I
THE COWS THAT ANTS MILK

DON'T let my title startle you; it was Linnæus himself who first invented it. Everybody knows the common little "green-flies" or "plant-lice" that cluster thick on the shoots of roses; and most people know that these troublesome small insects (from the human point of view) are the true source of that shining sweet juice, rather slimy and clammy, that covers so many leaves in warm summer weather, and is commonly called honey-dew. A good many people have heard, too, that ants use the tiny green creatures in place of cows, coaxing them with their feelers so as to make them yield up the sweet and nutritious juice which is the ants' substitute for butter at breakfast. But comparatively few are aware how strange and eventful is the brief life-history of these insignificant little beasts which we destroy by the thousand in our flower-gardens or conservatories with a sprinkle of tobacco-water. To the world at large, the aphides, as we call them, are mere nameless nuisances-pests that infest our choicest plants; to the eye of the naturalist, they are a marvellous and deeply interesting group of animals, with one of the oddest pedigrees, one of the queerest biographies, known to science.

I propose, therefore, in this paper briefly to recount their story from the cradle to the grave; or, rather, to be literally accurate, from the time when they first emerge from the egg to the moment when they are eaten alive (with some hundreds of their kind) by one or other of their watchful enemies. In this task I shall be aided not a little by the clever and vivid dramatic sketches of the Aphides at Home, which have been prepared for me by my able and watchful collaborator, Mr. Frederick Enock, an enthusiastic and observant naturalist, who thinks nothing of sitting up all night, if so he may catch a beetle's egg at the moment of hatching; and who will keep his eye to the microscope for twelve hours at a stretch, relieved only by occasional light refreshment in the shape of a sandwich, if so he may intercept some rare chrysalis at its moment of bursting, or behold some special grub spin the silken cocoon within whose case it is to develop into the perfect winged insect.

Rose-aphides, or "green-flies," as most people call them, are, to the casual eye, a mere mass of living "blight"-a confused group of tiny translucent insects, moored by their beaks or sucking-tubes to the shoots of the plant on which they have been born, and which they seldom quit unless forcibly ejected. For they are no Columbuses. The spray of rose-bush figured in sketch No. 1 shows a small part of one such numerous household in quiet possession of its family tree, and engaged, as is its wont, in sucking for dear life at the juices of its own peculiar food-plant. You will observe that they are clustered closest at the growing-point. Each little beast of this complex family is coloured protectively green, so as to be as inconspicuous as possible to the keen eyes of its numerous enemies; and each sticks to its chosen twig with beak and sucker as long as there is anything left to drink in it, only moving away on its six sprawling legs when its native spot has been drained dry of all nutriment.

We often talk metaphorically of vegetating: the aphis vegetates. Indeed, aphides are as sluggish in their habits and manners as it is possible for a living and locomotive animal to be: they do not actually fasten for life to one point, like oysters or barnacles; but they are born on a soft shoot of some particular plant; they stick their sucking-tube into it as soon as they emerge; they anchor themselves on the spot for an indefinite period; and they only move on to a new "claim" when sheer want of food or force majeure compels them. The winged members are an exception: they are founders of new colonies, and are now on their way to some undiscovered Tasmania.

And, indeed, as we shall see, these stick-in-the-mud creatures have yet, in the lump, a most eventful history-a history fraught with strange loves, with hairbreadth escapes, with remorseless foes, with almost incredible episodes. They have enemies enough to satisfy Mr. Rider Haggard or the British schoolboy. If you look at No. 2, you will see the first stage in the Seven Ages of a rose-aphis family. The cycle of their life begins in autumn, with the annual laying of the winter eggs; these eggs are carefully deposited on the leaf-buds of some rose-bush, by a perfect wingless female, at the first approach of the cold weather. I say a perfect wingless female, because, as I shall explain hereafter, most aphides (and especially all the summer crops or generations that appear with such miraculous rapidity on our roses and fruit-trees) are poor fatherless creatures; waifs and strays, budded out vegetatively like the shoots of a plant.

