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.