THE FLY-HUNT
After our list, in the last chapter, of the fare on which the Bembex
feed in the larval form, it behoves us to seek the motive that induces
these Wasps to adopt a method of victualling so exceptional among the
digger-insects. Why, instead of previously storing a sufficient
quantity of provisions on which the egg could be laid—which would
enable the mother to close the cell immediately afterwards and never to
return to it—why, I ask, does she tie herself down for a fortnight to
this incessant, toilsome coming and going from the burrow to the fields
and from the fields to the burrow, forcing her way each time through
the unstable sand, either to go hunting or to bring the larva her
latest capture? It is, first and foremost, a question of having fresh
victuals for her larva: an all-important question, for the grub
absolutely refuses any high or tainted game. Like the grubs of the
other Diggers, it wants fresh meat and nothing but fresh meat.
We have seen in the case of the Cerceres, the Sphex and the Ammophilæ
how the mother solves the problem of preserved food-stuffs, the problem
of stocking a cell with the requisite quantity of game for its future
occupant and keeping the meat fresh for whole weeks at a time; indeed,
it is something more than fresh, for the victims are kept in an almost
living state, except that they are incapable of movement, an essential
condition if the grub is to feed on them in safety. The miracle is
performed by the most cunning methods known to physiology. The poisoned
lancet is driven into the nerve-centres once or oftener, according to
the structure of the nervous system. Thus operated upon, the victim
retains all the attributes of life, short of the power of moving.
Let us see if the Bembex make use of this profound science of
slaughter. The Flies taken from between the legs of the kidnapper as
she enters her burrow present, in most cases, every appearance of
death. They are motionless; occasionally we can detect in a few of them
some faint convulsions of the tarsi, the last vestiges of a life that
is passing away. The same appearance of complete death is usually found
in the insects which are not actually killed but paralysed by the
adroit dagger-thrust of a Cerceris or a Sphex. The question whether
they are alive or dead can therefore be decided only according to the
manner in which the victims keep fresh.
Placed in little screws of paper or in glass tubes, the Crickets and
Grasshoppers of the Sphex, the caterpillars of the Ammophilæ, and the
Beetles and Weevils of the Cerceres preserve their flexibility of limb,
their freshness of colouring and the normal condition of their
intestines for weeks and months. They are not corpses but bodies sunk
in a lethargy from which there is no awaking. The Flies of the Bembex
behave quite differently. The Eristales, the Syrphi—in short, all those
whose livery is at all brightly coloured—soon lose the brilliancy of
their attire. The eyes of certain Gad-flies, magnificently gilded, with
three purple bands, very quickly grow pale and dim, like the eyes of a
dying man. All these Flies, large and small, when placed in little
paper bags through which the air circulates freely, dry up in two or
three days and become brittle; all, when preserved against evaporation
in glass tubes in which the air is stationary, go mouldy and decay.
They are dead, therefore, really and truly dead, when the Wasp brings
them to her larva. Should some of them still retain a remnant of life,
a few days or even hours put an end to their agony. Consequently, for
lack of talent in the use of her dagger or for some other reason, the
murderess kills her victims outright.
In view of this fact, that the prey is quite dead at the moment when it
is carried off, who would not admire the logic of the Bembex’
procedure? How methodical and consistent everything is in the actions
of the cunning Wasp! As the provisions cannot keep beyond two or three
days without going bad, they must not be stored entire in the first
stages of an infancy which will last at least a fortnight; and the
hunting and distribution must necessarily be done day by day, bit by
bit, as the larva grows up. The first ration, the one that receives the
egg, will last longer than the others; the budding grub will take
several days to eat its flesh. It must therefore be small, otherwise
the joint would begin to putrefy before it was all finished. This joint
therefore will not be a bulky Gad-fly or a corpulent Bombylius, but
rather a tiny Sphærophoria, or something similar, making a dainty meal
for the larva which is still so delicate. Later, getting bigger and
bigger in time, will come the larger joints of venison.
The burrow must be kept shut during the mother’s absence, to save the
larva from regrettable intrusions; nevertheless the entrance must be
one that can be opened very frequently and hurriedly, without much
difficulty, when the Wasp returns laden with her prey and watched by
the sharp eyes of daring parasites. These conditions could not be
obtained with a compact soil such as that in which the Digger-wasps
usually make their abodes: the door, left to itself, would stay open;
and so, each time, there would be the long and toilsome job of either
blocking up the entrance with earth and gravel or unblocking it, as the
case might be. The house therefore must be dug in ground with a very
loose surface, in fine dry sand, which will at once yield to the
slightest effort on the mother’s part and, as it slides down, will
close the door of its own accord, like a curtain which, when you thrust
it aside with your hand, lets you pass through and then falls back
again. There you have the series of actions as deduced by man’s reason
and as practised by the Wasp’s sagacity.
