Habit of making Slaves.—This habit, or instinct, obtains among at least three species of ant, viz., Formica rufescens, F. sanguinea, and strongylognathus. It was originally observed by P. Huber in the first-named species. Here the species enslaved is F. fusca, which is appropriately coloured black. The slave-making ants attack a nest of F. fusca in a body; there is a great fight with much slaughter, and, if victorious, the slave-makers carry off the pupæ of the vanquished nest in order to hatch them out as slaves. Mr. Darwin gives an account of a battle which he himself observed.[29]
When the pupæ hatch out in the nest of their captors, the young slaves begin their life of work, and seem to regard their master's home as their own; for they never attempt to escape, and they fight no less keenly than their masters in defence of the nest. F. sanguinea content themselves with fewer slaves than do F. rufescens; and the work that devolves upon the slaves differs according to the species which has enslaved them. In the nests of F. sanguinea the comparatively few captives are kept as household slaves; they never either enter or leave the nest, and so are never seen unless the nest is opened. They are then very conspicuous from the contrast which their black colour and small size present to the red colour and much larger size of F. rufescens. As the slaves are by this species kept strictly indoors, all the outdoor work of foraging, slave-capturing, &c., is performed by the masters; and when for any reason a nest has to migrate, the masters carry their slaves in their jaws. F. rufescens, on the other hand, assigns a much larger share of labour to the slaves, which, as we have already seen, are present in much larger numbers to take it. In this species the males and fertile females do no work of any kind; and the workers, or sterile females, though most energetic in capturing slaves, do no other kind of work. Therefore the whole community is absolutely dependent upon its slaves. The masters are not able to make their own nests or to feed their own larvæ. When they migrate, it is the slaves that determine the migration, and, reversing the order of things that obtains in F. sanguinea, carry their masters in their jaws. Huber shut up thirty masters without a slave and with abundance of their favourite food, and also with their own larvæ and pupæ as a stimulus to work; but they could not feed even themselves, and many died of hunger. He then introduced a single slave, and she at once set to work, fed the surviving masters, attended to the larvæ, and made some cells.
In order to confirm this observation, Lespès placed a piece of sugar near a nest of slave-makers. It was soon found by one of the slaves, which gorged itself and returned to the nest. Other slaves then came out and did likewise. Then some of the masters came out, and, by pulling the legs of the feeding slaves, reminded them that they were neglecting their duty. The slaves then immediately began to serve their masters with the sugar. Forel also has confirmed all these observations of Huber. Indeed, in the case of F. rufescens, the structure of the animal is such as to render self-feeding physically impossible. Its long and narrow jaws, adapted to pierce the head of an enemy, do not admit of being used for feeding, unless liquid food is poured into them by the mouth of a slave. This fact shows of how ancient an origin the instinct of slave-making must be; it has altered in an important manner a structure which could not have been so altered prior to the establishment of the instinct in question.
Mr. Darwin thus sums up the differences in the offices of the slaves in the nests of F. sanguinea and F. rufescens respectively:—
The latter does not build its own nest, does not determine its own migrations, does not collect food for itself or for its fellows, and cannot even feed itself; it is absolutely dependent on its numerous slaves. Formica sanguinea, on the other hand, possesses much fewer slaves, and in the early part of the summer extremely few; the masters determine when and where a new nest shall be formed, and when they migrate, the masters carry the slaves. Both in Switzerland and England the slaves seem to have the exclusive care of the larvæ, and the masters alone go on slave-making expeditions. In Switzerland the slaves and masters work together, making and bringing materials for the nest; both, but chiefly the slaves, tend and milk, as it may be called, their aphides; and thus both collect food for the community. In England the masters alone usually leave the nest to collect building materials and food for themselves, their slaves and larvæ. So that the masters in this country receive much less service from their slaves than they do in Switzerland.
Mr. Darwin further observes that 'this difference in the usual habits of the masters and slaves in the two countries probably depends merely on the slaves being captured in greater numbers in Switzerland than in England;' and records that he has observed in a community of the English species having an unusually large stock of slaves that 'a few slaves mingled with their masters leaving the nest, and marched along the same road to a tall Scotch fir tree, twenty-five yards distant, which they ascended together, probably in search of aphides or cocci.' And, according to Huber, the principal office of the slaves in Switzerland is to search for aphides.
Mr. Darwin also made the following observation:—'Desiring to ascertain whether F. sanguinea could distinguish the pupæ of F. fusca, which they habitually make into slaves, and which are an unwarlike species, from F. flava, which they rarely capture, and never without a severe fight,' he found 'it was evident that they did at once distinguish them;' for while 'they eagerly and instantly seized the pupæ of F. fusca, they were much terrified when they came across the pupæ, or even the earth from the nest, of F. flava, and quickly ran away; but in about a quarter of an hour, shortly after the little yellow ants had crawled away (from their nest having been disturbed by Mr. Darwin), they took heart and carried off the pupæ.'
Concerning the origin of this remarkable instinct, Mr. Darwin writes:—
As ants which are not slave-makers will, as I have seen, carry off pupæ of other species if scattered near their nests, it is possible that such pupæ originally stored as food might become developed, and the foreign ants thus unintentionally reared would then follow their proper instincts, and do what work they could. If their presence proved useful to the species which had seized them—if it were more advantageous to the species to capture workers than to procreate them—the habit of collecting pupæ, originally for food, might by natural selection be strengthened and rendered permanent for the very different purpose of raising slaves. When the instinct was once acquired, if carried out to a much less extent even than in our British F. sanguinea, which, as we have seen, is less aided by its slaves than the same species in Switzerland, natural selection might increase and modify the instinct, always supposing such modification to be of use to the species, until an ant was found as abjectly dependent on its slave as is the Formica rufescens.