Aphides and their enemies—Curious interrelations—The biter
bit—Altruistic development—Bread and beer protectors—Saved
by ladybirds.
WITH prolific powers which have been successful in arousing the interest
even of the late Professor Huxley, it is a comfort to think that the
numbers of the Aphides are always being kept down by the operation of
certain well-contrived causes, most of which take the shape of various
insect enemies. Were all of these, or perhaps were any one of them,
entirely removed, the whole world apparently might find itself deep
buried beneath a “star-y-pointing pyramid” of insect organisms, for what
cannot quintillions, in the hands of a competent mathematician, achieve?
The wonder certainly seems that any kind of check should be sufficient.
We owe our safety, in part, as might have been surmised, to a small
Ichneumon Fly, the traces of whose work may generally be seen in a
number of brown lifeless corpses, which are dotted about like so many
skeletons at the feast, amongst any collection of living Aphides. These,
if examined more closely, are seen to be mere empty sacs, each one
having at some part of it a quite circular aperture, through which the
issuing guest has escaped. But the amount of the good thus effected is
not to be estimated by the number of these shells, these nests from
which the bird has flown. Such are only in the last stage of things,
whilst almost all, including the healthiest-looking of the living
Aphides, are probably travelling along the same road, to arrive at the
same goal. All, or almost all, have within them a guest whose energies
are unceasingly devoted to absorbing the whole of their interior
arrangements into itself, and gradually taking their place. “In June,”
says Mr. Buckton, “during the hot weather, I have seen, at the same
time, as many as three of these flies on one rose sprig, each poised on
the back of an Aphis, which throws itself into many contortions for the
purpose of throwing off its enemy. The Ichneumon, however, remains fixed
on the back for ten or more minutes (as though enjoying the situation)
before the ovipositor is thrust under the skin of the victim and the egg
is laid. The Aphis appears to suffer at first but little, since it soon
resumes its occupation of pumping up the sap. A worm-like, or more
commonly, a maggot-like creature, according to the species of the
parasite, hatches from this egg, which revels in the organised
nutritious fluid elaborated by the Aphis. The greater part of the
abdomen is occupied by this maggot when it becomes full fed, and then it
may often be seen through the transparent integument, as a grub curled
into a semicircle. Finally, the Aphis dies, the grub ceases to feed, and
after a certain period of rest cuts out of the roof of its prison a
circular plate, like a trap-door, as regular in form as if a carpenter’s
centre-bit had been used. The emerging fly has four wings, long antennæ
composed of numerous joints, a wasp-like body and legs, and is in every
way suited for its marauding expeditions.”[43]
Other and more interesting dramas can take place within the body of an
Aphis; wheels within wheels, one masterpiece of economic contrivance
enclosing another, perfection more perfected. Along what path, indeed,
can the beauty and wisdom of Nature—those endless steps from endless
seeming halting-places that become, when reached, but so many points of
fresh departure—be run to earth, so to speak? The brain becomes, at
last, almost weary in the pursuit of wisdom’s ways, and even the
delighted spirit would fain cry, “Hold! Enough!” Thoughts like these are
powerfully excited by the following picture.
It is spring, early spring, and already the young Aphides have begun,
with gladness born of the opening year, to absorb the sweet sap from the
stems of the immature wheat-crop. Later, however, when July’s sun shines
brightly behind its cooling screen of clouds—for in England all climatic
extremes are tempered—they ascend in “numbers numberless” to attack the
ear itself. What, then, can save the hope of the husbandman? What but
_Ephedrus plagiator_, a small black-winged Ichneumon Fly that even now
is at work? As each individual Aphis clings to the wheat, it becomes, in
spite of its efforts to fill itself, hollower and hollower; its appetite
flags, and ere it can fatally affect the plant on which man’s life and
the machinations of the protectionist depend, it has become a mere brown
pupa-case for a body other than its own to develop in. A day or two, and
almost on every grain of the wheat hangs an insect, lifeless, but—oh,
floweret springing from the tomb!—life-filled. Hardly a living Aphis is
to be seen feeding amongst them. The wheat-crop has been saved. But the
march of events, thus unfolded, does not end here. Another drama has to
be played out ere the full life issues from the once living cradle whose
contents it has absorbed and become, nor will it bear the image of that
particular Ichneumon Fly that laid its little egg, some weeks ago, in
the body that seemed so designed for it. Like the Aphis, the Ichneumon,
too, must learn to live for others, thus rising through selfishness to a
purer and higher embodiment. If we pass a little later through the same
cornfield, another insect, differing from, yet of the same general type
as the prior parasite, may be seen running to and fro over the
wheat-ears, tapping each tenanted abode with tremulously quivering
antennæ, which, as well as the whole body, seem to vibrate with
excitement. It is looking for lodgings, but not every house so touched
can be utilised, for _Ceraphrus Carpenteri_—such is the new tenant’s
name—must first be satisfied that none of its own species have already
taken possession. Once assured on this point, however, its duty lies
