1911 Encyclopædia Britannica/Ant
ANT (O. Eng. aémete, from Teutonic a, privative, and maitan, cut or bite off, i.e. “the biter off”; aémete in Middle English became differentiated in dialect use to amete, then amte, and so ant, and also to emete, whence the synonym “emmet,” now only used provincially, “ant” being the general literary form). The fact that the name of the ant has come down in English from a thousand years ago shows that this class of insects impressed the old inhabitants of England as they impressed the Hebrews and Greeks. The social instincts and industrious habits of ants have always made them favourite objects of study, and a vast amount of literature has accumulated on the subject of their structure and their modes of life.
Characters.—An ant is easily recognized both by the casual observer and by the student of insects. Ants form a distinct and natural family (Formicidae) of the great order Hymenoptera, to which bees, wasps and sawflies also belong. The insects of this order have mandibles adapted for biting, and two pairs of membranous wings are usually present; the first abdominal segment (propodeum) becomes closely associated with the fore-body (thorax), of which it appears to form a part. In all ants the second (apparently the first) abdominal segment is very markedly constricted at its front and hind edges, so that it forms a “node” at the base of the hind-body (fig. 1), and in many ants the third abdominal segment is similarly “nodular” in form (fig. 3, b, c). It is this peculiar “waist” that catches the eye of the observer, and makes the insects so easy of recognition. Another conspicuous and well-known feature of ants is the wingless condition of the “workers,” as the specialized females, with undeveloped ovaries, which form the largest proportion of the population of ant-communities, are called. Such “workers” are essential to the formation of a social community of Hymenoptera, and their wingless condition among the ants shows that their specialization has been carried further in this family than among the wasps and bees. Further, while among wasps and bees we find some solitary and some social genera, the ants as a family are social, though some aberrant species are dependent on the workers of other ants. It is interesting and suggestive that in a few families of digging Hymenoptera (such as the Mutillidae), allied to the ants, the females are wingless. The perfect female or “queen” ants (figs. 1, 1, 3, a) often cast their wings (fig. 3, b) after the nuptial flight; in a few species the females, and in still fewer the males, never develop wings. (For the so-called “white ants,” which belong to an order far removed from the Hymenoptera, see Termite.)
Structure.—The head of an ant carries a pair of elbowed feelers, each consisting of a minute basal and an elongate second segment, forming the stalk or “scape,” while from eight to eleven short segments make up the terminal “flagellum.” These segments are abundantly supplied with elongate tooth-like projections connected with nerve-endings probably olfactory in function. The brain is well developed and its “mushroom-bodies” are exceptionally large. The mandibles, which are frequently used for carrying various objects, are situated well to the outside of the maxillae, so that they can be opened and shut without interfering with the latter. The peculiar form and arrangement of the anterior abdominal segments have already been described. The fourth abdominal segment is often very large, and forms the greater part of the hind-body; this segment is markedly constricted at its basal (forward) end, where it is embraced by the small third segment. In many of those ants whose third abdominal segment forms a second “node,” the basal dorsal region of the fourth segment is traversed by a large number of very fine transverse striations; over these the sharp hinder edge of the third segment can be scraped to and fro, and the result is a stridulating organ which gives rise to a note of very high pitch. For the appreciation of the sounds made by these stridulators, the ants are furnished with delicate organs of hearing (chordotonal organs) in the head, in the three thoracic and two of the abdominal segments and in the shins of the legs. 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 ), 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.”
Fig. 2.—Leaf-cutting and Foraging Ants. 1. Atta cephalus;
2. Eciton drepanophora; 3. Eciton erratica.
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 mould and bacterial growths, and causing the appearance, on the surface of their “mushroom garden,” of numerous small white bodies formed by swollen ends of the fungus hyphae. When the fungus is grown elsewhere than in the ants’ nest it produces gonidia instead of the white masses on which the ants feed, hence it seems that these masses are indeed produced as the result of some unknown cultural process. Other genera of South American ants—Apterostigma and Cyphomyrmex—make similar fungal cultivations, but they use wood, grain or dung as the substratum instead of leaf fragments. Each kind of ant is so addicted to its own particular fungal food that it refuses disdainfully, even when hungry, the produce of an alien nest.
