Popular Science Monthly/Volume 11/May 1877/On the Habits of Ants
|ON THE HABITS OF ANTS.|
THE anthropoid apes no doubt approach nearer to man in bodily structure than do any other animals; but when we consider the habits of ants, their social organization, their large communities, elaborate habitations, their roadways, their possession of domestic animals, and even in some cases of slaves, it must be admitted that they have a fair claim to rank next to man in the scale of intelligence. They present, moreover, not only a most interesting but also a very extensive field of study. In this country we have nearly thirty species; but ants become more numerous, in species as well as individuals, in warmer countries, and more than seven hundred kinds are known. Even this large number certainly is far short of those actually in existence.
I have kept in captivity nearly half of our British species of ants, and at the present moment have in my room more than thirty nests, belonging to about twenty species, some of which, however, are not English. No two species are identical in habits, and on various accounts their mode of life is far from easy to unravel. In the first place most of their time is passed underground; all the education of the young, for instance, is carried on in the dark. Again, ants are essentially gregarious; it is in some cases difficult to keep a few alive by themselves in captivity, and at any rate their habits under such circumstances are entirely altered. If, on the other hand, a whole community is kept, then the greater number introduces a fresh element of difficulty and complexity. Moreover, within the same species, the individuals seem to differ in character, and even the same individual will behave very differently under different circumstances. Although, then, ants have attracted the attention of many naturalists—Gould, De Geer, Swammerdam, Latreille, Leeuwenhoeck, Huber—and have recently been the object of interesting observations by Frederick Smith, Belt, Moggridge, Bates, Mayr, Emery, Forel, and others, they still present one of the most promising fields for observation and experiment.
The larvæ of ants, like those of bees and wasps, are small, white, legless grubs, somewhat conical in form, being narrower toward the head. They are carefully tended and fed, being carried about from chamber to chamber by the workers, probably in order to secure the most suitable amount of warmth and moisture. I have observed also that they are very often sorted according to age. It is sometimes very curious in my nests to see them divided into groups according to size, so that they remind one of a school divided into five or six classes. When full grown they turn into pupæ, sometimes naked, sometimes covered with a silken cocoon, constituting the so-called "ant-eggs." After remaining some days in this state, they emerge as perfect insects. In many cases, however, they would perish in the attempt, if they were not assisted, and it is very pretty to see the older ants helping them to extricate themselves, carefully unfolding their legs and smoothing out the wings, with truly feminine tenderness and delicacy.
Under ordinary circumstances an ants' nest, like a beehive, consists of three kinds of individuals: workers, or imperfect females (which constitute the great majority), males, and perfect females. There are, however, often several females in an ants' nest; while, as we all know, there is never more than one queen in a hive. The queens have wings, but after a single flight they tear off their own wings, and do not again quit the nest. In addition to the ordinary workers there is in some species a second, or rather a third, form of female. In almost any ants' nest we may see that the workers differ more or less in size. The amount of difference, however, depends upon the species. In Lasius niger, the small brown garden ant, the workers are, for instance, much more uniform than in the little yellow meadow ant, or in Atta barbara, where some of them are more than twice as large as others. But in certain ants there are differences still more remarkable. Thus, in a Mexican species, besides the common workers, which have the form of ordinary neuter ants, there are certain others in which the abdomen is swollen into an immense sub-diaphanous sphere. These individuals are very inactive, and principally occupied in elaborating a kind of honey. In the genus Pheidole—very common in Southern Europe—there are also two distinct forms without any intermediate gradations: one with heads of the usual proportion, and a second with immense heads provided with very large jaws. These latter are generally supposed to act as soldiers, and the size of the head enables the muscles which move the jaws to be of unusual dimensions, though the little ones are also very pugnacious. This differentiation of certain individuals so as to adapt them to special functions seems to me very remarkable; for it must be remembered that the difference is not one of age or sex.
The food of ants consists of insects—great numbers of which they destroy—of honey, honey-dew, and fruit; indeed, scarcely any animal or sweet substance comes amiss to them. Some species—such, for instance, as the small brown garden ant—ascend bushes in search of aphides. The ant then taps the aphis gently with her antennæ, and the aphis emits a drop of sweet fluid, which the ant drinks. Sometimes the ants even build covered ways up to and over the aphides, which, moreover, they protect from the attacks of other insects. Our English ants do not collect provision for the winter—indeed, their food is not of a nature which would admit of this. Some southern species, however, collect grain, occasionally in considerable quantities. Moreover, though our English ants cannot be said exactly to lay up stores, some at least do take steps to provide themselves with food in the future. The small yellow meadow ant (Lasius flavus), for instance, lives principally on the honey-dew of certain aphides which suck the roots of grass. The ants collect the aphides in the nest, not only watching over them themselves, but, as I have been able to satisfy myself, even over their eggs—an act which one is much tempted to refer to forethought, and which in such a case implies a degree of prudence superior to that of some savages. Besides these aphides, many other insects live in ants' nests. If they are to be regarded as domestic animals, then ants have more domestic animals than we have. The majority of these ant-guests are beetles. Some of them—as, for instance, the curious little Claviger—are quite blind, and are only found in ants' nests, the ant taking just as much care of them as of their own young. It is evident, therefore, that in some way they are useful or agreeable to the ants. The subject, however, is one as yet but little understood, and very difficult to study. Grimm and Lespés consider that some of these beetles secrete a sweet fluid like the aphides, and from analogy this seems probable. Other creatures which habitually live in ants' nests, like the little Beckia albinos or the blind woodlouse (Platyarthrus), perhaps make themselves useful as scavengers.
