In their impenetrable coat of mail the Coleoptera offer but one
vulnerable point to their dart-bearing foe. This defect in the cuirass
is known to the assassin, and the poisoned sting is there inserted,
striking at one blow the three centres of motion, the Weevil and
Buprestid, which alone have a nervous organisation sufficiently
centralised, being selected. But what happens when the insect wears no
armour and is soft-skinned, so that the Hymenopteron can pierce it
anywhere that the chances of the struggle may direct? Is there then a
choice as to where the blow is given? Like the assassin who strikes at
the heart to shorten the dangerous struggles of his victim, does the
Sphex follow the tactics of the Cerceris, and strike by preference at
the motor ganglia? If so, what happens when these are distant from one
another, acting so independently that paralysis of one does not affect
the others? These questions will be answered by the history of an
insect which hunts field crickets, Sphex flavipennis.
It is towards the end of July that this Sphex tears open the cocoon
which until then has protected it, and flies away from its subterranean
cradle. During the whole of August one constantly sees it seeking drops
of honey on the spiny heads of Eryngium campestre, the commonest of
such robust plants as brave the dog days. But this careless life is
brief, for in the earliest days of September the Sphex has begun the
hard existence of miner and hunter. It is usually on some small flat
spot on banks along a road that the dwelling is established, only there
must be two indispensable conditions—a sandy soil easy to work, and
sun. Beyond this no precaution is taken to shelter the domicile against
autumn rain and winter frost. A horizontal position, unsheltered,
beaten by rain and wind, suit the Sphex perfectly, so long as it is
exposed to the sun. But when the work is half-way through, if heavy
rain should come, it is sad to see next day galleries in course of
construction choked with sand and finally abandoned.
Rarely does the Sphex work in solitude; it is in small bands of ten,
twenty, or more excavators that the claim selected is worked. One must
have spent some days watching one of these colonies in order to form
any idea of the restless activity, the feverish haste, the abrupt
movements, of these hard-working miners. They rapidly attack the ground
with the rakes of their forefeet, canis instar, as Linnæus says. A
puppy does not show more energy in scratching up the ground in play. At
the same time each labourer hums a joyous song—shrill, high-pitched,
interrupted at short intervals, and modulated by vibrations of wings
and thorax. One would think they were a troop of merry comrades,
stimulating one another to work by a cadenced rhythm. Meanwhile, the
sand flies, falling in fine dust on their quivering wings, and the
heavier gravel, pulled out bit by bit, rolls far away. If a bit resist
too much, the insect goes at it with a high note, reminding one of the
cry with which a woodcutter accompanies the stroke of his axe. Under
the redoubled efforts of tarsi and mandibles the cavity is already
sketched out, and the Sphex can already dart into it. Then comes a
lively interchange of forward movements to detach material, and of
backward to brush out fragments. In this hurried coming and going the
Sphex does not so much walk as dart forward, as though impelled by a
spring. With panting abdomen, antennæ vibrating, the whole body moved
by a strong thrill, she springs forward and is out of sight. You still
hear the unwearied hum underground, and one sees from time to time hind
legs pushing backward a wave of sand to the mouth of the burrow. From
time to time labour underground is interrupted either that the Sphex
may dust herself in the sunlight, and get rid of grains of dust which
insinuate themselves into delicate joints and hamper the liberty of her
movements, or that she make a reconnaissance in the neighbourhood.
Notwithstanding these short interruptions, in a few hours the gallery
is hollowed out and the Sphex appears on her threshold, to voice her
triumph, and give the last touch to her labours by effacing some
inequality, or carrying away some particles of earth, the objection to
which only the eye of a Sphex could perceive.
