Animal Intelligence Part 7

I saw a large number of ants surrounding the dead ones. I determined to watch their proceedings closely.

I followed four or five that started off from the rest towards a hillock a short distance off, in which was an ants' nest. This they entered, and in about five minutes they reappeared, followed by others. All fell into rank, walking regularly and slowly two by two, until they arrived at the spot where lay the dead bodies of the soldier ante. In a few minutes two of the ants advanced and took up the dead body of one of their comrades; then two others, and so on, until all were ready to march. First walked two ants bearing a body, then two without a burden; then two others with another dead ant, and so on, until the line was extended to about forty pairs, and the procession now moved slowly onwards, followed by an irregular body of about two hundred ants. Occasionally the two laden ants stopped, and laying down the dead ant, it was taken up by the two walking unburdened behind them, and thus, by occasionally relieving each other, they arrived at a sandy spot near the sea. The body of ants now commenced digging with their jaws a number of holes in the ground, into each of which a dead ant was laid, where they now laboured on until they had filled up the ants' graves. This did not quite finish the remarkable circ.u.mstances attending this funeral of the ants. Some six or seven of the ants had attempted to run off without performing their share of the task of digging; these were caught and brought back, when they were at once attacked by the body of ants and killed upon the spot. A single grave was quickly dug, and they were all dropped into it.

The Rev. W. Farren White also, in his papers on ants published in the 'Leisure Hour' (1880), after alluding to the above case, corroborates it by some interesting observations of his own. He says:--

Several of the little s.e.xtons I observed with dead in their mandibles, and one in the act of burying a corpse... . I should mention that the dead are not interred without considerable difficulty, in consequence of the sides of the trays being almost perpendicular. The work of the s.e.xtons continued until no dead bodies remained upon the surface of the nest, but all were interred in the extramural cemeteries.

Afterwards I removed the trays, and turned the contents of the formicarium upside down, and then I placed six trays on the surface of the earth, two of which I filled with sugar for food. All six were used freely as cemeteries, being crowded with the corpses of the little people and their young, the larvae which had perished in the disruption of their home.

I have noticed in one of my formicaria a subterranean cemetery, where I have seen some ants burying their dead by placing earth above them. One ant was evidently much affected, and tried to exhume the bodies, but the united exertions of the yellow s.e.xtons were more than sufficient to neutralise the effort of the disconsolate mourner. The cemetery was now converted into a large vault, the chamber where the dead were placed, together with the pa.s.sage which led to it, being completely covered in.

_Habits Peculiar to Certain Species._

_Leaf-cutting Ants of the Amazon_ ([OE]codoma cephalotes).--The mode of working practised by these ants is thus described by Mr. Bates:--

They mount a tree in mult.i.tudes... . Each one places itself on the surface of a leaf, and cuts with its sharp scissor-like jaws a nearly semicircular incision on the upper side; it then takes the edge between its jaws, and by a sharp jerk detaches the piece.

Sometimes they let the leaf drop to the ground, where a little heap acc.u.mulates, until carried off by another relay of workers; but generally each marches off with the piece it has operated on, and as all take the same road to the colony, the path they follow becomes in a short time smooth and bare, looking like the impression of a cart-wheel through the herbage.

Each ant carries its semicircular piece of leaf upright over its head, so that the home-returning train is rendered very conspicuous. Nearer observation shows that this home-returning or ladened train of workers keeps to one side of the road, while the outgoing or empty-handed train keeps to the other side; so that on every road there is a double train of ants going in opposite directions. When the leaves arrive at the nest they are received by a smaller kind of workers, whose duty it is to cut up the pieces of leaf into still smaller fragments, whereby the leaves seem to be better fitted for the purpose to which, as we shall presently see, they are put. These smaller workers never take any part in the outdoor labours; but they occasionally leave the nest, apparently for the sole purpose of obtaining air and exercise, for when they leave the nest they merely run about doing nothing, and frequently, as if in mere sport, mount some of the semicircular pieces of leaf which the carrier ants are taking to the nest, and so get a ride home.

