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.
[Illustration: FIG. 26.---Honey-pot Ant.]