Fig. 60.—An ant “milking” a “plant-louse” or “green-fly” for honey-dew. The drop of honey-dew is seen exuding from one of the two long tubes or spouts (called “cornicles”) on the back of the plant-louse at a. These spouts are seen at the hinder part of the body in the drawings of the hop-louse (Figs. [55] to [59]). The ant is causing the aphis to pour out its honey-dew (in fact “milking” it) by “drumming” on the body of the plant-louse with its clubbed antennæ, and has taken the drop of honey-dew between its jaws. This drawing was made from life by the late Mr. Buckton, F.R.S., a great student of these creatures. The ant is that kind known as Myrinica rubra. The plant-louse is the Aphis sambuci or blight of the elder-tree.
Another curious production of the aphides—common on the leaves of elms and other trees infested by them—is known as “honey-dew.” It is sticky and sweet, and was supposed by old writers to have distilled from the stars, or otherwise to have dropped from heaven. It is this sweet secretion which has led to the establishment of a most curious friendship between ants and aphides, or plant-lice. It has long been known that an ant will approach an aphis, and tickle it, when at once the aphis exudes from its cornicles (see [Fig. 60]) a drop of sweet honey-dew, which the ant swallows—just as a man may milk a cow and drink the milk. And the resemblance goes further, for the ants take possession of certain aphides, and keep them either underground or in specially constructed chambers, where they can gain ready access to them and “milk” them for honey-dew. There has been a certain amount of exaggeration in the description of these facts by some of the older writers; but it is undoubtedly true that some species of ants keep special flocks or herds of aphides, and feed on their sweet secretion.
Other small insects nourish themselves on the enormous swarms of plant-lice in a less gentle way, but a way which man is very glad to see in active operation, namely, by biting them and sucking out their soft entrails—thus destroying them in great numbers. The lady-bird beetle is especially active in this matter, both when it is a grub and on attaining its adult form. A trustworthy observer saw as many as forty aphides consumed by a lady-bird in an hour. Where the plant-lice or aphides abound, there come also in countless swarms the beetles known as lady-birds. In the year 1869, such a cloud of these beetles passed over and settled on the fields and gardens of Kent, Sussex, and Surrey, as to cause something like terror; it was impossible to walk in the lanes without crushing hundreds under foot. But the little lady-birds are not like the terrible locust, which appears in millions and devours all vegetation before it; on the contrary, they are what are called “beneficials,” and come solely to feed on and destroy the plant-lice of the hops, plum trees, and apple trees. A first-rate hop crop in the year 1870 was the consequence of the abundance of lady-birds in 1869. It is this beneficent activity of the lady-birds which has given them their name. In Italy they are called Bestioline del Signore, also Madonnine, and Marioline, and in France Bête à Dieu. In English they are “our lady’s blessed bugs,” which save the crops from destruction.
The exertions of the aphides in pricking the plants they infest so as to get at their juices lead to the growth of galls on the leaves, and also on the rootlets of many plants, and often the leaves become rolled up into bag-like bodies filled with aphides. Many trees and smaller plants are killed by these attacks, but it is probable that where the plants have not been rendered delicate by nursing and cultivation, and where the aphides are not a strange foreign kind, introduced by man’s carelessness or by some rarely (if ever) occurring wind or flood, the aphides do not actually destroy any plants by their visitation, excepting the weaklings, and that their numbers are kept within bounds by their natural enemies the lady-birds and other such carnivorous insects.
We must now notice the most interesting of all the wonderful things which have been discovered about these tiny insects, which are even smaller than fleas. Any one who has a rose-garden and chooses to spend some hours a day in studying the “green-fly” can follow out the facts. They reproduce themselves—that is to say, propagate—with astounding rapidity. The great Linnæus, a hundred and fifty years ago, came to the conclusion, from his observation of one kind or species, that in one year a single aphis would produce a quintillion of descendants! Without insisting upon the exact numbers in different kinds of aphides, we may say that that is a fair indication of the rate at which they produce young. No sooner does a mother aphis produce some thirty or forty young, than in a few hours or days, according to the warmth of the season and the abundance of food, these young have grown to full size and themselves each produce the same number of young, and so on through the summer, and even into the autumn. Nineteen generations in sixteen weeks have been counted in some kinds of the plant-lice. Hence it is no wonder that these little creatures increase exceedingly and cover the leaves and shoots on which they feed; no wonder that they furnish a plentiful nourishment for the lady-birds which prey on them. But the most curious thing is this, that these abundant and rapidly reproducing broods of aphis are all females, and that they do not lay eggs, but extrude their young in a more or less complete state of development, that is to say, they are viviparous. They are all females! It is only late in the season that males are produced!
In fact, the summer broods of the “green-fly” and other aphides which do so much damage to rose bushes, hops, and other cultivated plants, are produced by females alone, without the intervention of a male. These minute insects present true instances of that very remarkable and interesting occurrence which is called “parthenogenesis,” or virginal propagation. It is further a noteworthy thing that the virginal aphis mothers do not lay or deposit eggs, but that the young grow from the eggs inside their mothers ([Fig. 61]), and are only extruded when they are complete little six-legged insects, capable of walking, and ready to feed themselves by stabbing the soft leaves of the plant on which they find themselves, and sucking up its juices. The summer aphides are spoken of as being both “viviparous” and “parthenogenetic.” The words are really useful, and we cannot get on without them.
No case is known to medical men or to naturalists of the birth of young from an unimpregnated or virgin mother among what are called the higher animals—those which are classed as vertebrates, and include mankind, mammals, birds, reptiles, batrachians, and fishes. But though uncommon, this virginal reproduction (or “parthenogenesis”) does occur constantly in a very few kinds of small insects and in some small shrimp-like creatures. It has excited the greatest interest amongst naturalists from the early days when it was first observed until the present, and it has been very carefully studied in the past thirty years.