From his continued observation of these ants, Bates concludes--and his opinion has been corroborated by that both of Belt and Muller--that the object of all this labour is highly interesting and remarkable. The leaves when gathered do not themselves appear to be of any service to the ants as food; but when cut into small fragments and stored away in the nests, they become suited as a nidus for the growth of a minute kind of fungus on which the ants feed. We may therefore call these insects the 'gardening ants,' inasmuch as all their labour is given to the rearing of nutritious vegetables on artificially prepared soil. They are not particular as to the material which they collect and store up for soil, provided that it is a material on which the fungus will grow. Thus they are very partial to the inside white rind of oranges, and will carry off the flowers of certain shrubs while leaving the leaves untouched. But, to quote again from Bates,--

They are very particular about the ventilation of their underground chambers, and have numerous holes leading up to the surface from them. These they open out or close up, apparently to keep up a regular degree of temperature below. The great care they take that the pieces of leaves they carry into the nest should be neither too dry nor too damp, is also consistent with the idea that the object is the growth of a fungus that requires particular conditions of temperature and moisture to ensure its vigorous growth. If a sudden shower should come on, the ants do not carry the wet pieces into the burrows, but throw them down near the entrances. Should the weather clear up again, these pieces are picked up when nearly dried, and taken inside: should the rain, however, continue, they get sodden down into the ground, and are left there. On the contrary, in dry and hot weather, when the leaves would get dried up before they could be conveyed to the nest, the ants, when in exposed situations, do not go out at all during the hot hours, but bring in their leafy burdens in the cool of the day and during the night. As soon as the pieces of leaves are carried in they must be cut up by the small cla.s.s of workers into little pieces. Some of the ants make mistakes, and carry in unsuitable leaves. Thus gra.s.s is always rejected by them, but I have seen some ants, perhaps young ones, carrying leaves of gra.s.s; but after a while these pieces are always brought out again and thrown away. I can imagine a young ant getting a severe ear-wigging from one of the major-domos for its stupidity.

When a nest is disturbed and the ma.s.ses of ant-food spread about, the ants are in great concern to carry every morsel of it under shelter again; and sometimes, when I had dug into a nest, I found the next day all the earth thrown out filled with little pits, that the ants had dug into it to get out the covered-up food.

When they migrate from one part to another, they also carry with them all the ant-food from their old habitations.

In Buchner's 'Geistesleben der Thiere' there is published an interesting description of the habits of these ants, which was communicated to the author by Dr. Fr. Ellendorf of Wiedenbruck, who has lived many years in Central America. Dr. Ellendorf says that--

It would be quite impossible for them to creep even through short gra.s.s with loads on their heads for miles. They therefore bite off the gra.s.s close to the ground for a breadth of about five inches, and throw it on one side. Thus a road is constructed, which is finally made quite smooth and even by the continual pa.s.sing to and fro of millions upon millions night and day... . If the road is looked down upon from a height with these millions thickly pressed together, and all moving along with their green bannerets over their heads, it looks as though a giant green snake were gliding slowly along the ground; and this picture is all the more striking in that all these bannerets are swaying backwards and forwards.[33]

This observer made the experiment of interrupting the advance of a column of these ants, with the interesting result which he describes:--

I wished to see how they would manage if I put an obstacle in their way. Thick high gra.s.s stood on either side of their narrow road, so that they could not pa.s.s through it with the load on their heads. I placed a dry branch, nearly a foot in diameter, obliquely across their path, and pressed it down so tightly on the ground that they could not creep underneath. The first comers crawled beneath the branch as far as they could, and then tried to climb over, but failed owing to the weight on their heads.

Meanwhile the unloaded ants from the other side came on, and when these succeeded in climbing over the bough there was such a crush that the unladen ants had to clamber over the laden, and the result was a terrible muddle. I now walked along the train, and found that all the ants with their bannerets on their heads were standing still, thickly pressed together, awaiting the word of command from the front. When I turned back to the obstacle, I saw with astonishment that the loads had been laid aside by more than a foot's length of the column, one imitating the other.

And now work began on both sides of the branch, and in about half an hour a tunnel was made beneath it. Each ant then took up its burden again, and the march was resumed in the most perfect order.

