Fig. 25.—Beetle soliciting food from Ant (after Wasmann).
Some of these insects, of which there are many species belonging to several orders, are parasitic; others appear to be hostile, and yet are able to maintain themselves in the nest; others simply live side by side with the ants, which seem to be neither hostile nor friendly to them. In some of these cases the biological purpose of the association is unknown, while in others the ant serves as a model which the associated insect mimics. Thus in the nest of an Indian ant (Sima rufa-nigra) occur a small wasp and a spider which, to some extent in form and more markedly in coloration, mimic their hosts. “Wherever you find this species in any numbers,” says Mr. Rothney,[95] “if you watch a few moments, you will see a mimicking spider, Salticus, running about among the ants, which it very closely resembles in appearance, much more so in life than in set specimens placed side by side; I have seen numbers on the most friendly footing with the ants, though I have never seen them enter their burrows.... They are, I should say, the only friends the ant has, with the exception of a sand-wasp, a new species of Rhinopsis since described by Mr. Cameron, which also very closely mimics the ant, and which, on first observing among the workers, I took to be the male.” But there are some beetles which are not only tolerated, but fed by the ants with which they live. In the case of the genera Atemeles and Lomechusa, which are always found in or near ants’ nests, the good offices are reciprocal, for the beetles “have patches of yellow hairs, and these secrete some substance with a flavour agreeable to the ants, which lick the beetles from time to time. On the other hand, the ants feed the beetles; this they do by regurgitating food, at the request of the beetle, on to their lower lip, from which it is then taken by the beetle. The beetles in many of their movements exactly resemble the ants, and their mode of requesting food, by stroking the ants in certain ways, is quite ant-like. So reciprocal is the friendship, that if an ant is in want of food the beetle will in its turn disgorge for the benefit of its host. The young of the beetles are reared in the nests by the ants, who attend to them as carefully as they do to their own young. The beetles are, however, fond of the ants’ larvæ as food, and, indeed, eat them to a very large extent, even when their own young are receiving food from the ants. Wasmann (to whom we are indebted for most of our knowledge on this subject) seems to be of opinion that the ants scarcely distinguish between the beetle larvæ and their own young; one unfortunate result for the beetle follows from this, viz. that in the pupal state the treatment that is suitable for the ant larvæ does not agree with the beetle larvæ. The ants are in the habit of digging up their own kind, and lifting them out and cleaning them during their metamorphosis: they do this also with the beetle larvæ, with fatal results; so that only those that have the good fortune to be forgotten by the ants complete their development.”[96]
Aphides, or plant-lice, yield to the solicitations of ants, which stroke them with their antennæ, by emitting a drop of sweet and viscid secretion, and it appears that the caress of the ant is the natural stimulus for the emission of the drop. Not only, however, do the ants go forth in search of aphides in their natural haunts, they bring them to the neighbourhood of the nest, and may even impound them by building a wall of earth round and over them. Huber stated that ants collected the eggs of the aphides and tended them in their nests, and the accuracy of the observation has been shown by Lord Avebury and others. “The aphid eggs are laid early in October, on the food plant of the insect. They are of no direct use to the ants, yet they are not left where they are laid, where they would be exposed to the severity of the weather and to innumerable dangers, but brought into the nests by the ants, and tended by them with the utmost care through the long winter months until the following March, when the young ones are brought out and again placed on the young shoots of the daisy.”[97] Dr. McCook noticed that ants, returning from the trees on which aphides abounded, fed others near the nests, and he regarded this as a case of division of labour, the foragers obtaining food for the nurses which remained in or near the nest.
A further division of labour, carried to lengths which seem almost absurd, is found in the honey-pot ant of the United States and Mexico. The juice on which these ants feed is obtained from an oak-gall. Foragers go forth at night and return distended with the sweet fluid, and, having fed the ordinary workers in the nest, apparently discharge the balance of their store into living honey-pots, which remain in the nest and preserve the food till it may be required by the members of the community. Their abdomens are enormously distended, they never leave the nest, and they seem to form a distinct caste, whose function it is to passively accumulate stores of reserve food for the community. Curiously enough the same peculiar social arrangement is found in different genera living as far apart as Mexico, Australia, and South Africa.
Fig. 26.—-Honey-pot Ant.
There is no doubt that in some cases the division of labour is not restricted to the individuals of the same species, but that other species are introduced into the nest to perform certain functions—thus giving rise to the so-called slavery among ants. This is carried to an extreme in the European species Formica rufescens, the males and queens of which do no work, while the sole function of the workers is to capture slaves of the smaller species Formica fusca. In association with this specialized mode of instinctive behaviour, “even their bodily structure has undergone a change; their mandibles have lost their teeth, and have become mere nippers, deadly weapons indeed, but useless except in war. They have lost the greater part of their instincts: their art—that is, the power of building; their domestic habits—for they take no care of their own young, all this being done by the slaves; their industry—they take no part in providing the daily supplies; if the colony changes the situation of its nest, the masters are all carried by the slaves to the new one; nay, they have even lost the habit of feeding.... I have had a nest of this species under observation for a long time, but never saw one of the masters feeding. I have kept isolated specimens for weeks, by giving them a slave for an hour or two a day to clean and feed them, and under these circumstances they remained in perfect health, while, but for the slaves, they would have perished in two or three days.”[98]