The Hymenopteron has partly revealed her secret by showing us where the
sting strikes. But does that explain the question? Not yet, by any
means. Let us retrace our steps, forget for a moment what the insect
has taught us, and consider the problem set before the Cerceris. The
problem is this: to lay up in an underground cell a certain number of
heads of game which may suffice to nourish the larva hatched from the
egg laid upon the heap of provender.
At first sight this storing of food appears simple enough, but
reflexion soon discovers graver difficulties. Our own game is brought
down by a shot and killed with horrible wounds. The Hymenopteron has
refinements unknown to us; she chooses to have her prey intact, with
all its elegance of form and colour. No broken limbs, no gaping wounds,
no hideous disembowelment. Her prey has all the freshness of the living
insect; she does not destroy an atom of the fine-coloured powder which
the mere contact of our fingers deflowers. If the insect were really
dead, really a corpse, how difficult it would be for us to obtain such
a result! Any one can slay an insect by stamping brutally on it, but to
kill it neatly leaving no sign is no easy operation, within every one’s
power. How many of us would be at our wits’ end if we had to kill on
the spot, without crushing it, a little creature so tenacious of life
that even beheaded it still goes on struggling! One must have been a
practical entomologist before thinking of asphyxiation, and here,
again, success would be doubtful with the primitive methods of vapour
of benzine or burnt sulphur. In this deleterious atmosphere the insect
struggles too long, and tarnishes its brightness. One must have
recourse to more heroic methods—for instance, to the terrible
exhalations of prussic acid slowly disengaging themselves from strips
of paper impregnated with cyanide of potassium, or better still, as
being without danger to the collector, to the thunderbolt of vapour of
bisulphide of carbon. It requires a real art, an art calling to its aid
the redoubtable arsenal of chemistry, to kill an insect neatly; to do
that is what the elegant method of the Cerceris brings about so
quickly, if we admit the stupid supposition that her prey really
becomes a dead body.
A dead body! But that is by no means the diet of the larvæ, little
ogres greedy for fresh meat, to whom game ever so slightly tainted
would inspire insurmountable disgust. They must have fresh meat with no
high taste—that first sign of decay. Yet the prey cannot be laid up
alive in the cell like animals destined to furnish fresh meat to the
crew and passengers of a vessel. What would become of a delicate egg
laid among living food? What would become of the feeble larva, a worm
bruised by the slightest thing among vigorous Coleoptera moving their
long spurred legs for whole weeks? It is absolutely necessary—and here
we seem caught in a blind alley—to obtain deathly immobility with the
freshness of life for the interior organs. Before such an alimentary
problem the best instructed man of the world would stand helpless—even
the practised entomologist would own himself at a loss. The larder of
the Cerceris would defy their reasoning powers.
Let us then imagine an academy of entomologists and physiologists, a
congress where the question should be discussed by Flourens, Majendies,
Claude Bernards. To obtain at once complete immobility and long
preservation of food, the first and most natural and simple idea would
be that of preserved meats. One would invoke some antiseptic liquid, as
the illustrious savant of the Landes did with regard to his Buprestids,
and attribute such virtue to the poisonous fluid of the Cerceris, but
this strange quality has yet to be proved. Gratuitous hypothesis
replacing the unknown quantity of the preserving liquid may perhaps be
the final verdict of the learned assembly, as it was that of the
naturalist of the Landes.
Should one insist and explain that the larvæ require not preserved food
which could never have the properties of flesh still palpitating, but
prey yet alive, so to say, in spite of complete absence of motion, the
learned Congress, after ripe consideration, will fall back upon
paralysis: “Yes, of course; the creature has to be paralysed without
being killed.” There is but one means of arriving at this result,
namely, to injure, cut, and destroy the nervous system of the insect in
one or more skilfully chosen points.