A migration of these ants is thus described by the same observer:--

The road led towards a cocoa plantation, and here I soon discovered the building which I afterwards visited daily. As I again went thither one day I was met, at a considerable distance from the nest, by a closely pressed column coming thence, and all the ants laden with leaves, beetles, pupae, b.u.t.terflies, &c.; the nearer I came to the nest, the greater was the activity. It was soon plain to me that the ants were in the act of leaving their dwelling, and I walked along the train to discover the new abode. They had gone for some distance along the old road, and had then made a new one through the gra.s.s to a cooler place, lying rather higher. The gra.s.s on the new road was all bitten off close to the ground, and thousands were busy carrying the path on to the new building. At the new home itself was an unusual stir of life. There were all sorts of labourers--architects, builders, carpenters, sappers, helpers. A number were busy digging a hole in the ground, and they carried out little pellets of earth and laid them together on end to make a wall. Others drew along little twigs, straws, and gra.s.s-stalks, and put them near the place of building. I was anxious to know why they had quitted their old home, and when the departure was complete, I dug it up with a spade. At a depth of about a foot and a half I found several tunnels of a large marmot species, the terror of cocoa planters, because in making their pa.s.sages they gnaw off the thickest roots of the cocoa plants. The interior of the ant-hill had apparently fallen in through these mines. Unfortunately I was unable to follow further the progress of the new building, for I was obliged to leave the next day for San Juan del Sur. When I returned at the end of a week the building was finished, and the whole colony was again busy with the leaves of the coffee plants.

_Harvesting Ants_ (Atta).--The ants which, so far as at present known, practise the peculiar and distinctive habits to be described under this division belong for the most part to one genus, _Atta_, which, however, comprises a number of species distributed in localised areas over all the four quarters of the globe. Hitherto nineteen species have been detected as having the habits in question. These consist of gathering nutritious seeds of gra.s.ses during summer, and storing them in granaries for winter consumption. We owe our present knowledge concerning these insects to Mr. Moggridge,[34] who studied them in the south of Europe, Dr. Lincec.u.m,[35] and Mr. MacCook,[36] who studied them in Texas, and Colonel Sykes[37] and Dr. Jerdon,[38] who made some observations upon them in India. They also occur scattered over a great part of Europe and in Palestine, where they were clearly known to Solomon and other cla.s.sical writers of antiquity,[39] whose claim to accurate observation, although long disputed (owing to the authority of Huber), has now been amply vindicated.

Mr. Moggridge, who was a careful and industrious observer, found the following points of interest in the habits of the European harvesters.

From the nest in various directions there proceed outgoing trains, which may be from twenty to thirty or more yards in length, and each consists of a double row of ants, moving, like the leaf-cutting ants, in opposite directions. Those in the outgoing row are empty-handed, while those in the incoming row are laden. But here the burdens are gra.s.s seeds. The roads terminate in the foraging ground, or ant-fields, and the insects composing the columns there become dispersed by hundreds among the seed-yielding gra.s.ses. The following is their method of collecting seeds; I quote from Moggridge:--

It is not a little surprising to see that the ants bring in not only seeds of large size and fallen grain, but also green capsules, the torn stalks of which show that they have been freshly gathered from the plant. The manner in which they accomplish this feat is as follows. An ant ascends the stem of a fruiting plant of shepherd's-purse (_Capsella bursa-pastoris_), let us say, and selects a well-filled but green pod about midway up the stem, those below being ready to shed their seeds at a touch. Then, seizing it in its jaws, and fixing its hind legs firmly as a pivot, it contrives to turn round and round, and so strain the fibres of the fruit-stalk that at length they snap. It then descends to the stem, patiently backing and turning upwards again as often as the clumsy and disproportionate burden becomes wedged between the thickly set stalks, and joins the line of its companions on their way to the nest. In this manner capsules of chickweed (_Stellaria media_) and entire calyces, containing the nutlets of calamint, are gathered; two ants also sometimes combine their efforts, when one stations itself near the base of the peduncle and gnaws it at the point of greatest tension, while the other hauls upon and twists it. I have never seen a capsule severed from its stalk by cutting alone, and the mandibles of this ant are perhaps incompetent to perform such a task. I have occasionally seen ants engaged in cutting the capsules of certain plants, drop them, and allow their companions below to carry them away; and this corresponds with the curious account given by aelian of the manner in which the spikelets of corn are severed and thrown down 'to the people below,' [Greek: to demo to kato].

The recognition of the principle of the division of labour which the latter observation supplies, is further proved by the following quotation from the same author. A dead gra.s.shopper which was being carried into their nest was--

Too large to pa.s.s through the door, so they tried to dismember it. Failing in this, several ants drew the wings and legs as far back as possible, while others gnawed through the muscles where the strain was greatest. They succeeded at last in thus pulling it in.

