennæ.
Sir John Lubbock has made an interesting addition to our knowledge respecting the habit in question, as practiced by a certain species of ant (Lasius flavus), which departs in a somewhat remarkable manner from the habit as practiced by other species. He says: "When my eggs hatched I naturally thought that the aphides belonged to one of the species usually found on the roots of plants in the nests of Lasius flavus. To my surprise, however, the young creatures made the best of their way out of the nest, and, indeed, were sometimes brought out of the nest by the ants themselves." Subsequent observation showed that these aphides, born from eggs hatched in the ants' nest, left the nest, or were taken from it, as soon as they were hatched, in order to live upon a kind of daisy which grew around the nest. Sir John then made out the whole case to be as follows:
As a supplement to this interesting observation, I may here append the following, which is due to Herr Nottebohm, who communicated it to Professor Büchner: This gentleman had a weeping-ash which was covered by millions of aphides. To save the tree, he one day in March cleaned and washed every branch and spray before the buds had burst, so removing all the aphides. There was no sign of the latter till the beginning of June, when he was surprised one fine sunny morning to see a number of ants running quickly up and down the trunk of the tree, each carrying up a single aphis to deposit it on the leaves, when it hurried back to fetch another. "After some weeks the evil was as great as ever. . . . I had destroyed one colony, but the ants replanted it by bringing new colonists from distant trees and setting them on the young leaves."
Aphides are not the only insects which are utilized by ants as cows. Gall-insects and cocci are kept in just the same way; but McCook observed that, where aphides and cocci are kept by the same ants, they are kept in separate chambers, or stalls. Caterpillars of the genus Lycæna have also been observed to be kept by ants for the sake of a sweet secretion which they supply.
Slavery.—The habit or instinct of keeping slaves obtains at least among three species of ant. It was first observed by P. Huber in Formica rufescens, which enslaves the species F. fusca, the members of which are appropriately colored black. The slave-making ants attack a nest of F. fusca in a body; there is a great fight with much slaughter, and, if victorious, the slave-makers carry off the pupæ of the vanquished nest in order to hatch them out as slaves. When these pupæ hatch out, the young slaves begin their life of work, and seem to regard their masters' home as their own, for they never attempt to escape, and they fight in defense of the nest should it be attacked. The work that devolves upon the slaves differs according to the species which has enslaved them. In the nests of F. sanguinea the comparatively few captives are kept exclusively as household slaves, all the out-door work of foraging, slave-capturing, etc., being performed by the masters; and when for any reason a nest has to migrate, the masters carry their slaves in their jaws. On the other hand, F. rufescens assigns a much larger share of work to the slaves, which they capture in much larger numbers to take it. In this species the masters do no work whatsoever, unless the capturing of slaves be regarded as such. Therefore the whole community is entirely dependent upon its slaves; the masters are not able to make their own nests, to feed their own larvæ, or even to feed themselves; they die of starvation in the midst of favorite food if a slave should not be present to hand it in proper form. In order to confirm this observation (originally due to Huber) Lespès placed a piece of moistened sugar near a nest of these slave makers. It was soon found by one of the slaves, which gorged itself and returned. Other slaves then came out and did likewise. Some of the masters next came out, and by pulling the legs of the feeding slaves reminded them that they were neglecting their duty. The slaves then immediately began to serve their masters to the sugar. Had they not done so, there is no doubt they would have been punished, for the masters bite the slaves when displeased with them. Forel and Darwin have also confirmed these observations of Huber. Indeed, the structure of the mouth in F. rufescens is such as to render self-feeding difficult, if not physically impossible. Its long and narrow jaws, admirably adapted to pierce the head of an enemy, do not admit of being used for feeding unless liquid food is poured into them from the mouth of a slave.
Ants do not appear to be the only animals of which ants make slaves; for there is at least one case in which these wonderful insects enslave insects of another species, which may therefore be said to stand to them in the relation of beasts of burden. The case to which I allude stands upon the authority of Audubon, who says that he has seen certain leaf-bugs used as slaves by ants in the forests of Brazil.
