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    10. Honey-Dew

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    Chapter 11
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    Place, the garden. Time, summer. Dramatis personæ, a couple of small
    brown garden-ants, and a lazy clustering colony of wee green
    'plant-lice,' or 'blight,' or aphides. The exact scene is usually on the
    young and succulent branches of a luxuriant rose-bush, into whose soft
    shoots the aphides have deeply buried their long trunk-like snouts, in
    search of the sap off which they live so contentedly through their brief
    lifetime. To them, enter the two small brown ants, their lawful
    possessors; for ants, too, though absolutely unrecognised by English law
    ('de minimis non curat lex,' says the legal aphorism), are nevertheless
    in their own commonwealth duly seised of many and various goods and
    chattels; and these same aphides, as everybody has heard, stand to them
    in pretty much the same position as cows stand to human herdsmen. Throw
    in for sole spectator a loitering naturalist, and you get the entire
    _mise-en-scène_ of a quaint little drama that works itself out a dozen
    times among the wilted rose-trees beneath the latticed cottage windows
    every summer morning.

    It is a delightful sight to watch the two little lilliputian proprietors
    approaching and milking these their wee green motionless cattle. First
    of all, the ants quickly scent their way with protruded antennæ (for
    they are as good as blind, poor things!) up the prickly stem of the
    rose-bush, guided, no doubt, by the faint perfume exhaled from the
    nectar above them. Smelling their road cautiously to the ends of the
    branches, they soon reach their own particular aphides, whose bodies
    they proceed gently to stroke with their outstretched feelers, and then
    stand by quietly for a moment in happy anticipation of the coming
    dinner. Presently, the obedient aphis, conscious of its lawful master's
    friendly presence, begins slowly to emit from two long horn-like tubes
    near the centre of its back a couple of limpid drops of a sticky pale
    yellow fluid. Honey-dew our English rustics still call it, because, when
    the aphides are not milked often enough by ants, they discharge it
    awkwardly of their own accord, and then it falls as a sweet clammy dew
    upon the grass beneath them. The ant, approaching the two tubes with
    cautious tenderness, removes the sweet drops without injuring in any way
    his little _protégé_, and then passes on to the next in order of his
    tiny cattle, leaving the aphis apparently as much relieved by the
    process as a cow with a full hanging udder is relieved by the timely
    attention of the human milkmaid.

    Evidently, this is a case of mutual accommodation in the political
    economy of the ants and aphides: a free interchange of services between
    the ant as consumer and the aphis as producer. Why the aphides should
    have acquired the curious necessity for getting rid of this sweet,
    sticky, and nutritious secretion nobody knows with certainty; but it is
    at least quite clear that the liquid is a considerable nuisance to them
    in their very sedentary and monotonous existence--a waste product of
    which they are anxious to disembarrass themselves as easily as
    possible--and that while they themselves stand to the ants in the
    relation of purveyors of food supply, the ants in return stand to them
    in the relation of scavengers, or contractors for the removal of useless
    accumulations.

    Everybody knows the aphides well by sight, in one of their forms at
    least, the familiar rose aphis; but probably few people ever look at
    them closely and critically enough to observe how very beautiful and
    wonderful is the organisation of their tiny limbs in all its exquisite
    detail. If you pick off one good-sized wingless insect, however, from a
    blighted rose-leaf, and put him on a glass slide under a low power of
    the microscope, you will most likely be quite surprised to find what a
    lovely little creature it is that you have been poisoning wholesale all
    your life long with diluted tobacco-juice. His body is so transparent
    that you can see through it by transmitted light: a dainty glass globe,
    you would say, of emerald green, set upon six tapering, jointed, hairy
    legs, and provided in front with two large black eyes of many facets,
    and a pair of long and very flexible antennæ, easily moved in any
    direction, but usually bent backward when the creature is at rest so as
    to reach nearly to his tail as he stands at ease upon his native
    rose-leaf. There are, however, two other features about him which
    specially attract attention, as being very characteristic of the aphides
    and their allies among all other insects. In the first place, his mouth
    is provided with a very long snout or proboscis, classically described
    as a rostrum, with which he pierces the outer skin of the rose-shoot
    where he lives, and sucks up incessantly its sweet juices. This organ is
    common to the aphis with all the other bugs and plant-lice. In the
    second place, he has half-way down his back (or a little more) a pair of
    very peculiar hollow organs, the honey tubes, from which exudes that
    singular secretion, the honey-dew. These tubes are not found in quite
    all species of aphides, but they are very common among the class, and
    they form by far the most conspicuous and interesting organs in all
    those aphides which do possess them.

