Falling in Love

10. Honey-Dew

Place, the garden. Time, summer. Dramatis person, 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-scne_ 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 antenn (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 _protg_, 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 antenn, 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. Andr 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

larv 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 _protgs_.

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 tropslums

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 larv so produced

themselves once more give origin to more larv, 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 larv 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.