Popular Science Monthly/Volume 54/November 1898/The Herds of the Yellow Ant
|THE HERDS OF THE YELLOW ANT.|
By JAMES WEIR, Jr., M. D.
AS far as my observations go, and they extend through a period of twenty years, the aphides which live on grapevines, and which are the peculiar cows of the common yellow ant (Lasius flavus) of our gardens, show many curious phases in their life history. Especially is this true of the manner in which they perpetuate their species.
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 antennae, 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.
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.
The winged females have both compound eyes and ocelli, or primitive eyes, yet they seek the under surface of the leaf, thus seeming to prefer the more subdued light to be found there. The young are always deposited on the under surface of the leaves; in a few days, however, they either migrate of their own accord to the more succulent stems or are carried thither by the ants, which never cease to watch over and care for them.
In order to test this guardianship, I have frequently wounded the vine below a colony of aphides, thus cutting off, to a certain extent, the flow of sap. The ants would soon discover this and would at once begin to move the-herd to another vine. The aphis is provided with boring and suction organs somewhat similar to those of the mosquito. In point of fact, it is interesting to note that the ancestors of the mosquito, in all probability, lived wholly on the juices of plants; hence, in this respect, the resemblance is more real than apparent. Aphides, also, like mosquitoes, have the curious habit of elevating their bodies, "standing on their heads," after they have become gorged with food; this can be observed in the drawings.
The cow of the yellow ant should not be confounded with her harmful cousin, Phylloxera vastatrix, the deadliest enemy of the grape. Fortunately for us, this last-mentioned aphis does not abound in the United States; in France and other European countries, however, phylloxera has occasioned the loss of millions of dollars. Notwithstanding the fact that the ants are exceedingly zealous in guarding their property, many of the aphides fall victims to the assaults of their enemies. The most cunning, insidious, and crafty of their foes are the ichneumon flies, three varieties of which are continually, during the summer months and m autumn, endeavoring to cradle their young in the bodies of the aphides.
One of these flies, which can be seen in the drawing, is quite large; I am inclined to believe, therefore, that their larvae do not pupate in the bodies of their hosts, but undergo further metamorphoses elsewhere. Another of these flies is very small, hardly larger, in fact, than the insect which it selects as a living cradle for its young. The ichneumon shows rare intelligence, inasmuch as she invariably deposits her eggs on the young members—the calves, as it were—of the herd; she seems to know that the older aphides would die before the ichneumon grubs arrived at a suitable age for pupation; hence she selects the young ones. She runs here and there about the colony until she finds a young aphis; then, curving her abdomen between her legs, she will quickly lay an egg on the body of the unconscious insect. When the egg hatches the larva feeds on the body of its host (carefully avoiding the vital organs, however) until the time arrives for it to undergo further metamorphoses. The animated cradle and cupboard eventually dies, but not until its queer baby has arrived at an age at which it has no further use for it.
The ants are fully aware of the fact that the ichneumon is a deadly enemy of their cows; hence, when one of these flies is seen hovering over the herd, they at once become alert and endeavor to chase her away whenever she alights. She manages, however, to elude them every now and then, and to lodge her fatal eggs on some of the tender young aphides.
Another implacable foe of these creatures is the larva of a neuropterous insect which in its perfect or mature form resembles the dragon fly. It is technically known as an aphis-lion, and differs very widely in habits from the common and well-known ant-lions. The last-mentioned grubs dig pits, at the bottoms of which they lie in wait for and seize their prey whenever it falls therein; the aphidian lion is, on the contrary, a bold and skillful hunter, and takes its prey wherever it may find it. It is an ugly, heavy, slothful-looking grub, yet it is remarkably agile. When darting upon its quarry (and it hunts the winged aphides only), its thick, clumsy-looking legs move with such rapidity that they can scarcely be seen. Its movements as well as its shape are decidedly lacertilian; in fact, when it is seen coursing over the grape leaves in pursuit of its prey, it reminds one irresistibly of the brilliant little lizards which are to be observed running here and there over stone walls, fences, and sunny woodland paths. This creature stalks its prey like the lycosids or hunting spiders, and fairly bounds upon it when it arrives within grasping distance. Its catlike movements when creeping up on its quarry are wonderful to behold, and indicate a very high degree of intelligence.