The same thing is strikingly shown by the following quotation from Lespes:--

If the road from the place where they are gathering their harvest to the nest is very long, they make regular depots for their provisions under large leaves, stones, or other suitable places, and let certain workers have the duty of carrying them from depot to depot.

Buchner (_loc. cit._ p. 101) also makes the following references to the statements of previous observers:--

The subterranean workers of this remarkable genus are very clever. The Rev. H. Clark reports from Rio de Janeiro, that the _Sa-ubas_ have made a regular tunnel under the bed of the river Parahyba, which is there as broad as the Thames at London, in order to reach a storehouse which is on the opposite bank. Bates tells us that close to the Magoary rice-mills, near Para, the ants bored through the dam of a large reservoir, and the water escaped before the mischief could be remedied. In the Para Botanical Gardens an enterprising French gardener did everything he could to drive the _Sa-ubas_ away. He lit fires at the chief entrances of their nests, and blew sulphur vapour into their galleries by means of bellows. But how astonished was Bates when he saw the vapour come out at no less a distance than seventy yards! Such an extension have the subterranean pa.s.sages of the _Sa-ubas_.

The recognition of the principle of the division of labour, which is shown by the above observations, is further corroborated by the following quotation from Belt:--

Between the old burrows and the new one was a steep slope. Instead of descending this with their burdens, they cast them down on the top of the slope, whence they rolled down to the bottom, where another relay of labourers picked them up and carried them to the new burrow. It was amusing to watch the ants hurrying out with bundles of food, dropping them over the slope, and rushing back immediately for more.

The same thing has been observed, as already stated, of the leaf-cutting ants--those engaged in cutting frequently throwing down the fragments of leaf which they cut to the carriers below. The prevalence of this habit among various species of ants therefore renders credible the following statements of Vincent Gredler of Botzen which are thus recorded in 'der Zool. Gart.,' xv. p. 434:--

In Herr Gredler's monastery one of the monks had been accustomed for some months to put food regularly on his window-sill for ants coming up from the garden.

In consequence of Herr Gredler's communications he took it into his head to put the bait for the ants, pounded sugar, into an old inkstand, and hung this up by a string to the cross-piece of his window, and left it hanging freely. A few ants were in with the bait.

These soon found their road out over the string with their grains of sugar, and so their way back to their friends. Before long a procession was arranged on the new road from the window-sill along the string to the spot where the sugar was, and so things went on for two days, nothing fresh occurring. But one day the procession stopped at the old feeding-place on the window-sill, and took the food thence, without going up to the pendent sugar-jar. Closer observation revealed that about a dozen of the rogues were in the jar above, and were busily and unwearyingly carrying the grains of sugar to the edge of the pot, and throwing them over to their comrades down below.

Many other instances of the division of labour might be given besides these, and those to be mentioned hereafter in other connections throughout the course of the present chapter; but enough has been said to show that the principle is unquestionably acted upon by sundry species of ants.

That ants are liable to make mistakes, and, when they do, that they profit by experience, is shown by the following experiment made by Moggridge; and many other instances might be given were it desirable:--

It sometimes happens that an ant has manifestly made a bad selection, and is told on its return that what it has brought home with much pains is no better than rubbish, and is hustled out of the nest, and forced to throw its burden away. In order to try whether these creatures were not fallible like other mortals, I one day took out with me a little packet of grey and white porcelain beads, and scattered these in the path of a harvesting train. They had scarcely lain a minute on the earth before one of the largest workers seized upon a bead, and with some difficulty clipped it with its mandibles and trotted back at a great pace to the nest. I waited for a little while, my attention being divided between the other ants who were vainly endeavouring to remove the beads, and the entrance down which the worker had disappeared, and then left the spot. On my return in an hour's time, I found the ants pa.s.sing unconcernedly by and over the beads which lay where I had strewed them in apparently undiminished quant.i.ties; and I conclude from this that they had found out their mistake, and had wisely returned to their accustomed occupations.

When the grain is thus taken into the nest, it is stored in regular granaries, but not until it has been denuded of its 'husks' or 'chaff.'

The denuding process is carried on below ground, and the chaff is brought up to the surface, where it is laid in heaps to be blown away by the wind.

It is a remarkable thing, and one not yet understood, why the seed, when thus stored in subterranean chambers just far enough below the surface to favour germination, does not germinate. Moggridge says that out of twenty-one nests and among many thousands of seeds that he examined, he only found twenty-seven cases of incipient germination. Moreover, all these cases occurred in months from November to February, while in the nests opened in October, March, April, and May, no sprouted seeds were discovered, though these are the months highly favourable to germination. He is at a loss to suggest the treatment to which the ants expose the seeds in order to prevent their sprouting. 'Apparently it is not that moisture or warmth or the influence of atmospheric air is denied to the seeds, for we find them in damp soil in genial weather, and often at but a trifling distance below the surface of the ground;'

and he has proved that the vitality of the seeds is not impaired, for he succeeded in raising crops of young plants from seeds removed from the granaries.

He also says,--

By a fortunate chance I have been able to prove that the seeds will germinate in an undisturbed granary when the ants are prevented from obtaining access to it: and this goes to show not only that the structure and nature of the granary chamber is not sufficient of itself to prevent germination, but also that the presence of the ants is essential to secure the dormant condition of the seeds.

I discovered in two places portions of distinct nests of _Atta structor_ which had been isolated owing to the destruction of the hollow wall behind which they lay, and then the granaries well filled up and literally choked with growing seeds, though the earth in which they lay completely enclosed and concealed them until by chance I laid them bare. In one case I knew that the destruction of the wall had only taken place ten days before, so that the seeds had sprouted in the interval.

My experiments also tend to confirm this, and to favour the belief that the non-germination of the seeds is due to some direct influence voluntarily exercised by the ants, and not merely to the conditions found in the nest, or to acid vapours which in certain cases are given off by the ants themselves.

These experiments consisted in confining a large number of harvesting ants with their queen and larvae in a gla.s.s test-tube partly filled with damp soil and various seeds, the whole being closed with a cork in the mouth of the test-tube. Under these circ.u.mstances the seeds all sprouted, showing that mere confinement in an atmosphere of exhalations from the ants did not prevent germination. Another series of experiments, undertaken at the suggestion of Mr. Darwin, on the effects of an atmosphere of formic acid, showed that although this vapour was very injurious to the seeds, it did not prevent their incipient germination. Therefore it yet remains to be ascertained why the seeds do not germinate in the granaries of the ants.

But in whatever way the ants manage to prevent germination, it is certain that they are aware of the importance in this connection of keeping the seeds as dry as possible; for Moggridge repeatedly observed that when the seeds which had been stored proved over-moist, the ants again took them out and spread them in the sun to dry, to be again brought into the nest after a sufficient exposure.

Lastly, he also repeatedly observed the most surprising and interesting fact that when, as we have seen was occasionally the case, the seeds did begin to germinate in the nests, the ants knew the most effective method of preventing the germination from proceeding; for he found that in these cases the ants gnawed off the tips of the radicles. This fact deserves to be considered as one of the most remarkable among the many remarkable facts of ant-psychology.

Pa.s.sing on now to the harvesting or agricultural ants of Texas, attention was first called to the habits of this insect by Mr. Buckley in 1860,[40] and by Dr. Lincec.u.m, who sent an account of his observations to Mr. Darwin, by whom they were communicated to the Linnaean Society in 1861. Five years later a paper was published in the Proceedings of the Academy of Natural Sciences of Philadelphia from the MS. of Dr.

Lincec.u.m. Lastly, in 1877 Mr. MacCook went to Texas expressly to study the habits of these insects, and he has recently embodied the results of his observations in a book of three hundred pages.[41] These observations are for the most part confirmatory of those of Lincec.u.m, and for this as well as for reasons to be deduced from the work itself, they deserve to be accepted as trustworthy, notwithstanding that in some cases they are provokingly incomplete. The following is an epitome of these observations.

The ants clear away all the herbage above their nest in the form of a perfect circle, or 'disk,' 15 or 20 feet in diameter, by carefully felling every stalk of gra.s.s or weed that may be growing thereon. As the nests are placed in thickly grown localities, the effect of these bald or shaven disks is highly conspicuous and peculiar, exactly resembling in miniature the clearings which the settlers make in the American backwoods. The disk, however, is not merely cleared of herbage, but also carefully levelled, all inequalities of the surface being reduced by building pellets of soil into the hollows to an extent sufficient to make a uniformly flat surface. The action of rain and the constant motion of mult.i.tudes of ants cause this flat surface to become hard and smooth. In the centre of the disk is the gateway of the nest. This may be either a simple hole or a hollow cone.