Fam. 7. Aphidae (Plant-lice or Green-fly.)—Minute Insects; as usually met with destitute of wings, though many individuals have two pairs of transparent wings. Antennae long, or moderately long, three- to seven-jointed; abdomen frequently with a pair of tubes (siphons), or short processes on the upper side of the fifth abdominal segment. Tarsi two-jointed, first joint sometimes excessively short.—These soft-skinned Insects are frequently called blight, and are so abundant in temperate climates that a garden, however small, is sure to afford abundance of specimens during the warm months of the year. This great abundance is due to peculiarities in the physiological processes that render these obscure little animals highly important creatures; the individual life for several generations is restricted to constant, or at any rate copious, imbibition of food, accompanied by an almost uninterrupted production of young by parthenogenetic females, the young so produced becoming rapidly (sometimes in the course of eight or ten days, but more usually in about twenty days) themselves devoted to a similar process; so that in the comparatively short period of a few months the progeny resulting from a single individual is almost innumerable. This remarkable state of affairs is accompanied by other peculiarities of physiology, with the result that the life-histories of successive generations become very diverse, and complex cycles of series of generations differing more or less from one another are passed through, the species finally returning to bi-sexual reproduction, and thus inaugurating another cycle of generations. The surprising nature of these facts has in the last 150 years caused an immense amount of discussion, but no satisfactory light has yet been thrown on the conditions that really give rise to the exceptional phenomena. These phenomena are (1) parthenogenesis; (2) oviparous and viviparous reproduction; (3) the production of generations of individuals in which the sexes are very unequally represented, males being frequently entirely absent; (4) the production of individuals differing as to the acquirement of wings, some remaining entirely apterous, while others go on to the winged form; (5) the production of individuals of the same sex with different sexual organs, and distinctions in the very early (but not the earliest) stages of the formation of the individual; (6) differences in the life-habits of successive generations; (7) differences in the habits of individuals of one generation, giving rise to the phenomenon of parallel series. All these phenomena may occur in the case of a single species, though in a very variable extent.
The simple form of Aphid life may be described as follows:—eggs are laid in the autumn, and hatch in the spring, giving rise to females of an imperfect character having no wings; these produce living young parthenogenetically, and this process may be repeated for a few or for many generations, and there may be in these generations a greater or less number of winged individuals, and perhaps a few males.[[517]] After a time when temperature falls, or when the supply of food is less in quantity, or after a period of deliberate abstention from food, sexual individuals are produced and fertilised eggs are laid which hatch in the spring, and the phenomena are repeated. In other cases these phenomena are added to or rendered more complicated by the intercalated parthenogenetic generations exhibiting well-marked metamorphosis, of kinds such as occur in apterous or in winged Insects; while again the habits of successive generations may differ greatly, the individuals of some generations dwelling in galls, while those of other generations live underground on roots.
Parthenogenesis.—Returning to the various kinds of peculiarities we have enumerated on the preceding page, we may remark that the phenomena of parthenogenesis have been thoroughly established as occurring in Aphidae since Bonnet discovered the fact 150 years ago; and though they have not been investigated in much detail it is known that the parthenogenesis is usually accompanied by the production of young all of the female sex. In other cases males are parthenogenetically produced; but whether these males come from a female that produces only that sex is not yet, so far as the writer knows, established. A note by Lichtenstein[[518]] suggests that usually only one sex is produced by a parthenogenetic female, but that both sexes are sometimes so produced. There is not at present any species of Aphid known to be perpetuated by an uninterrupted series of parthenogenetic generations. It was formerly supposed that there are no males at all in Chermes, but, as we shall subsequently show, this was erroneous. It has, however, been observed that a series of such generations may be continued without interruption for a period of four years, and we have no reason to suppose that even this could not be much exceeded under favourable conditions. The parthenogenetic young may be produced either viviparously or oviparously, according to species.