Why does the spoiler kill the captured prey instead of simply
paralysing it? Is it for want of skill in the use of her sting? Is it
because of some difficulty due to the structure of the Flies or to the
methods employed in the chase? I must begin by confessing that I have
failed in my attempts to place Flies, without killing them, in that
state of complete immobility to which it is so easy to reduce a
Buprestis, a Weevil or a Scarab by injecting a tiny drop of ammonia
with a needle into the thoracic ganglia. In making the experiment, it
is difficult to render the insect motionless; and, by the time that it
has ceased to move, death has actually occurred, as is proved by its
speedy corruption or desiccation. But I have too much confidence in the
resources of instinct and have witnessed the ingenious solution of too
many problems to believe that a difficulty which baffles the
experimenter can bring the insect to a standstill. Therefore, without
throwing doubt upon the Bembex’ talents as a slaughterer, I should be
inclined to look for other reasons.
Perhaps the Fly, so thinly covered, so devoid of any plumpness, in a
word, so lean, could not, if paralysed by the sting, resist evaporation
long enough and would shrivel up during the two or three weeks of
waiting. Consider the puny Sphærophoria, the larva’s first mouthful.
How much liquid has that body to satisfy the needs of evaporation? An
infinitesimal drop, a mere nothing. The abdomen is a thin strip; its
two sides touch. Can such game as this form the basis of preserved
food, seeing that evaporation would dry up its juices in a few hours
when these are not renewed by nutrition? It is doubtful, to say the
least.
Let us examine the method of hunting, so as to throw some final light
on the subject. In the quarry removed from between the legs of the
Bembex, it is not rare to observe signs of a hurried capture, made
anyhow, according to the chances of a rough-and-tumble fight. The Fly
sometimes has her head turned the wrong way round, as though the
spoiler had wrung her neck; her wings are crushed; her fur, when she
possesses any, is ruffled. I have seen some that had their bellies
ripped open by their assailant’s mandibles and had lost their legs in
the battle. As a rule, however, the victim is intact.
No matter: considering the nature of the game, endowed with good wings
for flying, the capture must take place with a suddenness that makes it
hardly possible, I should say, to obtain paralysis unaccompanied by
death. A Cerceris face to face with her clumsy Weevil, a Sphex
grappling with the fat Cricket or the portly Ephippiger, an Ammophila
holding her caterpillar by the skin of its neck, all three have an
advantage over a prey which is too slow in its movements to avoid
attack. They can take their time, select at their ease the mathematical
spot where the sting is to penetrate, and lastly go to work with the
precision of an anatomist probing with his scalpel the patient who lies
before him on the operating-table. But with the Bembex it is a very
different matter: at the least alarm, the game nimbly makes off; and,
once on the wing, it can defy its pursuer. The Wasp has to pounce upon
her prey unawares, without considering how she shall attack or
calculating her blows, just as the Goshawk does when hunting in the
fallows. Mandibles, claws, sting, every weapon must be employed
simultaneously in the fierce fray so as to put an end as early as
possible to a contest in which the least hesitation would give the
victim time to escape. If these conjectures are borne out by the facts,
the Bembex’ prize can be nothing but a corpse or at most a mortally
wounded prey.
Well, my conjectures are correct: the Bembex delivers her attack with a
dash which would do credit to a bird of prey. To surprise the Wasp
hunting is not an easy thing; were we never so well armed with
patience, we should watch in vain in the neighbourhood of the burrow:
the favourable opportunity would not present itself, for the insect
flies far away and there is no possibility of following it in its rapid
evolutions. Its tactics would doubtless be unknown to me but for the
assistance of a utensil from which I would certainly never have
expected such a service. I am speaking of my umbrella, which I used as
a protection against the sun in the sand of the Bois des Issarts.
I was not the only one to profit by its shade; I was generally
surrounded by numerous companions. Gad-flies of various species would
take refuge under the silken dome and sit peacefully on every part of
the tightly-stretched cover. I was rarely without their society when
the heat became overpowering. To while away the hours when I had
nothing to do, it amused me to watch their great gold eyes, which shone
like carbuncles under my canopy; I loved to follow their solemn
progress when some part of the ceiling became too hot and obliged them
to move a little way on.
One day, bang! The tight cover resounded like the skin of a drum.
