CHAPTER VIII
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.