Forel observed that after a nest of _Formica pratensis_ had been separated into two nests placed at a considerable distance from one another so as to have no intercommunication, the members yet recognised one another as parts of the same family after the lapse of more than a month; but another observation showed that after some years of separation they were no longer so recognised. Although it is now well ascertained that ants are able to distinguish the individuals belonging to their own nests and colonies from those that, though of their own species, are not so related to them, yet it is not known by what means the recognition is effected; there is, however, some reason to suppose that it {155}is by something of the nature of odour. One observer has noticed that if an ant fall into water it is on emerging at first treated as if it were a stranger by its own friends; but other naturalists have found this not to be the case in other species. Contact with corrosive sublimate deprives ants for a time of this power of recognising friends, and under its influence they attack one another in the most ferocious mariner. The nests and colonies of the species of Camponotides we have considered are all constructed by societies comprising a great number of individuals; there are, however, in the tropics numerous species that form their nests on foliage, and some of these contain only a few individuals. The leaf-nests (Fig. 64) of certain species of _Polyrhachis_ are said to be formed of a paper-like material, and to contain each a female and about 8 or 10 worker ants. Forel[65] has examined nests of several Indian species, and finds they differ from those of other ants in consisting of a single cavity, lined with silk like that of a spider. These nests are further said to be constructed so as to render them either inconspicuous or like other objects on the leaves; _P. argentea_ covers its small dwelling with little bits of vegetable matter, and a nest of _P. rastella_ was placed between two leaves in such a manner as to be entirely hidden. All the species of the genus do not, however, share these habits, _P. mayri_ making a card-nest, like _Dolichoderus_ and some other ants. The species of the genus _Polyrhachis_ are numerous in the tropics of the Old World. [Illustration: Fig. 64—Nest of _Polyrhachis_ sp. (After Smith.)]