If the question be thus left in hands unfamiliar with the secrets of a
delicate anatomy it will not have advanced far. What is the arrangement
of this nervous system which must be paralysed without killing the
insect? First, where is it? In the head no doubt and along the back,
like the brain and spinal marrow in the superior animals. “A grave
mistake!” our congress would reply; the insect is so to say an animal
reversed, which walks on its back—that is, it has the spinal marrow
below instead of above, all along breast and stomach; therefore on the
lower surface alone can the operation to paralyse the insect be
performed.
This difficulty removed, a far graver one presents itself. Armed with
his scalpel, the anatomist can direct its point where he will in spite
of obstacles which he may have to set aside. The Hymenopteron has no
choice. Its victim is a solidly cuirassed beetle, its lancet a dart,
extremely delicate, which the horny mail would certainly turn aside.
Only certain points are vulnerable to the frail tool, namely, the
joints, protected simply by a membrane with no power of resistance. But
the joints of the limbs, although vulnerable, do not in the least
fulfil the necessary conditions, for through these the utmost that
could be obtained is local paralysis, not one affecting the whole
organism of motion. Without any prolonged struggle, without repeated
operations, which, if too numerous, might endanger the victim’s life,
the Hymenopteron has, if possible, to abolish all motive power at one
blow. Therefore she must direct her dart at the nervous centres, the
source of the power of motion whence radiate the nerves running up to
the various organs of movement. Now these sources of locomotion, these
nervous centres, consist of a certain number of ganglia, more numerous
in the larva, less so in the perfect insect, and arranged on the median
line of the under surface in a string of beads more or less distant and
connected by a double ribbon of nervous tissue. In all insects which
have reached the perfect state the ganglia called thoracic, i.e. those
furnishing nerves to wings and feet and governing their movements, are
three in number. Here are the points to be struck: if their action can
be in any way destroyed, the possibility of movement is destroyed also.
Two ways of reaching these motive centres offer themselves to the
feeble dart of the Hymenopteron; one, the joint between neck and
corslet; the other the spot where the latter joins the continuation of
the thorax, between the first and second pair of feet. The way through
the neck does not answer; it is too far from the ganglia, which lie
near the base of the feet which they animate. The blow must be dealt at
the other spot, and through that only. Thus would an academy decide
where Claude Bernards illuminated the question by their profound
science. And it is precisely there, between the first and second pairs
of feet on the median line of the under surface, that the Cerceris
plunges her lancet. By what learned intelligence must she be inspired!
To choose as the spot in which to plant her sting the one vulnerable
point, the point which only a physiologist versed in the anatomy of
insects could determine beforehand is by no means enough; the
Hymenopteron has a far greater difficulty to overcome, and she
overcomes it with a mastery which fills one with amazement. We said
that the nervous centres controlling the organs of motion in an insect
are three. These are more or less distant from each other, but
sometimes, though rarely, near together. They possess a certain
independence of action, so that an injury to one does not cause, at all
events immediately, more than paralysis of members connected with it,
while the other ganglia and their corresponding members are not
affected by it. To reach these three sources of motion one after the
other, the second farther off than the first, and the last farther
still, and by a single way, between the first and second pairs of feet,
seems impossible for the sting, which is too short, and besides, so
difficult to aim well in such conditions. True, certain Coleoptera have
the three ganglia of the thorax almost touching, and others have the
two last completely united, soldered, smelted together. It is also
recognised that in proportion as the different nervous centres combine
and centralise, the characteristic functions of animality become more
perfect, and also, alas, more vulnerable. Those Coleoptera with centres
of motion so near that they touch or even gather into one mass, and so
are made part of each other, would be instantly paralysed by one sting;
or if several were needed, at all events the ganglia to be paralysed
are all collected under the point of the dart.
Now which are the Coleoptera so specially easy to paralyse? That is the
question. The lofty science of a Claude Bernard, floating in the
fundamental generalities of organisation and life, is no longer enough
for us; it is unable to inform and guide us in this entomological
selection. I appeal to every physiologist under whose eye these lines
may fall. Without having recourse to his book-shelves, could he name
the Coleoptera where such a nervous centralisation is found, and even
with the help of his library, could he instantly lay his hand on the
information wanted? The truth is, we are entering on the minute details
of the specialist; the highway is quitted for a path known to few.