Perhaps an oak had dropped an acorn on the umbrella. Presently, one
after the other, bang, bang, bang! Can some practical joker have come
to disturb my solitude and fling acorns or little pebbles at my
umbrella? I leave my tent and inspect the neighbourhood: nothing! The
same sharp sound is repeated. I look up at the ceiling, and the mystery
is explained. The Bembex of the vicinity, who all consume Gad-flies,
had discovered the rich provender that was keeping me company and were
impudently penetrating my shelter to seize the Flies on the ceiling.
Things were going to perfection: I had only to sit still and look.
Every moment a Bembex would enter, swift as lightning, and dart up to
the silken ceiling, which resounded with a sharp thud. Some rumpus was
going on aloft, where the eye could no longer distinguish between
attacker and attacked, so lively was the fray. The struggle did not
last for an appreciable time: the Wasp would retire forthwith with a
victim between her legs. The dull herd of Gad-flies, at this sudden
irruption which slaughtered them one after the other, drew back a
little all round, without quitting the treacherous shelter. It was so
hot outside! Why get excited?
Obviously, this suddenness of attack, followed by the swift removal of
the prey, does not allow the Bembex to regulate her dagger-play. The
sting no doubt performs its office, but it is directed without
precision at those spots which the hazards of the fight place within
its reach. I have seen Bembex, to finish off their half-killed
Gad-flies still struggling in the assassin’s grasp, munch the head and
thorax of the victims. This habit in itself proves that the Wasp wants
a genuine corpse and not a paralysed prey, since she ends the Fly’s
agony with so little ceremony. All things considered, therefore, I
think that, on the one hand, the nature of the prey, which dries up so
quickly, and, on the other hand, the difficulty of making such rapid
attacks, explain why the Bembex serve up dead prey to their larvæ and
consequently cater for them from day to day.
Let us watch the Wasp as she returns to the burrow with her capture
held under her abdomen between her legs. Here comes one, the Tarsal
Bembex (B. tarsata), who arrives laden with a Bee-fly. The nest is
situated at the sandy foot of a steep bank. The huntress announces her
approach by a shrill humming, which has something plaintive about it
and which continues until the insect sets foot to earth. We see the
Bembex hover above the bank and then dip straight down, very slowly and
cautiously, all the time emitting her shrill hum. Should her keen eye
descry anything unusual, she slackens her descent, hovers for a second
or two, goes up again, comes down again and flies away, swift as an
arrow. After a few moments, here she is once more. Hovering at a
certain height, she appears to be inspecting the locality, as if from
the top of an observatory. The vertical descent is resumed with the
most cautious slowness; finally, the Wasp alights with no hesitation
whatever at a spot which to my eye has naught to distinguish it from
the rest of the sandy surface. At that instant the plaintive whimper
ceases.
The insect, no doubt, has landed more or less on chance, since the most
practised eye cannot distinguish one spot from the other on that
expanse of sand; it has alighted somewhere near its home, of which it
will now seek the entrance, concealed after its last exit not only by
the natural falling-in of the materials but also by the Wasp’s own
careful sweeping. But no: the Bembex does not hesitate at all, does not
grope about, does not seek. By common consent the antennæ are looked
upon as organs for guiding insects in their searches. At this moment of
the return to the nest, I see nothing particular in the play of the
antennæ. Without once letting go her prey, the Bembex scratches a
little in front of her, at the very spot where she has alighted, gives
a push with her head and straightway enters, with the Fly under her
abdomen. The sand falls in, the door closes and the Wasp is at home.
It makes no difference that I have seen the Bembex return to her nest
hundreds of times; it is always with fresh astonishment that I behold
the keen-sighted insect find without hesitation a door whose presence
there is nothing to indicate. This door, in fact, is hidden with
jealous care, not now, after the Bembex has gone in—for the
obliterating sand does not become quite level of its own weight, but
leaves perhaps a slight depression, or an incompletely blocked
porch—but certainly after she comes out, for, when starting on an
expedition, she never fails to put a finishing touch to the result of
the natural landslip. Wait for her departure and you shall see her,
before flying off, sweep the front of the door and level it with
scrupulous care. When she is gone, I defy the most penetrating eye to
find the entrance. To discover it again, when the sandy expanse was of
any size, I had to resort to a kind of triangulation; and how often,
after a few hours’ absence, did not my combinations of triangles and my
efforts of memory prove to be at fault! All that remained was the
stake, a grass-stalk planted on the threshold; and even this method was
not always effective, for the insect, with its passion for continually
improving the outside of the nest, often made the bit of straw
disappear from sight.