If a colony of aphides be kept under observation during the summer, it will be noticed that several kinds of individuals arise within it. By far the largest number of the herd will be made up of the wingless agamic form—that is, of females which reproduce without mating. In the species under consideration the young (during the summer) are born alive, and do not come from eggs.
So rapid is the increase of these insects that overproduction would soon kill off the colony by starvation were it not held in check by the peculiar anatomical and physiological metamorphoses which make their appearance in the offspring from time to time. Every now and then young ones are born which, in the course of time, evolve two pairs of wings; these winged individuals (which are also females and agamic) abandon the colony and produce elsewhere, in turn, wingless and single-winged individuals. In autumn both males and females are born; these mate and the females lay eggs.
These creatures secrete a honeylike fluid which exudes from two tubelike teats on the back of the sixth abdominal segment. Ants are passionately fond of this honey-milk and jealously guard and protect the herds of aphidian cows which produce it. They can be seen at all hours of the day, busily engaged in milking their queer kine. They will gently stroke the aphides with their antennæ, thus inducing a free flow of the nectar from the abdominal tubules. Calves effect a like result by nudging their mothers with their heads; the cows "give down" their milk when thus assaulted by the calves.
Ichneumon Fly laying Eggs on Aphides.
In autumn, as soon as ovipositing has begun, the ants gather the aphis eggs as fast as they are laid and carry them into their nests. Here they remain, carefully guarded and protected by the ants, until they hatch out in the spring, when the young agamic females are carried out by their foster mothers and placed upon the tender grape shoots or buds. This year the ants brought out the young aphides, which formed the herds kept under observation this summer, on the 18th of March. On the 19th they carried them back into the nest; this occurred in the forenoon, when the sun was shining and the air was warm and balmy. About 12 M. it began to storm, and became quite cold; that night there was a hard frost. The ants evidently knew that this change in the weather was about to occur; therefore they removed their property to a warm and sheltered place. I have often watched the ants in autumn when the aphides were ovipositing; the former would caress the latter, and seemingly would endeavor to stimulate and cheer them during the operation. As soon, however, as the eggs were deposited, the ants would seize and carry them into the nest; the aphis mother was left, without any compunctions whatever, to die during the first frost! Her life work had ended, and the economy of Nature needed her no longer.
Ants are always on the lookout for the new colonies which are continually being started by the winged females. As soon as one of these new herds is found by an ant, she returns to the nest and notifies her companions. One or two ants then accompany her to her newly found treasure, which in future is always, night and day, under their watchful care. As the herd increases in numbers, additional herdsmen, or rather dairymaids, are called into service.
Associated with this species are commonly to be found other species of aphides, notably the one which secretes, or rather excretes, a white powdery substance which is to be seen on their backs in soft, plumose masses. On microscopic examination this substance is found to be fairly teeming with microbia. These microbes, at the first glance, seem to be of different species; maturer judgment, however, declares them to be but metamorphic forms of the same individual.
A "white" aphis can be seen in the photograph at the base of the upper leaf stem. With a small pocket lens the details of this creature's structure can be easily made out. There are several of these aphides on the vine, but the one mentioned is the largest and the most conspicuous. They are much larger than the nectar-producers, are oval in shape, and distinctly flattened. In fact, in general outline they are strikingly like that bête noire of all good housekeepers—the Cimex lectularius. The ants frequently congregate about these little creatures and appear to be paying them some kind of court. When I cut the vine for photographic purposes, six or eight ants were standing about the large individual mentioned above; they soon became aware that some dire calamity had happened, or was about to happen, either to their beloved herd or to themselves, and, becoming frightened, soon abandoned cattle and pasture and fled away in panic terror. I had hoped to photograph them in situ, but found this to be impossible with the instruments at my disposal.
The ants do not "milk" these white aphides, neither do they eat the excrementitious substance on their backs. The white individuals, however, seem to be factors in the social economy of the herd, for the ants move them, on occasions, along with the herd to other pastures. Once I saw an ant pick up a white aphis and carry it to a leaf some little distance away from the colony; she then returned, picked up a gravid nectar-producing aphis, and carried her to the spot where she had left the white individual. In a few days a fine herd of "milkers" was to be seen grazing in the new field. I judge from this that these white aphides are in some way useful to, if not absolutely necessary for, the welfare of the herd.