Of the many tribes of Sphegidæ visited by me, there is one of which I
retain a specially lively recollection, on account of its singular
installation upon the edge of a high road, where were little heaps of
mud thrown up from side ditches by the cantonnier’s shovel. One, well
sun-dried, had a conical shape like a sugar-loaf over fifteen inches
high. The situation pleased the Sphegidæ, who had established a more
populous community than I have ever again met with. From base to summit
the cone of dried mud was pierced with burrows, giving it the
appearance of a huge sponge. In every story was feverish animation, and
a busy coming and going which brought to mind the scene in some great
workshop when orders are pressing. Crickets were being dragged by the
antennæ up the slopes of the conical city; there was storing of
provisions in the larders of the cells; dust was pouring from galleries
in process of construction; at intervals the grimy faces of the miners
appeared at mouths of passages—there was a constant going and coming.
Now and then, in a short interval of leisure, a Sphex ascended the top
of the cone, perhaps to take a general and well-satisfied view from
this belvedere. What a tempting sight!—one to make me long to carry
away the entire city with its inhabitants. It was useless to try; the
mass was too heavy. One cannot take up a village by the roots to plant
it elsewhere.
Let us look at the Sphex at work in flat ground, as is much more
frequently the case. As soon as the burrow is hollowed out, the chase
begins. Let us profit by the absence of the Hymenopteron in search of
game, and take a look at her dwelling. The spot chosen by a Sphex
colony is generally horizontal, though the ground is not so level but
that there are little mounds crowned by a tuft of grass or thrift, or
inequalities consolidated by the slender roots of the vegetation which
covers them. It is on the sides of such furrows that the Sphex places
her den. For two or three inches in depth the gallery is horizontal,
serving as an approach to the hidden shelter for the provender and the
larvæ. In this vestibule the Sphex takes refuge in bad weather, rests
there at night, and occasionally by day for a few instants, showing
only her expressive face and impudent eyes. Beyond the vestibule an
abrupt turn descends more or less obliquely to a depth of two or three
inches more, ending in an oval cell rather larger in diameter, whose
axis lies parallel with the horizontal gallery. The cell walls are not
covered with any particular cement, but in spite of their bareness they
have evidently been the object of most careful labour. The sand is
heaped and levelled on the floor, on the ceiling, and sides, so as to
do away with the risk of landslips, or any roughness which might injure
the delicate skin of the larva. This cell communicates with the passage
by a narrow entrance, just wide enough to allow the Sphex, burdened
with prey, to enter. When this first cell is furnished with an egg and
necessary provisions, the Sphex walls up the entrance, but does not yet
abandon her burrow. A second cell is hollowed beside the first, and
provisioned in the same manner; there is then a third made, and
sometimes a fourth. Only then does the Sphex cast back into the burrow
the rubbish heaped at the entrance, completely effacing all outward
trace of her work. Three cells are usually found in each burrow, rarely
two, and yet more rarely four. As one learns by dissecting the insect,
one may estimate the number of eggs laid at about thirty, which would
make the number of burrows needed ten. Now these are hardly begun
before September, and are finished before the end of the month.
Consequently the Sphex cannot devote more than two or three days at
most to each burrow and its stores. Evidently the active little
creature has not a minute to lose, when in so short a time she has to
hollow out the lair, procure a dozen crickets, sometimes brought from a
distance through endless difficulties, to store them, and finally to
stop up the burrow. Moreover, there are days when wind makes hunting
impossible; rainy days or overcast ones suspending all work. The Sphex
cannot give to her building the enduring solidity that Cerceris
tuberculata gives to its deep galleries. This species transmit their
solid abodes from one generation to another, each year hollowed more
deeply, so that I was often bathed in perspiration when I tried to
reach them, and frequently my efforts and my implements proved useless.
The Sphex inherits nothing, and must herself do everything, and that
rapidly. Her dwelling is but a tent, hastily erected and moved on the
morrow. In compensation the larvæ, covered but by a thin layer of sand,
know how to supply the shelter which their mother has not given them;
they can clothe themselves with a double and triple waterproof
covering, far superior to the thin cocoon of the Cerceris.