Nor are ants without their enemies. In addition to birds and other larger foes, if you disturb a nest of the brown ants at any time during the summer, you will probably see some very small flies hovering over them, and every now and then making a dash at some particular ant. These flies belong to the genus Phora, and to a species hitherto unnamed, which Mr. Verrall has been good enough to describe for me. They lay their eggs on the ants, inside which the larvæ live. Other species of the genus are in the same way parasitic on bees. On the 14th of October last I observed that one of my ants had a mite attached to the underside of its head. The mite, which is still in the same position, is almost as large as the head. The ant cannot remove it herself. She has never come out of the nest, so that I could not do it for her, and none of her own companions from that day to this have thought of performing this kind office.
In character the different species of ants differ very much from one another. F. fusca, the one which is preëminently the enslaved ant, is, as might be expected, extremely timid; while the nearly allied F. cinerea has, on the contrary, a considerable amount of individual audacity. F. rufa, the horse ant, according to M. Forel, is especially characterized by the want of individual initiative, and always moves in troops; he also regards the genus Formica as the most brilliant, though some others excel it in other respects, as, for instance, in the sharpness of their senses. F. pratensis worries its slain enemies; F. sanguinea never does. The slave-making ant (P. rufescens) is, perhaps, the bravest of all. If a single individual finds herself surrounded by enemies, she never attempts to fly, as any other ant would, but transfixes her opponents one after another, springing right and left with great agility, till at length she succumbs, overpowered by numbers. M. scabrinodis is cowardly and thievish; during wars among the larger species they haunt the battle-fields and devour the dead. Tetramorium is said to be very greedy; Myrmecina very phlegmatic.
In industry ants are not surpassed even by bees and wasps. They work all day, and in warm weather, if need be, even at night too. I once watched an ant from six in the morning, and she worked without intermission till a quarter to ten at night. I had put her to a saucer containing larvæ, and in this time she carried off no less than a hundred and eighty-seven to the nest. I once had another ant, which I employed in my experiments, under observation several days. When I came up to London in the morning, and went to bed at night, I used to put her in a small bottle, but the moment she was let out she began to work again. On one occasion I was away from home for a week. On my return I let her out of the bottle, placing her on a little heap of larvæ about three feet from the nest. Under these circumstances I certainly did not expect her to return. However, though she had thus been six days in confinement, the brave little creature immediately picked up a larva, carried it off to the nest, and after half an hour's rest returned for another.
We have had hitherto very little information as to the length of life in ants. So far, indeed, as the preparatory stages are concerned, there is little difficulty in approximately ascertaining the facts—namely, that while they take only a few weeks in summer, in some species, as our small yellow meadow ants, the autumn larvæ remain with comparatively little change throughout the winter. It is much more difficult to ascertain the length of life of the perfect insect, on account of their gregarious habits, and the difficulty of recognizing individual ants. It has, however, generally been supposed that they live about a season, and this is probably the case; but I have still some workers of F. cinerea, which I captured at Castellamare in November, 1875, and some of F. sanguinea and F. fusca since September in that year. They must now, therefore, be at least a year and a half old. I have also some queens of F. fusca which have been with me since December, 1874, and still seem in perfect health. If they lived much longer, and could compare their experiences, ants would, from their immense numbers, even in temperate regions, contend with mankind on no such very unequal terms.
The behavior of ants to one another differs very much according as they are alone or supported by numerous companions. An ant which would run away in the first case, will fight bravely in the second.
It is hardly necessary to say that, as a general rule, each species lives by itself. There are, however, some interesting exceptions. The little Stenamma Westwoodii is found exclusively in the nests of the much larger F. rufa and the allied F. pratensis. We do not know what the relations between the two species are. The Stenammas, however, follow the Formicas when they change their nest, running about among them and between their legs, tapping them inquisitively with their antennæ, and even sometimes climbing on to their backs, as if for a ride, while the large ants seem to take little notice of them. They almost seem to be the dogs—or rather cats—of the ants. Another small species, Solenopsis fugax, which makes its chambers and galleries in the walls of the nests of larger species, is the bitter enemy of its hosts. The latter cannot get at them, because they are too large to enter the galleries. The little Solenopsis, therefore, are quite safe, and, as it appears, make incursions into the nurseries of the larger ant, and carry off the larvæ as food. It is as if we had small dwarfs, about eighteen inches to two feet long, harboring in the walls of our houses, and every now and then carrying off some of our children into their horrid dens.