plain before it, and bending its antennæ against the wheat-ear, so as to
form a fulcrum, it turns the tip of its abdomen towards that of the dead
Aphis, and with its ovipositor commences to saw through the skin. As
much as ten minutes may be occupied in the accomplishment of this task,
for the sarcophagus that has thus to be pierced is hard, and the
ovipositor, though short, is not stout, but slenderly formed. But there
is no flagging of energy, and at length, when efforts steadily continued
have been crowned with success, the same deft instrument is again
employed to pierce the sleeping _ephedrus_, and a second egg is
deposited in this second cradle. To this new tenant the former one must
now yield up the juices of its body, even as those of the Aphis were
freely rendered unto it. It must die in its turn, but by its death
another lives, and thus the physical act of aggression, which we call
selfishness, becomes the seed-bed, as it were, or forcing-house—the
food-plant, to use an entomological simile—of a moral altruism. True,
the Aphis may at first struggle, the maggot, pierced by the ovipositor,
may flinch for a moment, but after that there is complete passivity,
without which there can be no complete acquiescence. Self-absorption,
that is the moral of it all; for the true self of the Aphis, which is
not represented by the outer husk, is absorbed into the Ichneumon, and
so in regard to the latter. Thus, throughout the animal kingdom we must
look to the inner, and not to the outer, significance. What matters it,
though Nature be “red in tooth and claw,” if the fierce rendings of the
outer integuments are but as the first gropings towards interior rest
and calm? And should we not, in the lower walks of life, look to the
soul through the body, and see in processes which, with ourselves, might
seem to represent the flesh only, a blending which but anticipates the
more complete separation? Thus, and thus only, as it appears to me, can
we impart beauty to a scheme which, without this key, must appear
selfish and unsatisfactory. The key may be hard to find, but when we
once hold it we need no longer repine.
Besides the Wheat Aphis, which but for such arrangements as have here
been glanced at, would almost deprive us of bread, we have the Hop
Aphis, a species the dread of which is still more strongly disseminated
amongst the masses of this country, inasmuch as its interference would
be with the supply of beer. No wonder, then, that the little ladybird is
beloved by all, since, but for its efforts, many a poor man might have
to live in a state of enforced sobriety, which, in its turn, must
deleteriously affect that position of respect and esteem which many
illustrious and highly placed individuals now hold in the hearts of the
people, so that a general disturbal of habits and ideas, amounting
almost to moral chaos, would attend any serious diminution in the
numbers of this insect, England’s true guardian angel. But it were
unjust to claim an undue share in the merit of recognising work like
this. Appreciation of such services is, as one might expect, widely
spread, and is expressed in such popular names as, for instance, in
Lombardy “Bestioline del Signore,” in Tuscany “Madonnine” or
“Marioline,” in France “Bête” or “Vache à Dieu,” and in Germany
“Sonnenkäfer.” The first-named countries, indeed, are not, or used not
to be, beer-drinking, so that unless this little madonna is a patroness
of the vine too, they are not so easy to understand. It may, however, be
incidentally mentioned that the ladybirds are good friends to the
orchards, and destroy many thousands of apple- or plum-eating Aphides.
Aphides, in fact, of some kind or other, are what they have come into
the world to destroy. It is their mission, their epos.
The following account of the habits of these beneficent creatures,
principally in the above connection, is given by Mr. Buckton, a profound
student of insect life-histories: “The food of Coccinella (the ladybird)
consists almost exclusively of Aphides. Their marvellous voracity is
shown equally in their larval and their winged condition. The former
stage may be commonly seen throughout early summer as slaty-grey or
brown six-footed creatures, covered with tufted tubercles, and provided
with mandibles efficient both for holding and sucking out the juices of
their victims. In some years the imagos (or grown insects) are
wonderfully numerous, and when they take wing form vast swarms which
travel great distances. In the year 1869 such a cloud passed over a
large part of Kent, Sussex, and Surrey,” and their effect (of the same
genial nature as that of Bacchus wandering through the earth) was soon
seen in the good hop crop of the following year. “Although the
Coccinella is not restricted to the Hop Aphis for its food, it
frequently follows its migrations, and travels on the same winds. Whilst
feeding, the Aphis is held and manipulated by the jaws or palpi of the
Coccinella, and the devouring operation proceeds amidst the struggles of
the victims from the apex of the abdomen to the thorax, which parts,
together with the head and legs, are finally rejected.”[43] This
process—which is by no means confined to insect life, but extends
upwards from it even into the highly organised mammalia—does not seem to
be a pleasant one to witness, for Mr. Buckton remarks upon it: “We may
express some hope, in sympathy with the Aphis, that the automatic theory
of animal life may here find some place, and that reflex action may
explain the fact that, under the microscope, the mutilated remains of
the Aphis, without stomach and without internal organs, have been seen
to walk away and live after the operation for a considerable time!
