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INTELLIGENCE OF ANTS.
I HAVE now presented some of the most curious and interesting facts concerning the intelligence of ants in general; I shall next proceed to state some of the more remarkable facts concerning the intelligence of certain species of ants in particular.
Leaf-cutting Ants of the Amazon.—The mode of working practised by these ants is thus described by Bates :
They mount a tree in multitudes. . . . Each one places itself on the surface of a leaf, and cuts with its sharp scissors-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 accumulates, 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.
Other observers have since said that this herbage is regularly felled by the ants in order to make a road. Each ant carries its semicircular piece of leaf upright over its head, so that the home-returning train is rendered very conspicuous. Keener observation shows that this home-returning, or load-carrying, train of workers keeps to one side of the road, while the outgoing, or empty-handed, train keeps to the other side; so that on every road there is a double train of ants going in opposite directions. When the leaves arrive at the nest they are received by a smaller kind of worker, whose duty it is to cut up the pieces into still smaller fragments, whereby the leaves seem to be better fitted for the purpose to which, as we shall presently see, they are put. These smaller workers never take any part in the outdoor labour; but they occasionally leave the nest, apparently for the sole purpose of obtaining air and exercise, for when they leave the nest they merely run about doing nothing, and frequently, as in mere sport, mount some of the semicircular pieces of leaf, which the carrier ants are taking to the nest, and so get a ride home.
From his continued observation of these ants Bates concludesand his opinion has been corroborated by that both of Belt and Müller-that the object of all this labour is a highly remarkable one. The leaves when gathered do not themselves appear to be of any service to the ants as food; but when cut into small fragments, and stored away in the nests, they become suited as a nidus for the growth of a minute kind of fungus on which the ants feed. We may therefore call these insects' gardening ants,' inasmuch as all their 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-orange peel, certain flowers, &c., being equally acceptable to them. But they are very particular regarding the ventilation of their underground store-houses, on a suitable degree of which the successful growth of the fungus presumably depends. They therefore have numerous holes or ventilating shafts which lead up to the surface from the store-houses or underground gardens, and these they either open or close according to the horticultural requirements as regards temperature and moisture. If the leaves are either too damp or too dry, they will not grow the fungus, and therefore in gathering the leaves the ants are very particular that they should neither be the one nor the other. Thus Bates observed :
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 conreyed 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.
Dr. Ellendorf made the experiment of interrupting the advance of a column of these ants, with the interesting result which he thus describes in a letter to Büchner:
Thick dry grass stood on either side of their narrow road, so that they could not pass 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 pass 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 was astonished to see 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
its burden again, and the march was resumed in the most perfect order.
The operations here described show clearly that these ants act upon the principle of the division of labour. In this connection I may also quote an observation of Belt, which shows this fact in perhaps even a stronger light. He says :
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.
Ants of this genus are very clever at making tunnels. The Rev. H. Clark says that in one case they have made a tunnel of enormous length under the river Parahylia, where this is as broad as the Thames at London—their object being to reach a store-house which is on the opposite bank. This statement is not to be considered so incredible as it at first sight unquestionably appears, for Bates has seen the subterranean passages of these ants extending to a distance of seventy yards.
Harvesting Ants.—The harvesting ants belong almost exclusively to a single genus, which however comprises a number of species distributed in localised areas over all the four quarters of the globe. Their distinctive habits consist in gathering nutritious seeds of grasses during summer, and storing them in granaries for winter consumption. We owe our present knowledge concerning these insects mainly to Moggridge, who studied them in the South of Europe, Lincecum and M'Cook, who studied them in Texas ; Colonel Sykes and Dr. Jerdon also made some observations upon them in India. They likewise occur in Palestine, where they were clearly known to Solomon and other writers of antiquity, whose claim to accurate observation in this matter has within the last few years been amply vindicated, after having been for many years discredited, on account chiefly of the adverse statements of Huber.
Moggridge found that from the nest in various directions there proceed outgoing trains, which may be thirty or more yards in length, and each consisting of a double row of ants moving in opposite directions. Like the leaf-cutting ants, those composing the outgoing train are empty-handed, while those composing the incoming train are laden. But here the burdens are grass-seeds. At their terminations in the foraging ground, or ant-fields, the insects composing these columns disperse by hundreds among the seed-yielding grasses. They then ascend the stems of the grasses, and, seizing the seed or capsule in their jaws, fix their hind legs firmly as a pivot, round which they turn and turn till the stalk is twisted off. The ant then descends the stem, 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 to the nest. . . . Two ants sometimes combine their efforts, when one stations itself near the base of the peduncle, and graws it at the point of
greatest tension, while the other hauls upon and twists it. ... 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 Ælian of the manner in which the spikelets of corn are severed and thrown down to the people below.'