    The life-history of the rose-aphis, small and familiar as is the insect
    itself, forms one of the most marvellous and extraordinary chapters in
    all the fairy tales of modern science. Nobody need wonder why the blight
    attacks his roses so persistently when once he has learnt the unusual
    provision for exceptional fertility in the reproduction of these insect
    plagues. The whole story is too long to give at full length, but here is
    a brief recapitulation of a year's generations of common aphides.

    In the spring, the eggs of last year's crop, which have been laid by the
    mothers in nooks and crannies out of reach of the frost, are quickened
    into life by the first return of warm weather, and hatch out their brood
    of insects. All this brood consists of imperfect females, without a
    single male among them; and they all fasten at once upon the young buds
    of their native bush, where they pass a sluggish and uneventful
    existence in sucking up the juice from the veins on the one hand, and
    secreting honey-dew upon the other. Four times they moult their skins,
    these moults being in some respects analogous to the metamorphosis of
    the caterpillar into chrysalis and butterfly. After the fourth moult,
    the young aphides attain maturity; and then they give origin,
    parthenogenetically, to a second brood, also of imperfect females, all
    produced without any fathers. This second brood brings forth in like
    manner a third generation, asexual, as before; and the same process is
    repeated without intermission as long as the warm weather lasts. In each
    case, the young simply bud out from the ovaries of the mothers, exactly
    as new crops of leaves bud out from the rose-branch on which they grow.
    Eleven generations have thus been observed to follow one another rapidly
    in a single summer; and indeed, by keeping the aphides in a warm room,
    one may even make them continue their reproduction in this purely
    vegetative fashion for as many as four years running. But as soon as
    the cold weather begins to set in, perfect male and female insects are
    produced by the last swarm of parthenogenetic mothers; and these true
    females, after being fertilised, lay the eggs which remain through the
    winter, and from which the next summer's broods have to begin afresh the
    wonderful cycle. Thus, only one generation of aphides, out of ten or
    eleven, consists of true males and females: all the rest are false
    females, producing young by a process of budding.

    Setting aside for the present certain special modifications of this
    strange cycle which have been lately described by M. Jules Lichtenstein,
    let us consider for a moment what can be the origin and meaning of such
    an unusual and curious mode of reproduction.

    The aphides are on the whole the most purely inactive and vegetative of
    all insects, unless indeed we except a few very debased and degraded
    parasites. They fasten themselves early in life on to a particular shoot
    of a particular plant; they drink in its juices, digest them, grow, and
    undergo their incomplete metamorphoses; they produce new generations
    with extraordinary rapidity; and they vegetate, in fact, almost as much
    as the plant itself upon which they are living. Their existence is
    duller than that of the very dullest cathedral city. They are thus
    essentially degenerate creatures: they have found the conditions of life
    too easy for them, and they have reverted to something so low and simple
    that they are almost plant-like in some of their habits and
    peculiarities.

    The ancestors of the aphides were free winged insects; and, in certain
    stages of their existence, most living species of aphides possess at
    least some winged members. On the rose-bush, you can generally pick off
    a few such larger winged forms, side by side with the wee green wingless
    insects. But creatures which have taken to passing most of their life
    upon a single spot on a single plant hardly need the luxury of wings;
    and so, in nine cases out of ten, natural selection has dispensed with
    those needless encumbrances. Even the legs are comparatively little
    wanted by our modern aphides, which only require them to walk away in a
    stately sleepy manner when rudely disturbed by man, lady-birds, or other
    enemies; and indeed the legs are now very weak and feeble, and incapable
    of walking for more than a short distance at a time under exceptional
    provocation. The eyes remain, it is true; but only the big ones: the
    little ocelli at the top of the head, found amongst so many of their
    allies, are quite wanting in all the aphides. In short, the plant-lice
    have degenerated into mere mouths and sacks for sucking and storing food
    from the tissues of plants, provided with large honey-tubes for getting
    rid of the waste sugar.

