A SCIENTIFIC SLAUGHTERER
The wasp has told us part of her secret by showing us the spot which
her sting touches. Does this solve the question? Not yet, nor by a long
way. Let us go back for a moment, forget what the insect has just
taught us and, in our turn, set ourselves the problem of the Cerceris.
The problem is this: to store underground, in a cell, a big enough pile
of game to feed the larva which will be hatched from the egg laid on
the heap.
At first sight this victualling seems simple enough; but a little
reflection shows that it is attended by very grave difficulties. Our
own game, for instance, is brought down by a shot from a gun; it is
killed with horrible wounds. The Wasp has refinements of taste unknown
to us: she must have the prey intact, with all its elegance of form and
colouring, no broken limbs, no gaping wounds, no hideous
disembowelling. Her victim has all the freshness of the live insect; it
retains, without the loss of a single speck, that fine tinted bloom
which is destroyed by the mere contact of our fingers. If the insect
were dead, if it were really a corpse, how great would be our
difficulty in obtaining a like result! Each of us can kill an insect by
brutally crushing it under foot; but to kill it neatly, with no sign of
injury, is not an easy operation, is not an operation which any one can
perform. How many would be utterly perplexed if they were called upon
to kill, then and there, without crushing it, a hardy little insect
which, even when you cut off its head, goes on struggling for a long
time after! One has to be a practical entomologist to think of the
various ways of asphyxiation; and even here success would be doubtful
with primitive methods, such as the fumes of benzine or burning
sulphur. In this unwholesome atmosphere the insect flounders about too
long and loses its glory. We must have recourse to more heroic
measures, such as the terrible exhalations of prussic acid emanating
slowly from strips of paper steeped in cyanide of potassium, or else
and better still, as being free from danger to the insect-hunter, the
all-powerful fumes of bisulphide of carbon. It is quite an art, you
see—and an art which has to call to its aid the formidable arsenal of
chemistry—to kill an insect neatly, to do what the Cerceris performs so
quickly and so prettily, that is, if we are stupid enough to assume
that her captured prey actually becomes a corpse.
A corpse! But that is by no means the fare prescribed for the larvæ,
those little ogres clamouring for fresh meat, whom game ever so
slightly high would inspire with insurmountable disgust. They want meat
killed that day, with no suspicion of taint, the first sign of
corruption. Nevertheless, the prey cannot be packed into the cell
alive, as we pack the cattle destined to furnish fresh meat for the
passengers and crew of a ship. What indeed would become of the delicate
egg laid among live provisions? What would become of the feeble larva,
a tiny grub which the least touch would bruise, among lusty Beetles who
would go on kicking for weeks with their long, spurred legs? We need
here two things which seem utterly irreconcilable: the immobility of
death combined with the sweet wholesomeness of life. Before such a
dietetic problem the most deeply read layman would stand powerless; the
practical entomologist himself would own himself beaten. The Cerceris’
larder would defy their reasoning power.
Let us then suppose an academy of anatomists and physiologists; let us
imagine a congress at which the question is raised among such men as
Flourens, [9] Magendie [10] and Claude Bernard. [11] If we want to
obtain both complete immobility of the victim and also its preservation
during a long period without going bad, the simplest and most natural
idea which comes to us is that of tinned foods. Our congress would
suggest the use of some preserving liquid, just as the famous Landes
scientist did when he was confronted with his Buprestes; they would
attribute exquisite antiseptic virtues to the Wasp’s poison-fluid; but
these strange virtues would still remain to be proved. And perhaps the
conclusion of that learned assembly, like the conclusion of the sage of
the Landes, would be a purely gratuitous supposition which would simply
substitute one unknown quantity for another, giving us in the place of
the mystery of those uncorrupted tissues the mystery of that wonderful
preserving fluid.
If we insist, if we point out that the larvæ need, not preserved food,
which could never possess the properties of still palpitating flesh,
but something that shall be just as if it were live prey, despite its
complete inertia, the learned congress, after due reflection, will fix
on paralysis:
‘Yes, that’s it, of course! The creature must be paralysed; it must be
deprived of movement, without being deprived of life.’
There is only one way of achieving this result: to injure, cut or
destroy the insect’s nervous system in one or more skilfully-selected
places. But, even at that stage, if left in hands unfamiliar with the
anatomical secrets of a delicate organism, the question would not have
advanced much further. What in fact is the disposition of this nervous
system which has to be smitten if we would paralyse the insect without
at the same time killing it? And, first of all, where is it? In the
head, no doubt, and down the back, like the brain and the spinal marrow
of the higher animals.
