Supplement to Harvesting Ants and Trap-Door Spiders

by John Traherne Moggridge; Octavius Pickard-Cambridge

I discovered in two places portions of distinct nests of Atta structor which had been isolated owing to the destruction of the terrace-wall behind which they lay, and there the granaries were 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 this 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.

In order to put this latter point to the test of experiment, I confined about a hundred harvesting ants (A. structor), with their queen and several larvæ, in a glass test-tube eight inches long and one inch in diameter, closed with a cork and filled up to within about an inch of the cork with damp sandy soil, most of which was taken from the ants' nest.

I added six peas, six cress and six millet, and then kept the tube tightly corked for nine days, only once removing the cork for a few seconds in order to sprinkle a little water on the ants, which were evidently in need of it. On the ninth day I turned out the contents of the tube and found that all the peas, millet and cress, had germinated and were growing strongly. One of the cress, however, had had its root, which lay across the gallery constructed by the ants, gnawed off; four clover seeds, which had come with the soil taken from the nest, and which had formed part of the ants' stores, had germinated also. Here the small quantity of air contained in the test-tube must certainly have become saturated with any vapour which the ants may be supposed to give off, and we cannot therefore accept this as the cause of the dormant condition of the granary seeds.

I made other experiments in which harvesting ants were imprisoned along with various seeds in small, cylindrical, closed vessels containing a little damp sand. Here the vessels were frequently rolled from side to side or shaken, during the twenty-two hours for which the experiment lasted, so as to excite the ants and make them give off such odours as they possessed, but no trace of injurious influence was produced upon the seeds, which germinated and grew normally afterwards.

At Mr. Darwin's suggestion I made a long series of experiments with formic acid, in which measured quantities, pure or diluted, were placed in a watch-glass on damp sand and surrounded by seeds, the whole being enclosed in a covered tumbler, so that the effects produced on the seeds by the vapour rising from the acid might be noted. Similar seeds were sown at the same time and in the same way, but without the acid, so as to permit of comparison. These experiments have afforded some interesting results,[116] but do not supply any positive data which might help us to discover the secret of the ants. They narrow, indeed, the area in which search can profitably be made, indicating as they do that the vapour of formic acid is incapable of rendering the seeds dormant after the manner of the ants, and showing, on the contrary, that its influence is always injurious to the seeds, even when present only in excessively minute quantities.

[116] I hope shortly to offer these observations, together with another series of a similar nature in which my friend Mr. J. B. Andrews has taken part, to the Linnean Society.

It appears to me now that the most promising field for experiments made with a view to clearing up this difficulty, is that afforded by the closer investigation of the phenomena of normal germination, and by a study of the conditions under which seeds remain dormant, as they are occasionally known to do, in situations which our general experience would have selected as favourable to germination.

I have good hopes, also, that when we come to know more of the habits of harvesting ants in tropical countries, and when naturalists have excavated and described their subterranean stores—a thing which has not yet been done as far as I know—we may gather fresh indications to guide us in our search.

I am puzzled to account for the fact, which I have seen stated by more than one observer in India, that the ants there have a habit of bringing out large quantities of grain and seed and laying them in heaps outside their nests at the commencement of the wet season. Dr. King, the director of the Royal Botanic Gardens at Calcutta, has told me that when in the Gwalior territory during the beginning of the rainy season, he saw heaps of seeds, principally those of a leguminous plant (Alyssocarpus), piled up round the entrances to the ants' nests, and that it was precisely at that time that flocks of a rock-grouse (Pterocles exustus) first made their appearance. They fed freely upon the seeds, and Dr. King found the crops of some of these birds, which he had shot, filled with them.