Some ants press weaker species of their kind into unmerited captivity.
In one familiar instance the relatively large oppressor (_Polyergus
rufescens_) is of reddish color and well endowed in the matter of jaws,
while the enslaved species (_Formica fusca_) is small and dark. Regular
slave raids are made from time to time, when, after stubborn resistance,
the pupæ and older larvæ of the weaker form are carried away to lead a
life of bondage, to which, indeed, they take very kindly. This kind of
social economy has indeed become an absolute necessity to the slavers,
which have quite lost the power of feeding their own young, while some
such species cannot feed themselves.
A most extraordinary state of things occurs in the case of a small kind
of ant (_Anergates_) which possesses no workers of its own, but lives
within the communities of another species (_Tetramorium cæspitosum_)
entirely made up of workers.
Some ants, such as the little black species (_lasius niger_) common in
gardens, use as part of their food a sweet fluid that exudes from plant
lice (_aphides_), and keep these insects as we keep kine. The captives
are fed, sheltered, and jealously guarded. Fenced enclosures are
constructed for them on plants in the vicinity of the nest, with which
they are connected by covered roads. During winter the fragile eggs of
the plant lice are taken underground and sedulously cared for.