Part 11

As for the manner of freeing a tree once invaded by this insect, the most simple plan is to rub the trunk and the branches, in order to crush the insects, or to employ a brush or broom.

We spoke above of the reproduction of the aphis, but without entering into any particular details; we will now touch upon this question, one of the most interesting in natural history.

It was at the time when Réaumur was writing his immortal "Histoire des Insectes," when Trembley was publishing his admirable researches on the freshwater Hydra, whose wonderful vitality we have mentioned in our work on Zoophytes and Molluscs,[29] that another naturalist astonished the learned world by his experiments on the reproductions of plant-lice. This naturalist, whose name will live quite as long as those of Réaumur and of Trembley, was Charles Bonnet, of Geneva.

[29] "The Ocean World."

Charles Bonnet made the extraordinary discovery that aphides can increase and multiply without the intervention of the sexes. An isolated specimen can produce a series of generations of its kind. We will relate the curious experiments of the Genevese naturalist. He placed in a flower-pot, filled with mould, a phial full of water, and put into this phial a little branch of spindle, having only five or six leaves, and perfectly free from any insect. On one of these leaves he placed a plant-louse, which was born under his own eyes, of a wingless mother. He then covered the branch with a glass shade, whose rim fitted exactly into the top of the flower-pot. Having taken these precautions, Charles Bonnet was perfectly certain of being able to observe his prisoner at his ease. He could keep it under his eye and under his hand, with more certitude and security than was the mythological Danaë, shut up, by order of Acrisius, in a tower of bronze.

"I took care," says Charles Bonnet, "to keep a correct journal of the life of my insect. I noted down its least movements; nothing it did seemed to me indifferent. Not only did I observe it every day from hour to hour, beginning generally at four or five o'clock in the morning, and only leaving off at about nine or ten at night; but I even looked many times in the same hour, and always with the magnifying glass, to render my observation more exact, and to learn the most secret actions of my little lonely one. But if this continual application cost me some trouble, and bored me not a little, in amends I had some cause for self-applause and for having subjected myself to all this trouble.... My plant-louse changed its skin four times: on the 23rd, in the evening; on the 26th, at two in the afternoon; on the 29th, at seven o'clock in the morning; and on the 31st, at about seven o'clock in the evening.... Happily delivered from these four illnesses through which it was obliged to pass, it at last reached that point to which, by my care, I had been trying to bring it. It had become a perfect plant-louse. On the 1st of June, at about seven o'clock in the evening, I saw, with great satisfaction, that it had given birth to another; from that time I thought I ought to look upon it as a female. From that day up to the 20th inclusive, she produced ninety-five little ones, all alive and doing well, the greatest number of which were born under my own eyes!"[30]

[30] "Traité d'Insectologie, ou Observations sur les Pucerons," pp. 28-38. 1re partie, 18mo. Paris, 1745.

He very soon made some other experiments on the aphis of the elder-tree, so as to assure himself if the generations of plant-lice, reared successively in solitude, preserved the same property of procreating without copulation.

"On the 12th of July," says he, "about three o'clock in the afternoon, I shut up a plant-louse that had just been born under my eyes. On the 20th of the same month, at six o'clock in the morning, it had already produced three little ones. But I waited till the 22nd towards noon before I shut up a plant-louse of the second generation, because I could not manage earlier to be present at the birth of one of those produced by the mother I had condemned to live in solitude. I always continued to observe the same precaution. I shut up only those plant-lice which were born under my very eyes. A third generation began on the 1st of August; it was on this day that the plant-louse I had shut up on the 22nd of July gave birth to this generation. On the 4th of August, about one o'clock in the afternoon, I put into solitary confinement a plant-louse of the third generation. On the ninth of the same month, at six in the evening, a fourth generation, due to this last one, had already seen the light: it had given birth to four little ones. On the same day, towards midnight, all intercourse with its own species was forbidden to the plant-louse of the fourth generation born at that hour. On the 18th, between six and seven o'clock in the morning, I found this last in the company of four little ones to which it had given birth."[31]

[31] "Traité d'Insectologie," &c., pp. 67-69.

In this case, the want of food caused the death of the isolated individual of the fifth generation, and the experiment was brought to a close.

Bonnet then tried experiments on the plantain aphis, following them up during five consecutive generations, which succeeded each other without interruption, in the space of three months.

After having stated the extraordinary facts, which he relates with the most perfect simplicity, Charles Bonnet, examining at the end of the fine season specimens of the winged oak-tree aphis, was able to be present at their nuptials. He preserved the females with great care, and saw, not without profound astonishment, that they gave birth, not to small living insects, as was the case in the first experiments, but to eggs of a reddish colour, which were stuck fast to each other, on the stem or stalk of the plant.