But here comes a Sphex with noisy hum, returning from the chase. She
pauses on a neighbouring bush, holding in her mandibles one of the
antennæ of a big cricket, weighing far more than herself. Tired out by
the weight, she rests a moment, then grasps her captive between her
feet, and with a supreme effort flies right across the ravine between
her and her abode. She alights heavily on the flat ground where I am
watching, in the very middle of a Sphex village. The rest of the
journey is made on foot, the Sphex, not in the least intimidated by my
presence, comes astride her victim, holding her head proudly aloft
while she drags along the cricket between her feet by one of its
antennæ held in her jaws. If the soil be bare there is no difficulty,
but should a network of grass spread its runners across the way, it is
curious to see the astonishment of the Sphex at finding her efforts
baffled by this little obstacle—curious to witness her marches and
countermarches and repeated attempts until the difficulty is surmounted
either by the aid of her wings or a well-planned détour. The cricket is
at last conveyed to its destination and placed so that its antennæ come
exactly to the mouth of the burrow. Then the Sphex abandons it and
descends in haste to the bottom of the cave. A few seconds later she
puts her head out with a little cry of joy. The antennæ of the cricket
are within reach; she seizes them and promptly conveys it down to her
den.
I still ask myself in vain why these complicated manœuvres at the
moment of conveying the cricket into the burrow. Why, instead of going
down alone and returning to resume the prey left on the threshold, does
not the Sphex drag it into the gallery, as she did in the open air,
since the space is wide enough, or take it with her while she enters
backwards? The various predatory Hymenoptera which I have been able to
observe all drag their prey at once to the bottom of their cells,
holding it underneath them by their mandibles and intermediary feet.
Léon Dufour’s Cerceris does indeed somewhat complicate her movements,
since after putting down her Buprestis for a moment at the door of her
underground abode, she instantly goes backward into the gallery, seizes
her victim with her mandibles and drags it down; but that is very
unlike the tactics adopted in a like case by the Sphex. Why this
domiciliary visit, which invariably precedes the introduction of the
prey? May it not be that before descending hampered by a load, the
Sphex thinks it prudent to give a look round the bottom of her dwelling
to make sure that all is in order and to drive out, if necessary, some
impertinent parasite which may have slipped in during her absence?
Several Diptera, predatory flies, especially Tachinidæ, watch at the
doors of all the hunting Hymenoptera, spying out the favourable moment
to lay their eggs on other people’s game, but none penetrate into the
dwelling, nor venture into the dark passages, where, if by ill-luck the
owner caught them, they might have to pay dearly for their audacity.
The Sphex, like others, pays her tribute to the predatory Tachinidæ,
but they never enter her burrow to commit their misdeeds. Besides, have
they not all the time they need to lay their eggs on the cricket? If
they look sharp, they may very well profit by the Sphex’s momentary
absence from her victim to confide their posterity to it. What yet
greater danger menaces the Sphex which renders this preliminary descent
to the bottom of the burrow such an imperious necessity?
The one observed fact which can throw any light on the problem is this.
Amid a colony of Sphegidæ in full activity, whence all other
Hymenoptera are habitually excluded, I one day surprised a sportsman of
a different kind, Tachytes nigra, carrying one by one, without any
haste and with the greatest composure, amid the crowd where he was but
an intruder, grains of sand, little bits of dry stalk, and other small
materials, to stop up a burrow of the same shape and size as the
neighbouring ones of the Sphegidæ. This labour was pursued too
conscientiously to admit of any doubt as to the presence of the
worker’s egg in the underground dwelling. A Sphex with anxious
movements, apparently the legitimate owner of the burrow, never failed
each time that the Tachytes entered the gallery to dart in pursuit, but
emerged swiftly, as if frightened, followed by the other, who continued
her task unmoved. I visited this burrow, the evident cause of strife
between them, and found a cell provisioned with four crickets.
Suspicion almost gave place to certainty, for this allowance far
exceeded the needs of a Tachytes’ larva, which is at least one-half
smaller than the Sphex. The calm insect whose care to stop up the
burrow at first suggested that it was the owner was really a usurper.
How comes it that the Sphex, larger and more robust than her adversary,
allows herself to be robbed with impunity, limiting herself to a
fruitless pursuit, and flying like a coward when the intruder, who
seems not even to perceive her, turns round to come out of the burrow?
