Not far from Avignon, on the right bank of the Rhône opposite the mouth
of the Durance, is one of my favourite points for the observations
about to be recorded. It is the Bois des Issarts. Let no one deceive
himself as to the value of the word “bois”—wood, which usually gives
the idea of a soil carpeted with fresh moss and the shade of lofty
trees, through whose foliage filters a subdued light. Scorching plains,
where the cicada grinds out its song under pale olives, know nothing of
such delicious retreats full of shade and coolness.
The Bois des Issarts is composed of thin and scattered groups of ilex,
which hardly lessen the force of the sun’s rays. When I established
myself during the dog days in July and August, I used to settle myself
at some spot in the wood favourable for observations. I took refuge
under a great umbrella, which later lent me most unexpected aid of
another kind, very valuable too, as my story will show in good time. If
I had neglected to equip myself with this article, embarrassing enough
in a long walk, the only way to avoid sunstroke was to lie at full
length behind some heap of sand, and when my temporal arteries beat
intolerably, the last resource was to shelter my head at the mouth of a
rabbit hole. Such are the means of getting cool in the Bois des
Issarts.
The soil, unoccupied by any woody vegetation, is almost bare and
composed of a fine, arid, very light sand, heaped by the wind in little
hillocks where the stems and roots of the ilex hinder its blowing
about. The slope of such hillocks is generally very smooth, from the
extreme lightness of the material, which runs down into the least
depression, thus restoring the regularity of the surface. It is enough
to thrust a finger into the sand, and then to withdraw it in order
immediately to cause a downfall, which fills up the cavity and
re-establishes the former state of things without leaving any trace.
But at a certain depth, varying according to the more or less recent
date of the last rains, the sand retains a dampness which keeps it
stable, and lends a consistency allowing of slight excavations without
roof and walls falling in. A burning sun, a radiant blue sky, sand
slopes yielding without the least difficulty to the strokes of the
Hymenopteron’s rake, abundant game for the larvæ, a peaceful site
rarely troubled by the foot of the passer-by,—all unite here in this
paradise of the Bembex. Let us see the industrious insect at work.
If the reader will come under my umbrella, or profit by my rabbit
burrow, this is the sight which will meet him towards the end of July.
A Bembex (B. rostrata) arrives of a sudden and alights without
hesitation or investigation at a spot which, as far as I see, differs
in nothing from the rest of the sandy surface. With her front tarsi,
which, armed with stiff rows of hairs, suggest at once broom, brush,
and rake, she begins to dig a subterranean dwelling, standing on her
four hind feet, the two last slightly apart, while the front ones
alternately scratch and sweep the loose sand. The precision and
rapidity of the action could not be greater were the circular movement
of the tarsi worked by a spring. The sand, shot backward under the
creature, clears the arch of its hind legs, trickling like a liquid in
a continuous thread, describing a parabola and falling some eight
inches away. This dusty jet, constantly fed for five or ten minutes, is
enough to show with what dizzy rapidity the tools are used. I could
quote no second example of equal swiftness, which yet in no way
detracts from the elegance and free movements of the insect as it
advances and retires, now on one side, now on another, without allowing
the parabola of sand to stop.
The soil hollowed is of the lightest kind. As the Hymenopteron
excavates, the sand near falls and fills the cavity. In the landslip
are mingled little bits of wood, decayed leaf-stalks, and grains of
gravel larger than the rest. The Bembex picks these up in her
mandibles, and, moving backward, carries them to a distance, returning
to sweep again, but always lightly, without attempting to penetrate
into the earth. What is the object in this surface labour? It would be
impossible to learn from a first glance, but after spending many days
with my dear Hymenoptera, and grouping together the scattered results
of my observations, I think I divine the motive of these proceedings.
The nest is certainly there—underground, at the depth of a few inches:
in a little cell, dug in cool firm sand, is an egg, perhaps a larva,
which the mother feeds daily with flies, the invariable food of Bembex
larvæ. She must be able at any moment to penetrate to this nest,
carrying on the wing, between her feet, the nursling’s daily ration,
just as a bird of prey arrives at its eyrie carrying game for its brood
in its claw. But while the bird returns to a nest on some inaccessible
shelf of rock, without any difficulty beyond the weight of its prey,
the Bembex must undertake each time the hard work of mining, opening
afresh a gallery blocked and closed by ever-sliding sand in proportion
as she proceeds. The only stable part of this underground abode is the
spacious cell inhabited by the larva amid the remains of a fortnight’s
feast; the narrow vestibule entered by the mother to go down to the
cell, or come forth for the chase, gives way each time, at all events
at the upper end, built in dry sand, rendered even looser by her
constant goings and comings. Thus at each entrance or exit the
Hymenopteron must clear out a passage. The exit offers no difficulties,
even should the sand have the same consistency as when first stirred;
the insect’s movements are free; it is safe under cover, can take its
time and use tarsi and mandibles at its leisure. Going in is another
matter. The Bembex is embarrassed by her prey, pressed to her body by
her feet, so that there is no free use of the mining tools. What is
more serious is that impudent parasites—veritable bandits in ambush—are
crouching here and there about the burrow watching her difficult
entrance to hurriedly drop their egg on the game just as it disappears
into the gallery. If they succeed, the son of the house, the
Hymenopteron’s nursling, will perish, starved by greedy guests.