Oviparous and viviparous reproduction.—The distinction between these two processes has been extensively discussed, some naturalists maintaining that they are thoroughly distinct ab initio. This view, however, cannot be sustained. The best authorities are agreed that in the earliest processes of individualisation the ovum, and the pseudovum[[519]] giving rise to a viviparous individual, are indistinguishable. Leydig, Huxley, Balbiani, and Lemoine are agreed as to this. Nevertheless, differences in the development occur extremely early. The nature of these differences may be briefly described by saying that in the viviparous forms the embryonic development sets in before the formation of the egg is properly completed. Balbiani says, "In fact at this moment [when the viviparous development is commencing] the germ [pseudovum] is far from having obtained the development it is capable of, and from having accumulated all the matter necessary for the increase of the embryo, so that the evolution of the former coincides, so to speak, with that of the latter. On the other hand, in the true ovum the two processes are chronologically separate, for the rudiment of the new individual never appears before the egg has completed the growth of its constituent parts."[[520]] As regards the difference in structure of the organs of viviparously and oviparously producing individuals, it is sufficient to remark that they are not of great importance, being apparently confined to certain parts remaining rudimentary in the former. Leydig, indeed, found an Aphis in which certain of the egg-tubes contained eggs in various stages of development, and others embryos in all stages.[[521]]
As regards the physiology of production of winged and wingless individuals there has been but little exact inquiry. Vast numbers of individuals may be produced without any winged forms occurring, while on the other hand these latter are occasionally so abundant as to float about in swarms that darken the air; the two forms are probably, however, determined by the supply of food. The winged forms are less prolific than the apterous forms; and Forbes has noticed in Aphis maidi-radicis, where the generations consist partly of apterous and partly of winged individuals, that when the corn begins to flag in consequence of the attacks of the Aphis, then the proportion of winged individuals becomes large.[[522]] The appearance of winged individuals is frequently accompanied by a peculiar change of habit; the winged individuals migrating to another plant, which in many cases is of a totally different botanical nature from that on which the apterous broods were reared: for instance Aphis mali, after producing several apterous generations on apple, gives rise to winged individuals that migrate to the stems of corn or grass, and feeding thereon commence another cycle of generations. The study of this sort of Aphis-migration is chiefly modern, but many very curious facts have already been brought to light; thus Drepanosiphum platanoides, after producing a certain number of viviparous generations on maple (Acer), quits this food-plant for another, but after two or three months returns again to the maple, and produces sexual young that lay eggs.[[523]] Histories such as this are rather common. Even more interesting are the cases of those species that, after some weeks of physiological activity on a plant, pass into a state of repose on the same plant, and then after some weeks produce sexual young. On the whole, it would appear that the appearance of winged forms is a concomitant of decreasing nutrition. It is a very remarkable fact that the sexually perfect females are invariably apterous, and this is frequently also the case with the males. It is also highly remarkable that the sexually perfect individuals are of comparatively small size. There are at least three kinds of males in Aphidae—1, winged males; 2, wingless males with mouth well developed; 3, wingless small males with mouth absent. As regards some of these points the conditions usual in Insect life are reversed.[[524]] Huxley inclined to treat all these products of a fertilised egg, that are antecedent to another process of gamogenesis (i.e. production with fertilisation), as one zoological individual: in that case the Aphis zoological individual is winged before attaining the mature state, and is wingless and smaller when mature. Some species may have as a rule two, others three, winged generations in a year.
Fig. 285—Chermes abietis; hibernating female or "winter-mother." Europe. Much magnified. (After Cholodkovsky.)
Parallel series.—In certain cases individuals of one generation assume different habits, and so set up the phenomenon known as parallel series. This has been recently investigated in the genus Chermes by Blochmann, Dreyfus, and Cholodkovsky. This latter savant informs us[[525]] that a wingless parthenogenetic female of Chermes hibernates on a fir-tree—Picea excelsa—and in the spring lays numerous eggs; these hatch, and by the effects of suction of the Chermes on the young shoots, galls are formed (Fig. 286), in which the Insects are found in large numbers; when they have grown the galls open, and allowing the Insects to escape these moult and become winged females. They now take on different habits; some of them remain on the Picea, lay their eggs thereon, and out of these there are produced young that grow into hibernating females, which next spring produce galls as their grandmothers did; but another portion migrates to the Larch (Larix); here eggs are laid, from which proceed wingless parthenogenetic females, that hibernate on their new or secondary plant, and in the following spring lay their eggs and give rise to a dimorphic generation, part of them becoming nymphs and going on to the winged condition, while the other part remain wingless and lay eggs, that give rise to yet another wingless generation; in fact, a second pair of parallel series is formed on the new plant, of which one is wingless, and exclusively parthenogenetic, and continues to live in this fashion for an indefinite period on the secondary plant, while the other part becomes winged; these latter are called sexuparous, and go back to the Picea, and there lay eggs, that give rise to the sexual forms. If we would summarise these facts with a view to remembering them, we may say that a migration of a part of a generation from the Picea was made with a view of producing a sexual generation, but that only a portion of the migrants succeeded in effecting the object of the migration, and this only in their third generation. Thus portions remained on the Picea, producing unisexual (female) individuals, and a portion of those that emigrated to the Larix remained thereon, producing also unisexual (female) individuals, while the others returned to the Picea and produced a sexual generation. How long the production of the unisexual generations may continue has not been determined.
Phylloxera.—The Phylloxera, that has caused such an enormous amount of damage in the Old World during the last thirty years, is a small Aphid that was introduced from North America into Europe. In North America it is not so injurious as it is in Europe, owing, no doubt, to slight distinctions in the conditions of life in the two hemispheres, as one of which may be mentioned that in Europe a larger proportion of the individuals produced appear to confine themselves to feeding on the roots, P. vastatrix being one of the species that lives both in galls on leaves, and underground on the roots. The species is one that exhibits in their most complex form the peculiar phenomena of Aphid life we have already mentioned. It has probably only one congener, Phylloxera quercus, and of this Lichtenstein says that in its cycle, from the starting-point of the winter-egg to the assumption of the sexual condition, it exhibits a series of no less than twenty-one forms.[[526]] The life of Phylloxera vastatrix apparently differs essentially from what we have described in Chermes, inasmuch as the migrations are only between leaf and root of the same plant—the vine—and not from one species of plant to another. Some authorities treat Phylloxera and Chermes as a separate family under the name of Phylloxeridae.
