There is no doubt that in some cases the division of labour is not restricted to the individuals of the same species, but that other species are introduced into the nest to perform certain functions--thus giving rise to the so-called slavery among ants. This is carried to an extreme in the European species _Formica rufescens_, the males and queens of which do no work, while the sole function of the workers is to capture slaves of the smaller species _Formica fusca_. In association with this specialized mode of instinctive behaviour, “even their bodily structure has undergone a change; their mandibles have lost their teeth, and have become mere nippers, deadly weapons indeed, but useless except in war. They have lost the greater part of their instincts: their art--that is, the power of building; their domestic habits--for they take no care of their own young, all this being done by the slaves; their industry--they take no part in providing the daily supplies; if the colony changes the situation of its nest, the masters are all carried by the slaves to the new one; nay, they have even lost the habit of feeding.... I have had a nest of this species under observation for a long time, but never saw one of the masters feeding. I have kept isolated specimens for weeks, by giving them a slave for an hour or two a day to clean and feed them, and under these circumstances they remained in perfect health, while, but for the slaves, they would have perished in two or three days.”[98] In this matter, we have in different species successive stages in the development of the instinctive behaviour which is thus carried so far in _Formica rufescens_. Our English ants, of the species _Formica sanguinea_, have fewer slaves and are less dependent on them; they can feed and forage for themselves, and during migration carry their slaves--which are of the same species as in the other case--instead of being carried by them. In the nests of the common wood ant or horse ant (_Formica rufa_) there are occasionally a few slaves. Lord Avebury thinks it likely that they are developed from larvæ or pupæ, originally taken for food, which have by chance come to maturity in the nest of their captors.