    Now, the greater the amount of food any animal gets, and the less the
    amount of expenditure it performs in muscular action, the greater will
    be the surplus it has left over for the purposes of reproduction. Eggs
    or young, in fact, represent the amount thus left over after all the
    wants of the body have been provided for. But in the rose-aphis the
    wants of the body, when once the insect has reached its full growth, are
    absolutely nothing; and it therefore then begins to bud out new
    generations in rapid succession as fast as ever it can produce them.
    This is strictly analogous to what we see every day taking place in all
    the plants around us. New leaves are produced one after another, as fast
    as material can be supplied for their nutrition, and each of these new
    leaves is known to be a separate individual, just as much as the
    individual aphis. At last, however, a time comes when the reproductive
    power of the plant begins to fail, and then it produces flowers, that is
    to say stamens (male) and pistils (female), whose union results in
    fertilisation and the subsequent outgrowth of fruit and seeds. Thus a
    year's cycle of the plant-lice exactly answers to the life-history of an
    ordinary annual. The eggs correspond to the seeds; the various
    generations of aphides budding out from one another by parthenogenesis
    correspond to the leaves budded out by one another throughout the
    summer; and the final brood of perfect males and females answers to the
    flower with its stamen and pistils, producing the seeds, as they produce
    the eggs, for setting up afresh the next year's cycle.

    This consideration, I fancy, suggests to us the most probable
    explanation of the honey-tubes and honey-dew. Creatures that eat so much
    and reproduce so fast as the aphides are rapidly sucking up juices all
    the time from the plant on which they fasten, and converting most of the
    nutriment so absorbed into material for fresh generations. That is how
    they swarm so fast over all our shrubs and flowers. But if there is any
    one kind of material in their food in excess of their needs, they would
    naturally have to secrete it by a special organ developed or enlarged
    for the purpose. I don't mean that the organ would or could be developed
    all at once, by a sudden effort, but that as the habit of fixing
    themselves upon plants and sucking their juices grew from generation to
    generation with these descendants of originally winged insects, an organ
    for permitting the waste product to exude must necessarily have grown
    side by side with it. Sugar seems to have been such a waste product,
    contained in the juices of the plant to an extent beyond what the
    aphides could assimilate or use up in the production of new broods; and
    this sugar is therefore secreted by special organs, the honey-tubes. One
    can readily imagine that it may at first have escaped in small
    quantities, and that two pores on their last segment but two may have
    been gradually specialised into regular secreting organs, perhaps under
    the peculiar agency of the ants, who have regularly appropriated so many
    kinds of aphides as miniature milch cows.

    So completely have some species of ants come to recognise their own
    proprietary interest in the persons of the aphides, that they provide
    them with fences and cow-sheds on the most approved human pattern.
    Sometimes they build up covered galleries to protect their tiny cattle;
    and these galleries lead from the nest to the place where the aphides
    are fixed, and completely enclose the little creatures from all chance
    of harm. If intruders try to attack the farmyard, the ants drive them
    away by biting and lacerating them. Sir John Lubbock, who has paid great
    attention to the mutual relations of ants and aphides, has even shown
    that various kinds of ants domesticate various species of aphis. The
    common brown garden-ant, one of the darkest skinned among our English
    races, 'devotes itself principally to aphides which frequent twigs and
    leaves'; especially, so far as I have myself observed, the bright green
    aphis of the rose, and the closely allied little black aphis of the
    broad bean. On the other hand a nearly related reddish ant pays
    attention chiefly to those aphides which live on the bark of trees,
    while the yellow meadow-ants, a far more subterranean species, keep
    flocks and herds of the like-minded aphides which feed upon the roots of
    herbs and grasses.

    Sir John Lubbock, indeed, even suggests--and how the suggestion would
    have charmed 'Civilisation' Buckle!--that to this difference of food and
    habit the distinctive colours of the various species may very probably
    be due. The ground which he adduces for this ingenious idea is a capital
    example of the excellent use to which out-of-the-way evidence may be
    cleverly put by a competent evolutionary thinker. 'The Baltic amber,' he
    says, 'contains among the remains of many other insects a species of
    ant intermediate between our small brown garden-ants and the little
    yellow meadow-ants. This is possibly the stock from which these and
    other allied species are descended. One is tempted to suggest that the
    brown species which live so much in the open air, and climb up trees and
    bushes, have retained and even deepened their dark colour; while others,
    such as the yellow meadow-ant, which lives almost entirely below ground,
    have become much paler.' He might have added, as confirmatory evidence,
    the fact that the perfect winged males and females of the yellow
    species, which fly about freely during the brief honeymoon in the open
    air, are even darker in hue than the brown garden-ant. But how the light
    colour of the neuter workers gets transmitted through these dusky
    parents from one generation to another is part of that most insoluble
    crux of all evolutionary reasoning--the transmission of special
    qualities to neuters by parents who have never possessed them.

