Page:EB1911 - Volume 02.djvu/97

 The hinder abdominal segments and the stings of the queens and workers resemble those of other stinging Hymenoptera. But there are several subfamilies of ants whose females have the lancets of the sting useless for piercing, although the poison-glands are functional, their secretion being ejected by the insect, when occasion may arise, from the greatly enlarged reservoir, the reduced sting acting as a squirt.

Nests.—The nests of different kinds of ants are constructed in very different situations; many species (Lasius, for example) make underground nests; galleries and chambers being hollowed out in the soil, and opening by small holes on the surface, or protected above by a large stone. The wood ant (Formica rufa, fig. 1) piles up a heap of leaves, twigs and other vegetable refuse, so arranged as to form an orderly series of galleries, though the structure appears at first sight a chaotic heap. Species of Camponotus and many other ants tunnel in wood. In tropical countries ants sometimes make their nests in the hollow thorns of trees or on leaves; species with this habit are believed to make a return to the tree for the shelter that it affords by protecting it from the ravages of other insects, including their own leaf-cutting relations.

Early Stages.—The larvae of ants (fig. 3, e) are legless and helpless maggots with very small heads (fig. 3, f&#8202;), into whose mouths the requisite food has to be forced by the assiduous “nurse” workers. The maggots are tended by these nurses with the greatest care, and carried to those parts of the nest most favourable for their health and growth. When fully grown, the maggot spins an oval silken cocoon within which it pupates (fig. 3, g). These cocoons, which may often be seen carried between the mandibles of the workers, are the “ants’ eggs” prized as food for fish and pheasants. The workers of a Ceylonese ant (Oecophylla smaragdina) are stated by D. Sharp to hold the maggots between their mandibles and induce them to spin together the leaves of trees from which they form their shelters, as the adult ants have no silk-producing organs.

Origin of Societies.—Ant-colonies are founded either by a single female or by several in association. The foundress of the nest lays eggs and at first feeds and rears the larvae, the earliest of which develop into workers. C. Janet observed that in a nest of Lasius alienus, established by a single female, the first workers emerged from their cocoons on the 102nd day. These workers then take on themselves the labour of the colony, some collecting food, which they transfer to their comrades within the nest whose duty is to tend and feed the larvae. The foundress-queen is now waited on by the workers, who supply her with food and spare her all cares of work, so that henceforth she may devote her whole energies to egg-laying. The population of the colony increases fast, and a well-grown nest contains several “queens” and males, besides a large number of workers. One of the most interesting features of ant-societies is the dimorphism or polymorphism that may often be seen among the workers, the same species being represented by two or more forms. Thus the British “wood ant” (Formica rufa) has a smaller and a larger race of workers (“minor” and “major” forms), while in Ponera we find a blind race of workers and another race provided with eyes, and in Atta, Eciton and other genera, four or five forms of workers are produced, the largest of which, with huge heads and elongate trenchant mandibles, are known as the “soldier” caste. The development of such diversely-formed insects as the offspring of the unmodified females which show none of their peculiarities raises many points of difficulty for students in heredity. It is thought that the differences are, in part at least, due to differences in the nature of the food supplied to larvae, which are apparently all alike. But the ovaries of worker ants are in some cases sufficiently developed for the production of eggs, which may give rise parthenogenetically to male, queen or worker offspring.

Food.—Different kinds of ants vary greatly in the substances which they use for food. Honey forms the staple nourishment of many ants, some of the workers seeking nectar from flowers, working it up into honey within their stomachs and regurgitating it so as to feed their comrades within the nest, who, in their turn, pass it on to the grubs. A curious specialization of certain workers in connexion with the transference of honey has been demonstrated by H. C. McCook in the American genus Myrmecocystus, and by later observers in Australian and African species of Plagiolepis and allied genera. The workers in question remain within the nest, suspended by their feet, and serve as living honey-pots for the colony, becoming so distended by the supplies of honey poured into their mouths by their foraging comrades that their abdomens become sub-globular, the pale intersegmental membrane being tightly stretched between the widely-separated dark sclerites. The “nurse” workers in the nest can then draw their supplies from these “honey-pots.” Very many ants live by preying upon various insects, such as the British “red ants” with well-developed stings (Myrmica rubra), and the notorious “driver ants” of Africa and America, the old-world species of which belong to Dorylus and allied genera, and the new-world species to Eciton (fig. 2, 2, 3). In these ants the difference between the large, heavy, winged males and females, and the small, long-legged, active workers, is so great, that various forms of the same species have been often referred to distinct genera; in Eciton, for example, the female has a single petiolate abdominal segment, the worker two. The workers of these ants range over the country in large armies, killing and carrying off all the insects and spiders that they find and sometimes attacking vertebrates. They have been known to enter human dwellings, removing all the verminous insects contained therein. These driver ants shelter in temporary nests made in hollow trees or similar situations, where the insects may be seen, according to T. Belt, “clustered together in a dense mass like a great swarm of bees hanging from the roof.”

1911 Britannica-Ant-Foraging Ants.png

The harvesting habits of certain ants have long been known, the subterranean store-houses of Mediterranean species of Aphaenogaster having been described by J. T. Moggridge and A. Forel, and the complex industries of the Texan Pogonomyrmex barbatus by H. C. McCook and W. M. Wheeler. The colonies of Aphaenogaster occupy nests extending over an area of fifty to a hundred square yards several feet below the surface of the ground. Into these underground chambers the ants carry seeds of grasses and other plants of which they accumulate large stores. The species of Pogonomyrmex strip the husks from the seeds and carry them out of the nest, making a refuse heap near the entrance. The seeds are harvested from various grasses, especially from Aristida oligantha, a species known as “ant rice,” which often grows in quantity close to the site selected for the nest, but the statement that the ants deliberately sow this grass is an error, due, according to Wheeler, to the sprouting of germinating seeds which the ants have turned out of their store-chambers.

Perhaps no ants have such remarkable habits as those of the genus Atta,—the leaf-cutting ants of tropical America (fig. 2, 1). There are several forms of worker in these species, some with enormous heads, which remain in the underground nests, while their smaller comrades scour the country in search of suitable trees, which they ascend, biting off small circular pieces from the leaves, and carrying them off to the nests. Their labour often results in the complete defoliation of the tree. The tracks along which the ants carry the leaves to their nests are often in part subterranean. H. C. McCook describes an almost straight tunnel, nearly 450 ft. long, made by Atta fervens.

Within the nest, the leaves are cut into very minute fragments and gathered into small spherical heaps forming a spongy mass, which—according to the researches of A. Möller—serves as the substratum for a special fungus (Rozites gongylophora), the staple food of the ants. The insects cultivate their fungus, weeding out