Automatically the Coccinella furbishes up its jaws and antennæ in
readiness for another meal. From thirty to forty Aphides may thus be
consumed in one hour.”[43]
Automatically perhaps—that is to say, between the lines—we may gather
Mr. Buckton’s opinion of the automatic theory. There are some theories
which seem held, like dykes or barriers, to prevent the sea from getting
in. One doesn’t want the sea to get in, because it would swamp such a
lot of things, which, although quite artificial, one is not prepared to
part with, but one doesn’t believe in the barrier except for that
particular purpose to which it is applied. The automatic theory in
regard to animals is a case in point. Scientific men make use of it in
order to keep out another, which they don’t want to have to admit,
though they do, as a fact, believe in it. This, again, one can read
between the lines whenever they give any account of their observations
on this or that animal, whether it be dog or elephant, ants or something
much lower down in the scale—rotifers, for instance, or amœba in the
ocean of a watch-glass. One sees what they really mean very well then,
though they may not themselves be aware of it, but they are never in the
least _convincing_ when they air their automatic theory. Aphides, as may
have been gathered incidentally, feed wholly upon the juices or sap of
plants. Active _ab ovo_ (which means from the egg), “their occupation,”
says Mr. Buckton, “is to grow as fast as possible,” and with a view to
this end the rostrum or beak, with its enclosed sucking or pumping
apparatus, is fully developed from the very commencement, “often,
indeed, to such an abnormal extent that it forms an awkward appendage,
trailing behind the body whilst walking.”[43] The insect does not,
however, walk much, but, settling itself down on the twig or stem where,
perhaps, it first saw the light, pierces the bark with the instrument
thus provided for it, and commences to suck up the sap into its mouth.
This is not a process which can be indulged in with impunity to the
plant, especially since Aphides reside in great societies upon the same
one, and turn their attention to every part of it, not even excepting
the roots. Troops of small Aphides, in fact, have sometimes been found
in the pips of large codling apples. In consequence of this excessive
drain upon their fluids, which is as though our own blood were to be
sucked, plants thus invaded by Aphides become greatly weakened, and
their young shoots and leaves have a distorted appearance in
consequence. Others, either through this cause alone, or in consequence
of some poison or acid injected by the Aphis, have gall-like excresences
produced upon them. These have a hollow interior, into which the Aphides
penetrate, and there take up their residence. Such swellings thus become
their houses, and therefore, since it is a great advantage to the
Aphides to be sheltered in this way, it is possible that some special
instinct through the exercise of which the tree is thus affected, may
have been implanted in them by the action of natural selection.
Aphides are often spoken of by entomologists as if a very high degree of
interest attached to them, and, no doubt, in many respects this is the
case. As we have seen, they exhibit certain phenomena of _corporealism_
(which _did_ interest Professor Huxley) to a greater extent perhaps than
any other creature, though of this I am not at all sure; but after all
one soon gets over the wonder of that, especially since there is no
realising it, and then it does not seem to raise a creature to a very
high level of interest. Again, to quote authority, “there is a most
curious alternation of broods in these insects, some forms being winged
and with separate sexes, and others wingless or apterous and capable of
producing their kind for an indefinite number of generations before a
sexual brood is again developed. In fact, the anomalies of members of
this family are endless, and it would require volumes to epitomise even
the comparatively little which has already been discovered with
reference to their habits and transformations.”[44] Still, for all this,
it is difficult to look long at an Aphis, or a collection of Aphides
upon a rose tree or any other of the plants they affect, without getting
heartily tired of them, and for me, as perhaps for most people, the
principal interest about these sluggish creatures lies in the relations
which have become established, without any intelligent efforts on their
own part, between them and ants—but it will be best to reserve the
discussion of this subject for the following chapter.
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Footnote 43:
Buckton, Monograph of the British Aphides, vol. i
Footnote 44:
The Concise Knowledge Library, Natural History, pp. 601, 602.
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