Most ants, indeed, will carry off the larvæ and pupæ of others if they get a chance; and this explains, or at any rate throws some light upon, that most remarkable phenomenon, the existence of slavery among ants. If you place a number of larvæ and pupæ in front of a nest of the horse ant, for instance, they are soon carried off; and those which are not immediately required for food remain alive for some days, though I have never been able to satisfy myself whether they are fed by their captors. Both the horse ant and the slave ant (F. fusca) are abundant species, and it must not unfrequently occur that the former, being pressed for food, attack the latter and carry off some of their larvæ and pupæ. Under these circumstances it occasionally happens that the pupæ come to maturity in the nests of the horse ant, and nests are sometimes, though rarely, found in which with the legitimate owners there are a few F. fuscas. With the horse ant this is, however, a very rare and exceptional phenomenon; but with an allied species, F. sanguinea, a species which exists in our southern counties and throughout Europe, it has become an established habit. The F. sanguineas make periodical expeditions, attack neighboring nests of F. fusca, and carry off the pupæ. When the latter come to maturity, they find themselves in a nest consisting partly of F. sanguineas, partly of F. fuscas—the results of previous expeditions. They adapt themselves to circumstances, assist in the ordinary household duties, and, having no young of their own species, feed and tend those of the F. sanguinea. But though the F. sanguineas are thus aided by the F. fuscas, they have not themselves lost the instinct of working. It seems not improbable that there is some division of functions between the two species, but we have as yet no distinct knowledge on this point, and at any rate the F. sanguineas can "do" for themselves, and carry on a nest, if necessary, without slaves.
In another species, however, Polyergus rufescens, which is not British, this is not the case. They present a striking lesson of the degrading tendency of slavery, for they have become entirely dependent on their slaves. Even their bodily structure has undergone a change: their mandibles have lost their teeth, and have become mere nippers—deadly weapons, indeed, but useless except in war. They have lost the greater part of their instincts: their art, that is, the power of building; their domestic habits, for they take no care of their own young, all this being done by the slaves; their industry—they take no part in providing the daily supplies; if the colony changes the situation of its nest, the masters are all carried by the slaves to the new one; nay, they have even lost the habit of feeding. Huber placed thirty of them, with some larvæ and pupæ, and a supply of honey, in a box.
This observation has been fully confirmed by other naturalists. However small the prison, however large the quantity of food, these stupid creatures will starve in the midst of plenty rather than feed themselves. I have had a nest of this species under observation for a long time, but never saw one of the masters feeding, I have kept isolated specimens for weeks by giving them a slave for an hour or two a day to clean and feed them, and under these circumstances they remained in perfect health, while but for the slaves they would have perished in two or three days. I know no other case in Nature of a species having lost the instinct of feeding.
In P. rufescens, the so-called workers, though thus helpless and stupid, are numerous, energetic, and in some respects even brilliant. In another slave-making species, however, Strongylognathus, the workers are much less numerous, and so weak that it is an unsolved problem how they contrive to make slaves.
Lastly, in a fourth species, Anergetes atratulus, the workers are absent, the males and females living in nests with workers belonging to another ant, Tetramorium cœspitum. In these cases the Tetramoriums, having no queen, and consequently no young of their own, tend the young of the Anergetes. It is, therefore, a case analogous to that of Polyergus, but it is one in which slave-owning has almost degenerated into parasitism. It is not, however, a case of true parasitism, because the Tetramoriums take great care of the Anergetes, and, if the nest is disturbed, carry them off to a place of safety.
M. Forel, in his excellent work on ants, has pointed out that very young ants devote themselves at first to the care of the larvæ and pupae, and that they take no share in the defense of the nest or other out-of-door work until they are some days old. This seems natural, because at first their skin is comparatively soft; and it would clearly be undesirable to undertake rough work or run into danger until their armor had had time to harden. There are, however, reasons for thinking that the division of labor is carried still further. I do not allude merely to those cases in which there are completely different kinds of workers, but even to the ordinary workers. In L. flavus, for instance, it seems probable that the duties of the small workers are somewhat different from those of the large ones, though no such division of labor has yet been detected. In F. fusca I made an observation which surprised me very much. In the autumn of 1875 I noticed an ant out feeding alone. The next day the same ant was out by herself, and I could easily recognize her because by some accident she had lost the claws of one of her hind-feet. My attention being roused, I watched the nest for some weeks, and saw this same ant out repeatedly, but no other. This winter I have kept two nests under close observation—that is, I arranged with my daughters and their governess, Miss Wendland, most conscientious observers, that we should look at the nest once every hour throughout the day, and this has been done since the middle of November, with a few exceptions not enough to affect the conclusion. The former nest contains about two hundred, the second about four hundred individuals; but as they are somewhat torpid, and there are no larvæ to be fed, much food is not required. In each case only two or three individuals came out for food, each about twice a day, though some days they did not come out at all. Thinking that possibly these specimens were unusually voracious, or in some other way abnormal, I imprisoned the foragers belonging to one of the nests. The following day two others came out for food, and continued coming for several days. I then imprisoned them also, when two others came out—showing, I think, that the community requires food, and that it was the functions of certain individuals to obtain it.