‘You make a grave mistake,’ our congress would say. ‘The insect is like
an inverted animal, walking on its back; that is to say, instead of
having the spinal marrow on the top, it has it below, along the breast
and the belly. The operation on the insect to be paralysed must
therefore be performed on the lower surface and on that surface alone.’
This difficulty once removed, another arises, equally serious in a
different way. Armed with his scalpel, the anatomist can direct the
point of his instrument wherever he thinks fit, in spite of obstacles,
for these he can eliminate. The Wasp, on the contrary, has no choice.
Her victim is a Beetle in his stout coat of mail; her lancet is her
sting, an extremely delicate weapon which would inevitably be stopped
by the horny armour. Only a few points are accessible to the fragile
implement, namely, the joints, which are protected merely by an
unresisting membrane. Moreover, the joints of the limbs, though
vulnerable, do not in the least fulfil the desired conditions, for the
utmost that could be obtained by means of them would be a partial
paralysis and not a general paralysis affecting the whole of the motor
organism. Without a prolonged struggle, which might be fatal to the
patient, without repeated operations, which, if too numerous, might
jeopardize the Beetle’s life, the Wasp has, if possible, to suppress
all power of movement at one blow. It is essential, therefore, that she
should aim her sting at the nervous centres, the seat of the motor
faculties, whence radiate the nerves scattered over the several organs
of movement. Now these sources of locomotion, these nervous centres,
consist of a certain number of nuclei or ganglia, more numerous in the
larva, less numerous in the perfect insect and arranged along the
median line of the lower surface in a string of beads more or less
distant one from the other and connected by a double ribbon of the
nerve-substance. In all the insects in the perfect state, the so-called
thoracic ganglia, that is to say, those which supply nerves to the
wings and legs and govern their movements, are three in number. These
are the points to be struck. If their action can be destroyed, no
matter how, the power of movement will be destroyed likewise.
There are two methods of reaching these motor centres with the Wasp’s
feeble instrument, the sting: through the joint between the neck and
the corselet; and through the joint between the corselet and the rest
of the thorax, in short, between the first and second pair of legs. The
way through the joint of the neck is hardly suitable: it is too far
from the ganglia, which are near the base of the legs which they endow
with movement. It is at the other point and there alone that the blow
must be struck. That would be the opinion of the academy in which the
Claude Bernards were treating the question in the light of their
profound knowledge. And it is here, just here, between the first and
second pair of legs, on the median line of the lower surface, that the
Wasp inserts her dirk. By what expert instinct is she inspired?
To select, as the spot wherein to drive her sting, the one vulnerable
point, the point which none save a physiologist versed in insect
anatomy could determine beforehand: even that is far from being enough.
The Wasp has a much greater difficulty to surmount; and she surmounts
it with an ease that stupefies us. The nerve-centres governing the
locomotory organs of the insect are, we were saying, three in number.
They are more or less distant from one another; sometimes, but rarely,
they are close together. Altogether they possess a certain independence
of action, so that an injury done to any one of them induces, at any
rate for the moment, the paralysis only of the limbs that correspond
with it, without affecting the other ganglia and the limbs which they
control. To strike in succession these three motor centres, each
farther back than the one before it, and to do so between the first and
second pair of legs, seems an impracticable operation for such a weapon
as the Wasp’s sting, which is too short and is besides very difficult
to guide under such conditions. It is true that certain Beetles have
the three ganglia of the thorax very near together, almost touching,
while others have the last two completely united, soldered, welded
together. It is also a recognized fact that, in proportion as the
different nervous nuclei tend towards a closer combination and greater
centralization, the characteristic functions of animal nature become
more perfect and consequently, alas, more vulnerable. Here we have the
prey which the Cerceris really needs. Those Beetles with motor centres
brought close together or even gathered into a common mass, making them
mutually dependent on one another, will be at the same instant
paralysed with a single stroke of the dagger; or, if several strokes be
needed, the ganglia to be stung will at any rate all be there,
collected under the point of the dart.
Which Beetles are they, then, that constitute a prey so eminently
convenient for paralysing? That is the question. The lofty science of a
Claude Bernard, concerning itself only with the fundamental
generalities of organism and life, would not suffice here; it could
never tell us how to make this entomological selection. I appeal to any
physiologist under whose eyes these lines may come. Without referring
to his library, could he name the Beetles in whom that centralization
of the nervous system occurs; and, even with the aid of his books,
would he at once know where to find the desired information? The fact
is that, with these minute details, we are now entering the domain of
the specialist; we are leaving the public road for the path known to
the few.
