After this bill of fare for Bembecids in the larva state, we must seek
the motive which causes these Hymenoptera to adopt a mode of storage
exceptional among Fossors. Why, instead of laying up sufficient food
and dropping an egg on it, which would allow the cell to be closed at
once without need of returning, does the Hymenopteron oblige itself to
come and go perpetually for a fortnight from the fields to the burrow
and back again, toiling every time through the sand to issue forth and
hunt, or bring back prey? The explanation is that the food must be
fresh—an all-important matter, for the grub absolutely refuses game
which is at all high, with a hint of decay; like the larvæ of all
Fossors, it must have fresh provisions—always fresh provisions.
We have seen in the case of the Cerceris, Sphex, and Ammophila how the
mother resolves the feeding problem, by placing beforehand in the cell
a sufficient quantity of game, and also that of keeping it for weeks
perfectly fresh—nay, almost alive, though motionless—in order to secure
the safety of the grub which feeds on the prey. This marvel is brought
about by the most skilful means known to physiology. The poisoned sting
is sent into the nerve centres once or oftener, according to the
construction of the nervous system, and the victim retains all which we
call life, except power of motion.
Let us see if the Bembex practises this deep science of murder. Diptera
taken from between the feet of their captor as the latter enters the
burrow mostly seem quite dead. They are motionless; only in rare cases
are there some slight convulsions of the tarsi—the last vestiges of
life soon to be extinct. The same appearance of complete death is
found, as a rule, in insects not really killed but paralysed by the
skilful stab of a Cerceris or a Sphex. The question as to life or death
can, therefore, only be decided by the manner in which the victims keep
fresh.
Placed in little paper twists or glass tubes, the Orthoptera of the
Sphex, the caterpillars of the Ammophila, the Coleoptera of the
Cerceris, preserve flexibility of limb and freshness of colour, and the
normal state of their intestines, for weeks and months. They are not
corpses, but bodies plunged in a lethargy from which there will be no
awakening. The Diptera of the Bembex behave quite otherwise. Eristalis,
Syrphus,—in short, all which are brightly coloured,—soon lose their
brilliance; the eyes of certain gadflies, magnificently gilded, and
with three purple bands, soon grow pale and dim, like the gaze of a
dying man. All these Diptera, great and small, placed in paper twists
where air circulates, dry up and grow brittle in two or three days,
while all kept from evaporation in glass tubes, where the air is
stagnant, grow mouldy and decay. So they are dead—really dead—when
carried to the larva. If some few preserve a little life, a few days, a
few hours ends all. Not being clever enough to use its sting, or for
some other reason, the assassin kills its victims outright.
Knowing this complete death of the prey at the moment when it is
seized, who would not admire the logic of the Bembecid’s manœuvres? How
methodical all is, and how one thing brings about another in all which
the wary Hymenopteron does! As the food could not be stored without its
decaying at the end of two or three days, it cannot be laid in
wholesale at the beginning of a phase of life destined to last at least
a fortnight, and there must be a hunt and distribution of provisions
day by day, in proportion to the larva’s growth. The first ration—that
on which the egg is laid—will last longer than the others, and must be
small, for the little grub will take several days to eat it, and if too
big it would go bad before it was finished. Therefore it will not be a
huge gadfly or a corpulent Bombylius, but a small Sphærophoria, or
something of that kind, as a tender meal for a still frail larva.
Later, and gradually larger, will come the bigger joints.
In the mother’s absence the burrow must be closed to prevent awkward
intrusions, but the entrance must be one opened quickly, without
serious difficulty, when the Hymenopteron returns loaded with prey, and
laid in wait for by audacious parasites. These conditions would be
wanting in a tenacious soil, such as that in which the mining
Hymenoptera habitually establish themselves. The wide-open entrance
would each time require long and painful labour, whether to close it
with earth or gravel, or to clear it. The domicile, therefore, must be
hollowed in earth with a very light surface, in dry, fine sand,
yielding at once to the least effort of the mother, and which slips and
closes the entrance like floating tapestry, which, pushed back by the
hand, allows entrance and then drops back. Such is the sequence of
acts, deduced by human reason, and put into practice by the wisdom of
the Bembex.