One of the most interesting problems about ants is, of course, to determine the amount of their intelligence. In order to test this, it seemed to me that one way would be to ascertain some object which they would clearly desire, and then to interpose some obstacle which a little ingenuity would enable them to overcome. With this object in view, I placed food in a porcelain cup on a slip of glass surrounded by water, but accessible to the ants by a bridge, consisting of a strip of paper two-thirds of an inch long and one-third wide. Having then put a F. nigra from one of my nests to this food, she began carrying it off, and by degrees a number of friends came to help her. I then, when about twenty-five ants were so engaged, moved the little paper bridge slightly, so as to leave a chasm just so wide that the ants could not reach across. They came to the edge and tried hard to get over, but it did not occur to them to push the paper bridge, though the distance was only about one-third of an inch, and they might easily have done so. After trying for about a quarter of an hour they gave up the attempt, and returned home. This I repeated several times. Then, thinking that paper was a substance to which they were not accustomed, I tried the same with a bit of straw one inch long and one-eighth of an inch wide. The result was the same. I repeated this twice. Again I placed particles of food close to and directly over the nest, but connected with it only by a passage several feet in length. Under these circumstances it would be obviously a saving of time and labor to drop the food on to the nest, or at any rate to spring down with it, so as to save one journey. But, though I have frequently tried the experiment, my ants never adopted either of these courses. I arranged matters so that the glass on which the food was placed was only raised one third of an inch above the nest. The ants tried to reach down, and the distance was so small that occasionally, if another ant passed underneath just as one was reaching down, the upper one could step on to its back, and so descend; but this only happened accidentally, and they did not think of throwing the particles down, nor, which surprised me very much, would they jump down themselves.
I then placed a heap of fine mould close to the grass, but just so far that they could still not reach across. It would have been of course quite easy for any ant, by moving a particle of earth for a quarter of an inch, to have made a bridge by which the food might have been reached, but this simple expedient did not occur to them. On the other hand, I then put some provisions in a shallow box with a glass top, and a single hole on one side, and put some specimens of Lasius niger to the food. As soon as a stream of ants was at work, busily carrying supplies off to the nest, and when they had got to know the way thoroughly, I poured some fine mould in front of the hole so as to cover it up to a depth of about half an inch. I then took out the ants which were actually in the box. As soon as they had recovered from the shock of this unexpected proceeding on my part, they began to run all around and about the box, looking for some other place of entrance. Finding none, however, they began digging down into the earth just over the hole, carrying off the grains of earth one by one, and depositing them, without any order, all round at a distance of from half an inch to six inches, until they had excavated down to the doorway, when they again began carrying off the food as before. This experiment I repeated on the following days three or four times, always with the same result.
As evidence both of their intelligence and of their affection for their friends, it has been said by various observers that when ants have been accidentally buried they have been very soon dug out and rescued by their companions. Without for a moment doubting the facts as stated, we must remember the habits which ants have of burrowing in loose fresh soil, and especially their practice of digging out fresh galleries when their nests are disturbed. It seemed to me, however, that it would not be difficult to test whether the excavations made by ants under the circumstances were the result of this general habit, or really due to a desire to extricate their friends. With this view I tried (20th of August) the following experiments: I placed some honey near a nest of Lasius niger on a glass surrounded with water, and so arranged that in reaching it the ants passed over another glass covered with a layer of sifted earth about one-third of an inch in thickness. I then put some ants to the honey, and by degrees a considerable number collected round it. Then, at 1.30 p. m., I buried an ant from the same nest under the earth, and left her there till 5 p. m., when I uncovered her. She was none the worse, but during the whole time not one of her friends had taken the least notice of her.
Again, September 1st, I arranged some honey in the same way. At 5 p. m. about fifty ants were at the honey, and a considerable number were passing to and fro. I then buried an ant as before, of course taking one from the same nest. At 7 p. m. the number of ants at the honey had nearly doubled. At 10 p. m. they were still more numerous, and had carried off about two-thirds of the honey. At 7 a. m. the next morning the honey was all gone; two or three ants were still wandering about, but no notice had been taken of the prisoner, whom I then let out. In this case I allowed the honey to be finished, because I thought it might perhaps be alleged that the excitement produced by such a treasure distracted their attention; or even, on the principle of doing the greatest good to the greatest number, that they were intelligently wise in securing a treasure of food before they rescued their comrade, who, though in confinement, was neither in pain nor danger. So far as the above ants, however, are concerned, this cannot be urged. I may add that I repeated the same experiment several times, in some cases with another species, Myrmica ruginodis, and always with the same results.
Ants have been much praised on account of their affection for their friends. In this respect, however, they seem to vary greatly. At any rate, any one who has watched them much must have met with very contradictory facts. I have often put ants which were smeared with a sticky substance on the boards attached to my nests, and very rarely indeed did their companions take any notice of or seek to disentangle them.