    This last-mentioned yellow meadow-ant has carried the system of
    domestication further in all probability than any other species among
    its congeners. Not only do the yellow ants collect the root-feeding
    aphides in their own nests, and tend them as carefully as their own
    young, but they also gather and guard the eggs of the aphides, which,
    till they come to maturity, are of course quite useless. Sir John
    Lubbock found that his yellow ants carried the winter eggs of a species
    of aphis into their nest, and there took great care of them. In the
    spring, the eggs hatched out; and the ants actually carried the young
    aphides out of the nest again, and placed them on the leaves of a daisy
    growing in the immediate neighbourhood. They then built up a wall of
    earth over and round them. The aphides went on in their usual lazy
    fashion throughout the summer, and in October they laid another lot of
    eggs, precisely like those of the preceding autumn. This case, as the
    practised observer himself remarks, is an instance of prudence
    unexampled, perhaps, in the animal kingdom, outside man. 'The eggs 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 their nests by the ants, and tended by them with the utmost
    care through the long winter months until the following March, when the
    young ones are brought out again and placed on the young shoots of the
    daisy.' Mr. White of Stonehouse has also noted an exactly similar
    instance of formican providence.

    The connection between so many ants and so many species of the aphides
    being so close and intimate, it does not seem extravagant to suppose
    that the honey-tubes in their existing advanced form at least may be due
    to the deliberate selective action of these tiny insect-breeders.
    Indeed, when we consider that there are certain species of beetles which
    have never been found anywhere except in ants' nests, it appears highly
    probable that these domesticated forms have been produced by the ants
    themselves, exactly as the dog, the sheep, and the cow, in their
    existing types, have been produced by deliberate human selection. If
    this be so, then there is nothing very out-of-the-way in the idea that
    the ants have also produced the honey-tubes of aphides by their long
    selective action. It must be remembered that ants, in point of
    antiquity, date back, under one form or another, no doubt to a very
    remote period of geological time. Their immense variety of genera and
    species (over a thousand distinct kinds are known) show them to be a
    very ancient family, or else they would not have had time to be
    specially modified in such a wonderful multiformity of ways. Even as
    long ago as the time when the tertiary deposits of Oeningen and
    Radoboj were laid down, Dr. Heer of Zurich has shown that at least
    eighty-three distinct species of ants already existed; and the number
    that have left no trace behind is most probably far greater. Some of the
    beetles and woodlice which ants domesticate in their nests have been
    kept underground so long that they have become quite blind--that is to
    say, have ceased altogether to produce eyes, which would be of no use to
    them in their subterranean galleries; and one such blind beetle, known
    as Claviger, has even lost the power of feeding itself, and has to be
    fed by its masters from their own mandibles. Dr. Taschenberg enumerates
    300 species of true ants'-nest insects, mostly beetles, in Germany
    alone; and M. André gives a list of 584 kinds, habitually found in
    association with ants in one country or another. Compared with these
    singular results of formican selection, the mere production or further
    development of the honey-tubes appears to be a very small matter.

    But what good do the aphides themselves derive from the power of
    secreting honey-dew? For we know now that no animal or plant is ever
    provided with any organ or part merely for the benefit of another
    creature: the advantage must at least be mutual. Well, in the first
    place, it is likely that, in any case, the amount of sugary matter in
    the food of the aphides is quite in excess of their needs; they
    assimilate the nitrogenous material of the sap, and secrete its
    saccharine material as honey-dew. That, however, would hardly account
    for the development of special secretory ducts, like the honey-tubes, in
    which you can actually see the little drops of honey rolling, under the
    microscope. But the ants are useful allies to the aphides, in guarding
    them from another very dangerous type of insect. They are subject to the
    attacks of an ichneumon fly, which lays its eggs in them, meaning its
    larvæ to feed upon their living bodies; and the ants watch over the
    aphides with the greatest vigilance, driving off the ichneumons whenever
    they approach their little _protégés_.

    Many other insects besides ants, however, are fond of the sweet
    secretions of the aphides, and it is probable that the honey-dew thus
    acts to some extent as a preservative of the species, by diverting
    possible foes from the insects themselves, to the sugary liquid which
    they distil from their food-plants. Having more than enough and to spare
    for all their own needs, and the needs of their offspring, the
    plant-lice can afford to employ a little of their nutriment as a bribe
    to secure them from the attacks of possible enemies. Such compensatory
    bribes are common enough in the economy of nature. Thus our common
    English vetch secretes a little honey on the stipules or wing-like
    leaflets on the stem, and so distracts thieving ants from committing
    their depredations upon the nectaries in the flowers, which are intended
    for the attraction of the fertilising bees; and a South American acacia,
    as Mr. Belt has shown, bears hollow thorns and produces honey from a
    gland in each leaflet, in order to allure myriads of small ants which
    nest in the thorns, eat the honey, and repay the plant by driving away
    their leaf-cutting congeners. Indeed, as they sting violently, and issue
    forth in enormous swarms whenever the plant is attacked, they are even
    able to frighten off browsing cattle from their own peculiar acacia.