In color it is jet black; in fact, in certain lights it glistens like a jet jewel. It is about half an inch long and one sixteenth of an inch broad. On the mar-gins of its body, from its head to its tail, are rows of thornlike spines. Its masticatory organs, as well as its viscera,
are much more highly developed than are those of ant-lions. It is a brave little creature, and only succumbs to the ants (which make war on it wherever they find it, thus showing that they are fully aware of the fact that it is inimical to their herds) when life ceases. During the last twenty years I have frequently observed this larva, and have endeavored to follow it in its metamorphoses. I have succeeded only once, however, in carrying it through to its imago or perfect form. It is not described in any of my lists, and may be, therefore, a new species. There is another aphis-lion which in very many respects closely resembles the one just described. It is pictured by Professor Comstock, a modification of whose drawingis here produced. He writes of this creature as follows:
'"When the aphis-lion is full grown, it rolls itself up into a tiny ball and weaves around itself a glistening, white cocoon, which looks like a seed pearl." (This can be seen in the sketch near the base of the upper leaf.) "It may be supposed that while the aphis-lion is secluded in this pearly cell it repents its greedy, murderous ways, and changes in spirit; at least the body changes greatly, for, after a time, a circular lid is made in the cocoon, and out of it there emerges a beautiful, dainty creature, with delicate-veined, green wings, a palegreen body, slender brown antennæ, and a pair of large eyes that shine like melted gold. It is sometimes called golden-eyes, and sometimes a laced-wing fly, from its appearance."
This beautiful little insect evinces marvelous forethought in the matter of perpetuating her kind. She knows that her young are predaceous, devouring anything in the shape of an insect or an egg that they can secure; she is aware of the fact that, if she were to deposit her eggs, side by side, on a leaf, the first young aphis-lion hatched out would devour all of the remaining eggs. In order to guard against this, she spins a delicate but stiff stalk of hard silk, upon the tip of which she deposits an egg. By the side of this stalk she rears another, and another, and another, tipping each with an egg, until finally, when she has finished ovipositing, there appears a miniature grove of delicate silken stems, each one of which bears aloft on its summit a round and shining egg. When the first-born of this brood makes its appearance, it crawls down the stem to the surface of the leaf, and goes in search of food, utterly unconscious of the rich and toothsome feast just above its head on the tips of the other stalks!
Lubbock concludes, from certain experiments, that the yellow ant will not voluntarily drop from an elevation. Now, observations and experiments made by myself teach me that these ants (Lasius flavus) will drop from elevations when they wish to attain a certain object.
On one occasion one of the herds of aphides under observation was discovered by a wandering black ant (Lasius niger), which reported her discovery to her comrades. At once a marauding expedition was inaugurated by these cattle thieves, which fiercely attacked the yellow guardians of the herd. The black rievers swarmed up the grapevine, but were met by the brave yellow warriors, which valiantly withstood their attack. Finally, the yellow ants were in danger of being overwhelmed by numbers, when I suddenly perceived that they were being re-enforced. Closer examination revealed the fact that they were crawling up a neighboring vine and then dropping from an overhanging leaf on to the leaf on the stem of which the aphides were feeding. They could not reach the herd by way of the original path on account of the intervening army of black ants, hence their shrewd and most intelligent use of the neighboring vine and overhanging leaf. I am glad to report that the yellow ants were victorious, and that they completely routed the would-be robbers.
When alarmed, the yellow ant will draw in its legs and drop to the ground; moreover, this is characteristic of all vine-and tree climbing ants, Lubbock to the contrary notwithstanding. It stands to reason that past experiences must have taught them that they received no injury from involuntary tumbles; that they have evolved the habit of voluntarily throwing themselves from an elevation in order to attain certain objects does not seem to me, therefore, at all wonderful or extraordinary.