These insatiable depredators cause sometimes a still more remarkable alteration. On the leaves of elms are often seen bladders round and rosy, like little apples. On opening these bladders one finds that they are inhabited by a species of aphis. On the black poplar galls of different kinds grow, some from the leaf stalks, and others from the young stems. They are rounded, oblong, horned, and twisted into a spiral. Other galls show themselves on the leaf itself. They are all inhabited by plant-lice, differing from those of which we have given a description above, in the extremity of their abdomen not presenting the two remarkable horns to which we shall have later to call the attention of the reader. The body is generally covered with a long and thick down.
Of this genus, the species, alas! so unfortunately celebrated is the Apple-tree Aphis (Myzoxyle mali), which attacks that tree. This insect is of a dark russet brown, with the upper part of the abdomen covered with very long white down. Its presence was announced for the first time in England in 1789, and in France, in the department of the Côtes du Nord, in 1812. In 1818 it was found in Paris, in the garden of the École de Pharmacie. It had become common in 1822 in the departments of the Seine, the Somme, and the Aisne. In 1827 its presence in Belgium was announced.
The apple-tree aphis, according to M. Blot, can only exist on that tree. Carried away and placed on any other, it very soon perishes. It does not attack the blossom, the fruit, nor the leaves, but fixes itself on the lower part of the trunk, whence it propagates itself downwards as far as the roots, underneath the graftings, &c. It also likes to lodge in cracks of the trunk and large branches. But it always looks out for a southern, and avoids a northern aspect. It is not active, walks very little, and its dissemination from one place to another can only be explained by the facility with which so small an insect can be transported by the wind, its lightness being still more increased by the down which covers it.
The Myzoxyle mali renders the wood knotty, dry, hard, brittle, and brings on rapidly all the symptoms which characterise old age and decay in attacked trees. M. Blot recommends the following means for preserving the apple-tree from this insect: Employ for the seed-beds the pips of bitter apples only; give to the nursery and to the plants only as much shelter as is absolutely necessary; avoid those sites which are too low and too damp; encourage the circulation of air, and the desiccation of the soil; surround the foot of each apple-tree with a mixture of soot or of tobacco and fine sand.
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.
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]
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.