In tropical and subtropical America there are about one hundred species and varieties of ants which have most extraordinary habits, and are grouped together in the Myrmicine tribe Attii. These ants are usually rather small and dull colored, and, while they are powerful and industrious diggers, are not given to rapid movements as most ants are, but walk slowly and sedately about. When picked up they do not struggle as many other ants do, but feign death after the manner of certain well known beetles.

It was long noted that the Attii carried great quantities of leaves into their nests, and there was considerable doubt as to the use to which these were put, some observers believing that they were used immediately as food, and others contending that they served as roofing and carpets in the underground passageways. Belt, a naturalist who lived in Nicaragua, was probably the first to discover the secret of the leaves. Digging into one of the nests in his garden, he was surprised to find no great quantity of leaves in any of the passages, although ants were continually bringing them in at the entrance. The chambers were always partly filled with “a speckled, brown, flocculent, spongy-looking mass of a light and loosely connected substance.... This mass, which I have called the ant-food, proved on examination to be composed of minutely subdivided pieces of leaves, weathered to a brown color, and overgrown and lightly connected together by a minute white fungus that ramified in every direction throughout it.... When a nest is disturbed and the masses of ant-food spread about, the ants are in great concern to carry away every morsel of it under shelter again; and sometimes, when I dug into the 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.”

Further investigation brought Belt to the conclusion that the Attii do not eat leaves at all, but use them as manure to grow fungus on; and further, that they feed upon this fungus, and will eat nothing else. The Attii are, in Belt’s own phrase, “mushroom growers and eaters.” While leaves are the chief fertilizer, other substances are often found suitable for growing fungus on; flowers are sometimes used, and some species are particularly partial to pieces of orange peel. The temperature and ventilation of the subterranean gardens are matters of great importance, and there are many small holes which connect the larger chambers with the surface. These air-shafts are plugged and reopened at intervals, and by this means the temperature and ventilation are regulated.

Alfred Moeller was a naturalist who studied the Attii in Brazil, and published the results of his labors in 1893. He found that the gardens contain only one kind of fungus, all alien spores being carefully weeded out. The ants do not allow the fruits to develop, and this has made the classification of the fungi a very difficult matter. The fungi found in the Attii nests are different from any others known, but no one can tell whether they are really distinct species or merely modified forms of certain common moulds or mushrooms.

Von Ihering, in 1898, discovered that the virgin queen, when leaving the nest on her nuptial flight, always carries a little pellet of fungus in her mouth. After being fertilized by the male the queen shuts herself up in a little burrow and sets about the founding of a new colony. There are in this case no leaves available, and she starts the fungus growing upon some of her new-laid eggs, which she crushes for the purpose, and which seem to work quite as well as the usual vegetable fertilizer.

J. Huber, in 1905, studied the same problems which interested Von Ihering, and concluded that the fungus is not grown upon crushed eggs, but is nourished by the liquid excrement of the queen. He describes his observations as follows: “After watching the ant for hours she will be seen suddenly to tear a little piece of the fungus from the garden with her mandibles and hold it against the tip of her abdomen, which is bent forward for this purpose. At the same time she emits from her vent a clear yellowish or brownish droplet which is at once absorbed by the tuft of hyphae. Hereupon the tuft is again inserted, amid much feeling about with the antennae, in the garden, but usually not in the same spot from which it was taken, and is then patted into place by means of the fore feet.... According to my observations, this performance is repeated usually once or twice an hour, and sometimes, to be sure, even more frequently.” Although, according to Huber, the eggs are not used directly as fertilizer for the fungus, the same result is brought about indirectly, as the female is accustomed to feed upon her own new-laid eggs. Huber estimates that nine out of every ten eggs laid are eaten at once by the mother. The young larvae, too, are fed with eggs thrust directly into their mouths by the queen. When the adult workers appear, however, they live exclusively on the fungus which has been growing during their larval life, and feed the queen upon fungus also, while continuing to supply the larvae with their mother’s eggs. After a week or so the workers dig their way out of the chamber, bring in leaf-manure for the garden, and the fungus is no longer cared for by the queen, who now gives all her attention to the serious business of egg-laying. As the fungus becomes more abundant under this cultivation it is fed to the larvae also, and eggs are no longer used as food by any of the individuals in the hive.

The extraordinary habits of the Attine ants have fascinated many students, and a number of theories about their development have been advanced. Forel suggested that the ancestors of the present mushroom-growers must have lived in rotten wood, and fed upon the fungus which grew upon the moist walls of their nests, or upon insect excrement. Von Ihering thinks that they may have developed from the harvesting ants, which gradually acquired such an appetite for the fungus which happened to grow in their granaries that the original stores came to be used only as fertilizer. Wheeler points out that, besides the Attine ants, there are several kinds of beetles and termites which cultivate fungus upon their own excrement, and suggests that originally this was the method employed by the ants. Later on they came to use the excrement of other insects, and finally passed to the addition of leaves and other non-fecal vegetable matter.

As has been said above, the Attii are primarily tropical and subtropical insects, but a few species have come north into the United States. They are found chiefly in peninsular Florida, in southern Texas, and in Arizona, although one species has been reported as far north as southern New Jersey.