A short time afterwards, this illustrious observer was able to convince himself that the oak-tree plant-lice, whose nuptials he had witnessed in the autumn, present the same phenomena of solitary and viviparous propagation, already so often mentioned by him.

At last some new observations permitted him to establish beyond all doubt the connection of these facts, in appearance so contradictory. He discovered that, during the whole of the fine season, the plant-lice are solitary and viviparous, but that towards the autumn these creatures return to the ordinary course of things, and are propagated by eggs, whose development requires the co-operation of a male and female individual. These eggs are hatched in Spring, and produce only viviparous plant-lice. In the autumn the males and females show themselves, and from that moment ovipositing recommences. These curious facts, seen and published more than a century ago, have been verified many times since. In 1866 M. Balbiani asserted that the plant-lice are hermaphrodite, or of both sexes at the same time, which would explain the facts observed by Charles Bonnet. But the anatomical proofs appealed to by Balbiani in support of this idea are far from establishing the existence of this arrangement of sexes among them. The observations of Charles Bonnet produced profound astonishment among naturalists, and, in this respect, 1743 may be considered as a memorable year.

The simple statement of the few experiments which he made, and which we have cited, has sufficed to show how rapid is the multiplication of aphides. A single female produced generally 90 young ones; at the second generation these 90 produce 8,100; these give a third generation, which amounts to 729,000 insects; these, in their turn, become 65,610,000; the fifth generation, consisting of 590,490,000, will yield a progeny of 53,142,100,000; at the seventh, we shall thus have 4,782,789,000,000; and the eighth will give 441,461,010,000,000. This immense number increases immeasurably when there are eleven generations in the space of a year. Fortunately a great many carnivorous insects wage fierce war against the plant-lice, and destroy immense numbers of them. Thus they are kept in check, and prevented from multiplying inordinately. To show with what prodigious abundance the reproduction of these little but formidable parasites must go on, we will relate a fact which was made known to us by M. Morren, Professor in the University of Liége.

The winter of 1833-34 had been extremely warm and dry; whole months had passed without any rain. A well-known _savant_, Van Mons, had foretold, as early as the 12th of May, that all the vegetables would be devoured by plant-lice. On the 28th of September, 1834, at the moment when the cholera had began to spread its ravages over Belgium, all of a sudden a swarm of plant-lice showed themselves between Bruges and Ghent. They were to be seen the next day at Ghent, hovering about in troops, in such quantities that the daylight was obscured. Standing on the ramparts, one could no longer distinguish the walls of the houses in the town, so covered were they with plant-lice. The whole road from Antwerp to Ghent was rendered black by innumerable legions of them. They appeared everywhere quite suddenly. People were obliged to protect their eyes with spectacles and their faces with handkerchiefs, to keep off the painful and disagreeable tickling caused by them. The progress of these insects was interrupted by mountains, hills, even by undulations of land of very slight elevation, but sufficient to have an influence on the wind. M. Morren thinks that they came from a great distance, and that they arrived in Belgium by the sea-coast. Whatever be the explanation of the phenomenon, it establishes sufficiently the prodigious multiplication of these little insects.

There is another trait, and without doubt the most curious in the history of the aphides, to which we have still to call the attention of the reader: we mean the relations which exist between them and the ants.

No one can have failed to observe ants frequenting those places where plant-lice are gathered together in great numbers. Are ants simply friends of the plant-lice, as thought the ancients? or have their visits some selfish object?

Linnæus, Bonnet, and Pierre Huber thought that the ants did not pay these visits for nothing, and that they had some object in view. But what could they want of the plant-lice? It is to Pierre Huber we owe the solution of this mystery. This naturalist has made the most beautiful observations on the relations which exist between plant-lice and ants. They are detailed in a chapter of his admirable work, entitled "Recherches sur les Moeurs des Fourmis Indigènes."

The plant-lice have, as we have said, at the extremity of their abdomen, two small movable horns. These are in communication with a little gland which produces a sugary liquid. When one carefully observes plant-lice attached to the stem of a plant, one sees a little syrup droplet oozing out of the extremity of these tubes.

M. Morren, who has made some interesting observations on the anatomy and generation of the aphis, says that, having shut up females in wide-mouthed glass bottles, he saw the young, a little time after their birth, suck the sweet juice which exudes from the little tubes at the extremity of the mother's abdomen. This secretion seems, then, destined for the nourishment of the young in the first moments of their existence, before they are able to nourish themselves on vegetable juices. The saccharine fluid produced by the mother must be, then, a sort of milk intended for the nourishment of her young. This being settled, attend to what follows. In all places where plant-lice are assembled in great numbers it is easy to observe how excessively fond ants are of the sugary liquid destined for suckling the young. But how do the ants manage to get the plant-lice to allow themselves to be, as we may say, milked?