As further evidence that these insects well understand the advantages arising from the division of labour, I may quote one or two other observations. Thus Moggridge once saw a dead grasshopper carried into a nest of harvesting ants by the following means :
It was too large to pass 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 pulling it in.
Again, Lespis says of the harvesting ant that,
if the road from the place where they are gathering their harvest to the nest is very long, they make regular depóts for their provisions under large leaves, stones, or other suitable places, and let certain workers have the duty of carrying them from depôt to depôt.
No less, therefore, than the leaf-cutting ants already described, do these harvesting ants appreciate the benefits arising from the division of labour; and, as we shall presently see, there is a kind of ant exhibiting widely different habits, which shows appreciation of this principle in an even higher degree.
When the grain is taken into their nest by the harvesters, it is stored in regular granaries, but not until it has been denuded of its husks’ or chaff. The denuding process, which corresponds to threshing, is carried on below ground, and the chaff is brought up to the surface, where it is laid in heaps to be blown away by the wind. It is not yet understood why the seed, when thus stored in subterranean chambers just far enough below the surface to favour germination, does not germinate. Moggridge proved that the vitality of the seeds is not impaired, for he grew some plants from seeds taken from the granaries; and he also found that the seeds would germinate even in the granaries, if the ants were prevented from obtaining access to them for two or three days. The non-germination of the seeds must, therefore, be due to some influence exerted by the ants. Moggridge thought this influence might be the exhalations from the ants, and so tried enclosing some seeds in a bottled test-tube, containing also earth and ants. The seeds, however, sprouted; and even an atmosphere of formic acid vapour was found not to prevent germination. Probably, therefore, the ants in their granaries do something to the seeds for the express purpose of preventing germination; and, if so, it would be interesting to botanists to ascertain what this process
But, be this as it may, there is no doubt that the ants are fully aware of the importance in this connection of keeping their garnered seeds as dry as possible ; for when the latter prove over-moist after collection, or have been subsequently wetted by soaking rains, the insects bring them up to the surface and spread them out to dry, to be again brought into the nest after a sufficient exposure.
Lastly, Moggridge observed that the process, whatever it is, whereby the ants prevent germination, is not invariably successful, but that a small percentage of stored seeds sometimes do begin to germinate. When this was the case, he also observed the highly interesting fact that the ants then knew the most effective method of checking further germination, for he found that in these cases they gnawed off the tip of the sprouting radicle. This fact deserves to be considered as one of the most remarkable among
remarkable facts of ant psychology.
Passing on now to the harvesting ants of the New World, the insects here remove all the herbage above their nest in the form of a perfect circle, or “disk, fifteen to twenty feet in diameter. Every grass or weed within the disk is carefully felled, and, as the nests are situated in thickly-grown localities, the effect of the bald or shaven disk is highly conspicuous and peculiar, exactly resembling in miniature the clearings' which are made by settlers in the backwoods. The disk, however, is not merely cleared of herbage, but also carefully levelled-all inequalities of the surface being reduced by pellets of soil being built into the hollows to an extent sufficient to make a uniformly flat surface. In the centre of the disk is the gateway of the nest. From the disk in various directions there radiate out-roads or avenues, which are cleared and smoothed like the disk. These roads course through the thick grass, branching and narrowing as they go, till they eventually taper away. They are usually four to seven inches wide at their origin, and may be from 60 to 300 feet in length. Along these roads there is always passing during the daytime a constant double stream of ants, one being laden and the other not.
In their manner of gathering and garnering grain these harvesters resemble in general the harvesters of Europe; but it is alleged by Dr. Lincecum that in one respect their habits manifest an astonishing, and indeed well-nigh incredible, advance upon those of their European allies. For this observer, who, it must be remembered, was the first to call attention to these ants in the New World, and whose other observations, extending over a number of years, have since been fully confirmed—this observer states in the most positive terms that the ants actually sow the seeds of a certain plant called the ant-rice, for the purpose of subsequently reaping a harvest of grain; hence these ants have been called the 'agricultural ants. Now there is no doubt, from the subsequent observations of M'Cook and others, that the antdisks do very frequently support this peculiar kind of grass, and that the ants are particularly fond of its seed. Nevertheless, M'Cook did