I find the necessary documents in the fine work of M. E. Blanchard
(Annales des Sciences Naturelles, 3me série, tome v.) on the nervous
system of Coleoptera. There I find that this centralisation of nerve
power belongs especially to the Scarabæus, but most of these are too
large; the Cerceris could neither attack nor carry them away; besides,
many live in filth, where the cleanly Hymenopteron could not go to seek
them. Motive centres very close together are also found among the
Histers, which live on impurity, amid the smell of decay, and again
that will not do; also in the Scolytus, which is too small, and finally
in Buprestids and Weevils.
What unexpected light amid the pristine obscurities of the problem!
Amid the immense number of the Coleoptera which the Cerceris seem able
to prey upon, two groups alone, Weevils and Buprestids, fulfil the
indispensable conditions. They live far from decay and dung, which
perhaps cause invincible repugnance in this dainty Cerceris; they are
of most varied size, proportioned to that of their different captors,
which may thus choose according to their convenience. They are far more
vulnerable than all the others at the one point where the sting of the
Hymenopteron can penetrate successfully, for at that point, all easily
accessible to the dart, crowd the motor centres of feet and wings. At
this point the three thoracic ganglia of Weevils lie very close, the
hind two are contiguous. At that same spot in the Buprestids the second
and third are welded in one large mass a little distance from the
first. And as it is precisely Buprestids and Weevils which are hunted,
to the absolute exclusion of all other game, by the eight species of
Cerceris, whose food stores of Coleoptera have been ascertained, a
certain internal likeness, namely, in centralisation of the nervous
system must be the explanation why there are heaped in the dens of
various Cerceris victims, outwardly so unlike.
In this choice, upon which even transcendent knowledge could not
improve, such an assembly of difficulties is splendidly resolved, that
one asks if one be not the dupe of some involuntary illusion, and if
preconceived theories have not obscured the reality of facts, in short,
whether the pen has not described imaginary marvels. A scientific
result is only solidly established when confirmed by experiments
repeated in every possible way. Now let us submit to experimental proof
the physiological operation taught us by Cerceris tuberculata. If it be
possible to obtain artificially what the Hymenopteron obtained by her
sting, i.e. abolition of movement, and long preservation of the victim
in a state of perfect freshness; if it be possible to bring about this
wonder with the Coleoptera hunted by the Cerceris, or with those
possessing a like nervous centralisation, while one fails with those
whose ganglia are far apart, one must admit, however exacting one may
be in the matter of proof, that the Hymenopteron possesses in the
unconscious inspirations of instinct the resources of sublime science.
Let us see then what experiment shows. The manner of operation is very
simple. With a needle, or, better still, with the point of a fine steel
pen, we must introduce a tiny drop of some corrosive liquid into the
thoracic motive centres, pricking the insect slightly at the jointing
of the prothorax behind the first pair of feet. The liquid which I use
is ammonia, but it is evident that any other liquid whose action is
equally strong would produce the same results. The metal pen being
charged with ammonia as it might be with a droplet of ink, I give the
prick. The effects thus obtained differ enormously, according to
whether the experiment be made upon species with thoracic ganglia near
together or upon those where these same ganglia are far apart. With
regard to the first category, my experiments were made on Scarabæus, S.
sacer and S. longicollis; on a bronze Buprestis; and on weevils,
especially that Cleonus hunted by the heroine of these observations. In
the second category I have experimented on Caraboidea, Carabus,
Procrustes, Chlœnius, Sphodrus, Nebria; Longicornia, Saperda, and
Lamia; on Melasomes; Blaps, Scaurus, and Asida.
Among the Scarabæus class, the Buprestids, and the Weevils, the effect
is instantaneous. Every movement stops suddenly, without any
convulsion, as soon as the fatal drop has touched the nerve centres.
