When discussing social wasps the importance of trophallaxis, or the interchange of food between adult and larva, was stressed. Certain ant larvae supply their nurses with saliva while many exude a fatty secretion through the delicate general integument of the body, and larvae of the group Pseudomyrminae produce similar exudations from special papillae or appendages. These products are eagerly imbibed by the attendant ants, whose mater nal care for their brood is, at least in part, initiated and sustained by what they receive in return for their attentions.
The relations between ant communities of different species represent another aspect of social behaviour. In some cases two species may occupy a compound nest and live amicably together although their broods are kept separate. Relations of this char acter are of the nature of social symbiosis, but in other cases the behaviour of one species towards another is one of aggression rather than of mutual benefit. The peculiar behaviour that is known as slavery or dulosis is especially well exemplified in the European ant Formica sanguinea: its workers raid the nests of Formica fusca and other species, carrying off to their own nests pupae from which workers subsequently develop. Such workers live as slaves in the nests of their captors, but the latter are not wholly dependent upon them since independent slaveless nests also occur. Obligatory slave-makers or "amazons" are rep resented by another European ant (Polyergus rufescens) which is dependent upon its slaves, in whose nests its young queens estab lish their own brood. Polyergus invades a weak nest of Formica fusca and, of ter killing its queen, leaves the workers to feed and tend the amazon's brood. Polyergus workers are abundant but their sole part is to raid other F. fusca nests and bring back larvae and pupae for purposes of maintaining the slave population to the necessary level. Another European ant Anergates atratulus is a highly specialized social parasite with no workers, which lives at the expense of the species Tetramorium caespitum. The Anergates queen enters a nest of the latter and the host workers rear her offspring which are all males and females.
Passing now from the relations of ants as parasites and hosts of one another, there remain to be considered those cases in which they function as hosts for other creatures that live within their nests. These ant-guests or myrmecophiles include not only mem bers of nearly all orders of insects, but also various mites, spiders, millipedes and land crustaceans which collectively number quite 2,000 different species. The nature of the association between
these aliens and their hosts varies immensely. Some, such as certain bristle-tails, mites and rove-beetles are thieves or preda tors, giving nothing in return and devour dead or diseased ants or prey upon the brood. The bristle-tail Atelura is common in the nests of various European ants and according to Janet it obtains most of its food by imbibing it while it is being regurgi tated by one ant to another. Other insects such as the curious slug-like larvae of the fly Microdon live as scavengers and at least confer some benefit upon their hosts. The true guests, or symphiles, come under a different category to the foregoing and mainly consist of various beetles which all show remarkable adaptations to their mode of life. These insects are tended with the greatest fidelity by the ants who rear their larvae like their own brood, notwithstanding the fact that they devour the eggs and larvae of their host. There are also a large number of Hemiptera and a variety of butterfly caterpillars of the family Lycaenidae which supply either honey-dew or glandular products highly attractive to ants. In many cases these insects are har boured in the nests while in other instances, where the association is less intimate, the ants follow them about on plants solely for the products which they yield them.