    Aphides, then, are essentially degraded insects, which have become
    almost vegetative in their habits, and even in their mode of
    reproduction, but which still retain a few marks of their original
    descent from higher and more locomotive ancestors. Their wings,
    especially, are useful to the perfect forms in finding one another, and
    to the imperfect ones in migrating from one plant to its nearest
    neighbours, where they soon become the parents of fresh hordes in rapid
    succession. Hence various kinds of aphides are among the most dreaded
    plagues of agriculturists. The 'fly,' which Kentish farmers know so well
    on hops, is an aphis specialised for that particular bine; and, when
    once it appears in the gardens, it spreads with startling rapidity from
    one end of the long rows to the other. The phylloxera which has spoilt
    the French vineyards is a root-feeding form that attacks the vine, and
    kills or maims the plant terribly, by sucking the vital juices on their
    way up into the fresh-forming foliage. The 'American blight' on apple
    trees is yet another member of the same family, a wee creeping cottony
    creature that hides among the fissures of the bark, and drives its very
    long beak far down into the green sappy layer underlying the dead outer
    covering. In fact, almost all the best-known 'blights' and
    bladder-forming insects are aphides of one kind or another, affecting
    leaves, or stalks, or roots, or branches.

    It is one of the most remarkable examples of the limitation of human
    powers that while we can easily exterminate large animals like the wolf
    and the bear in England, or the puma and the wolverine in the settled
    States of America, we should be so comparatively weak against the
    Colorado beetle or the fourteen-year locust, and so absolutely powerless
    against the hop-fly, the turnip-fly, and the phylloxera. The smaller and
    the more insignificant our enemy, viewed individually, the more
    difficult is he to cope with in the mass. All the elephants in the world
    could have been hunted down and annihilated, in all probability, with
    far less labour than has been expended upon one single little all but
    microscopic parasite in France alone. The enormous rapidity of
    reproduction in the family of aphides is the true cause of our
    helplessness before them. It has been calculated that a single aphis may
    during its own lifetime become the progenitor of 5,904,900,000
    descendants. Each imperfect female produces about ninety young ones,
    and lives long enough to see its children's children to the fifth
    generation. Now, ninety multiplied by ninety four times over gives the
    number above stated. Of course, this makes no allowance for casualties
    which must be pretty frequent: but even so, the sum-total of aphides
    produced within a small garden in a single summer must be something very
    extraordinary.

    It is curious, too, that aphides on the whole seem to escape the notice
    of insect-eating birds very tolerably. I cannot, in fact, discover that
    birds ever eat them, their chief real enemy being the little lizard-like
    larva of the lady-bird, which devours them everywhere greedily in
    immense numbers. Indeed, aphides form almost the sole food of the entire
    lady-bird tribe in their earlier stages of existence; and there is no
    better way of getting rid of blight on roses and other garden plants
    than to bring in a good boxful of these active and voracious little
    grubs from the fields and hedges. They will pounce upon the aphides
    forthwith as a cat pounces upon the mice in a well-stocked barn or
    farmyard. The two-spotted lady-bird in particular is the determined
    exterminator of the destructive hop-fly, and is much beloved accordingly
    by Kentish farmers. No doubt, one reason why birds do not readily see
    the aphis of the rose and most other species is because of their
    prevailing green tint, and the close way in which they stick to the
    leaves or shoots on whose juices they are preying. But in the case of
    many black and violet species, this protection of imitative colour is
    wanting, and yet the birds do not seem to care for the very conspicuous
    little insects on the broad bean, for example, whose dusky hue makes
    them quite noticeable in large masses. Here there may very likely be
    some special protection of nauseous taste in the aphides themselves (I
    will confess that I have not ventured to try the experiment in person),
    as in many other instances we know that conspicuously-coloured insects
    advertise their nastiness, as it were, to the birds by their own
    integuments, and so escape being eaten in mistake for any of their less
    protected relatives.

