Certain genera—e.g. Aphaenogaster, Pheidole—exhibit great disparity in the forms of the workers, some of which are of size much superior to the others, and possess disproportionately large heads; these large individuals are found in the same nest as the smaller forms. All the intermediate forms may frequently be found, and at the same time, in the genus Aphaenogaster; but in Pheidole intermediates are of the utmost rarity.
The genus Cremastogaster is remarkable on account of the shape of the hind body and its articulation, which give the abdomen the appearance of being put on upside down. This mode of articulation may allow the Insect to threaten its enemies when they are in front of it; but it is doubtful whether the Cremastogaster possesses an effective sting.
Fig. 71—Cremastogaster tricolor, worker. A, with abdomen extended; B, uplifted.
ii. The group Attini is distinguished by the presence of a carina near the eye, by the antennae being inserted at a moderate distance from one another, by the clypeus being prolonged backwards between them; and by the absence of a sting. The group is not represented in Europe, but in Tropical America the ants belonging to it are amongst the most important of natural objects. The species of the genus Atta (usually styled Oecodoma) are the formidable leaf-cutting ants of America. They occur in enormous colonies in certain places, and will in a short time completely strip a tree of its leaves. As they appear to prefer trees of a useful kind, especially those planted by man, their ravages are often of the most serious nature. The natives, feeling it hopeless to contend with these Insect hordes, only too frequently abandon all attempts to cultivate the trees and vegetables the Insects are fond of. Both Bates and Belt have given accounts of some points in the economy of these ants. They are amongst the largest of the Formicidae, the females in some cases measuring about two and a half inches across their expanded wings; the males are much smaller, but are less dissimilar to their partners than is usual among ants. The workers, on the other hand, are so extremely different, that no one would suppose them to be at all related to the males and females (see vol. v. Fig. 339).
The mode of operation of these ants is to form paths from their formicary extending for a considerable distance in various directions, so that they have a ready access to any spot in a district of considerable extent; when a tree or bush is found bearing leaves suitable for their purposes, the worker ants ascend it in large numbers and cut up the leaves by biting out of them pieces similar in size and shape to a small coin; these pieces are then carried back in the jaws of the ants to their nests; the ant-paths are therefore constantly traversed by bands of little creatures carrying burdens homewards, or hurrying outwards in search of suitable trees.
The formicaries are of considerable size, and are described as consisting of low mounds of bare earth of considerable extent. Bates speaks of as great a circumference as forty yards; these accumulations of earth have frequently an appearance different from the adjoining soil, owing to their being formed of subsoil brought up from below; they are kept bare by the ants constantly bringing to and depositing on the surface fresh material resulting from their subterranean excavations. The true abodes, beneath the earth, are of greater extent than the mounds themselves, and extend to a considerable depth; they consist of chambers connected by galleries.
The leaf-cutting ants extend their range to North America, and M‘Cook has recently called attention to a case there in which A. fervens made an underground route at an average depth of 18 inches, and at an occasional depth of 6 feet, extending 448 feet entirely beneath the earth, after which it was continued for 185 feet to reach a tree which the ants were engaged in defoliating. This route, extending altogether to a length of more than 600 feet, presented only a very slight deviation from a straight line drawn between the point of departure and the object to be attained. By what sense this ant was enabled to make a subterranean tunnel in a straight line to a desired object situated at so great a distance, we know not.
The use the leaf-cutting ants make of the enormous amount of material they gather was for long a subject of debate, and has only recently been ascertained by the observations of Möller. After being carried to the nest the pieces of leaves are cut into small fragments by another set of workers and formed into balls, which are packed in various parts of the nest, and amongst which the mycelium of a fungus—Rozites gongylophora—ramifies. This fungus the ants cultivate in the most skilful manner: they manage to keep it clear from mouldiness and bacterial agents, and to make it produce a modified form of growth in the shape of little white masses, each one formed by an agglomeration of swellings of the mycelium. These form the chief food of the colony. Möller ascertained by experiment that the results were due to a true cultivation on the part of the ants: when they were taken away from the nests, the mycelium produced two kinds of conidia instead of the ant-food.
Many details of the economy of these leaf-cutting ants are still very imperfectly known. The large-headed forms, called soldiers, have been the subject of contradictory statements; Bates having concluded from his own observations that they are harmless, while Mr. J. H. Hart assures us that they are very fierce and vindictive, and inflict very serious wounds by biting (the Attini do not sting). We anticipate that the observations of both these naturalists will prove to be substantially correct, and that the differences in habits will be found to be owing to distinctions in the conditions of the community. In connection with this point we may remark that the function of the excessively large heads of certain kinds of soldier-ants is still obscure. In the East Indian Pheidologeton diversus the big soldiers are quite one hundred times as large as the smaller workers. As these latter bite viciously it would naturally be supposed that their gigantic confrères with enormous heads would be warriors of a most formidable nature; but, as a matter of fact, the giants are unable to bite even when they try to do so. Aitken has somewhere suggested that these enormous individuals play the part of state elephants; and we have been informed by Colonel Bingham that the small ants may frequently be seen riding in numbers on their unwieldy fellows. We anticipate however, that some other function will be found to exist for these forms with enormous heads. An examination of their organs of sense and of voice is very desirable.