M‘Cook further observed that one of the oak-trees was reached by individuals from another nest, and that each of the two parties was limited to its own side of the tree, sentinels being placed on the limits to prevent the trespassing of an intruder; he also noticed that the ants saw an object when the distance became reduced to about an inch and a half from them. This species is considered to be wanting in individual courage; but when acting in combination of vast numbers it does so with intelligence and success. It does not make slaves, but it has been observed by Bignell and others that it sometimes recruits its numbers by kidnapping individuals from other colonies of its own species. Its nests are inhabited by forty or fifty species of guests of various kinds, but chiefly Insects. Another ant, _Myrmica laevinodis_, sometimes lives with it in perfect harmony, and _Formicoxenus {149}nitidulus_ lives only with _F. rufa_. Amongst the most peculiar of its dependants we may mention large beetles of the genera _Cetonia_ and _Clythra_, which in their larval state live in the hills of the wood-ant. It is probable that they subsist on some of the vegetable matter of which the mounds are formed. Adlerz has given some attention to the division of labour amongst the different forms of the workers of ants, and says that in _F. rufa_ it is only the bigger workers that carry building and other materials, the smaller individuals being specially occupied in the discovery of honey-dew and other Aphid products. In _Camponotus_ it would appear, on the other hand, that the big individuals leave the heavy work to be performed by their smaller fellows. The wood-ant and its near allies have been, and indeed still are, a source of great difficulty to systematists on account of the variation that occurs in the same species, and because this differs according to locality. Our European _F. rufa_ has been supposed to inhabit North America, and the interesting accounts published by M‘Cook of the mound-making ant of the Alleghanies were considered to refer to it. This Insect, however, is not _F. rufa_, as was supposed by M‘Cook, but _F. exsectoides_, Forel. It forms colonies of enormous extent, and including an almost incredible number of individuals. In one district of about fifty acres there was an Ant City containing no less than 1700 of these large ant-hills, each one teeming with life. It was found by transferring ants from one hill to another that no hostility whatever existed between the denizens of different hills; the specimens placed on a strange hill entered it without the least hesitation. Its habits differ in some particulars from those of its European congener; the North American Insect does not close the formicary at night, and the inquilines found in its nest are very different from those that live with _F. rufa_ in Europe. Whether the typical wood-ant occurs in North America is doubtful, but there are races there that doubtless belong to the species.