I find the necessary information in M. Émile Blanchard’s fine work on
the nervous system of the Coleoptera. [12] I see there that this
centralization of the nervous system is the prerogative, in the first
place, of the Scarabæidæ, or Chafers; but most of these are too large:
the Cerceris could perhaps neither attack them nor carry them away;
besides, many of them live in the midst of ordure where the Wasp,
herself so cleanly, would refuse to go in search of them. Motor centres
very close together are found also in the Histers, who live on carrion
and dung, in an atmosphere of loathsome smells, and who must therefore
be eliminated; in the Scolyti, who are too small; and lastly in the
Buprestes and the Weevils.
What an unexpected light amid the original darkness of the problem!
Among the immense number of Beetles whereon the Cerceres might seem
able to prey, only two groups, the Weevils and the Buprestes, fulfil
the indispensable conditions. They live far removed from stench and
filth, two qualities perhaps invincibly repugnant to the dainty
huntress; their numerous representatives vary considerably in size, in
much the same way as their kidnappers, who can thus pick and choose the
victims that suit them; they are far more vulnerable than any of the
others at the one point where the Wasp’s dart can penetrate, for at
this point the motor centres of the feet and wings are crowded
together, all easily accessible to the sting. At this point, in the
Weevils, the three thoracic ganglia are very close together, the last
two even touching; at the same point, in the Buprestes, the second and
third are mingled in one large mass, very near the first. And it is
just Buprestes and Weevils that we see hunted, to the absolute
exclusion of all other game, by the eight species of Cerceres whose
provisions have been found to consist of Beetles! A certain inward
resemblance, that is to say, the centralization of the nervous system,
must therefore be the reason why the lairs of the different Cerceres
are crammed with victims bearing no outward resemblance whatever.
The most exalted knowledge could make no more judicious choice than
this, by which so great a collection of difficulties is magnificently
solved that we wonder if we be not the dupes of some involuntary
illusion, whether preconceived theoretic notions have not obscured the
actual facts, whether, in short, the pen have not described imaginary
marvels. No scientific conclusion is firmly established until it has
received confirmation by means of practical tests, carried out in every
variety of way. We will therefore subject to experimental proof the
physiological operation of which the Great Cerceris has just apprised
us. If it be possible to obtain artificially what the Wasp obtains with
her sting, namely, the abolition of movement and the continued
preservation of the patient in a perfectly fresh condition; if it be
possible to work this wonder with the Beetles hunted by the Cerceris,
or with those presenting a similar nervous centralization, while we are
unsuccessful with Beetles whose ganglia are far apart, then we shall be
bound to admit, however hard to please we may be in the matter of
tests, that in the unconscious inspiration of her instinct the Wasp has
all the resources of consummate art. Let us see what experiment has to
tell us.
The operating method is of the simplest. It is a question of taking a
needle, or, better and more convenient, the point of a fine steel nib,
and introducing a tiny drop of some corrosive fluid into the thoracic
motor centres, by pricking the insect slightly at the junction of the
prothorax, behind the first pair of feet. The fluid which I employ is
ammonia; but obviously any other liquid as powerful in its action would
produce the same results. The nib being charged with ammonia as it
might be with a very small drop of ink, I give the prick. The effects
obtained differ enormously, according to whether we experiment upon
species whose thoracic ganglia are close together or upon species in
which those same ganglia are far apart. In the first class, my
experiments were made on Dung-beetles: the Sacred Scarab [13] and the
Wide-necked Scarab; on Buprestes: the Bronze Buprestis; lastly, on
Weevils, in particular on the Cleonus hunted by the heroine of this
essay. In the second class, I experimented on Ground-beetles: Carabi,
Procrustes, Chlænii, Sphodri, Nebriæ; on Longicornes: Saperdæ and
Lamiæ; on Melasoma-beetles: Cellar-beetles, Scauri, Asidæ.
In the Scarabæi, the Buprestes and the Weevils the effect is
instantaneous: all movement ceases suddenly, without convulsions, so
soon as the fatal drop has touched the nerve-centres. The Cerceris’ own
sting produces no more speedy annihilation. There is nothing more
striking than this immediate immobility provoked in a powerful Sacred
Beetle.