Guests of Ants.—Many ants feed largely and some almost entirely on the saccharine secretions of other insects, the best known of which are the Aphides (plant-lice or “green-fly”). This consideration leads us to one of the most remarkable and fascinating features of ant-communities—the presence in the nests of insects and other small arthropods, which are tended and cared for by the ants as their “guests,” rendering to the ants in return the sweet food which they desire. The relation between ants and aphids has often been compared to that between men and milch cattle. Sir J. Lubbock (Lord Avebury) states that the common British yellow ants (Lasius flavus) collect flocks of root-feeding aphids in their underground nests, protect them, build earthen shelters over them, and take the greatest care of their eggs. Other ants, such as the British black garden species (L. niger), go after the aphids that frequent the shoots of plants. Many species of aphid migrate from one plant to another at certain stages in their life-cycle when their numbers have very largely increased, and F. M. Webster has observed ants, foreseeing this emigration, to carry aphids from apple trees to grasses. It has been shown by M. Büsgen that the sweet secretion (honey-dew) of the aphids is not derived, as generally believed, from the paired cornicles on the fifth abdominal segment, but from the intestine, whence it exudes in drops and is swallowed by the ants.
Besides the aphids, other insects, such as scale insects (Coccidae), caterpillars of blue butterflies (Lycaenidae), and numerous beetles, furnish the ants with nutrient secretions. The number of species of beetles that inhabit ants’ nests is almost incredibly large, and most of these are never found elsewhere, being blind, helpless and dependent on the ants’ care for protection and food; these beetles belong for the most part to the families Pselaphidae, Paussidae and Staphylinidae. Spring-tails and bristle-tails (order Aptera) of several species also frequent ants’ nests. While some of these “guest” insects produce secretions that furnish the ants with food, some seem to be useless inmates of the nest, obtaining food from the ants and giving nothing in return. Others again play the part of thieves in the ant society; C. Janet observed a small bristle-tail (Lepismima) to lurk beneath the heads of two Lasius workers, while one passed food to the other, in order to steal the drop of nourishment and to make off with it. The same naturalist describes the association with Lasius of small mites (Antennophorus) which are carried about by the worker ants, one of which may have a mite beneath her mouth, and another on either side of her abdomen. On patting their carrier or some passing ant, the mites are supplied with food, no service being rendered by them in return for the ants’ care. Perhaps the ants derive from these seemingly useless guests the same satisfaction as we obtain by keeping pet animals. Recent advance in our knowledge of the guests and associates of ants is due principally to E. Wasmann, who has compiled a list of nearly 1500 species of insects, arachnids and crustaceans, inhabiting ants’ nests. The warmth, shelter and abundant food in the nests, due both to the fresh supplies brought in by the ants and to the large amount of waste matter that accumulates, must prove strongly attractive to the various “guests.” Some of the inmates of ants’ nests are here for the purpose of preying upon the ants or their larvae, so that we find all kinds of relations between the owners of the nests and their companions, from mutual benefit to active hostility.
Among these associations or guests other species of ants are not wanting. For example, a minute species (Solenopsis fugax) lives in a compound nest with various species of Formica, forming narrow galleries which open into the larger galleries of its host. The Solenopsis can make its way into the territory of the Formica to steal the larvae which serve it as food, but the Formica is too large to pursue the thief when it returns to its own galleries.
Slaves.—Several species of ants are found in association with another species which stands to them in the relation of slave to master. Formica sanguinea is a well-known European slave-making ant that inhabits England; its workers raid the nests of F. fusca and other species, and carry off to their own nests pupae from which workers are developed that live contentedly as slaves of their captors. F. sanguinea can live either with or without slaves, but another European ant (Polyergus rufescens) is so dependent on its slaves—various species of Formica—that its workers are themselves unable to feed the larvae. The remarkable genus Anergates has no workers, and its wingless males and females are served by communities of Tetramorium cespitum (fig. 3).