"It had been already noticed," says this celebrated observer, "that the ants waited for the moment at which the plant-lice caused this precious manna to come out of their abdomen, which they immediately seized. But I discovered that this was the least of their talents, and that they also knew how to manage to be served with this liquid at will. This is their secret. A branch of a thistle was covered with brown ants and plant-lice. I observed the latter for some time, so as to discover, if possible, the moment when they caused this secretion to issue from their bodies; but I remarked that it very rarely came out of its own accord, and that the plant-lice, which were at some distance from the ants, squirted it out with a movement resembling a kick.

"How did it happen, then, that the ants wandering about on the thistle were nearly all remarkable for the size of their abdomens, and were evidently full of some liquid? This I discovered by narrowly watching one ant, whose proceedings I am going to describe minutely. I saw it at first passing, without stopping, over some plant-lice, which did not seem in the least disturbed by its walking over them; but it soon stopped close to one of the smallest, which it seemed to coax with its antennæ, touching the extremity of its abdomen very rapidly, first with one of its antennæ and then with the other. I saw with surprise the liquid come out of the body of the plant-louse, and the ant forthwith seize upon the droplet and convey it to its mouth. It then brought its antennæ to bear upon another plant-louse, much larger than the first; this one, caressed in the same manner, yielded the nourishing fluid from its body in a much larger dose. The ant advanced and took possession of it. It then passed to a third, which it cajoled as it had the preceding ones, giving it many little strokes with its antennæ near the hinder extremity of its body; the liquid came out immediately, and the ant picked it up.... A small number of these repasts are sufficient to satisfy the ant's appetite. (See Fig. 91.)

"It does not appear that it is out of importunity that these insects obtain their nourishment from the plant-lice.

"The neighbourhood of ants is agreeable to plant-lice, since those which could get out of the way of their visits, viz., the winged plant-lice, prefer to remain amongst them, and to lavish upon them the superabundance of their nourishment."[32]

[32] "Recherches sur les Moeurs des Fourmis Indigènes," pp. 181-186. 8vo. Paris, 1810.

What we have just related applies not only to the brown (_Formica brunnea_), but also to the tawny ant (_Formica flava_), to the ashy black (_Formica nigra_), to the fuliginous (_Formica fuliginosa_), and to a great many more.

The Red Ant (_Formica rufa_) is singularly adroit in seizing the droplet left it by the plant-louse. According to Pierre Huber, it employs its antennæ, which swell somewhat towards their extremities, in conveying this droplet to its mouth, and causes it to enter it by pressing it first on one side, then on the other, using its antennæ as if they were fingers. The greater number of ants seek them on those plants on which they usually fix themselves--the lowest herbs, as well as the highest trees. There are some, however, which never leave their place of abode, and never go out to the chase. These are the little ants, of a pale yellow colour, rather transparent, and covered with hairs, and which are extremely numerous in our meadows and orchards. These subterranean creatures are very noxious to the farmer. Pierre Huber often wondered how they subsisted, and with what food they could provision themselves, without quitting their gloomy habitations. Having one day turned up the earth of which a habitation was composed, in order to discover if any treasure were to be found stowed away there, he found nothing but plant-lice. Of these the greater number were fixed to the roots of the trees which hung down from the roof of their subterranean nest; others were wandering about among the ants. These latter, moreover, set about milking their nurses, as usual, and with the same success. To verify his discovery, he dug up a great number of nests of the yellow ant, and invariably found aphides in them. So as to study the relations which must exist between these insects, he shut up ants with their friends, the plant-lice, in a glazed box, placing at the bottom of the box, earth, mixed with the roots of some plants, whose branches vegetated outside the box. He watered this ant-hill from time to time, and thus both the animals and the plants found in his apparatus sufficient nourishment.

"The ants," he says, "did not endeavour in the least to make their escape. They seemed to want for nothing, and to be quite content. They tended their larvæ and females with the same affection they would have shown in their usual ant-hill; they took great care of the plant-lice, and never did them any harm. These, on the other hand, did not seem to fear the ants; they allowed themselves to be moved about from one place to another, and when they were set down they remained in the place chosen for them by their guardians. When the ants wished to move them to a fresh place, they began by caressing them with their antennæ, as if to request them to abandon their roots or to withdraw their trunk from the cavity in which it was inserted; then they took them gently above or below the abdomen with their jaws, and carried them with the same care they would have bestowed on the larvæ of their own species. I saw the same ant take three plant-lice in succession, each bigger than itself, and carry them away into a dark place.... However, the ants do not always act so gently towards them. When they fear that they may be carried off by ants of another kind, and living near their habitation, or when one opens up too suddenly the turf under which they are hidden, they seize them up in haste and carry them off to the bottom of their little cavern. I have seen the ants of two different ant-hills fighting for their plant-lice. When those belonging to one ants' nest could enter the nest of the others, they took them away from their rightful owners, and often these took possession of them again in their turn; for the ants know well the value of these little animals, which seem made on purpose for them,--they are the ants' treasures. An ants' nest is more or less rich, according as it is more or less stocked with plant-lice. The plant-lice are its cattle, its cows, its goats. One would never have thought that the ants were a pastoral people!"[33]

[33] "Recherches," &c., pp. 192-194.