I then tried the following experiment: A number of the small yellow ants (L. flavus) were out feeding on some honey. I took five of them, and also five others of the same species, but from a different nest, chloroformed them, and put them close to the honey, and on the path which the ants took in going to and from the nest, so that these could not but see them. The grass on which the honey was placed was surrounded by a moat of water. This, then, gave me an opportunity of testing both how far they would be disposed to assist a helpless fellow-creature, and what difference they would make between their nest-companions and strangers from a different community. The chloroformed ants were put down at ten in the morning. For more than an hour, though many ants came up and touched them with their antennæ, none of them did more. At length one of the strangers was picked up, carried to the edge of the glass, and quietly thrown, or rather dropped, into the water. Shortly afterward a friend was taken up and treated the same way. By degrees they were all picked up and thrown into the water. One of the strangers was, indeed, taken into the nest, but in about half an hour she was brought out again and thrown into the water like the rest. I repeated this experiment with fifty ants, half friends and half strangers. In each case twenty out of the twenty-five ants were thrown into the water as described. A few were left lying where they were placed, and these also, if we had watched longer, would no doubt have been also treated in the same way. One out of the twenty-five friends, and three out of the twenty-five strangers, were carried into the nest, but they were all brought out again and thrown away like the rest. Under such circumstances, then, it seems that ants make no difference between friends and strangers.
It may, however, be said in this experiment that, as ants do not recover from chloroform, and these ants were therefore to all intents and purposes dead, we should not expect that much difference would be made between friends and strangers. I therefore tried the same experiment, only, instead of chloroforming the ants, I made them intoxicated. This was a rather more difficult experiment. No ant would voluntarily degrade herself by getting drunk, and it was not easy in all cases to hit off the requisite degree of this compulsory intoxication. In all cases they were made quite drunk, so that they lay helplessly on their backs. The sober ants seemed much puzzled at finding their friends in this helpless and discreditable condition. They took them up and carried them about for a while in a sort of aimless way, as if they did not know what to do with their drunkards, any more than we do. Ultimately, however, the results were as follows: The ants removed twenty-five friends and thirty strangers. Of the friends, twenty were carried into the nest, where no doubt they slept off the effect of the spirit—at least, we saw no more of them—and five were thrown into the water. Of the strangers, on the contrary, twenty-four were thrown into the water; only six were taken into the nest, and four of these were shortly afterward brought out again and thrown away.
The difference in the treatment of friends and strangers was, therefore, most marked.
Dead ants, I may add, are always brought out of the nest, and I have more than once found a little heap on one spot, giving it almost the appearance of a burial-ground.
I have also made some experiments on the power possessed by ants of remembering their friends. It will be recollected that Huber gives a most interesting account of the behavior of some ants, which, after being separated for four months, when brought together again, immediately recognized one another, and "fell to mutual caresses with their antennæ." Forel, however, regards these movements as having indicated fear and surprise rather than affection, though he also is quite inclined to believe, from his own observation, that ants would recognize one another after a separation of some months. The observation recorded by Huber was made casually; and neither he nor any one else seems to have taken any steps to test it by subsequent experiments. The fact is one, however, of so much interest, that it seemed to me desirable to make further experiments on the subject. On the 4th of August, 1875, therefore, I separated one of my nests of F. fusca into two halves, which I kept entirely apart.
I then from time to time put an ant from one of these nests into the other, introducing also a stranger at the same time. The stranger was driven out, or sometimes even killed. The friend, on the contrary, was never attacked, though I am hound to say that I could see no signs of any general welcome, or that she was taken any particular notice of.
I will not trouble you with all the evidence, but will content myself with one case.
On the 12th November last—that is to say, after the ant had been separated for a year and three months—I put a friend and a stranger into one of the divisions. The friend seemed quite at home. One of the ants at once seized the stranger by an antenna, and began dragging her about. At—
11.45.—The friend is quite at home with the rest. The stranger is being dragged about.
12.—The friend is all right. Three ants now have hold of the stranger by her legs and an antenna.
1.30.—Do. One now took hold of the friend, but soon seemed to find out her mistake and let go again.
1.45.—The friend is all right. The stranger is being attacked. The friend also has been almost cleaned; while on the stranger the color has been scarcely touched.
2.15.—Two ants are licking the friend, while another pair are holding the stranger by her legs.
2.30.—The friend is now almost clean, so that I could only just perceive any color. The stranger, on the contrary, is almost as much colored as ever. She is now near the door, and I think would have come out, but two ants met her and seized her.
3.—Two ants are attacking the stranger. The friend was no longer distinguishable from the rest.
6.—The stranger now escaped from the nest, and I put her back among her own friends.
The difference of behavior to these two ants was most marked. The friend was gradually licked clean, and, except for a few moments, and that evidently by mistake, was never attacked. The stranger, on the contrary, was not cleaned, was at once seized, was dragged about for hours with only a few minutes' interval, by one, two, or three assailants, and at length made her escape from the nest at a time when no other ant was out.
In most species of ants the power of smell is very keen. I placed ants on a strip of paper, each end of which was supported on a pin, the foot of which was immersed in water. They then ran backward and forward along the paper, trying to escape. If a camel's-hair pencil be suspended just over the paper, they pass under it without taking any notice of it; but if it be scented, say with lavender-water, they at once stop when they come near it, showing in the most unmistakable manner that they perceive the odor. This sense appears to reside, though not perhaps exclusively, in the antennæ. I tethered, for instance, a large specimen of Formica ligniperda with a fine thread to a board, and when she was quite quiet I approached a scented camel's-hair pencil slowly to the tip of the antenna, which was at once withdrawn, though the antenna took no notice of a similar pencil, if not scented.
