THE RED SLAVE MAKERS
The European ant known as _Formica sanguinea_ is blood-red in color,
and is one of the most industrious, versatile, and belligerent insects
known to man. This species, according to Wheeler, “assails any intruder
with its mandibles, simultaneously turning the tip of its gaster
forward and injecting formic acid into the wound.”
Although _sanguinea_ is widely known as a slave-holding species, it
is by no means wholly dependent upon its slaves, but is quite able
to dig its own nest, gather food and rear young without the aid of
any slaves at all. “There is,” said Wheeler, “nothing to show that
the slaves contribute anything more to the communal activities than
would be contributed by an equal number of small _sanguinea_ workers.”
Many observers have reported slaveless colonies of _sanguinea_ which
seemed to be flourishing, and Wasmann found that the youngest colonies
contain, as a rule, more slaves than the older nests. He also reported
an inverse ratio between the number of slaves and the size of the
colony, some of the very largest being practically slaveless.
The slave-hunting expeditions of the _sanguinea_ are said to occur
only two or three times a year, and the general procedure is described
by Wheeler as follows: “The army of workers usually starts out in the
morning and returns in the afternoon, but this depends on the distance
of the _sanguinea_ nest from the nest to be plundered. Sometimes the
slavemakers postpone their sorties till three or four o’clock in the
afternoon. On rare occasions they may pillage two different colonies in
succession before going home. The _sanguinea_ army leaves its nest in
a straggling, open phalanx sometimes a few meters broad and often in
several companies or detachments. These move to the nest to be pillaged
over the directest route permitted by the often numerous obstacles in
their path. As the forefront of the army is not headed by one or a few
workers that might serve as guides, but is continually changing, some
dropping back while others move forward to take their places, it is
not easy to understand how the whole body is able to go so directly to
the nest of the slave species, especially when this nest is situated,
as is often the case, at a distance of fifty or a hundred meters. We
must suppose that the colony has acquired a knowledge of the precise
location of the various nests of the slave species within an area of
a hundred meters or more of its own nest. This knowledge is probably
acquired by scouts leaving the nest singly and from time to time for a
period of several weeks, and these scouts must be sufficiently numerous
to determine the movements of the whole worker body when it leaves the
nest. This presupposes not only a high development of memory, but some
form of communication, for the nest attacked is usually one of many
lying in different directions from the _sanguinea_ nest.
“When the first workers arrive at the nest to be pillaged, they do
not enter at once, but surround it and wait for the other detachments
to arrive. In the meantime the _fusca_ or _rufibarbis_ scent their
approaching foes and either prepare to defend their nest or seize their
young and try to break through the cordon of _sanguinea_ and escape.
They scramble up the grass-blades with their larvae and pupae in their
jaws or make off on the ground. The sanguinary ants, however, intercept
them, snatch away their charges, and begin to pour into the entrance of
the nest. Soon they issue forth one by one with the remaining larvae
and pupae and start for home. They turn and kill the workers of the
slave-species only when these offer hostile resistance. The troop of
cocoon-laden _sanguinea_ straggle back to their nest, while the bereft
ants slowly enter their pillaged formicary and take up the nurture of
the few remaining young or await the appearance of future broods.
“Forel is of the opinion that many of the young brought home by the
sanguinea are eaten, for the number of those which eventually hatch and
become auxiliaries is very small compared with the number pillaged
during the course of the summer. Wasmann believes, however, that the
forays take place for the specific purpose of obtaining young to rear.
This seems to be disproved by the fact that even small _sanguinea_
colonies are quite able to get along without slaves and by the
insignificant number of these individuals in many nests. Darwin has
interpreted the surviving and adopted workers as a kind of by-product,
or as representing food which the ants failed to eat at the proper
time, and such they would appear to be in the adult colony, though, as
we shall see, they have an additional significance as the result of an
instinct inherited by the _sanguinea_ workers from their queen. That
the foray is, to some extent at least, due to the promptings of hunger,
seems to be shown by the fact that _sanguinea_ sometimes plunders the
nests of ants which it could not adopt as slaves.”
Wasmann describes the military expeditions of the so-called sanguine
slavemakers (_F. sanguinea_), which generally hunt in companies of
from twenty to fifty workers, “with the purpose not only of stealing
the neuter pupae of the slave species, but often also of pillaging
the nests of smaller ants belonging to the genus _Lasius_, the
larvae, pupae and winged individuals of which are carried off to be
devoured. During the time of the nuptial flight of _Lasius niger_, many
_sanguinea_ colonies are hunting in the vicinity of their nest for the
heavy _Lasius_ females which drop to the ground. Then either singly
or with united forces these robbers pull their victims into their
strongholds, where they are mercilessly slaughtered. On the afternoon
of August 24, 1888, I witnessed such a typical hunting expedition of
several _sanguinea_ colonies near Exaten, Holland, on the outskirts of
a fir plantation. The road passing the nests was covered far and wide
with _sanguineas_ rushing upon every _Lasius_ female that dropped from
the air, as upon a welcome booty. Within the space of an hour I counted
more than one hundred females of _Lasius niger_ that fell victims to
the hunters.”
There are several species and sub-species of _sanguinea_ in the United
States, and the habits of these differ in several particulars from
those of their European relatives. Wheeler reports that although he
has found plenty of slaveless colonies, most nests contain slaves in
much greater number than do similar colonies in Europe. He thinks this
due in part to the fact that the American species make more frequent
raids, and partly also because the species chosen as slaves are “much
more cowardly and docile” than the victims of the slave-hunters of
the Old World. The actual tactics employed in the raids do not differ
essentially from those of the European species.
It was long supposed that new colonies of the _sanguinea_ were founded
in this wise: When the queen descends from her nuptial flight she
either brings up a brood of her own like many common ants, or she is
adopted into a nest of one of the slave species. On either of these
suppositions it is difficult to explain how the slave-making instincts
could be transmitted to the workers, because the latter have no
offspring and the queen was supposed to lack the slaving instincts. In
1906, Wheeler cleared the matter up by introducing a _sanguinea_ queen
into a nest containing workers, larvae, and cocoons of one of the slave
species. She was immediately attacked, but beat off her assailants,
killed a number of them, and captured a large number of cocoons, which
she carried into a separate chamber and defended against all comers.
Here she waited until the workers emerged from the captured cocoons;
these workers, of course, attached themselves to her and soon gained
possession of the whole nest. This experiment shows clearly that the
_sanguinea_ queen really possesses all the slave-making tendencies
exhibited by the workers in their raiding, and solves the problem of
the inheritance of these instincts.