Wars.—On the wars of ants a great deal might be said, as the facts of interest in this connection are very numerous; but for the sake of brevity I shall confine myself to giving only a somewhat meager account. One great cause of war is the plundering of ants' nests by the slave-making species. Observers all agree that, in the case of the so called Amazon slave-making ant, this plundering is effected by a united march of the whole army composing a nest, directed against some particular nest of the species which they enslave. According to Lespès and Forel, single scouts or small companies are first sent out from the nest to explore in various directions for a suitable nest to attack. These scouts afterward serve as guides to the marauding excursion. When the scouts have been successful in discerning a suitable nest to plunder, and have completed their strategical investigations of the locality to their satisfaction—the latter process being often a laborious one, as it has special reference to the entrances of the nest, which are purposely made difficult to find by their architects—they return to their own fortress. Forel has seen them then walk about on the surface of this underground fortress for a long time, as if in consultation, after which some of them entered and again came out leading the host of warriors; these streamed from all the gateways, and ran about tapping each other with their heads and antennæ. They then formed into a column, composed of between one and two thousand individuals, and set out in orderly march to pillage the nest which had been examined by the scouts. According to Lespès, the column is about five metres long and fifty centimetres wide, marches at the rate of a metre per minute, and, on account of the distance which may have to be traversed, the march sometimes lasts for more than an hour. When they arrive at their destination a fierce battle begins, which, after raging for a time with much slaughter on both sides, generally, though not invariably, ends in the robbers gaining an entry. A barricade conflict then takes place below-ground, and, if the attack proves successful, the slave making ants again stream out of the plundered nest, each ant carrying a stolen pupa. The Amazons can not climb, and this fact being known to the other ants, when they find that victory is on the side of the enemy, they devote themselves to saving what treasure they can by carrying their pupæ up the grasses and bushes surrounding the nest. When the marauders have obtained all the booty that they can, they set off on their homeward march, each carrying a pupa. They do not always follow the shortest road, but return exactly on the track by which they came, no doubt being guided entirely by the scent left on the ground from their previous march. When they arrive home they commit the pupæ to the care of the slaves. Forel found that a particular colony of slave-makers watched by him sent out forty-four marauding expeditions in thirty days, of which number twenty-eight were completely successful, nine partially so, and the remainder failures. The average booty obtained by a successful expedition was one thousand pupæ, so that during a single summer the total number of pupæ captured by this colony might be put down at forty thousand.
Forel further tried the following experiment: He kept nests of two species of slave-making ants in two separate sacks, and when he saw that an expedition of a third species (Amazons) had found a slave nest to plunder, and were fairly on their march toward it, he turned out one of his sacks upon the nest. A fight at once began between the slave-ants and sanguine ants which he had turned loose upon them. Then the vanguard of the Amazons came up; but, when they saw that the sanguines were already on the field, they drew back and awaited the approach of the main army. In close order this whole array then precipitated itself upon the already struggling host of sanguine ants. The latter, however, repulsed the attack, and the Amazons retired to reform. This done they made a second assault, which appearing as if it would end successfully, Forel, to complicate matters, poured upon the field his second sack containing the third species of slave-makers. All three species then fought together, till at last victory declared itself on the side of the Amazons. After overcoming their enemies they paused for a breathing-space before beginning the work of plunder. They then ravished the nest of the slave-ants, which, however, fought desperately, so that it seemed as though they courted death. They even followed the Amazons right up to their own nest, harassing them all the way. On arriving at the nest of the Amazons the slaves of the latter came out and assisted their masters to fight. These slaves were of two species—one being the same as that which was being plundered, so that these slaves were fighting for their masters against their own kind. Altogether, therefore, in that day's warfare there were six different species of ants engaged—three in alliance, and the rest in mutual antagonism.
The military tactics employed by the sanguine ants above mentioned are different from those employed by the Amazons. They do not seek to carry the fortress of the slave-ants by storm, but lay a regular siege, forming a complete circle round the nest, and facing it with jaws held fiercely open and antennæ thrown back. Being individually large and strong, they are able thus to confine the whole nest of slave-ants. A special guard is set upon the entrances of the nest, and this allows all slave-ants not carrying pupæ to pass, while it stops all the slave-ants which carry pupæ. The siege lasts till most of the slave-ants have thus been allowed to pass out, while all the pupæ are left behind. The forces then close in upon the entrances and completely rifle the nest of its pupæ—a few companies, however, being told off to pursue any slave-ants which may possibly have succeeded here and there in escaping with a pupa.