On the other hand, as regards their sense of hearing, the case is very different. Approaching an ant which was standing quietly, I have over and over again made the loudest and most shrill noises I could, using a penny pipe, a dog-whistle, a violin, as well as the most piercing and startling sounds I could produce with my own voice, without effect. At the same time I by no means would infer from this that they are really deaf, though it certainly seems that their range of hearing is very different from ours. We know that certain allied insects produce a noise by rubbing one of their abdominal rings against another. Landois is of opinion that ants also make sounds in the same way, though these sounds are inaudible to us. Our range is, however, after all, very limited, and the universe is probably full of music which we cannot perceive. There are, moreover, in the antennæ of ants certain curious organs which may perhaps be of an auditory character. There are from ten to a dozen in the terminal segment of Lasius flavus, the small meadow ant, and, indeed, in most of the species which I have examined, and one or two in each of the short intermediate segments. These organs consist of three parts: a small, spherical cup opening to the outside, a long, narrow tube, and a hollow body shaped like an elongated clock-weight. They may serve to increase the resonance of sounds, acting, in fact, to use the words of Prof. Tyndall, who was good enough to look at them with me, like microscopic stethoscopes.
The organs of vision are in most ants very complex and conspicuous. There are generally three eyes arranged in a triangle on the top of their heads, and on each side a large compound eye containing sometimes more than two thousand facets between them. Nevertheless, the sight of ants does not seem to be very good. In order to test how far ants are guided by vision, I made the following experiments: I placed a common lead-pencil on a board, fastening it upright, so as to serve as a landmark. At the base I then placed a glass containing food, and then put a L. niger to the food; when she knew her way from the glass to the nest and back again perfectly well, she went quite straight backward and forward, I then took an opportunity when the ant was on the glass, and moved the glass with the ant on it about three inches. Now, under such circumstances, if she had been much guided by sight, she could not of course have had any difficulty in finding her way to the nest. As a matter of fact, however, she was entirely at sea, and, after wandering about for some time, got back to the nest by another and very round-about route. I then again varied the experiment as follows: I placed the food in a small china cup on the top of the pencil, which thus formed a column seven and a half inches high. When the ant once knew her way, she went very straight to and from the nest. This puzzled her very much; she went over and over the spot where the pencil had previously stood, retraced her steps several times almost to the nest, and then returned along the old line, showing great perseverance, if not much power of vision. I then moved the pencil six inches. She found the pencil at last, but only after many meanderings.
I then repeated the observation on three other ants, with the same result; the second was seven minutes before she found the pencil, and at last seemed to do so accidentally; the third actually wandered about for no less than half an hour, returning up the paper bridge several times.
Let us compare this relatively to man. An ant measuring say one-sixth of an inch, the pencil, being seven inches high, is consequently forty-two times as long as the ant. It bears, therefore, somewhat the same relation to the ant as a column two hundred and fifty feet high does to a man. The pencil having been moved six inches, it is as if a man in a country he knew well would be puzzled at being moved a few hundred feet, or, if put down in a square containing less than an acre, could not find a column two hundred and fifty feet high, that is to say, higher than the Duke of York's column.
Another evidence of this consists in the fact that if, when my L. nigers were carrying off food placed in a cup on a piece of board, I turned the board round so that the side which had been turned toward the nest was away from it, and vice versa, the ants always returned over the same track on the board, and consequently directly away from home. If I moved the board to the other side of my artificial nest, the result was the same. Evidently they followed the road, not the direction.
It is remarkable that we do not even now know exactly how an ants' nest is begun. Whether they always commence as a colony from some older establishment; whether wandering workers who chance to find a queen under certain circumstances remain with her and begin a new nest; or whether the queen ant, like the queen wasp, forms a cell for herself, and then brings up a few workers, who afterward take upon themselves the labors of the family, as yet we know not. When once started, the communities last for years, being kept up by a succession of individuals. The queens themselves rarely or never quit the nest, but receive their food from the workers, and indeed appear to do nothing except lay eggs.
A nest of ants must not be confused with an ant-hill in the ordinary sense. Very often, indeed, a nest has only one dwelling, and in most species seldom more than three or four. Some, however, form numerous colonies. M. Forel even found a case in which one nest of F. exsecta had no less than two hundred colonies, and occupied a circular space with a radius of nearly two hundred yards. Within this area they had exterminated all the other ants, except a few nests of Tapinoma erraticum, which survived, thanks to their great agility. In these cases the number of ants thus associated together must have been enormous. Even in single nests Forel estimates the numbers at from 5,000 to 500,000.
In their modes of fighting, different species of ants have their several peculiarities. Some, also, are much less military than others. Myrmecina Latreillii, for instance, never attack, and scarcely even defend themselves. Their skin is very hard, and they roll themselves into a ball, not defending themselves even if their nest is invaded: to prevent which, however, they make the entrances small, and often station at each a worker, who uses her head to stop the way. The smell of this species is also, perhaps, a protection. Tetramorium cæspitum has the habit of feigning death. This species, however, does not roll itself up, but merely applies its legs and antennæ closely to the body.
Formica rufa, the common horse ant, attacks in serried masses, seldom sending out detachments, while single ants scarcely ever make individual attacks. They rarely pursue a flying foe, but give no quarter, killing as many enemies as possible, and never hesitating, with this object, to sacrifice themselves for the common good.