Is it with insects as with men, the first quality needed for success is
audacity—audacity—audacity? Certainly the usurper had no lack of it. I
can still see that Tachytes, imperturbably calm, going and coming
before the meek Sphex, which stamped with impatience, but did not
venture to fall upon the thief.
Let us add that in other circumstances I have repeatedly found this
Hymenopteron, I suppose to be a parasite—this Tachytes nigra, dragging
a cricket by one of its antennæ. Was it a prey lawfully acquired? I
would fain think so, but the indecision of the insect which strayed
about the ruts in the paths as if seeking a convenient burrow always
left me suspicious. I have never been present when it burrowed, if
indeed it ever does undertake that labour, and what is more, I have
seen it abandon its game to decay, perhaps not knowing what to do with
it for want of a hole where to put it. Such wastefulness seems to
indicate goods ill-gotten, and I ask myself if the cricket were not
stolen when the Sphex left it on her threshold? I also suspect Tachytes
obsoleta, banded with white round the abdomen like Sphex albisecta,
which nourishes its larvæ with crickets such as are hunted by the
latter. I have never seen it digging galleries, but I have caught it
dragging crickets that the Sphex would not have disdained. This
similarity of food in species of different genera makes me doubtful
whether the booty were lawfully come by. Let me add, however, to atone
in some measure for the injury which my suspicions may do to the
character of the genus, that I have seen the perfectly lawful capture
of a little cricket yet wingless by Tachytes tarsina, and have also
seen it hollow cells and store them with prey bravely acquired. Thus I
have only suspicions to offer as to why the Sphex persists in
descending to the bottom of her hole before carrying in prey. Is there
some other end besides that of dislodging a parasite which may have got
in during the owner’s absence? I despair of finding out; who can
interpret the thousand manœuvres of instinct? Poor human reason which
cannot even explain the wisdom of a Sphex!
At all events, it is proved that these manœuvres are singularly
invariable, àpropos of which I will mention an experiment which greatly
interested me. At the moment when the Sphex makes her domiciliary
visit, I take the cricket and put it some way off. The Sphex comes up,
utters her usual cry, looks round with astonishment, and seeing the
game too far off, comes out to seize and put it in the right position.
Then she goes down again without the cricket. Same manœuvre on my part,
same disappointment when she reappears. Again the prey is brought to
the mouth of the hole, and again the Sphex goes down alone, and so on
as long as my patience holds out. Forty times on end have I tried the
experiment on the same individual; her persistence vanquished mine, and
her tactics never varied.
Having proved the inflexible pertinacity of all the Sphegidæ in one
colony on whom I cared to experiment, I could not but perplex myself
over it. “Does then the insect obey a fixed tendency which
circumstances cannot modify?” I asked myself. “Are its actions all done
by rule, and is it unable to acquire the least experience from its own
proceedings?” Later observations modified this too absolute judgment.
The following year, at the proper time, I visited the same spot. The
new generation had inherited for their burrows the place chosen by the
preceding ones; it had also faithfully inherited their tactics, for the
cricket experiment gave the same results. Such as were the Sphegidæ of
the past year such are those of the present one, equally persistent in
a fruitless attempt. My error grew confirmed until good luck brought me
to another colony in a different place. I renewed my experiments. After
two or three trials with the old, well-known result, the Sphex got
astride of the cricket, seized its antennæ with her mandibles, and
dragged it at once into the burrow. Who looked a fool then? The
experimenter baffled by the clever Hymenopteron. At the other holes her
neighbours, some sooner, some later, found me out, and went down with
their prey instead of persisting in leaving it on the threshold to
seize it later. What is the meaning of this? This colony, descended
from another stock, for sons return to the spot selected by their
forefathers, is cleverer than the one observed last year. Craft is
inherited; there are sharper-witted tribes and duller ones, apparently
according to the faculties of their forefathers. With Sphegidæ, as with
us, the kind of intellect changes with the province. Next day I tried
the cricket experiment in another locality, and it invariably
succeeded. I had come upon a dense-minded tribe, a true colony of
Bœotians, as in my earlier observations.