About this strange retrogressive mode of reproduction, however, I shall have more to tell you in due time by-and-by; for the present, we will confine ourselves to the immediate history of the autumn brood, which is regularly produced in the legitimate fashion, as the result of an ordinary insect marriage between perfectly developed males and females. As October approaches, a special generation of such perfect males and females is produced by the unwedded summer green-flies; and the females of this brood, specially told off for the purpose, lay the winter eggs, which are destined to carry on the life of the species across the colder months, when no fresh shoots for food and drink are to be found in the frozen fields or gardens.

The eggs, so to speak, must be regarded as a kind of deferred brood, to bridge over the chilly time when living aphides cannot obtain a livelihood in the open. In No. 2 we see, above, a rose-twig with its leaf-buds, which are undeveloped leaves, inclosed in warm coverings, and similarly intended to bridge over the winter on behalf of the rose-bush. On this twig, then, we have the winter eggs of the aphis, mere dots represented in their natural size; they are providently laid on the bud, which in early spring will grow out into a shoot, and thus supply food at once for the young green-flies as they hatch and develop. So beautifully does Nature in her wisdom take care that blight in due season shall never be wanting to our Marshal Niels and our Gloires de Dijon!

In the same sketch, too, we have, below, a pathetic illustration, greatly magnified, of the poor old worn-out mother, a martyr to maternity, laying her last egg in the crannies of the bud she has chosen. I say "a martyr to maternity" in solemn earnest. You will observe that she is a shrivelled and haggard specimen of over-burdened motherhood. The duties of her station have clearly been too much for her. The reason is that she literally uses herself up in the production of offspring; which is not surprising, if you consider the relative size of egg and egg-layer. When this model mother began to lay, I can assure you she was fat and well-favoured, as attractive a young green-fly as you would be likely to come across in a day's march on the surface of a rose-twig. But once she sets to work, she lays big eggs with a will (big, that is to say, compared with her own size), till she has used up all her soft internal material; and when she has finished, she dies-or, rather, she ceases to be; for there is nothing left of her but a dried and shrivelled skin.

During the winter, indeed-in cold climates at least-the race of aphides dies out altogether for the time being, or only protracts an artificial existence in the heated air of green-houses and drawing-rooms. The species is represented at such dormant periods by the fertilised eggs alone, which lie snug among the folds or scales of the buds till March or April comes back again to wake them. Then, with the first genial weather, the eggs hatch out, and a joyous new brood of aphides emerges. And here comes in one of the greatest wonders; for these summer broods do not consist, like their parents in autumn, of males and females, but of imperfect mothers-all mothers alike, all brotherless sisters, and all budding out young as fast as they can go, without the trouble and expense of a father. They put forth their progeny as a tree puts forth leaves, by mere division. The new broods thus produced are budded out tail first, as shown in No. 3, so that all the members of the family stand with their heads in the same direction, the mother moving on as her offspring increases; and since each new aphis instantly begins to fix its proboscis into the soft leaf-tissue, and in turn to bud out other broods of its own, you need not wonder that your favourite roses are so quickly covered with a close layer of blight in genial weather.

To say the truth, the rate of increase in aphides is so incredibly rapid, that one dare hardly mention it without seeming to exaggerate. A single industrious little green-fly, which devotes itself with a quiet mind to eating and reproduction, may easily within its own lifetime become the ancestor of some billions of great-grandchildren. It is not difficult to see why this should be so. The original parent buds out little ones from its own substance at a prodigious rate; and each of these juniors, reaching maturity at a bound, begins at once to bud out others in turn, so that as long as food and fine weather remain the population increases in an almost unthinkable ratio. Of course, it is the extreme abundance of food and the ease of living that result in this extraordinary rate of fertility; the race has no Malthus to keep it in check-each aphis need only plunge its beak into the rose-shoots or leaves and suck; it can get enough food without the slightest trouble...