The minute “green-flies” which attack all kinds of plants, and among which are ranked the hop-louse or hop-blight, the rose aphis or green-fly of rose trees, the woolly blight or aphis of apple trees and pear trees, and the terrible vine-killer—the Phylloxera vastatrix—form a special group of bug-like insects known as the Aphides. They have soft cylindrical bodies, six legs, sometimes two pairs of transparent wings, sometimes none, and a sharp beak (in some kinds this is one and a half times as long as the body), with which they prick the soft parts of plants, when they suck up the juices which issue from the wound (Fig. 59). There is in the temperate regions of the world a special kind of aphis or plant-louse peculiar to each of many kinds of flowering plants, including most trees. A very complete, illustrated account of the kinds or species of British aphides, amounting to some two hundred, was produced by the late Mr. Buckton, F.R.S., and published by the Ray Society.

There are many facts of extraordinary interest about these tiny swarming insects. In the first place, they are closely related to the minute scale-insects or CoccidÆ, several species of which produce the celebrated lac of lacquer-work and the dyes known as lake, cochineal, and kermes, the latter a dye manufactured in South Europe and used to colour wool and cloth crimson before cochineal reached us from Mexico. The CoccidÆ include also the “mussel-scale” and other destructive diseases of fruit trees. A beautiful purple colour can be extracted from crushed masses of some kinds of aphides (as well as from CoccidÆ), and has been used as a dye. The aphides have very generally a green colour, like many insects (caterpillars and leaf insects) which pass their lives upon green leaves and feed on them. It is often supposed that this green colour is merely the green colouring matter (so-called chlorophyll) of the leaf, taken up by the insects in feeding on the leaf. But this is not so; it is a peculiar substance derived in a crude state from the plant-juice, but digested in the stomach and completed in the insects’ blood and tissues. Then, again, the aphides produce curious secretions, often in great abundance, which surround them as the lac surrounds the lac-insect. The threads which are produced in such abundance, by the woolly aphis of apple trees, as to look like masses of cotton wool adhering to the twigs of the tree, are of this nature.

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.

In order to appreciate this matter it is necessary to know the chief facts about the ordinary process of reproduction in animals and plants. All animals and plants are built up of minute particles of living matter called “cells” (see p. 170). Really, these are not cells, or hollow boxes, or cases. We use the word “cell” for the contents of a cell. Each is a droplet of protoplasm or living matter lying in a small or large envelope or case of dead matter which it has produced around itself (Fig. 61). Observers using their microscopes saw at first only the case, and called it a “cell,” and the word “cell” is now used almost universally for the soft stuff within the cell (see p. 173). Each soft cell of “plasm” or “protoplasm” has a very special structure. The existence in it of a central kernel, or “nucleus” of peculiarly active substance, is the most obvious feature. These “cells” are so small (for instance, those which build up the human body) that from one to two thousand could be placed side by side on a line an inch long. They are the “units” which make up the body of an animal or plant, just as bricks and planks and rods make up a building constructed by human contrivance. Two most important things about them are—first, that each is always the seat of chemical activity, absorbing liquid material, changing it and either fixing it or throwing it out in a new chemical condition; and, second, that as a result each cell grows, and after a very little growth divides into two. This “dividing into two” is immensely important, for in this way the number of cells forming a very young or small animal or plant is increased from a few thousands to many millions whilst the organism grows. And not only that, but we find on tracing the young animal or plant back to its beginning as an individual that it actually started as a single cell. The germ of every living thing, then, is a single nucleated particle of protoplasm—a cell which we call the “egg-cell,” because “eggs” are merely shells and packing to hold and protect this all-important egg-cell.

Every individual flower, tree, insect, snail, fish, and man started as a single egg-cell, which became detached from the mother’s body. Take the case of a common marine animal, the star-fish. At the breeding season, early in the year, the female star-fish discharges thousands of these egg-cells into the sea-water. Each floats separately in a delicate case of its own. Before any one of those floating egg-cells can commence to divide so as to build up a new mass of cells—a new young star-fish—it must undergo the process of “fertilisation.” That is to say, its substance must fuse with that of a “sperm-cell.” These “sperm-cells” are discharged into the sea-water in countless thousands by the male star-fishes. They are excessively minute, actively wriggling threads, swollen out at one end to form a little knob, the “nucleus” of the sperm-cell (see p. 134 for figures of the spermatozoa, and eggs of the oyster). The water is rendered cloudy by the abundance of these microscopic filaments, which are called “spermatozoa.” One sperm-cell, or spermatozoon comes into contact, in the sea-water, with each of the discharged floating egg-cells. It burrows into it and fuses or melts and mixes with the substance of the egg-cell. The whole process is easy to watch with a microscope, and I am writing of what I, in common with many others, have actually seen.