Why does the spoiler kill the prey instead of paralysing it? Is it want
of skill with the sting? Is it a difficulty arising from the
organisation of the Diptera or from the manœuvres of the chase? I must
own, at once, that I have failed to put a Dipteron, without killing it,
into that state of complete immobility into which it is so easy to
plunge a Buprestis, a Weevil, or a Scarabæus, by injecting a little
drop of ammonia, on the point of a needle, into the thoracic ganglia.
It is difficult to render your subject motionless; when it no longer
moves, actual death has occurred, as is proved by its speedy decay or
desiccation. But I have too much confidence in the resources of
instinct,—I have seen the ingenious solution of too many problems,—to
believe that a difficulty, though insurmountable for the experimenter,
can baffle an insect; therefore, without casting doubt on the Bembex’s
capacity for murder, I should be inclined to seek other motives.
Perhaps the Dipteron, so thinly cuirassed, of so little substance,—so
lean, in short,—could not, when paralysed by a sting, resist
evaporation, and would dry up in two or three weeks. Consider the
slender Sphærophoria—the larva’s first mouthful. What is there in this
body to evaporate? An atom—a mere nothing. The body is a thin strip—its
two walls touch. Could such prey form a basis for preserved food when a
few hours would evaporate its juices, unrenewed by nutrition? To say
the least, it is doubtful.
Let us proceed to consider the manner of hunting, by way of throwing a
final light on the subject. In prey withdrawn from the clasp of a
Bembex, one may not infrequently observe indications of a capture made
in haste, as best might be, in the chances of a wild struggle.
Sometimes the Dipteron has its head turned backward, as if its neck had
been twisted, its wings are crumpled, and its hairs, if it have any,
are ruffled. I have seen one with the body ripped open by a bite from
the mandibles, and legs lost in the battle. Usually, however, the prey
is intact.
No matter. Considering that the game has wings prompt in flight, the
capture must be made with a suddenness which it seems to me hardly
allows of obtaining paralysis without death. A Cerceris with its heavy
weevil, a Sphex engaged with a corpulent grasshopper or a paunched
ephippiger, an Ammophila holding its caterpillar by the nape of its
neck, have all three the advantage over a prey too slow to avoid
attack. They may take their time, choose at leisure the exact spot
where the sting shall penetrate, and, in short, can act with the
precision of a physiologist who uses his scalpel on a patient laid upon
the operating table; but for the Bembex it is another matter. At the
least alarm the prey is off, and its power of wing defies that of the
pursuer. The Hymenopteron must pounce on its prey, without measuring
its attack or calculating its blow, like a hawk hunting over the
fallows. Mandibles, claws, sting—all weapons—must be used at the same
moment in the hot battle, to end as fast as possible a struggle in
which the least indecision would give the prey time to escape. If these
conjectures agree with facts, the Bembex can only secure a dead body,
or, at all events, a prey wounded to death.
Well, my calculations are right. The Bembex attacks with an energy
which would do honour to a bird of prey. To surprise one on the chase
is no easy matter, and it would be useless to lay in a stock of
patience and watch near the burrow, for the insect flies to a distance,
and it is impossible to follow its rapid evolutions, and doubtless its
manœuvres would be still unknown to me but for the help of an article
from which I should assuredly never have expected a like
service—namely, the umbrella which served me as a tent amid the sands
of Issarts.
I was not the only one to profit by its shade; my companions were
usually numerous. Gadflies of different kinds would take refuge under
the silken canopy, and roost peacefully here and there on the outspread
silk, rarely failing to appear when the heat was overpowering. To pass
the hours when I was unemployed, I used to observe with pleasure their
great gilded eyes shining like carbuncles under my canopy, or their
grave movements when some spot of their ceiling became too much heated,
and they were forced to move a little way.
