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orange (Citrus aurantium) and the citron (Citrus medicus), on the other hand, are only found where they have been planted and protected by man; and, were he to give up their cultivation, the only species that would ultimately withstand the attacks of the ants, and obtain a permanent footing in Central America, would be the lime. The reason why the lime is not so subject to the attacks of the ants is unknown; and the fact that it is so is another instance of how little we know why one species of a particular genus should prevail over another nearly similar form. A little more or less acridity, or a slight chemical difference in the composition of the tissues of a leaf, so small that it is inappreciable to our senses, may be sufficient to ensure the preservation or the destruction of a species throughout an entire continent.
The ravages of this ant are so great that it may not be without interest for me to enter upon some details respecting the means I took to protect my own garden against their attacks, especially as the continual warfare I waged against them for more than four years made me acquainted with much of their wonderful economy.
In June 1869, very soon after the formation of my garden, the leaf-cutting ants came down upon it, and at once commenced denuding the young bananas, orange and mango trees of their leaves. I followed up the paths of the invading hosts to their nest, which was about one hundred yards distant, close to the edge of the forest. The nest was not a very large one, the low mound of earth covering it being about four yards in diameter. At first I tried to stop the holes up, but fresh ones were immediately opened out: I then dug down below the mound, and laid bare the chambers beneath, filled with
ant-food and young ants in every stage of growth; but I soon found that the underground ramifications extended so far, and to so great a depth, while the ants were continually at work making fresh excavations, that it would be an immense task to eradicate them by such means; and notwithstanding all the digging I had done the first day, I found them the next as busily at work as ever at my garden, which they were rapidly defoliating. At this stage, our medical officer, Dr. J. H. Simpson,* came to my assistance, and suggested pouring carbolic acid, mixed with water, down their burrows. The suggestion proved a most valuable one. We had a quantity of common brown carbolic acid, about a pint of which I mixed with four buckets of water, and, after stirring it well about, poured it down the burrows; I could hear it rumbling down to the lowest depths of the formicarium four or five feet from the surface. The effect was all that I could have wished: the marauding parties were at once drawn off from my garden to meet the new danger at home. The whole formicarium was disorganised. Big fellows came stalking up from the cavernous regions below, only to descend again in the utmost perplexity.
Next day I found them busily employed bringing up the ant-food from the old burrows, and carrying it to a new one a few yards distant; and here I first noticed a wonderful instance of their reasoning powers. Between the old burrows and the new one was a steep slope.
* This gentleman, beloved by all who knew him, of rare talent, and with every prospect of a prosperous career before him, died at Jamaica from hydrophobia, between two and three months after being bitten by a small dog that had not itself shown any symptoms of that disease.
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. They also brought out great numbers of dead ants that the fumes of the carbolic acid had killed. A few days afterwards, when I visited the locality again, I found both the old burrows and the new one entirely deserted, and I thought they had died off; but subsequent events convinced me that the survivors had only moved away to a greater distance.
It was fully twelve months before my garden was again invaded. I had then a number of rose-trees and also cabbages growing, which the ants seemed to prefer to everything else. The rose-trees were soon defoliated, and great havoc was made amongst the cabbages. I followed them to their nest, and found it about two hundred yards from the one of the year before. I poured down the burrows, as before, several buckets of water with carbolic acid. The water is required to carry the acid down to the lowest chambers. The ants, as before, were at once withdrawn from my garden; and two days afterwards, on visiting the place, I found all the survivors at work on one track that led directly to the old nest of the year before, where they were busily employed making fresh excavations. Many were bringing along pieces of the ant-food from the old to the new nests; others carried the undeveloped white pupa and larvæ. It was a wholesale and entire migration; and the next day the formicarium down which I had last
poured the carbolic acid was entirely deserted. I afterwards found that when much disturbed, and many of the ants destroyed, the survivors migrate to a new locality. I do not doubt that some of the leading minds in this formicarium recollected the nest of the year before, and directed the migration to it.
Don Francisco Velasquez informed me, in 1870, that he had a powder which made the ants mad, so that they bit and destroyed each other. He gave me a little of it, and it proved to be corrosive sublimate. I made several trials of it, and found it most efficacious in turning a large column of the ants. A little of it sprinkled across one of their paths in dry weather has a most surprising effect. As soon as one of the ants touches the white powder, it commences to run about wildly, and to attack any other ant it comes across. In a couple of hours, round balls of the ants will be found all biting each other; and numerous individuals will be seen bitten completely in two, whilst others have lost some of their legs or antennæ. News of the commotion is carried to the formicarium, and huge fellows, measuring three-quarters of an inch in length, that only come out of the nest during a migration or an attack on the nest or one of the working columns, are seen stalking down with a determined air, as if they would soon right matters. As soon, however, as they have touched the sublimate, all their stateliness leaves them : they rush about; their legs are seized hold of by some of the smaller ants already affected by the poison; and they themselves begin to bite, and in a short time become the centres of fresh balls of rabid ants. The sublimate can only be used effectively in dry weather. At Colon I found the Americans using coal
tar, which they spread across their paths when any of them led to their gardens. I was also told that the Indians prevent them from ascending young trees by tying thick wisps of grass, with the sharp points downwards, round the stems. The ants cannot pass through the wisp, and do not find out how to surmount it, getting confused amongst the numberless blades, all leading downwards. I mention these different plans of meeting and frustrating the attacks of the ants at some length, as they are one of the greatest scourges of tropical America, and it has been too readily supposed that their attacks cannot be warded off. I myself was enabled, by using some of the means mentioned above, to cultivate successfully trees and vegetables of which the ants were extremely fond.
Notwithstanding that these ants are so common throughout tropical America, and have excited the attention of nearly every traveller, there still remains much doubt as to the use to which the leaves are put. Some naturalists have supposed that they use them directly as food; others, that they roof their underground nests with them. I believe the real use they make of them is as a manure, on which grows a minute species of fungus, on which they feed;—that they are, in reality, mushroom growers and eaters. This explanation is so extraordinary and unexpected, that I may be permitted to enter somewhat at length on the facts that led me to adopt it. When I first began my warfare against the ants that attacked my garden, I dug down deeply into some of their nests. In our mining operations we also, on two occasions, carried our excavations from below up through very large formicariums, so that