The Bembex seems aware of this danger, and arranges so as to enter
quickly, without serious obstacles, so that the sand blocking the door
should yield to a mere push from her head, aided by a rapid sweep of
the forelegs. To this end she, so to say, sifts the materials round her
abode. In leisure moments, when the sun shines and the larva has its
food, and does not need her care, the mother rakes before her door, and
puts on one side all the tiny bits of wood, of over-large gravel or
leaves, which might get on her path and bar the passage at the perilous
moment of return. The Bembex which we saw so hard at work was busy
sifting so as to make access to her abode easier; the materials of the
vestibule are examined, minutely sorted, and cleared of every
encumbrance. Who can tell whether the rapid labour and joyous activity
of the insect do not express in their own way her maternal satisfaction
and happiness in caring for the roof of the cell which has received the
precious trust of the egg? As the Bembex confines herself to exterior
household cares without seeking to penetrate the sand, everything must
be in order within, and there is nothing pressing to do. We may wait,
but for the time the insect will teach us nothing more. Let us
therefore examine the underground dwelling.
By lightly scratching the bank with the blade of a knife just where the
Bembex was oftenest seen, one soon discovers the entrance hall, which,
blocked as it is for part of its length, is none the less recognisable
by the special look of the materials moved about. This passage, a
finger’s-breadth in size, rectilinear or winding, longer or shorter,
according to the nature of the ground, measures eight to twelve inches.
It leads to a single chamber, hollowed in damp sand, with walls
undaubed with mortar, which might prevent landslips and lend polish to
the rough surface. Enough if the ceiling lasts while the larva is being
fed up. Future falling-in matters little when the larva is enclosed in
its stout cocoon—a kind of strong box, which we shall see in process of
construction. In workmanship the cell is as rustic as possible, being
merely a rude excavation with no well-determined form, low roofed, and
of a size which might hold two or three nests.
Within lies one head of game—one only—quite small and quite
insufficient for the voracious nursling for whom it is destined. It is
a golden green-fly, Lucilia Cæsar, a dweller in tainted meat, and is
quite motionless. Is it really dead or only paralysed? This will be
cleared up later. Just now let us observe the cylindrical egg upon its
side, white, slightly curved, and a couple of millimetres in length. It
is a Bembex egg. As we have foreseen from the mother’s behaviour, there
is no pressing household business; the egg is laid and a first ration
provided for the needs of the feeble larva, which ought to hatch in
twenty-four hours. For some time the Bembex need not re-enter her hole,
confining herself to keeping a good lookout in the neighbourhood, or
possibly making new burrows and laying there egg after egg, always in a
separate cell.
This peculiarity of beginning to lay in food by a single small piece of
game is not peculiar to Bembex rostrata; all the other species do the
same. Open any cell after the egg is laid, and you always find it glued
to the side of a Dipteron—all the food there is; moreover, this first
ration is invariably small, as if the mother had sought some specially
tender mouthful for her frail nursling. Another motive, the freshness
of the food, may also have guided her choice. Later we will look
further into the matter. This first ration—always a moderate one—varies
much, according to the frequency of such or such a kind of game in the
neighbourhood. It is sometimes a Lucilia Cæsar, sometimes a Stomoxys,
or some small Eristalis, or a delicate Bombylius clad in black velvet,
but the commonest is a Sphærophoria with a slender abdomen. This fact
(and it has no exception) of storing the nest with but a single
Dipteron,—a ration far too meagre for a larva with a voracious
appetite,—at once puts us on the track of the most remarkable habit of
the Bembecidæ. Hymenoptera whose larvæ live on prey heap into each cell
the whole number of victims needed by the grub, which is hatched and
lives alone,—an egg having been laid on one fly and the dwelling closed
up. The larva has before it its whole store of food. But the Bembex is
an exception to this rule. First a head of game is brought to the cell
and an egg dropped on it. Then the mother leaves the burrow, which
closes of its own accord; besides which she takes care to rake the
surface smooth, and hide the entrance from every eye but her own.