One day—ping! ping! the tense silk was resounding like the parchment of
a drum. Perhaps an acorn has fallen on my umbrella. Soon after, close
together, came ping! ping! Has some idle jester come to disturb my
solitude, and fling acorns or little pebbles on my umbrella? I came out
of my tent and inspected the neighbourhood. Nothing! The blow was
repeated. I looked upward, and the mystery was explained. The Bembecids
of the neighbourhood, which prey on gadflies, had found out the rich
store of food which was keeping me company, and were darting
audaciously under my shelter to seize the gadflies on the ceiling.
Nothing could have been better. I had only to keep quiet and observe.
Every moment a Bembex entered like a sudden flash, and darted up to the
silken ceiling, which resounded with a dull thud. A tumult went on
aloft, in which one could not distinguish attacker from attacked, so
lively was the mêlée. The struggle was very brief; almost at once the
Hymenopteron retired with a captive between its feet. The dull band of
gadflies drew a little back all round on this sudden irruption, which
decimated them, but without leaving the treacherous shelter. It was so
hot outside; wherefore move? Plainly, such swift attack and prompt
departure with the prey does not allow the Bembex to use a poignard
according to rules. The sting no doubt fulfils its office, but is
directed with no precision towards such spots as are exposed by the
chances of the combat. To slay outright the half-murdered gadfly, still
struggling between the feet of its assassin, I have seen the Bembex
chew the head and thorax of her victim. This habit, peculiar to the
Bembecids, shows that the Bembex desires death, not paralysis, since
she ends the life of the Diptera with so little ceremony. Everything
considered, I think that on the one side the nature of the prey, so
quickly dried up, and on the other, the difficulties of so vehement an
attack, are the reasons why the Bembecids serve up dead prey to their
larvæ, and consequently provide it daily.
Let us follow the Hymenopteron when it returns with its captive closely
clasped to the burrow. Here is one—B. tarsata—coming loaded with a
Bombylius. The nest is placed at the sandy foot of a vertical slope,
and the approach of the Bembex is announced by a sharp humming,
somewhat plaintive, and only ceasing when the insect has alighted. One
sees her hover above the bank, then descend, following the vertical
line slowly and cautiously, still emitting the sharp hum. If her keen
gaze should discover anything unusual, she delays her descent, hovers a
moment, ascends again, redescends, then flies away, swift as an arrow.
In a few moments she returns. Hovering at a certain height she appears
to inspect the locality, as if from the top of an observatory. The
vertical descent is resumed with most circumspect deliberation;
finally, she alights without hesitation at a spot which to my eye has
nothing to distinguish it from the rest of the sandy surface. The
plaintive note ceases at once. She must have alighted somewhat by
chance, since the most practised eye could not distinguish one spot
from another on the sandy tract. She will have dropped down somewhere
near her hole, whose entrance she will now seek, marked since her last
exit not only by the natural falling in of materials, but by her
scrupulous sweeping. No! she does not hesitate in the least—does not
feel about—does not seek. All have agreed that the organs fitted to
direct insects in their researches reside in the antennæ. At the moment
of returning to the nest I see nothing special in their play. Without
once losing hold of the prey the Bembex scratches a little in front of
her just where she alighted, pushes with her head, and straightway
enters clasping the Dipteron to her body. The sand falls in, the door
closes, and the Hymenopteron is at home.
I have watched the Bembex return home a hundred times, yet it is always
with fresh astonishment that I see the keen-sighted insect at once
detect an entrance which nothing indicates, and which indeed is
jealously hidden—not indeed when she has entered (for the sand, more or
less fallen in, does not become level, and now leaves a slight
depression, now a porch incompletely obstructed), but always after she
comes out, for when going on an expedition she never neglects to efface
the traces of the sliding sand. Let us await her departure, and we
shall see that she sweeps before her door and levels everything
scrupulously. When she is gone, I defy the keenest eye to rediscover
the entrance. To find it when the sandy tract was of some extent I was
forced to have recourse to a kind of triangulation, and how often did
my triangle and efforts of memory prove vain after a few hours’
absence! I was obliged to have recourse to a stake—in other words, a
grass stalk planted before the entrance—a means not always effectual,
for it often disappeared during the frequent settings to rights of the
outside of the Bembex’s nest.