Works of Grant Allen, page 1008
In No. 4, again, we have a portrait from life of such a winged female, the mother of a numerous fatherless progeny; for both winged and wingless forms are produced through the summer. She is round and well-fed, as becomes a matron. Observe in particular the curious pair of tubes on the last few rings of her back; these are the organs for secreting nectar or honey-dew, a point about which I shall have a good deal more to say presently. A winged female like this may fly away to another rose-bush to become the foundress of a distant colony. The same illustration also shows, in a greatly enlarged form, her beak or sucking apparatus, which consists of four sharp lance-like siphons, enclosed in a protective sheath or proboscis, and admirably adapted both for piercing the rose-twig and for draining the juices of your choicest crimson ramblers. The aphis sticks in the point as if it were a needle, and then sucks away vigorously at the rose-tree’s life-blood. You can watch her so any day with a common small magnifier, and see how, like the lady at Mr. Stiggins’ tea meeting, she “swells wisibly” in the process. Indeed, aphides are always beautiful objects for the microscope or pocket lens, with their pale, transparent green bodies, their bright black eyes, their jointed hairy legs, their delicate feelers, and their marvellous honey-tubes; and it will not be my fault if you still continue to regard them as nothing more than the “nasty blight” that destroys your roses.
Do not for a moment suppose, however, that you and your gardener, with his spray and his tobacco-water, are the only enemies the rose-aphis possesses. The name of her foes is legion. She is devoured alive, from without and from within, by a ceaseless horde of aggressive belligerents. The most destructive of these enemies are no doubt the lady-birds, which, both in their larval and their winged forms, live almost entirely on various kinds of green-fly. This practical fact in natural history is well known to hop-growers, for the dreaded “fly” on hops is an aphis; its abundance or otherwise governs the hop market, and Kentish farmers are keenly aware that a certain particular lady-bird eats the “fly” by millions, on which account they protect and foster the lady-bird, thus leaving the two insects, the parasite and the carnivore, to fight it out in their own way between them.
But No. 5 introduces us to a still more insidious though less dangerous foe: an internal parasite which lays its eggs inside the body of the bud-producing female. There the grub hatches out, and proceeds to eat up its unwilling hostess, alive, from within. In the sketch, we have an illustration, below, of an aphis which has thus been compelled to take in a stranger to board and lodge in her stomach; while the top figure shows how the lodger, after eating his hostess out, eats himself out into the open air through her empty skin. If you look out closely for such haunted green-flies, inhabited by a parasite — most often an ichneumon fly — you will find them in abundance on the twigs of rose-bushes. They have a peculiar swollen, quiescent look, and a brownish colour.
No. 6 shows us another such fierce enemy at work. This formidable insect tiger is the larva of the wasp-fly; he is a savage carnivore, who moors himself by his tail end, stretches out to his full length, and swoops down upon his unsuspecting prey from above; and being blessed with a good appetite, he can get rid of no fewer than 120 aphides in an hour. As he probably eats all day, with little intermission for rest and digestion, this gives a grand total of about 1500 or 1600 victims at a sitting. However, the remaining aphides go on budding away as fast as ever to make up the deficiency, so the loss to the race is by no means irreparable. “Il n’y a pas d’homme nécessaire,” Napoleon used to say; and the principle is even more true as applied to the green-flies. If a few millions die, their place is soon filled again.
Look once more at No. 6, and you will see that while the tiger-like enemy is engaged in hoisting and devouring one unfortunate aphis, its neighbour below, heedless of the tragedy, is quietly engaged in blowing-off honey-dew.
This blowing-off of honey-dew leads me on direct to the very heart of my subject; for it is as manufacturers of honey-dew and as cows to the ants that aphides base their chief claim to attention. If they did not produce this Turkish delight of the insect world, nobody would have troubled to study them so closely. Let us go on to see, then, what is the origin and meaning of this curious and almost unique secretion.