The sting of the Cerceris does not produce prompter extinction. Nothing
can be more striking than this sudden immobility in a vigorous
Scarabæus sacer, but the likeness between the effects produced by the
dart of the Cerceris and the steel pen charged with ammonia does not
stop here. Scarabæids, Buprestids, and Weevils artificially stung, in
spite of their complete immobility, preserve for three weeks, one
month, or even two, the perfect flexibility of every joint and the
normal freshness of the interior organs. With them defecation takes
place on the first days as in the normal condition, and movement can be
excited by the Voltaic current. In a word, they behave exactly as do
Coleoptera sacrificed by the Cerceris. There is complete identity
between the state into which she plunges her victims and that produced
at will by injecting ammonia into the nerve centres of the thorax. Now,
as it is impossible to attribute the perfect preservation of the insect
during so long a time to the drop injected, one must altogether reject
the notion of an antiseptic fluid, and grant that in spite of utter
immobility the creature is not really dead. A spark of life exists,
keeping the organs for some time in normal freshness, but dying out by
degrees and leaving them at last subject to corruption. Moreover, the
ammonia in some cases produces extinction of movement in the feet only,
and then the deleterious action of the fluid having doubtless not
extended far enough, the antennæ preserve some mobility, and one sees
that the creature, even a month after inoculation, draws them back
quickly at the least touch—an evident proof that life has not
completely abandoned the inert body. This movement is not rare with
Weevils wounded by the Cerceris.
Injection of ammonia always stops motion at once in Buprestids,
Weevils, and Scarabæus, but it is not always possible to put the
creature into the state just described. If the wound be too deep, or
the little drop instilled be too strong, at the end of two or three
days the victim really dies, and after two or three days there is but a
decaying body. If, on the contrary, the prick be too slight, it
recovers the power of motion, at least partially, after being inanimate
for more or less time. The Cerceris herself may operate clumsily, just
like man, for I have seen this kind of resurrection in a victim struck
by the dart of a Hymenopteron. Sphex flavipennis, whose history will
presently occupy us, heaps in her dens young crickets struck by her
venomed lancet. From one of her holes I have taken three poor crickets
whose extreme flabbiness would, in any other circumstances, have
denoted death. But here, again, death was only apparent. Placed in a
bottle, these crickets kept quite fresh but motionless for nearly three
weeks, after which two grew mouldy, while the third came partly to
life—that is to say, it regained motion of the antennæ, mouth-parts,
and, which is more remarkable, of the first two pairs of feet. If even
the skill of the Hymenopteron sometimes fails to benumb a victim for
good and all, can one expect constant success with the rough
experiments of man?
In Coleoptera of the second category—those where the ganglia of the
thorax are distant one from another—the effect of ammonia is quite
different. Those which show themselves least vulnerable are the
Caraboidea. A puncture which would instantly have annihilated motion in
the large Scarabæus sacer, in the middle size Caraboidea only causes
violent, disordered convulsions. By degrees the creature quiets down,
and after some hours’ rest resumes its habitual movements as if nothing
had happened to it. If the experiment be repeated on it twice, thrice,
even four times, the results are the same, until the wound becomes too
serious, and it dies outright, as is proved by the drying up and
putrefaction which soon follow.
The Melasomes and the Longicorns are more sensitive to the action of
ammonia. The injection of a small corrosive drop quickly renders them
motionless, and after some twitching they seem dead. But the paralysis
which would have persisted in Weevils, Scarabids, and Buprestids is but
momentary. Before long motion reappears as energetic as before. It is
only when the dose of ammonia is of a certain strength that movement
does not reappear. But then the creature is really dead, and
putrefaction rapidly comes on. It is then impossible to cause complete
and persistent paralysis in Coleoptera with ganglia far apart by the
means so efficacious in those with ganglia near together. At the utmost
one can only obtain momentary paralysis, which passes quickly away. The
demonstration is decisive. Cerceris which prey on Coleoptera conform in
their choice to what the most learned physiology and finest anatomy
alone can teach. It would be vain to endeavour to see nothing here but
chance agreement; it is not chance which explains such harmony.