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).
[Illustration: 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.