If you examine the leaves of a lime-tree or a rose-bush in warm summer weather you will find them covered all over with a soft sticky substance, sweet to the taste, and spread in a thin layer upon the surface of the foliage. This sweet stuff is honey-dew, and it is manufactured solely by various kinds of aphides, without whose trade-mark none other is genuine. Why do they make it? Not, you may be sure, out of pure unselfish moral desire to benefit the ants and other beasts that like it. In the animal world, nothing for nothing is the principle of conduct. The true secret of the origin of honey-dew appears to be this. Aphides live entirely off a light diet of vegetable juices; now, these juices are rich in compounds of hydrogen and carbon, especially sugar (or rather, to be strictly scientific, glucose), but are relatively deficient in nitrogenous materials, which last are needed as producers of movement by all animals, however sluggish. In order, therefore, to procure enough nitrogenous matter for its simple needs, your aphis is obliged to eat its way through a quite superfluous amount of sweets, or of sugar-forming substances. It is almost as though we ourselves had to swallow daily a barrel of treacle so as to reach at the bottom an ounce of beefsteak. To get rid of this surplus of sugar (or rather, undigested glucose) almost all aphides (for they are a large family, with many separate kinds) have acquired a pair of peculiar organs, known as honey-tubes, on the backs of their bodies. Sometimes, when distended with superfluous food, they simply blow out the honey-dew secreted by these tubes on to the leaves below them.
The aphis in No. 6 is represented at the moment when it is thus ridding itself of its excessive sweetness. But honey-dew is sticky, and apt to get in the way; it may clog one’s legs, or interfere with one’s proboscis: so the aphides prefer as a rule to retain it prudently till some friendly animal, with a taste for sweets, steps in to relieve them of the unpleasant tension. The animal which especially performs this kind office for the rose-aphis is the garden ant; and No. 7 represents such an ant in the very act of tapping and caressing an aphis with its feelers, in order to make her yield up on demand her store of honey. The process is ordinarily described as “milking.”
You must understand, of course, that neither aphis nor ant is actuated by purely philanthropic considerations; this is a case of mutual accommodation. The aphis wants to get rid of a troublesome waste product which is apt to clog it. The ant wants to secure that waste product as a valuable foodstuff. Hence, from all time, an offensive and defensive alliance of the profoundest type has been mutually struck up between ants and aphides. How far this alliance has gone is truly wonderful. The ants not merely “milk” the aphides, but actually collect them together in herds and keep them in parks as domestic animals. Nay, more; as Sir John Lubbock has pointed out, different kinds of ants domesticate different breeds of aphides, as each is suited to the other’s conditions. The common black garden ant attends chiefly to the aphides which frequent twigs and leaves, such as this very rose-aphis — for the black ant is a rover and a good tree-climber; he is much given to exploring expeditions over the surface of plants in search of honey, and he is not particular whether he happens to gather it from flowers or from insects. The brown ant, on the other hand, goes in rather for such species of aphides as frequent the crannies in the bark of trees; while the little yellow ant, an almost subterranean race, living underground among the grass roots in meadows, “keeps flocks and herds” (says Lubbock) “of the root-feeding aphides.” All these facts you can verify for yourself with very little trouble.
It is most interesting to watch a black ant on the prowl after honey-dew. He is evidently led on to the herd by smell, for he mounts the stem where the aphides live in a business-like way, and goes straight to the point, as if he knew what he was after. When he finds an aphis that looks likely, he strokes and caresses her gently with his antennæ; (as you see in the sketch), coaxing her to yield up the coveted nectar. The aphis, on her side, glad to receive his polite attentions, and accustomed to the signal, exudes a clear drop of her surplus sweet, which the ant licks up with its jaws greedily. But ants do much more than this in the way of aiding and protecting their “cows.” They really appropriate them. Often they build, with mud, covered ways or galleries up to their particular herds, and erect earthen cowsheds above them; they also fight in defence of their flocks, as a Zulu will fight for his oxen, or an Arab for his camels. Their foresight is almost human: for when the winter eggs are laid, the ants will transport them into their nest, to keep them safe against frost; and when summer comes again, they will carry them out with care, and place them in the sun to hatch on the proper food-plant. Could man himself show greater prudence and forethought than these mites of herdsmen?