Formica sanguinea, on the contrary, at least in their slave-making expeditions, attempt rather to terrify than to kill. Indeed, when they are invading a nest, they do not attack the flying inhabitants unless they are attempting to carry off pupæ, in which case they are forced to abandon the pupæ. When fighting, they attempt to crush their enemies with their mandibles.
Formica exsecta is a delicate but very active species. They, also, advance in serried masses, but in close quarters they bite right and left, dancing about to avoid being bitten themselves. When fighting with larger species they spring on to their backs, and then seize them by the neck or by an antenna. They also have the instinct of combining in small parties, three or four seizing an enemy at once, and then pulling different ways, so that she, on her part, cannot get at any one of her foes. One of them then jumps on her back and cuts, or rather saws, off her head. In battles between this ant and the much larger F. pratensis, many of the latter may be seen, each with a little F. exsecta on her back, sawing off her head from behind.
One might, at first sight, be disposed to consider that the ants with stings must have a great advantage over those with none. In some cases, however, the poison is so strong that it is sufficient for it to touch the foes to place them hors de combat, or at least to render them incapacitated, with every appearance of extreme pain. Such species have the abdomen unusually mobile.
The species of Lasius make up in numbers what they want in strength. Several of them seize an enemy at once, one by each of her legs or antennæ, and, when they have once taken hold, they will suffer themselves to be cut in pieces rather than let go.
Polyergus rufescens, the celebrated slave-making or Amazon ant, has a mode of combat almost peculiar to herself. The jaws are very powerful and pointed. If attacked—if, for instance, another ant seizes her by a leg—she at once takes her enemy's head into her jaws, which generally makes her quit her hold. If she does not, the Polyergus closes her mandibles, so that the points pierce the brain of her enemy, paralyzing the nervous system. The victim falls in convulsions, setting free her terrible foe. In this manner a comparatively small force of Polyergus will fearlessly attack much larger armies of other species, and suffer themselves scarcely any loss.
Much of what has been said as to the powers of communication possessed by bees and ants depends on the fact that, if one of them in the course of her rambles has discovered a supply of food, a number of others soon find their way to the store. This, however, does not necessarily imply any power of describing localities. If the bees or ants merely follow their more fortunate companion, or if they hunt her by scent, the matter is comparatively simple; if, on the contrary, the others have the route described to them, the case becomes very different. To determine this, therefore, I have made a great number of experiments, of which, however, I will here only mention a few. Under ordinary circumstances, if an ant discovers a stock of food, she carries as much as possible away to the nest, and then returns for more, accompanied generally by several friends. On their return, these bring others, and, in this way, a string of ants is soon established. Unless, therefore, various precautions are taken—and this, so far as I know, has never been done in any of the previous observations—the experiment really tells very little.
I therefore made the following arrangement: One of my nests of the small brown garden ant, Lasius niger, was connected with a board, on which I was in the habit of placing a supply of food and water. At a short distance from the board I placed two glasses (b b'), and on b I placed some food. I then connected the glass b with the board a by three slips of paper, c, d, e, and put an ant to the food. She carried off a supply to the nest, returning for more, and so on. Several friends came with her, and I imprisoned them till the experiment was over. When she had passed several times over the paper bridges, I proceeded as follows: Any friends who came with her were excluded from the bridges when she was on them. If she was not there, as soon as a friend arrived at the bridge c, I took up e in my fingers and rubbed it lightly, with a view of removing or blurring the scent; and as soon as the ant arrived on d I took up the bridge c, and put it across the chasm from d to b'. Now, if the ant went by description, she would of course cross e to b. If, on the
other hand, she went by scent, then she would be at the least as likely to go over c to b'. The results were that, out of about one hundred and twenty friends who passed over d, only twenty went to the food, while nearly one hundred passed over c to the empty glass. In this case the friends generally came more or less in sight of one another to the bridge c, and, once there, could hardly avoid arriving either at b or b'. I therefore modified the experiment as follows: I established and endowed an ant as before, imprisoning the friends who came with her. When she got to know her way thoroughly, I allowed her to return to the nest on her own legs, but as soon as she emerged again I took her up and transferred her to the food.
Under these circumstances, as will be seen, very few ants indeed ever found their way to the food. I began this at 5.30, when she returned to the nest. At 5.34 she came out with no less than ten friends, and was then transferred to the food. The others wandered about a little, but by degrees returned to the nest, not one of them finding her way to the food. The first ant took some food, returned, and again came out of the nest at 5.39 with eight friends, when exactly the same happened. She again came out—
(39 journeys: 11 alone, 28 with 120 friends.)
Thus, during these two hours, more than one hundred and twenty-ants came out of the nest, in company with the one under observation. She knew her way perfectly, and it is clear that, if she had been left alone, all these ants would have accompanied her to the store of food. Three of them were accidentally allowed to do so, but of the remainder only five found their way to the food; all the others, after wandering about awhile, returned empty-handed to the nest.