The egg-cell after this process consists really of the substance of two equal cells—the egg-cell and the sperm-cell—completely fused so as to form a single cell, having a single “nucleus,” which has resulted from the fusion of the nucleus of the egg-cell with that of the sperm-cell. Now, and not before, the egg-cell can divide, take up nourishment, and continue to divide and grow, so as to form a constantly increasing mass of young cells, a young animal which gradually assumes the form of a star-fish. All animals, and plants, too, reproduce themselves in this way. When the animal or plant is not aquatic in its habits the sperm-cell and the egg-cell cannot be discharged and take their chance of coming into contact with one another outside the parent’s body; the sperm-cells are, in such cases, received into a chamber of the egg-producing parent’s body, and there the fusion of the egg-cells with them, one sperm-cell to one egg-cell, takes place. Parthenogenesis then consists in the omission of the fusion of a sperm-cell with the egg-cell. The egg-cell develops, divides again and again, and produces the young animal without the addition to it of a sperm-cell—without, in fact, being “fertilised,” as it is called. That is what happens in the summer broods of the little plant-lice or aphides (Fig. 57). When, however, the cold weather comes the virgin mothers suddenly produce two kinds of young—males as well as females—and then the solitary winter egg, which the late autumn females lay to last through winter until spring, is fertilised by a sperm-cell derived from the late produced autumn male (Fig. 56) in the ordinary way.

Another parthenogenetic animal is the rare little fresh-water shrimp called Apus, which goes on multiplying in this manner in wayside ponds for years, thousands of female individuals being produced in successive seasons, laying their eggs and carrying on the race for an indefinite time until at last—one fine day—we do not know why then and not before, that rare creature a male Apus is hatched. Why these and one or two other such small shrimps and insects are able to set aside the almost universal law as to the necessity for fertilisation of the egg-cell by a sperm-cell, naturalists have not yet found out. It is quite certain that these exceptional creatures have been derived from ancestors which had their eggs fertilised in the regular way, and that this elimination of the male is a special device, an innovation.

There are incomplete attempts at it in other insects. Thus it has been discovered that the queen bee produces only females from the eggs which are fertilised before she lays them. When the stock of sperm-cells which she received from a drone in her nuptial flight is exhausted, or if we carefully remove by a painless operation the internal sac in which they are stored, the eggs are no longer fertilised, but they are not rendered sterile or abortive. They develop into drones! And drones or male bees are produced in no other way, and only drones are so produced, never worker-females (so-called neuters) nor queens.

Another curious fact is that in rearing moths in captivity some naturalists have quite unexpectedly found that when they have hatched out female moths from the chrysalids and kept them from the moment of hatching quite apart from the male moths (which are of another size and colour, and easily distinguished), these females will sometimes lay eggs—unfertilised eggs—which give birth to caterpillars, which feed and complete all their changes. The second generation of moths so produced are male and female, but the females, being kept apart again, produce a parthenogenetic brood, and the process has been repeated to a third generation. These instances are very rare. The remarkable thing about them is that, apparently, the parthenogenesis is only due to the experimental interference of an entomologist, and that unless some such accident had befallen the moths, the eggs would have been fertilised in the usual way, since there was no deficiency of male moths. These facts have led to many interesting speculations, and are particularly curious in regard to the inquiry as to what determines the sex of offspring, about which sensational announcements are sometimes made in the foreign correspondence columns of our newspapers. Here we find the parthenogenetic eggs of the moths producing both males and females, those of the aphides and the pond-shrimp producing predominantly females, and those of the queen bee producing exclusively males (drones). Biologists have not yet arrived at a solution of the problem raised by these divergent results.

It is necessary, in regard to this subject, to remember that many lower animals and plants can reproduce or propagate by separating “buds,” or large bits of their bodies, built up of thousands of cells, and, therefore, not to be confused with the single egg-cell. The egg-cell is a cell specially prepared for fusion with a sperm-cell, necessitating—except in very rare instances—the union in the new individual or young of living material from two separate parental organisms, and, therefore, in many cases, from two widely separate lines of ancestry. A snippet, or bit cut from a begonia leaf, will produce a new individual plant; a bit cut or torn from a polyp will similarly give rise to a new individual: but the parthenogenetic egg is not to be confused with these masses of cells. It is a true egg-cell which might have been fertilised, and it is found in animals such as insects and crustaceans, which are more highly elaborated in structure than any which, like the polyps and zoophytes, multiply by buds and cuttings.