But this is not the only resemblance between the effects produced by
the Wasp’s sting and those resulting from the nib poisoned with
ammonia. The Scarabs, Buprestes and Beetles artificially stung,
notwithstanding their complete immobility, preserve for three weeks, a
month or even two the perfect flexibility of all their joints and the
normal freshness of their internal organs. Evacuation takes place with
them during the first days as in the normal state; and movements can be
induced by the electric battery. In a word, they behave exactly like
the Beetles immolated by the Cerceris; there is absolute identity
between the state into which the kidnapper puts her victims and that
which we produce at will by injuring the thoracic nerve-centres with
ammonia. Now, as it is impossible to attribute the perfect preservation
of the insect for so long a period to the tiny drop injected, we must
reject altogether any notion of an antiseptic fluid and admit that,
despite its perfect immobility, the insect is not really dead, that it
still retains a glimmer of life, which for some time to come keeps the
organs in their normal condition of freshness, but gradually fades out,
until at last it leaves them the prey of corruption. Besides, in some
cases, the ammonia does not produce complete annihilation of movement
except in the insect’s legs; and then, as the deleterious action of the
liquid has doubtless not extended far enough, the antennæ preserve a
remnant of mobility and we see the insect, even more than a month after
the inoculation, draw them back quickly at the least touch: a
convincing proof that life has not entirely deserted the inanimate
body. This movement of the antennæ is also not uncommon in the Weevils
wounded by the Cerceris.
In every case the injection of ammonia at once stops all movement in
Scarabs, Weevils and Buprestes; but we do not always succeed in
reducing the insect to the condition just described. If the wound be
too deep, if the drop administered be too strong, the victim really
dies; and, in two or three days’ time, we have nothing but a putrid
body before us. If the prick, on the other hand, be too slight, the
insect, after a longer or shorter period of deep torpor, comes to
itself and at least partially recovers its power of motion. The
assailant herself may sometimes operate clumsily, just like man, for I
have noticed this sort of resurrection in a victim stung by the dart of
a Digger-wasp. The Yellow-winged Sphex, whose story will shortly occupy
our attention, stacks her lairs with young Crickets first pricked with
her poisoned lancet. I have extracted from one of those lairs three
poor Crickets whose extreme limpness would, in any other circumstances,
have denoted death. But here again death was only apparent. Placed in a
flask, these Crickets kept in very good condition, perfectly motionless
all the time, for nearly three weeks. In the end, two went mouldy, and
the third partly revived, that is to say, he recovered the power of
motion in his antennæ, in his mouth-parts and, what is more remarkable,
in his first two pair of legs. If the Wasp’s skill sometimes fails to
benumb the victim permanently, one can hardly expect invariable success
from man’s rough experiments.
In the Beetles of the second class, that is to say, those whose
thoracic ganglia are some distance apart, the effect of the ammonia is
quite different. The least vulnerable are the Ground-beetles. A
puncture which would have produced instant annihilation of movement in
a large Sacred Beetle produces nothing but violent and disordered
convulsions in the medium-sized Ground-beetles, be they Chlænius,
Nebria or Calathus. Little by little the insect quiets down and, after
a few hours’ rest, its usual movements are resumed as though it had met
with no accident whatever. If we repeat the experiment on the same
specimen, twice, thrice, or four times over, the results remain the
same, until the wound becomes too serious and the insect actually dies,
as is proved by its desiccation and putrefaction, which follows soon
after.
The Melasoma-beetles and Longicornes are more sensitive to the action
of the ammonia. The injection of the corrosive drop pretty quickly
renders them motionless; and, after a few convulsions, the insect seems
dead. But this paralysis, which would have persisted in the
Dung-beetles, the Weevils and the Buprestes, is only temporary here:
within a day, motion is once more apparent, as energetic as ever. It is
only when the dose of ammonia is of a certain strength that the
movements fail to reappear; but then the insect is dead, quite dead,
for it soon begins to decay. It is impossible, therefore, to produce
complete and persistent paralysis in Beetles that have their ganglia
far apart by the same measures which proved so efficacious in Beetles
with ganglia close together: the utmost that we can obtain is a
temporary paralysis whose effects pass off within a day.
The demonstration is conclusive; the Cerceres that prey on Beetles
conform in their selection to what could be taught only by the most
learned physiologists and the finest anatomists. One would vainly
strive to see no more in this than casual coincidences: it is not in
chance that we shall find the key to such harmonies as these.