Two or three days pass: the egg hatches and the small larva eats up its
choice ration. Meanwhile, the mother remains near: one may see her
licking the sugary exudations on the flower-heads of Eryngium campestre
for nourishment, then settling with enjoyment on the burning sand,
whence she doubtless surveys the exterior of her dwelling, or she sifts
the sand at its entrance, then flies off and vanishes—perhaps to
excavate other cells to be stored in a like manner. But however
prolonged her absence, she does not forget the young larva so scantily
provided for; maternal instinct teaches her the hour when the grub has
finished its food and needs new sustenance. Then she comes back to the
nest whose invisible entrance she knows right well how to find, and
penetrates the hollow—this time laden with a larger prey. This
deposited, she goes out again, and awaits outside the time for a second
expedition. It soon comes, for the larva shows a devouring appetite.
Again the mother arrives with fresh provender.
During almost a fortnight, while the larva is growing, the meals follow
each other thus, one by one, as it needs them, and so much the nearer
together as the nursling grows stronger. Toward the end of the
fortnight the mother requires all her activity to supply the glutton’s
appetite as it crawls heavily amid the remains of its repasts—wings,
feet, and horny rings of abdomens. Each moment she returns with a new
capture or comes forth for the chase. In short, the Bembex brings up
her family from hand to mouth without storing provisions, like the bird
which brings a beakful of food to the little ones still in the nest.
Among the numerous proofs of this method of upbringing—one very
singular in a Hymenopteron which feeds its family on prey—I have
already mentioned the presence of the egg in a cell where but one
little fly is found as provender—always one—never more. Another proof
is the following one, which does not require any special moment for its
ascertainment.
Let us examine the burrow of a Hymenopteron, which provides beforehand
for its larvæ. If we choose the moment when the insect enters with a
captive, we shall find in the cell a certain number of victims already
stored, but never a larva—not even an egg, for this is only laid when
the provisions are complete. The egg deposited, the cell is closed, and
the mother returns no more. It is, therefore, only in burrows where the
mother’s visits are no longer needed that one can find larvæ amid the
larger or smaller heap of food. Visit, on the other hand, the dwelling
of a Bembex as she enters with the produce of her chase, and you are
sure to find a larva, larger or smaller, amid the remains of food
already devoured. The ration now brought is to continue a repast which
has been going on for several days, and is to be prolonged upon the
produce of future expeditions. If we can make this examination towards
the end of the larva’s upbringing,—an advantage which I have enjoyed at
pleasure,—we shall find upon a great heap of fragments a portly larva,
to which the mother is still bringing food. The Bembex only ceases to
do so and to leave the cell definitely when the larva, distended by a
wine-coloured pap, refuses to eat, and reclines, thoroughly stuffed, on
the remains of wings and feet of the game which it has devoured.
Each time that she penetrates into the burrow on returning from the
chase, the mother brings but a single fly. Were it possible by means of
the remains contained in a cell where the larva is full grown to count
the victims served up, one would at least know how often the
Hymenopteron visited its burrow after the egg is laid. Unfortunately,
these broken meats—munched and munched again in moments of scarcity—are
for the most part unrecognisable. But on opening a cell with a less
advanced nursling, one can examine the provisions, some of the prey
being yet whole or nearly so, and others, more numerous, being trunks
in sufficiently good preservation to be distinguishable. Incomplete as
it is, the enumeration thus obtained strikes one with surprise, as
showing what activity the Hymenopteron must display to satisfy the
demands of such a table. Here is one of the bills of fare observed.
At the end of July around the larva of Bembex Julia, which had almost
reached the third of its full size, I found the prey of which the
following is the list:—Six Echinomyia rubescens—two whole and four in
pieces; four Syrphus corollæ—two whole, two in fragments; three Gonia
atra—all intact, and one just brought by the mother, which had enabled
me to discover the burrow; two Pollenia ruficollis—one whole, one
attacked; a Bombylius reduced to pulp; two Echinomyia intermedia in
bits; and finally two Pollenia floralis, also in bits—total, twenty.
Certainly we have here a bill of fare as abundant as varied, but as the
larva had only attained to a third of its complete size, the entire
bill of fare might well amount to sixty articles.
