It is important to remember that among bees social life exists without that peculiar interchange of nutriment between the adults and their brood, which is so characteristic of social wasps. It is possible that the habit of using an abundant and highly nutritious food, and the storage of the same in reserve within the nest, ensures sufficient nourishment at all times for both adults and brood and thus renders exploitation of the latter by their nurses of little advantage.
Ants.—It is among ants that social life in insects attains its highest expression and, it may be added, all ants are social in habit. Structurally they are easily recognized by their elbowed antennae and the conspicuous "waist" formed by a pronounced constriction of the abdomen where it joins the thorax. Except in certain individuals to be mentioned later, wings are absent and the mouth-parts are adapted both for biting and for taking liquid food. Ants live in societies which inhabit nests of various kinds: many species construct their abodes in the soil, galleries and chambers being hollowed out underground. Others form mound nests which are composed not only of excavated soil, but also of heaped-up masses of straws, twigs, pine-needles, leaves, etc., so arranged to form an orderly series of galleries and chambers. Perhaps the largest number of nests are simply excavated beneath stones or logs. In the tropics many ants take advantage of cavities found in stems, petioles, thorns, bulbs, etc.: others form suspended nests attached to trees and constructed of earth, carton or silk. The tropical ant Oecophylla smaragdina forms leaf nests, the leaves being fastened together by silk. Doflein and others have shown that the silk is provided by the larvae which are held by the jaws of the worker ants and used, as it were, as shuttles in weaving the silken threads, which bind the leaves together.
The larvae of ants are legless creatures with a small head and 13 trunk segments. When they become fully grown some ants construct cocoons which contain the pupae, such cocoons regu larly being sold for bird-food under the name of "ants' eggs": other ants pupate without any such protection. No cells are con structed to protect the brood, the latter simply being contained in special chambers of the nest where they are assiduously nursed.
With the exception of the parasitic species, all ants possess a sharply defined worker caste composed of individuals devoid of wings with a relatively simply formed thorax, reduced compound eyes and with the simple eyes minute or wanting. As in other social Hymenoptera these workers are normally sterile females but on occasion their ovaries develop sufficiently to produce parthenogenetic eggs which may give rise to male or, in some cases, worker offspring. In some ants the workers are all alike, but in others they may be differentiated into large-headed or major individuals often termed soldiers, and small-headed indi viduals or minor workers : in other species there may be grades of individuals between these extremes. The males and normal fe males are winged insects with well-developed eyes but either sex may develop into abnormal forms of several types.
Under ordinary circumstances the queens cast their wings shortly after mating and the loss of the flight organs results in the degeneration of the voluminous wing-muscles, which finally break down into material which serves to nourish the developing eggs. When the eggs are mature and the first batch is laid, the queen attends the growing larvae, feeding them with her own saliva until they pupate. The workers which emerge then take charge, feeding the queen and themselves besides tending the new brood, thus enabling the parent to devote herself entirely to egg-laying.
Winged males and females develop in large numbers in strong colonies and, given favourable weather conditions, they leave the nest and either pair with their brothers or sisters as the case may be or with individuals from other colonies. This flight provides for the dissemination of the species since the fertilized daughter queens usually found their nests some distance from the maternal colony.
The feeding habits of ants are both varied and complicated and it is noteworthy that hunting, pastoral and agricultural modes of life appear to have succeeded one another in these insects, as they are believed to have done in the case of man. As Wheeler points out, the most primitive species are carnivorous or, in other words, hunters of other insects. This habit is well ex hibited in the tropical "legionary" or "driver" ants (Dorylinae) which do not construct permanent nests but wander about from place to place. These ants make sorties, often at night, for predatory and migratory purposes attacking insects, spiders and even vertebrate animals—especially if the latter be penned up. There are other groups of ants which represent the pastoral stage, such insects living mainly upon saccharine matter obtained partly from plants, but more especially as honey-dew exuded from Hempiterous insects. Many ants have learned how to induce these insects to void the honey dew by stroking them with the antennae and even to keep and protect them—Aphides (q.v.) are especially sought after for this purpose and Linnaeus was amply justified in calling them the dairy cattle of the ants. This pastoral habit has led to remark able specialization in species known as "honey-ants" which have discovered the advantage of storing honey-dew when it is available in quantity. Since ants, however, have not the art of making receptacles, they have adopted the curious method of using the crops of certain workers or soldiers for the purpose of food-storage (fig. I). Individuals thus functioning are termed repletes, which become so inflated that they are quite unable to walk, and assume the role of animated casks or carboys. When hungry the ants stroke the repletes and receive from them droplets of regurgitated honey-dew collected during times of plenty. The habit above described has been observed in ants living in desert or other arid regions of Africa, Australia and North America, while a more direct vegetarian adaptation is found in certain other ants inhabit ing much the same terrain. In such regions insect food is com monly scarce and certain ants resort to collecting and feeding upon plant seeds. These harvesting ants collect, husk and store the seeds in special granaries: in some species the soldiers or major workers appear to function solely as seed-crushers, thus allowing the softer parts to be available for consumption by the smaller individuals. The harvesting ants can hardly, as Wheeler says, be regarded as true agriculturists since they neither sow nor cultivate the plants from which they gather the seeds. There are, however, other ants which form the group of the Attiini, found in America from Long Island, N.Y., to Argentina, which may be looked upon as cultivators. They are fungus-growers and fungus-eaters and the fungi concerned are cultivated in gardens or special chambers of the nest. According to Moeller these gardens are practically pure cultures of the fungi, which are "weeded" and tended by the worker ants.