Wars are not confined to species of ants having slave-making habits. The agricultural ants likewise at times have fierce contests with one another. The importance of seeds to these insects, and the consequent value which they set upon them, induce the animals, when supplies are scarce, to plunder one another's nests, prolonged warfare being the result. Thus Moggridge says: "By far the most savage and prolonged contests which I have witnessed were those in which the combatants belonged to two different colonies of the same species. . . . The most singular contests are those which are waged for seeds by A. barbara, when one colony plunders the stores of an adjacent nest belonging to the same species; the weaker nest making prolonged, though, for the most part, inefficient attempts to recover their property." In one case the predatory war lasted for forty-six days, during which time it became evident that the attacking nest was the stronger, for
Lastly, McCook records the history of an interesting engagement which he witnessed between two nests of Tetramorium cæspitum in the streets of Philadelphia, and which lasted for nearly three weeks. Although all the combatants belonged to the same species, friends were always distinguished from foes, however great the confusion of the fight. This fact is always observable in the case of battles between nests of the same species, and McCook thinks that the distinction appears to be effected in some way by contact of antennæ.
Keeping Pets.—Many species of ants display the curious habit of harboring in their nests sundry kinds of other insects, which, so far as observation extends, are of no benefit to the ants, and which have therefore been regarded by observers as mere domestic pets. These pets are, for the most part, species which occur nowhere else except in ants' nests, and each species of pet is peculiar to certain species of ant. Beetles and crickets seem to be the more favorite kinds of insects, and these live on the best terms with their hosts, playing round the nests in fine weather, and retiring into them in stormy weather, while allowing the ants to carry them from place to place during migrations. It is evident, therefore, that ants not only tolerate these insects, but foster them; and, as it seems absurd to credit the ants with any mere fancy or caprice, such as that of keeping pets, it is perhaps safest to suppose that these insects, like the aphides, are of some use to their masters, although we are not yet in a position to surmise what this use can be.
Sleep and Cleanliness.—It is probable that all ants enjoy periods of true slumber alternating with those of activity; but actual observations on this subject have only been made in the case of two or three species. McCook says that the harvesting ants of Texas sleep so soundly that they may be pretty severely stroked with a feather without being aroused; but they are immediately awakened by a sharp tap. On awakening they often stretch their limbs in a manner precisely resembling that of warm-blooded animals, and even yawn—the latter action being "very like that of the human animal; the mandibles are thrown open with the peculiar muscular strain which is familiar to all readers; the tongue is also sometimes thrust out." The ordinary duration of sleep in this species is about three hours.
Invariably on awakening, and often at other times, the ants perform, like many other insects, elaborate processes of washing and brushing. But, unlike other insects, ants assist one another in the performance of their toilet. The author just quoted describes the whole process in the genus Atta. The cleanser begins with washing the face of her companion, and then passes on to the thorax, legs, and abdomen.
Bates also has described similar facts with regard to ants of another genus—the Ecitons.
Play and Leisure.—The life of ants is not all work, or, at least, is not so in all species. Huber describes regular gymnastic sports as practiced by the species pratensis. They raise themselves on their hind-legs to wrestle and throw pretended antagonists with their forelegs, run after each other and seem to play at hide and seek. When one is victorious in a display of strength, it often seizes all the others in the ring, and tumbles them about like nine-pins. Forel has amply confirmed these observations of Huber, and says that the chasing, struggling, and rolling together upon the ground, pulling each other in and out of the entrances, etc., irresistibly reminded him of romping boys at play. "I understand," he says, "that the matter must seem wonderful to those who have not witnessed it, particularly when we remember that sexual attraction can here play no part."
McCook and Bates also give similar accounts of the habits of play and leisure among species of the Western Hemisphere.
Funerals.—The habit of carrying their dead out and away from their nests is very general, if not universal, among ants; and, being no doubt due to sanitary requirements, has probably been developed as a beneficial instinct by natural selection. McCook says of the agricultural ants:
Experiments made on ants kept in confinement showed that the desire to remove dead companions was one of the strongest that they exhibited:
This author also records in his recently published work an interesting piece of information to which he was led by Mrs. Treat: