Animal Intelligence Part 9

Concerning the other habits of this species, the same author writes:--

The eyes in the Ecitons are very small, in some of the species imperfect, and in others entirely absent; in this they differ greatly from the _Pseudomyrma_ ants, which hunt singly and which have the eyes greatly developed. The imperfection of eyesight in the Ecitons is an advantage to the community, and to their particular mode of hunting. It keeps them together, and prevents individual ants from starting off alone after objects that, if their eyesight was better, they might discover at a distance; the Ecitons and most other ants follow each other by scent, and, I believe, they can communicate the presence of danger, of booty, or other intelligence, to a distance by the different intensity or qualities of the odours given off. I one day saw a column of _Eciton hamata_ running along the foot of a nearly perpendicular tramway cutting, the side of which was about six feet high. At one point I noticed a sort of a.s.sembly of about a dozen individuals that appeared in consultation. Suddenly one ant left the conclave, and ran with great speed up the perpendicular face of the cutting without stopping. It was followed by others, which, however, did not keep straight on like the first, but ran a short way, then returned, then again followed a little further than the first time. They were evidently scenting the trail of the pioneer, and making it permanently recognisable. These ants followed the exact line taken by the first one, although it was far out of sight. Wherever it had made a slight _detour_ they did so likewise. I sc.r.a.ped with my knife a small portion of the clay on the trail, and the ants were completely at fault for a time which way to go. Those ascending and those descending stopped at the sc.r.a.ped portion, and made short circuits until they hit the scented trail again, when all their hesitation vanished, and they ran up and down it with the greatest confidence. On gaining the top of the cutting, the ants entered some brushwood suitable for hunting. In a very short s.p.a.ce of time the information was communicated to the ants below, and a dense column rushed up to search for their prey. The Ecitons are singular amongst the ants in this respect, that they have no fixed habitations, but move on from one place to another, as they exhaust the hunting grounds around them. I think _Eciton hamata_ does not stay more than four or five days in one place. I have sometimes come across the migratory columns; they may easily be known. Here and there one of the light-coloured officers moves backwards and forwards directing the columns. Such a column is of enormous length, and contains many thousands if not millions of individuals. I have sometimes followed them up for two or three hundred yards without getting to the end.

They make their temporary habitations in hollow trees, and sometimes underneath large fallen trunks that offer suitable hollows. A nest that I came across in the latter situation was open at one side. The ants were cl.u.s.tered together in a dense ma.s.s, like a great swarm of bees, hanging from the roof but reaching to the ground below. Their innumerable long legs looked like brown threads binding together the ma.s.s, which must have been at least a cubic yard in bulk, and contained hundreds of thousands of individuals, although many columns were outside, some bringing in the pupae of ants, others the legs and dissected bodies of various insects. I was surprised to see in this living nest tubular pa.s.sages leading down to the centre of the ma.s.s, kept open just as if it had been formed of inorganic materials. Down these holes the ants who were bringing in booty pa.s.sed with their prey. I thrust a long stick down to the centre of the cl.u.s.ter, and brought out clinging to it many ants holding larvae and pupae, which probably were kept warm by the crowding together of the ants. Besides the common dark-coloured workers and light-coloured officers, I saw here many still larger individuals with enormous jaws. These they go about holding wide open in a threatening manner.

It was this ant which, as previously stated, showed sympathy and fellow-feeling with companions in difficulties.

The habits of _E. drepanophora_ are closely similar to those of the species already described; and, indeed, except in matters of detail, all the species of Ecitons have much the same habits. Mr. Bates records an interesting observation which he made on one of the moving columns of this species. He says: 'When I interfered with the column or abstracted an individual from it, news of the disturbance was quickly communicated to a distance of several yards to the rear, and the column at that point commenced retreating.' The main column is in this species narrower, viz., 'from four to six deep,' but extends to a great length, viz., half a mile or more. It was this species of Eciton that the same naturalist describes as enjoying periods of leisure and recreation in the 'sunny nooks of the forest.'

Next we have to consider _E. praedator_, of which the same observer writes:--

This is a small dark reddish species, very similar to the common red stinging ant of England. It differs from all other Ecitons in its habit of hunting, not in columns, but in dense phalanxes consisting of myriads of individuals, and was first met with at Ega, where it is very common. Nothing in insect movements is more striking than the rapid march of these large and compact bodies. Wherever they pa.s.s, all the rest of the animal world is thrown into a state of alarm. They stream along the ground and climb to the summits of all the lower trees, searching every leaf to its apex, and whenever they encounter a ma.s.s of decaying vegetable matter, where booty is plentiful, they concentrate, like other Ecitons, all their forces upon it, the dense phalanx of s.h.i.+ning and quickly-moving bodies, as it spreads over the surface, looking like a flood of dark red liquid. They soon penetrate every part of the confused heap, and then, gathering together again in marching order, onward they move.

All soft-bodied and inactive insects fall an easy prey to them, and, like other Ecitons, they tear their victims in pieces for facility of carriage. A phalanx of this species, when pa.s.sing over a tract of smooth ground, occupies a s.p.a.ce of from four to six square yards; on examining the ants closely they are seen to move, not all together in one straightforward direction, but in variously spreading contiguous columns, now separating a little from the general ma.s.s, now reuniting with it. The margins of the phalanx spread out at times like a cloud of skirmishers from the flanks of an army. I was never able to find the hive of this species.

Lastly, there are two species of Eciton totally blind, and their habits differ from those of the species which we have hitherto considered.

Bates writes of them:--

The armies of _E. vastator_ and _E. erratica_ move, as far as I could learn, wholly under covered roads, the ants constructing them gradually but rapidly as they advance. The column of foragers pushes forward step by step, under the protection of these covered pa.s.sages, through the thickets, and on reaching a rotting log, or other promising hunting-ground, pour into the crevices in search of booty. I have traced their arcades, occasionally, for a distance of one or two hundred yards; the grains of earth are taken from the soil over which the column is pa.s.sing, and are fitted together without cement. It is this last-mentioned feature that distinguishes them from the similar covered roads made by termites, who use their glutinous saliva to cement the grains together. The blind Ecitons, working in numbers, build up simultaneously the sides of their convex arcades, and contrive, in a surprising manner, to approximate them and fit in the key-stones without letting the loose uncemented structure fall to pieces. There was a very clear division of labour between the two cla.s.ses of neuters in these blind species. The large-headed cla.s.s, although not possessing monstrously lengthened jaws like the worker-majors in _E. hamata_ and _E.

drepanophora_, are rigidly defined in structure from the small-headed cla.s.s, and act as soldiers, defending the working community (like soldier termites) against all comers. Whenever I made a breach in one of their covered ways, all the ants underneath were set in commotion, but the worker-minors remained behind to repair the damage, whilst the large-heads issued forth in a most menacing manner, rearing their heads and snapping their jaws with an expression of the fiercest rage and defiance.

_Annornia arcens._--This is the so-called 'driver' or 'marching' ant of West Africa, which in habits and intelligence closely resembles the military ants of the other hemisphere. I shall therefore not wait again to describe these habits in detail. Like the Ecitons, the marching ants of Africa have no fixed nest, but make temporary halts in the shade of hollow trees, overhanging rocks, &c. They march in large armies, and, like the Ecitons, always in the form of a long close column; but in this case the relative position of the carriers of spoil and larvae is reversed, for while these occupy the middle place the soldiers and officers march on either side. These have large heads armed with powerful jaws, and never take part in carrying; their function is to maintain order, act as scouts, and attack prey. The habits of these ants resemble most closely those of the blind Ecitons in that they very frequently, and indeed generally, build covered ways; they do so apparently in order to protect themselves from the heat of the African sun. Their line of march is therefore marked by a continuous arch or tunnel, which is always being constructed by the van of the column. The structure is made of earth moulded together by saliva, and is very quickly built. But it is only built in places where the line of march is exposed to the sunlight; at night, or in the shadow of trees or long gra.s.s, it is not made. If their camp is flooded by a tropical rainstorm, the ants congregate in a close ma.s.s, with the younger ants in the centre; they thus form a floating island.

It is remarkable that ants of different hemispheres should manifest so close a similarity with respect to all these wonderful habits. The Cha.s.seur ants of Trinidad, and, according to Madame Merian, the ants of visitation of Cayenne, also display habits of the same kind.

_General Intelligence of Various Species._

Many of the foregoing facts display an astonis.h.i.+ng degree of intelligence as obtaining among ants; for I think that however much lat.i.tude we may be inclined to allow to 'blind instinct' in the way of imitating actions elsewhere due to conscious purpose, some at least of these foregoing facts can only be fairly reconciled with the view that the insects know what they are doing and why they are doing it. But as I am myself well aware of the difficulty that arises in all such cases of drawing the line between purposeless instinct and purposive intelligence, I have thought it desirable to reserve for this concluding division of the present chapter several isolated facts which have been observed among sundry species of ants, and which do not seem to admit of being reasonably comprised under the category of instinctive action, if by the latter we mean action pursued without knowledge of the relation between the means adopted and the ends attained.

It will be remembered that our test of instinctive as distinguished from truly intelligent action is simply whether all individuals of a species perform similar adaptive movements under the stimulus supplied by similar and habitual circ.u.mstances, or whether they manifest individual and peculiar adaptive movements to meet the exigencies of novel and peculiar circ.u.mstances. The importance of this distinction may be rendered manifest by the following ill.u.s.trations.

[Ill.u.s.tration: Fig. 8.]

We have already seen that the ants which Sir John Lubbock observed display many and complex instincts, which together might seem to justify us in antic.i.p.ating that animals which present such wonderful instincts must also present sufficient general intelligence to meet simple though novel exigencies by such simple adaptations as the unfamiliar circ.u.mstances require. Yet experiments which he made in this connection seem to show that such is not the case, but that these ants, with all their wealth of instinctive endowments, are utterly dest.i.tute of intelligent resources; they have abundance of common and detailed knowledge (supposing the adaptations to be made consciously) how to act under certain complex though familiar circ.u.mstances, but appear quite unable to originate any adaptive action to obviate even the simplest conceivable difficulty, if this is of a kind which they have not been previously accustomed to meet. Thus, on a horizontal rod B supported in a saucer of water S, and therefore inaccessible to the ants from beneath, he placed some larvae A. On the nest N he then placed a block of wood C D, constructed so that the portion D should touch the larvae at A. When the ants had made a number of journeys over C D A and back again, he raised the block C D so that there was an interval 3/10 of an inch between the end of the block D and the larvae at A.

The ants kept on coming, and tried hard to reach down from D to A, which was only just out of their reach... . After a while they all gave up their efforts and went away, losing their prize in spite of most earnest efforts, because it did not occur to them to drop 3/10 of an inch. At the moment when the separation was made there were fifteen ants on the larvae. These could, of course, have returned if one had stood still and allowed the others to get on its back.

This, however, did not occur to them; nor did they think of letting themselves drop from the bottom of the paper (P) on to the nest. Two or three, indeed, fell down, I have no doubt by accident; but the remainder wandered about, until at length most of them got into the water.

In another experiment he interposed a light straw bridge on the way between the nest and the larvae, and when the ants had well learnt the way, he drew the bridge a short distance towards the nest, so that a small chasm was made in the road. The ants tried hard and ineffectually to reach across it, but it did not occur to them to _push_ the straw into its original position.

The following experiment is still more ill.u.s.trative of the absence of intelligence, because the adjustive action required would not demand the exercise of such high powers of imagination and abstraction as would have been required for the moving forwards of the paper drawbridge.

To test their intelligence I made the following experiments: I suspended some honey over a nest of _Lasius flavus_ at a height of about 1/2 an inch, and accessible only by a paper bridge more than 10 feet long. Under the gla.s.s I then placed a small heap of earth. The ants soon swarmed over the earth on to the gla.s.s, and began feeding on the honey. I then removed a little of the earth, so that there was an interval of about 1/3 of an inch between the gla.s.s and the earth; but though the distance was so small, they would not jump down, but preferred to go round by the long bridge. They tried in vain to stretch up from the earth to the gla.s.s, which, however, was just out of their reach, though they could touch it with their antennae; but it did not occur to them to heap the earth up a little, though if they had moved only half a dozen particles of earth they would have secured for themselves direct access to the food. This, however, never occurred to them. At length they gave up all attempts to reach up to the gla.s.s, and went round by the paper bridge. I left the arrangement for several weeks, but they continued to go round by the long paper bridge.

Another and somewhat similar experiment consisted in placing an upright stick A, supporting at an angle another stick B, which nearly but not quite touched the ground at C. At the end of the stick B there were placed some larvae in a horizontal gla.s.s cell at D. Into this cell were also placed a number of ants along with the larvae. The drop from D to C was only 1/2 an inch; 'still, though the ants reached over and showed a great anxiety to take this short cut home, they none of them faced the leap, but all went round by the sticks, a distance of nearly 7 feet.'

Sir John then reduced the interruption to 2/5 of an inch, so that the ants could even touch the gla.s.s cell with their antennae; yet all day long the ants continued to go the long way round rather than face the drop. Next, therefore, he took still longer sticks and tapes, and arranged them as before, only horizontally instead of vertically. He also placed some fine earth under the gla.s.s cell containing the larvae.

The ants as before continued to go the long way round (16 feet), though the drop could not have hurt either themselves or the larvae, and though even this drop might have been obviated by heaping up the fine earth into a little mound 1/8 of an inch high, so as to touch the gla.s.s cell.

It is desirable, however, here to state that all species of ants do not show this aversion to allowing themselves to drop through short distances; for Moggridge describes the harvesting ants of Europe as seeming rather to enjoy acrobatic performances of this kind; and the same fact is recorded by Belt of the leaf-cutting ants of the Amazons.

Dr. Bastian, in his work on 'Brain as an Organ of Mind,' suggests that the 'seeming lack of intelligence betrayed by our English ants, from their disinclination to take a small leap, may be due simply to their defective sight' (pp. 241-2). But even this consideration does not extenuate the stupidity of the ants which failed to heap up the fine earth to reach the gla.s.s cell which they were able to touch with their antennae.

That the species of ants on which Sir John Lubbock experimented were not, however, quite dest.i.tute of intelligence is proved by the result of the following experiment:--

I put some provisions in a shallow box with a gla.s.s top and a single hole in one side; I then put some specimens of _Lasius niger_ to the food, and soon a stream of ants was at work busily carrying supplies off to the nest. When they had got to know their way thoroughly, and from thirty to forty were so occupied, I poured some fine mould in front of the hole, so as to cover it to a depth of about 1/2 an inch. I then took out the ants which were actually in the box. As soon as the ants had recovered from the shock of this unexpected proceeding on my part, they began to run all round 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 1/2 to 6 inches, until they had excavated down to the doorway, when they again began carrying off the food as before.

This experiment was several times repeated on _L. niger_ and on _L.

flavus_, always with the same result.

Thus, then, we may conclude that the reasoning power of these ants, although shown by the first experiments to be almost _nil_, is shown by this experiment to be not quite _nil_; for the attempt to meet the exigencies of the case by first going round the box to seek another entrance, before taking the labour to remove the earth from the known entrance, implies a certain rudimentary degree of adaptive capacity which belongs to the category of the rational.

Another point of considerable interest, as bearing on the general intelligence of ants, is one that was brought out as the result of a laborious series of hourly observations, extending without intermission from 6.30 A.M. to 10 P.M. for a period of three months. The object of these observations was to ascertain whether the principle of the division of labour is practised by the ants. The result of these observations was to show that during the winter-time, when the ants are not active, certain individuals are told off to forage for supplies, and that when any casualty overtakes these individuals, others are told off to supply their places. Thus, in the words of Sir John Lubbock's a.n.a.lysis of his lengthy tables,--

The feeders at the beginning of the experiment were those known to us as Nos. 5, 6, and 7. On the 22nd of November a friend, registered as No. 8, came to the honey, and again on the 11th December; but with these two exceptions the whole of the supplies were carried in by Nos. 5 and 6, with a little help from No. 7.

Thinking now it might be alleged that possibly these were merely unusually active or greedy individuals, I imprisoned No. 6 when she came out to feed on the 5th.

As will be seen from the table, no other ant had been out to the honey for some days; and it could therefore hardly be accidental that on that very evening another ant (then registered as No. 9) came out for food. This ant, as will be seen from the table, then took the place of No. 6 (No. 5 being imprisoned). On the 11th January No. 9 took in all the supplies, again with a little help from No. 7. So matters continued until the 17th, when I imprisoned No. 9, and then again, _i.e._ on the 19th, another ant (No. 10) came out for the food, aided, on and after the 22nd, by another (No.

11). This seems to me very curious. From the 1st November to the 5th January, with two or three casual exceptions, the whole of the supplies were carried in by three ants, one of whom, however, did comparatively little. The other two are imprisoned, and then, but not till then, a fresh ant appears on the scene. She carries in the food for a week, and then she being imprisoned, two others undertake the task. On the other hand, in nest 1, when the first foragers were not imprisoned, they continued during the whole time to carry in the necessary supplies.

The facts, therefore, certainly seem to indicate that certain ants are told off as foragers, and that during winter, when but little food is required, two or three such foragers are sufficient to provide it.

Although Sir John Lubbock's ants showed such meagre resources of intelligent adjustment, other species of ants, which we have already had occasion to consider, appear to be as remarkable in this respect as they are in respect of their instinctive adjustments. Unfortunately observations on this subject are very spa.r.s.e, but such as they are they hold out a strong inducement for any one who has the opportunity to experiment with the view of testing the intelligence of those species in connection with which the following observations have been made.

Reaumur states that ants will make no attempt to enter an inhabited beehive to get at the contained honey, knowing that the bees will slaughter them if they do so. But if the hive is uninhabited, or the bees all dead, the ants will swarm into the hive as long as any honey is to be found there.

P. Huber records that a wall which had been partly erected by ants was observed by him--

As though it were intended to support the still unfinished arched roof of a large room, which was being built from the opposite side. But the workers which had begun the arch had given it too low an elevation for the wall on which it was to rest, and if it had been continued on the same lines it would have met the part.i.tion wall halfway up, and this was to be avoided. I had just made this criticism to myself, when a new arrival, after looking at the work, came to the same conclusion. For it began at once to destroy what had been done, and to heighten the wall on which it was supported, and to make a new arch with the materials of the old one under my very eyes. When the ants begin an undertaking it seems exactly as if an idea slowly ripened into execution in their minds.

Thus if one of them finds two stalks lying crosswise on the nest, which make possible the formation of a room, or some little rafters which suggest the walls and the corners, it first observes the various parts accurately, and then quickly and neatly heaps little pellets of earth in the inters.p.a.ces and alongside the stalks. It brings from every side materials that seem appropriate, and sometimes takes such from the uncompleted works of its companions, so much is it urged on by the idea which it has once conceived, and by the desire to execute it. It goes and comes and turns back again, until its plan is recognisable by the others.

Ebrard, in his 'Etudes de Moeurs' (p. 3), gives the following remarkable instance of the display of intelligence of _F. fusca_:--

The earth was damp and the workers were in full swing. It was a constant coming and going of ants, coming forth from their underground dwelling, and carrying back little pellets of earth for building. In order to concentrate my attention I fixed my gaze on the largest of the rooms which were being built, wherein several ants were busy. The work had made considerable progress; but although a projection could be plainly seen along the upper edge of the wall, there remained an inters.p.a.ce of about twelve or fifteen millimetres to fill in. Here would have been the place, in order to support the earth still to be brought in, to have had recourse to those pillars, b.u.t.tresses, or fragments of dried leaves, which many ants are wont to use in building. But the use of this expedient is not customary with the ants I was observing (_F. fusca_). Our ants, however, were sufficient for the occasion. For a moment they seemed inclined to leave their work, but soon turned instead to a gra.s.s-plant growing near, the long narrow leaves of which ran close together. They chose the nearest, and weighted its distal end with damp earth, until its apex just bent down to the s.p.a.ce to be covered.

Unfortunately the bend was too close to the extremity, and it threatened to break. To prevent this misfortune, the ants gnawed at the base of the leaf until it bent along its whole length and covered the s.p.a.ce required. But as this did not seem to be quite enough, they heaped damp earth between the base of the plant and that of the leaf, until the latter was sufficiently bent. After they had thus attained their object, they heaped on the b.u.t.tressing leaf the materials required for building the arched roof.

The characteristic _trait_ of the building of ants, says Forel, is the almost complete absence of an unchangeable model, peculiar to each species, such as is found in wasps, bees, and others. The ants know how to suit their indeed little perfect work to circ.u.mstances, and to take advantage of each situation. Besides, each works for itself and on a given plan, and is only occasionally aided by others when these understand its plan. Naturally many collisions occur, and some destroy that which others have made. This also gives the key to understanding the labyrinth of the dwelling. For the rest, it is always those workers which have discovered the most advantageous method, or which have shown the most patience, which win over to their plan the majority of their comrades and at last the whole colony, although not without many fights for supremacy. But if one succeeds in obtaining a second to follow it, and this second draws the others after it, the first is soon lost again in the crowd.

Espinas also observed ('Thierischen Gesellschaften,' German translation, 1879, p. 371) that each single ant made its own plan and followed it until a comrade, which had caught the idea, joined it, and then they worked together in the execution of the same plan.

Moggridge says of the harvesters of Europe,--

I have observed on more than one occasion that when in digging into an ants' nest I have thrown out an _elater_ larva, the ants would cl.u.s.ter round it and direct it towards some small opening in the soil, which it would quickly enlarge and disappear down. At other times, however, the ants would take no notice of the _elater_, and it is my belief that the attentions paid to it on former occasions were purely selfish, and that they intended to avail themselves of the tunnel thus made down into the soil, with the view of reopening communications with the galleries and granaries concealed below, the approaches to which had been covered up. I have frequently watched the ants make use of these pa.s.sages mined by the _elater_ on these occasions.

And again, as showing apparently intelligent adaptation of their usual habits to altered circ.u.mstances, he gives an account of the behaviour of these ants when a great crowd of them were confined by him in a gla.s.s jar containing earth. He says:--

On the following morning the openings were ten in number, and the greatly increased heaps of excavated earth showed that they must probably have been at work all night. The amount of work done in this short time was truly surprising, for it must be remembered that, eighteen hours before, the earth presented a perfectly level surface, and the larvae and ants, now housed below, found themselves prisoners in a strange place, bounded by gla.s.s walls, and with no exit possible.

It seems to me that the ants displayed extraordinary intelligence in having thus at a moment's notice devised a plan by which the superabundant number of workers could be employed at one time without coming in one another's way. The soil contained in the jar was of course less than a tenth part of that comprised within the limits of an ordinary nest, while the number of workers was probably more than a third of the total number belonging to the colony. If therefore but one or two entrances had been pierced in the soil, the workers would have been for ever running against one another, and a great number could never have got below to help in the all-important task of preparing pa.s.sages and chambers for the accommodation of the larvae. These numerous and funnel-shaped entrances admitted of the simultaneous descent and ascent of large numbers of ants, and the work progressed with proportionate rapidity. After a few days only three entrances, and eventually only one, remained open.

Concerning the harvesting ant of Texas, the following quotation may be made, under the present head, from MacCook. After remarking that these ants always select sunny places wherein to build their nests, or disks, he goes on to say that within a few paces of his tent--