The verification of this magnificent sum-total is easily obtained. I
myself will undertake the maternal cares of the Bembex, and feed the
larva until it is thoroughly satisfied. I place the cell in a little
cardboard box furnished with a layer of sand. On this bed is placed the
larva with due regard to its delicate epidermis. Around it, without
omitting a single fragment, I arrange the provender with which it was
supplied, and return home with the box still in my hand, to avoid any
shake which might turn it topsy-turvy and endanger my charge during a
journey of several miles. Any one who had seen me on the dusty road to
Nîmes, exhausted with fatigue and bearing religiously in my hand, as
the only result of my painful journey, a wretched grub, distending
itself with a heap of flies, would assuredly have smiled at my
simplicity. The journey was achieved without hindrance; when I got home
the larva was peacefully consuming its flies as if nothing had
happened. On the third day the provisions taken from the burrow were
finished, and the grub with its pointed mouth was searching in the heap
of remains without finding anything to its taste. The dry, horny,
juiceless pieces which it got hold of were rejected with disgust. The
moment had come for me to continue the food supply. The first Diptera
within reach must content my prisoner; I slew them by squeezing them
between my fingers, but did not crush them. Three Eristalis tenax
composed the first ration, together with a Sarcophaga. In twenty-four
hours all were devoured. The next day I provided two Eristalis and four
house-flies. This sufficed for that day, but nothing was left over. I
went on thus for a week, giving the grub each morning a larger ration.
On the ninth day it refused to eat and began to spin its cocoon. The
bill of fare for the week’s high feeding amounted to sixty-two items,
chiefly Eristalis and house-flies, which, added to the twenty items
found entire or in fragments in the cell, formed a total of eighty-two.
Possibly I may not have brought up my larva with the wholesome
frugality which the mother would have shown; there may have been some
waste in the daily rations, provided all at once and left entirely to
the discretion of the grub. I fancied that in some particulars things
did not go on exactly as in the cell, for my notes have such details
as: “In the alluvial sands of the Durance I discovered a burrow into
which Bembex oculata had taken a Sarcophaga agricola. At the bottom of
the gallery was a larva, numerous fragments, and some Diptera
entire—namely, four Sphærophoria scripta, one Onesia viarum, and two
Sarcophaga agricola, counting that which the Bembex had brought under
my very eyes.” Now it must be remarked that one half of this game, the
Sphærophoria, was quite at the bottom of the cell—under the very jaws
of the larva, while the other half was still in the gallery—on the
threshold of the cell—consequently out of the grub’s reach, as it could
not leave its place. It would seem that when game abounds, the mother
disposes provisionally of her captures on the threshold of the cell,
and forms a reserve on which she draws as need arises, especially on
rainy days, when all labour is at a standstill. This economy in
distributing food would prevent the waste unavoidable with my larva
perhaps too sumptuously treated. I subtract then from the sum obtained,
and reduce it to sixty pieces of medium size, between that of the
house-fly and Eristalis tenax. This would be about the number of
Diptera given by the mother to the larva when the prey is middle-sized,
as is the case with all the Bembecids of my district except B. rostrata
and B. bidentata, which especially favour the gadfly. For these the
number of slain would be from one to two dozen, according to the size
of the Dipteron, which varies greatly in the gadfly species.
In order not to return to the kind of provisions, I give a list of the
Diptera observed in the burrows of the six kinds of Bembex, which are
the subject of this essay.
(1) B. olivacea, Rossi. Once only have I seen this species, at
Cavaillon, preying on Lucilia Cæsar. The five next are common round
Avignon.
(2) B. oculata, Jur. The Dipteron upon which the egg is laid is
generally a Sphærophoria, especially S. scripta; sometimes it is a
Geron gibbosus. Further provender consisted in Stomoxys calcitrans,
Pollenia ruficollis, P. rudis, Pipiza nigripes, Syrphus corollæ, Onesia
viarum, Calliphora vomitoria, Echinomyia intermedia, Sarcophaga
agricola, Musca domestica. The usual food was Stomoxys calcitrans, of
which I have found fifty or sixty in a single burrow.
(3) Bembex tarsata, Lat. It, too, lays its egg on Sphærophoria scripta;
but it also hunts Anthrax flava, Bombylius nitidulus, Eristalis æneus,
E. sepulchralis, Merodon spinipes, Syrphus corollæ, Helophilus
trivittatus, Zodion notatum. Its favourite prey consists in Bombylius
and Anthrax.
(4) Bembex Julii (a new species). The egg hatches either on a
Sphærophoria or a Pollenia floralis, and the provender is a mixture of
Syrphus corollæ, Echinomyia rubescens, Gonia atra, Pollenia floralis,
P. ruficollis, Clytia pellucens, Lucilia Cæsar, Dexia rustica,
Bombylius.
(5) Bembex rostrata. This is above all a captor of gadflies. It lays
its egg on a Syrphus corollæ, or a Lucilia Cæsar, but then only brings
to the larva large game belonging to the various kinds of Tabanus.
(6) Bembex bidentata. Another ardent hunter of gadflies. I have never
seen it with other game, and do not know on what the egg is laid.
This variety of provisions shows that the Bembecids have no exclusive
tastes, and attack one and all of the species of Diptera which are
offered by the chances of the chase. They seem, however, to have some
favourites—one species especially choosing Bombylius, another Stomoxys,
and a third and fourth, Gadflies.