“The eggs,” says Sir John Lubbock, “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, exposed to the severity of the weather and to innumerable dangers, but brought into the nests, and tended with the utmost care through the long winter months till the following March,” when they are brought out again and placed on their special food-plant.
Lubbock even notes that ants have domesticated a far larger variety of other animals than we ourselves have. Our list includes at best the horse, the dog, the cat, the cow, the camel, the sheep, the llama, the alpaca, the goat, the hen, the duck, the goose, the bee, the silkworm, and a dozen or so others; while ants have domesticated no fewer than 584 different kinds of crustaceans and insects, including beetles, flies, and mites, some of which have lived for so many generations in the dark galleries of the ant-hills that they have become totally blind, as happens almost always, in the long run, with underground animals.
During the live-long summer the aphides go on, eating and drinking, budding out new broods with inexhaustible fertility. They settle down calmly on the spot where they were born, they stick to it for life, and they seldom move away from their native twig unless somebody pushes them, for though they have legs, they do not care to use them except on extreme provocation. But when autumn arrives “a strange thing happens.” Broods of perfect winged males and wingless females are then produced; and the males of these, like almost all other insects, take a marriage flight, find their predestined mates, and become with them the parents of the dormant eggs which outlive the year, and carry on the race to the succeeding summer. While warm weather lasts, few or no males are budded out; it is only when the cold threatens to destroy the entire colony that little husbands are born, so as to give rise to eggs which may bridge over the gulf between summer and summer. If you keep the insects warm, however, and supply them with abundant food (as in a conservatory), they will go on producing imperfect females and fatherless broods, without intermission, for many years together. The egg-laying generation is thus shown to be merely a device for meeting the adverse chances of winter; the budding process suffices well enough, as long as warmth and food render the possibility of freezing or starvation unimportant.
On the other hand, the eggs and the brood born from them revert to the earlier habit of the race, when it was still an active, free-flying type, before it had been demoralised by acquiring its sedentary, parasitic habits. They hatch out into active little six-footed or six-legged larvæ, which again, in some cases, give rise to very similar chrysalis forms, and finally develop into the “viviparous” or budding females. Whenever a species earns its livelihood with too little exertion, it invariably degenerates, and often grows small, unintelligent, and vastly prolific; for superior races have relatively small families, while inferior races reproduce by the million. The mites which infest cheese and other foodstuffs are an exactly analogous case to that of the aphides, for they are degenerate spiders, grown small and prolific through the excessive ease of life afforded them by always settling in a cheese, all ready-made food for them, without the trouble or exertion of hunting.
Creatures which reproduce at such a rate, however, invariably pay the penalty for their rapid increase by an equally rapid and enormous death-rate; were it otherwise, the offspring of a single pair of codfish (with their million eggs) would soon turn the sea into one solid mass of cod; while the descendants of a single viviparous aphis would cover the earth with a ten feet thick layer of teeming green-flies. However, Nature has remedies in store for them. Storms of rain and hail kill myriads of aphides; sudden changes of weather wilt them and nip them up; innumerable enemies make an honest livelihood out of them. Another of these ubiquitous foes is graphically represented in No. 8 — the grub of the lace-wing fly, a sort of insect old-clothes man, which covers its back with the cast-off skins of its discarded victims. This is a clever device to enable it to escape observation. The larva, which is a fat and juicy morsel, catches aphides wholesale, and sucks their life-blood; when he has drained them dry, he hoists up their skins on to his back with his jaws, by way of overcoat. Then the hooks or spines on his back (shown above) hold them in place for a time, while the larva bends over and spins a few threads of web across them, to weave them into a neat and compact garment. Thus securely clad, he is hidden from view: he looks much like a twig covered with aphides, and avoids to some extent the too pressing attentions of his own enemies. Observe in this sketch the characteristic unconcern of the aphis who is destined to be his next victim.
Birds also destroy large numbers of aphides. You can see them picking them off in the bean-fields in summer.