    On the other hand, it seems pretty clear that certain plants have
    efficiently armed themselves against the aphides, in turn, by secreting
    bitter or otherwise unpleasant juices. So far as I can discover, the
    little plunderers seldom touch the pungent 'nasturtiums' or tropsælums
    of our flower-gardens, even when these grow side by side with other
    plants on which the aphides are swarming. Often, indeed, I find winged
    forms upon the leaf-stem of a nasturtium, having come there evidently in
    hopes of starting a new colony; but usually in a dead or dying
    condition--the pungent juice seems to have poisoned them. So, too,
    spinach and lettuce may be covered with blight, while the bitter
    spurges, the woolly-leaved arabis, and the strong-scented thyme close by
    are utterly untouched. Plants seem to have acquired all these devices,
    such as close networks of hair upon the leaves, strong essences, bitter
    or pungent juices, and poisonous principles, mainly as deterrents for
    insect enemies, of which caterpillars and plant-lice are by far the most
    destructive. It would be unpardonable, of course, to write about
    honey-dew without mentioning tobacco; and I may add parenthetically that
    aphides are determined anti-tobacconists, nicotine, in fact, being a
    deadly poison to them. Smoking with tobacco, or sprinkling with
    tobacco-water, are familiar modes of getting rid of the unwelcome
    intruders in gardens. Doubtless this peculiar property of the tobacco
    plant has been developed as a prophylactic against insect enemies: and
    if so, we may perhaps owe the weed itself, as a smokable leaf, to the
    little aphides. Granting this hypothetical connection, the name of
    honey-dew would indeed be a peculiarly appropriate one. I may mention in
    passing that tobacco is quite fatal to almost all insects, a fact which
    I present gratuitously to the blowers of counterblasts, who are at
    liberty to make whatever use they choose of it. Quassia and aloes are
    also well-known preventives of fly or blight in gardens.

    The most complete life-history yet given of any member of the aphis
    family is that which M. Jules Lichtenstein has worked out with so much
    care in the case of the phylloxera of the oak-tree. In April, the winter
    eggs of this species, laid in the bark of an oak, each hatch out a
    wingless imperfect female, which M. Lichtenstein calls the foundress.
    After moulting four times, the foundress produces, by parthenogenesis, a
    number of false eggs, which it fastens to the leaf-stalks and under side
    of the foliage. These false eggs hatch out a larval form, wingless, but
    bigger than any of the subsequent generations; and the larvæ so produced
    themselves once more give origin to more larvæ, which acquire wings, and
    fly away from the oak on which they were born to another of a different
    species in the same neighbourhood. There these larvæ of the second crop
    once more lay false eggs, from which the third larval generation is
    developed. This brood is again wingless, and it proceeds at once to bud
    out several generations more, by internal gemmation, as long as the warm
    weather lasts. According to M. Lichtenstein, all previous observations
    have been made only on aphides of this third type; and he maintains that
    every species in the whole family really undergoes an analogous
    alternation of generations. At last, when the cold weather begins to set
    in, a fourth larval form appears, which soon obtains wings, and flies
    back to the same kind of oak on which the foundresses were first hatched
    out, all the intervening generations having passed their lives in
    sucking the juices of the other oak to which the second larval form
    migrated. The fourth type here produce perfect male and female insects,
    which are wingless, and have no sucking apparatus. The females, after
    being impregnated, lay a single egg each, which they hide in the bark,
    where it remains during the winter, till in spring it once more hatches
    out into a foundress, and the whole cycle begins over again. Whether all
    the aphides do or do not pass through corresponding stages is not yet
    quite certain. But Kentish farmers believe that the hop-fly migrates to
    hop-bines from plum-trees in the neighbourhood; and M. Lichtenstein
    considers that such migrations from one plant to another are quite
    normal in the family. We know, indeed, that many great plagues of our
    crops are thus propagated, sometimes among closely related plants, but
    sometimes also among the most widely separated species. For example,
    turnip-fly (which is not an aphis, but a small beetle) always begins its
    ravages (as Miss Ormerod has abundantly shown) upon a plot of charlock,
    and then spreads from patches of that weed to the neighbouring turnips,
    which are slightly diverse members of the same genus. But, on the other
    hand, it has long been well known that rust in wheat is specially
    connected with the presence of the barberry bush; and it has recently
    been proved that the fungus which produces the disease passes its early
    stages on the barberry leaves, and only migrates in later generations to
    the growing wheat. This last case brings even more prominently into
    light than ever the essential resemblance of the aphides to
    plant-parasites.
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