I conclude, then, that when large numbers of ants come to food they follow one another, being also to a certain extent guided by scent. The fact, therefore, does not imply any considerable power of intercommunication. There are, moreover, some circumstances which seem to point in an opposite direction. For instance, I have already mentioned that if a colony of Polyergus changes the situation of its nest, the masters are all carried to the new one by the slaves. Again, if a number of F. fusca are put in a box, and in one corner a dark place of retreat is provided for them with some earth, one soon finds her way to it. She then comes out again, and, going up to one of the others, takes her by the jaws. The second ant then rolls her-self into a heap, and is carried off to the place of shelter. They then both repeat the same manœuvre with other ants, and so on until all their companions are collected together. Now, it seems to me difficult to imagine that so slow a course would be adopted if they possessed any power of communicating description.
On the other hand, they certainly can, I think, transmit simpler ideas. In support of this I may adduce the following experiment: Two strips of paper were attached to the board just mentioned (p. 54), and parallel to one another, and at the other end of each I placed a piece of glass. In the glass at the end of one tape I placed a considerable number (three to six hundred) of larvæ. In the second I put two or three larvæ only. I then took two ants, and placed one of them to the glass with many larvæ, the other to that with two or three. Each of them took a larva and carried it to the nest, returning for another, and so on. After each journey I put another larva in the glass with only two or three larvæ, to replace that which had been removed. Now, if other ants came under the above circumstances as a mere matter of accident, or accompanying one another by chance, or if they simply saw the larvæ which were being brought, and consequently concluded that they might themselves also find larvæ in the same place, then the numbers going to the two glasses ought to be approximately equal. In each case the number of journeys made by the ants would be nearly the same; consequently, if it were a matter of smell, the two routes would be in the same condition. It would be impossible for an ant, seeing another in the act of bringing a larva, to judge for itself whether there were few or many larvæ left behind. On the other hand, if the strangers were brought, then it would be curious to see whether more were brought to the glass with many larvæ than to that which only contained two or three. I should also mention that every stranger was imprisoned until the end of the experiment. I will select a few of the results:
Experiment 1.—Time occupied, one hour. The ant with few larvæ made six visits, and brought no friends. The one with many larvæ made seven, and brought eleven friends.
Experiment 3.—Time occupied, three hours. The ant with few larvæ made twenty-four journeys, and brought five friends. The one with many larvæ made thirty-eight journeys, and brought twenty-two friends.
Experiment 5.—Time occupied, one hour. The ant with few larvæ made ten journeys, and brought three friends. The other made five journeys, and brought sixteen friends.
Experiment 9.—Time occupied, one hour. The ant with few larvæ made eleven journeys, and brought one friend. The one with many larvæ made fifteen journeys, and brought thirteen friends.
Experiment 10.—I now reversed the glasses, the same two ants being under observation; but the ant which in the previous observation had few larvæ to carry off now consequently had many, and vice versa. Time occupied, two hours. The ant with few larvæ made twenty-one journeys, and brought one friend. The one with many larvæ made twenty-two journeys, and brought twenty friends. These two experiments are, I think, especially striking.
Taken as a whole, I found that in about fifty hours the ants which had access to many larvæ brought two hundred and fifty-seven friends, while those visiting a glass with few larvæ only brought eighty-two. The result will appear still more striking if we remember that a certain number, say perhaps twenty-five, would have come to the larvæ anyhow, which would make the numbers two hundred and thirty-two as against fifty-seven, a very striking difference.
I have elsewhere discussed the relations of flowers to insects, and especially with bees, and particularly the mode in which the flowers were modified so that the bees might transfer the pollen from one flower to another. Ants are also of considerable importance to plants, especially in keeping down the number of insects which feed on them. So far as I know, however, there are no plants which are specially modified in order to be fertilized by ants; and, indeed, even to those small flowers which any little insect might fertilize, the visits of winged insects are much more advantageous, because, as Mr. Darwin has shown in his excellent work on cross and self fertilization of plants, it is important that the pollen should be brought, not only from a different flower, but also from a different plant, while creeping insects, such as ants, would naturally pass from flower to flower of the same plant.
Under these circumstances it is important to plants that ants should not obtain access to the flowers, for they would otherwise rob them of their honey without conferring on them any compensating advantage. Accordingly, we not only find in flowers various modes of attracting bees, but also of excluding ants; and in this way ants have exercised more influence on the vegetable kingdom than might be supposed. Sometimes, for instance, the flowers are protected by chevaux-de-frise of spines and fine hairs pointing downward (Carlina, Lamium); some have a number of glands secreting a glutinous substance, over which the ants cannot pass (Linnœa, gooseberry); in others the tube of the flower is itself very narrow, or is almost closed either by hairs or by internal ridges, which just leave space for the proboscis of a bee, but no more. Lastly, some, and especially pendulous flowers (Cyclamen, snowdrop), are so smooth and slippery that ants cannot easily enter them, but often slip off in the attempt, and thus are excluded, just as the pendulous nests of the weaver-birds preclude the entrance of snakes. This, however, is a large subject, into which I cannot now enter.
Let me, in conclusion, once more say that, as it seems to me, notwithstanding the labors of those great naturalists to whom I gratefully referred in commencing, there are in natural history few more promising or extensive fields for research than the habits of ants.—Fortnightly Review.
- Westwood, "Modern Classification of Insects," vol. ii., p. 225.
- Huber, "Natural History of Ants."