It is lucky for us that these insect pests have so abundant a supply of natural enemies; for man, by himself, is almost powerless against them. Strange to say, and paradoxical as it sounds, it is the smallest enemies that we always find most difficult to extirpate. Lions and tigers we can kill off without difficulty; they can be shot and exterminated. Wolves and hyenas give us a little more trouble; while against rabbits, our resources are taxed to the utmost. A plague of rats and mice, or of tiny field-voles, can hardly be combated with any hope of success; while locusts and Colorado beetles devastate our crops with practical impunity.
When it comes to aphides, we are quite unable to cope with the infinite numbers of our infinitesimal foes; and if we take the microscopic creatures which cause cholera, typhoid fever, and other zymotic diseases, we may keep out of their way, it is true, or may isolate the objects in which they breed and store their germs, but we are practically without means to kill or hurt them. The larger the foe, the more easily is he met; the smaller our enemy, the more difficult is he to extirpate. We killed off the American buffalo (or bison) in a single generation; a thousand years would probably fail to kill off the insignificant little aphides that infest our roses.
In the case of one member of the family at least the experiment has been tried on a gigantic scale in France, and as yet with comparatively small results. For the dreaded phylloxera which attacks the vines is, in fact, an aphis; and though immense rewards have been offered by the French Assembly for any good remedy against phylloxera, the only successful plan as yet proposed has been that of planting healthier and sturdier American vines, which resist the little beast a good deal better than the effete and worn-out European species. But many other members of the family wage war with distinguished success against the British farmer. The little black “colliers” which attack our bean crops are a species of aphis; so are the “blight” of apple-trees, the “fly” on turnips, and the most familiar parasites of the hop, the cabbage, the pear, and the potato. It is well for us, therefore, that the aphides have roused against them so many natural enemies among the birds and insects, or our crops would be destroyed by their persistent efforts. The ichneumon-flies alone kill their millions yearly; and the lady-birds well deserve their popular esteem for the good they do in keeping down the ever-increasing numbers of these voracious insects.
Yet, mischievous as they are, the tiny green aphides are well deserving of study, both for their personal beauty and their singular life-history. Everybody can observe them, because they are practically everywhere. If you have a garden, they swarm on every bush. If you grow flowers in your window, they live in every pot. If you content yourself with an occasional bunch of roses or geraniums, you will find them, if you look, sucking away contentedly on the leaves of the rosebuds. Even in London parks or squares you may watch the industrious ants creeping slowly up the stems to milk their wee green cows; you may see with the naked eye, or still better with a pocket lens, the grateful aphis exude a tiny drop of limpid honey from its translucent tubes, and the ant lick it up with unmistakable gusto. Go out into the parks or gardens and examine it for yourself; for every one of the facts I have mentioned in this paper can be verified with ease, if only you have patience.
II. A PLANT THAT MELTS ICE
IF you have ever visited the Alps in early spring, you will know well by sight the dainty little nodding bells of the alpine soldanella — twin flowers on one stalk, like fairy tocsins, which push their heads boldly through the ice of the névé, and form a border of blue blossoms on the edge of the snow-sheet. Most people, to be sure, visit the Alps in August; and they go too late. Autumn is the time when heather purples our bleak northern moors, but when the central mountain chain of Europe, so glorious in April, has become comparatively green and flowerless. If you wish to see what nature can do in the way of rock-gardens, however, you should go to Switzerland in early spring. It is then that blue gentians spread vast girdles of blossom over the alpine pastures; then that the green slopes on the mountain sides are yellowed by globe-flowers; then that the poet’s narcissus stars with its white petals and scents with its sweet perfume the rich meadows on the spurs of the lesser ranges. Higher up, sheets of creeping rock-plants, close clinging to the uneven surface, fall in great cataracts of pink and blue over the steep declivities. As the snow melts, upward, the flowers open in zones, one after another, upon the mountain sides, so that you can mark your ascent by the variations in the flora, and the different successive stages of development reached by the most persistent kinds at various levels.