Their hiding in the ants' nest is not voluntary; they are prisoners of war. The ants, after having hollowed out galleries in the midst of roots, make a foray upon the turf, and seize upon plant-lice scattered about here and there, bringing them with them, and collect them together in their nests. The captive insects take their wrongs with patience, and behave like philosophers under this new kind of life. They lavish on their masters, with the best grace in the world, the nutritious juices with which their bodies superabound. Charles Bonnet has stated some real wonders of the cleverness and industry of other ants which also make a provision of plant-lice.

"I discovered one day," says he, "a Euphorbia, which supported in the middle of its stem a small sphere, to which it served as the axis. It was a case which the ants had constructed of earth. They issued forth from this by a very narrow opening made in its base, descended the stem, and passed into a neighbouring ants' nest. I destroyed one part of this pavilion, built almost in the air, so that I might study the interior. It was a little room, the vault-shaped walls of which were smooth and even. The ants had profited by the form of the plant to sustain their edifice. The stalk passed up the centre of the apartment, and for its timber-work it had the leaves. This retreat contained a numerous family of plant-lice, to which the brown ants came peacefully, to make their harvest, sheltered from the rain, the sun, and from other ants. No insect could disturb them; and the plant-lice were not exposed to the attacks of their numerous enemies. I admired this trait of industry; and I was not long in finding it again, in a more interesting character, in ants of different species.

"Some red ants had built round the foot of a thistle a tube of earth, two inches and a half long by one and a half broad. The ants' nest was below, and communicated directly with the cylinder. I took the stalk, with what surrounded it, and all that the cylinder contained. That portion of the stem which was inside the earthen tube was covered with plant-lice. I very soon saw the ants coming out at the opening I had made at the base; they were very much astonished to see daylight at that place, and I saw that they lived there with their larvæ. They carried these with great haste to the highest part of the cylinder which had not been altered. In this retreat they were within reach of their plant-lice, and here they fed their young.

"In other places many stalks of the Euphorbia laden with plant-lice rose in the very centre of an ant-hill belonging to the brown ants. These insects, profiting by the peculiar arrangement of the leaves of this plant, had constructed round each branch as many little elongated cases; and it was here they came to get their food. Having destroyed one of these cells, the ants forthwith carried off into their nests their precious animals; a few days afterwards it was repaired under my eyes by these insects, and the herd were taken back to their pens.

"These cases are not always at a few inches from the ground. I saw one five feet above the soil, and this one deserves also to be described. It consisted of a blackish, rather short tube, which was built round a small branch of the poplar at the point where it left the trunk. The ants reached it by the interior of the tree, which was excavated, and without showing themselves, they were able to reach their plant-lice by an opening which they had made in the base of this branch. This tube was formed of rotten wood, of the vegetable earth of this very tree, and I saw many a time the ants bringing little bits in their mouths to repair the breaches I had made in their pavilion. These are not very common traits, and are not of the number of those which can be attributed to an habitual routine."[34]

[34] "Traité d'Insectologie," &c., pp. 198-201.

One day, Pierre Huber discovered in a nest of yellow ants a cell containing a mass of eggs having the appearance of ebony. They were surrounded by a number of ants, which appeared to be guarding them, and endeavouring to carry them off.

Huber took possession of the cell, its inhabitants, and of the little treasure it contained, and placed the whole in a box lid, covered with a piece of glass, so as to be more easily observed. He saw the ants approach the eggs, pass their tongues in between them, depositing on them a liquid. They seemed to treat these eggs exactly as they would have treated those of their own species; they felt them with their antennæ, gathered them together, raised them frequently to their mouths, and did not leave them for an instant. They took them up, and turned them over, and after having examined them with care, they carried them with extreme delicacy into the little box of earth placed near them.[35]

[35] "Recherches," &c., pp. 205, 206.

These were not, however, ants' eggs. They were the eggs of aphides. The young which were soon to be hatched were to give to the provident ants a reward for the attentions they had lavished upon them. How wonderful are the life and the habits of the plant-lice, and their relations to ants! But we should be led on too far, if we were to pursue these attractive details.