Formica, are large and shiny red or black, have extraordinary toothless,
sickle-shaped mandibles and conduct dramatic, well-organized slave
raids, usually during warm, sunny afternoons in July and August.
Typically, a slave-maker worker discovers a nest of the host species and
returns to its nest laying a chemical trail. Soon, slave makers pour from
their nest, and proceed to the target nest in a fast-moving (3 cm s−^1 ),
compact column, which can contain thousands of slave-maker workers.
Upon arriving at the target nest, sometimes as much as 150 m away, the
slave makers force an immediate entrance, seize brood and return home.
Resisting host-species workers are attacked and killed, the slave makers
piercing their heads with their specialized mandibles. The slave makers
do not forage for food, feed their brood or queen or build or maintain their
nests, and are entirely dependent on their slaves for the performance
of these tasks. Young Polyergus queens display similar aggressive
behaviours during colony foundation, as they attack small host-species
colonies, kill the host queen(s) and are subsequently adopted by the host
workers. Obligatory slavery has been discovered in three tribes, belonging
to two subfamilies of ants, and has apparently evolved repeatedly
in various phylogenetic lines, which include the genera Polyergus,
Rossomyrmex, Leptothorax, Harpagoxenus, Protomognathus, Chale-
poxenus, Epimyrma and Strongylognathus (Alloway, 1980, 1997;
Buschinger, 1986; Hölldobler and Wilson, 1990).Inquilinism
Inquilines are obligatory social parasites and spend their entire life cycle
within colonies of their host species, except when they leave to mate or
disperse to found new colonies (Hölldobler and Wilson, 1990; Bourke and
Franks, 1991). Typically, inquilines produce few, if any, workers, and the
workers that are produced contribute little to the economy of the colony.
Often, inquiline queens and host queens coexist within the same colony
and live in close association with one another. Indeed, in certain extreme
cases, the parasites have evolved to the point of being essentially
ectoparasitic and ride about on the bodies of their hosts, especially on the
host queens (see below). When the host queen(s) is present, she continues
to lay eggs, which are reared to produce new workers, but the parasite
queen(s) usually produces only reproductives. In some cases, the host
queen(s) is killed either by the parasites or by the host queen’s own
workers, in a manner similar to that seen during colony usurpation by the
queens of temporary social parasites or obligatory slave makers. Without
a resident host queen to lay eggs for the production of new workers,
such parasite colonies can exist only for the lifetime of the current host
workers, perhaps only a few years. None the less, even this limited life
expectancy can be sufficient for the production of large numbers of new
parasites. Interestingly, since many inquilines completely lack a worker
caste and, therefore, no longer exhibit any division of labour between
reproductive and non-reproductive conspecifics, they no longer satisfy
the definition of being highly social (i.e. eusocial) species and therein322 R.J. Stuart