|Fig. 3.—Ant, Tetramorium cespitum (Linn.), a, Female; b, female after loss of wings; c, male; d, worker; e, larva; g, pupa; f, head of larva more highly magnified. After Marlatt, Bull. 4 (n.s.) Div. Ent. U. S. Dept. Agriculture.|
Senses and Intelligence of Ants.—That ants possess highly developed senses and the power of communicating with one another has long been known to students of their habits; the researches of P. Huber and Sir J. Lubbock (Lord Avebury) on these subjects are familiar to all naturalists. The insects are guided by light, being very sensitive to ultra-violet rays, and also by scent and hearing. Recent experiments by A. M. Fielde show that an ant follows her own old track by a scent exercised by the tenth segment of the feeler, recognizes other inmates of her nest by a sense of smell resident in the eleventh segment, is guided to the eggs, maggots and pupae, which she has to tend, by sensation through the eighth and ninth segments, and appreciates the general smell of the nest itself by means of organs in the twelfth segment. Lubbock’s experiments of inducing ants to seek objects that had been removed show that they are guided by scent rather than by sight, and that any disturbance of their surroundings often causes great uncertainty in their actions. Ants invite one another to work, or ask for food from one another, by means of pats with the feelers; and they respond to the solicitations of their guest—beetles or mites, who ask for food by patting the ants with their feet. In all probability the actions of ants are for the most part instinctive or reflex, and some observers, such as A. Bethe, deny them all claim to psychical qualities. But it seems impossible to doubt that in many cases ants behave in a manner that must be considered intelligent, that they can learn by experience and that they possess memory. Lubbock goes so far as to conclude the account of his experiments with the remark that “It is difficult altogether to deny them the gift of reason ... their mental powers differ from those of men, not so much in kind as in degree.” Wasmann considers that ants are neither miniature human beings nor mere reflex automata, and most students of their habits will probably accept this intermediate position as the most satisfactory. C. L. Morgan sums up a discussion on Lubbock’s experiments in which the ants failed to utilize particles of earth for bridge-making, with the suggestive remark that “What these valuable experiments seem to show is that the ant, probably the most intelligent of all insects, has no claim to be regarded as a rational being.” Nevertheless, ants can teach “rational beings” many valuable lessons.
Bibliography.—The literature on ants is so vast that it is only possible to refer the reader to a few of the most important works on the family. Pierre Huber’s Traité des mœurs des fourmis indigènes (Genève, 1810) is the most famous of the older memoirs. H. W. Bates, A Naturalist on the Amazons; T. Belt, A Naturalist in Nicaragua; H. C. McCook, Agricultural Ant of Texas (Philadelphia, 1880); and A. Möller’s paper in Botan. Mitt. aus den Tropen, (1893), contain classical observations on American species. Sir J. Lubbock’s (Lord Avebury) Ants, Bees and Wasps (London 1882), dealing with British and European species, has been followed by numerous important papers by A. Forel and C. Emery in various Swiss and German periodicals, and especially by C. Janet in his Études sur les fourmis, les guêpes et les abeilles (Paris, &c., 1893–1904). Forel (Ann. Soc. Ent. Belg. xlvii., 1893, Journ. Bomnay N. H. Soc. 1900–1903, and Biologia Cent. Americana) and Emery (Zool. Jahrb. Syst. viii., 1896) have written on the classification of the Formicidae. Among recent American writers on habit may be mentioned W. M. Wheeler (American Naturalist, 1900–1902) and A. M. Fielde (Proc. Acad. Sci. Philadelphia, 1901); E. Wasmann (Kritisches Verzeichniss der myrmecophilen und termitophilen Arthropoden, Berlin, 1894, and 3me Congrès Intern. Zool. 1895) is the great authority on ant-guests and associates. D. Sharp’s general account of ants in the Cambridge Nat. Hist. (vol. vi., 1898) is excellent. For discussions on intelligence see A. Bethe, Journ. f. d. ges. Physiol. lxx. (1898); Wasmann, Die psychischen Fähigkeiten der Ameisen (Stuttgart, 1899); C. Ll. Morgan, Animal Behaviour (London, 1900.) (G. H. C.)