THE BEMBEX
One of my favourite spots for the observations which I will now
describe is not far from Avignon, on the right bank of the Rhône,
opposite the mouth of the Durance. It is the Bois des Issarts. Let not
the reader mistake the value of this word bois, which usually suggests
a carpet of cool moss and the shade of tall trees, with a dim light
filtering through the leaves. The scorched plains where the Cicada
grates out his ditty on the pale olive-tree know none of these
delicious retreats filled with cool shadow.
The Bois des Issarts is a coppice of holm-oaks no higher than one’s
head and sparingly distributed in scanty clumps which, even at their
feet, hardly temper the force of the sun’s rays. When I used to settle
myself in some part of the coppice suitable for my observations, on
certain afternoons in the dog-days of July and August, I had the
shelter of a large umbrella, which later, in the most unexpected
fashion, lent me a very precious aid of a different kind, as my story
will show in good time. If I neglected to furnish myself with this
embarrassing adjunct to a long walk, my only resource against sunstroke
was to lie down at full length behind some sandy knoll; and, when the
veins in my temples were throbbing to bursting point, my last hope lay
in putting my head down a Rabbit-burrow. Such are one’s means of
keeping cool in the Bois des Issarts.
The soil not occupied by those clumps of woody vegetation is almost
bare and consists of fine, dry, very loose sand, which the wind heaps
into little dunes wherever the stems and roots of the holm-oak
interfere with its dissemination. The sides of these sand-dunes are
generally very smooth, because of the extreme lightness of the
materials, which slide down into the smallest depression and of their
own accord restore the evenness of the surface. You need but push your
finger into the sand and take it out again to bring about an immediate
landslip which fills up the hole and restores things to their original
condition without leaving a visible trace. But, at a certain depth,
which varies according to the more or less recent date of the last
rains, the sand retains a lingering dampness which keeps it in its
place and gives it a consistency that enables it to have small
excavations made in it without a subsequent collapse of walls and roof.
A blazing sun, a gloriously blue sky, sandy slopes that yield without
the least difficulty to the strokes of the Wasp’s rake, game galore for
the grub’s food, a peaceful site hardly ever disturbed by the foot of
man: all the good things are combined in this Bembex paradise. Let us
watch the industrious insect at work.
If the reader will sit with me under the umbrella, or consent to share
my Rabbit-burrow, this is the sight which he is invited to behold, at
the end of July: a Bembex (B. rostrata) arrives suddenly, I know not
whence, and alights, without preliminary investigations or the least
hesitation, at a spot which to my eyes differs in no respect from the
rest of the sandy surface. With her fore-tarsi, which are armed with
rows of stiff hairs and suggest at the same time a broom, a brush and a
rake, she works at clearing her subterranean dwelling. The insect
stands on its four hind-legs, holding the two at the back a little wide
apart, while the front ones alternately scratch and sweep the shifting
sand. The precision and quickness of the performance could not be
greater if the circular movement of the tarsi were worked by a spring.
The sand, shot backwards under the abdomen, passes through the arch of
the hind-legs, gushes like a fluid in a continuous stream, describes
its parabola and falls to the ground some seven or eight inches away.
This spray of dust, kept up evenly for five or ten minutes at a time,
is enough to show the dazzling rapidity of the tools employed. I know
no other example of this swiftness, which nevertheless in no way
detracts from the easy grace and the free movement of the insect, as it
advances and retires first on this side, then on that, without
discontinuing its parabolic streams of sand.
The soil excavated is of the lightest kind. As the Wasp digs, the sand
near by slips back and fills the cavity. Amongst the rubbish that falls
are tiny bits of wood, decayed leaf-stalks and particles of grit larger
than the rest. The Bembex takes them up in her mandibles and carries
them away, moving backwards as she goes; then she returns to her
sweeping, but never going to any length and making no attempt to bury
herself underground. What is her object in thus labouring entirely on
the surface? It would be impossible to tell from this first glance;
but, after spending many days with my beloved Wasps and grouping
together the scattered facts resulting from my observations, I seem to
catch a glimpse of the reason for the present proceedings.
The Wasp’s nest is certainly there, a few inches below the ground; in a
little cell dug in the cool, firm sand lies an egg, perhaps a grub for
which the mother caters from day to day, bringing it Flies, the
unvarying food of the Bembex in their first state. The mother has to be
able at any moment to enter the nest, as she flies up carrying in her
legs the nurseling’s daily portion of game, even as the bird of prey
enters its eyrie with the food for its young in its talons. But, while
the bird returns to a home on some inaccessible ledge of rock, with no
difficulty to overcome but that of the weight and encumbrance of the
captured prey, the Bembex has each time to undertake rough miner’s work
and open up anew a gallery blocked and closed by the mere fact that the
sand gives way as the insect proceeds. In that underground dwelling,
the only room with steady walls is the spacious cell where the larva
lives amid the remnants of its fortnight’s feast; the narrow corridor
which the mother enters to reach the flat at the back or to come out
and go hunting collapses each time, at least in the front part dug out
of very dry sand, which repeated exits and entrances make looser still.
Each time therefore that the Wasp goes in or out, she has to clear
herself a passage through the débris.
Going out presents no difficulty, even should the sand retain the
consistency which it might have at the start, when first disturbed: the
insect’s movements are free, it is safe under cover, it can take its
time and use its tarsi and mandibles without undue hurry. Going in is a
very different matter. The Bembex is hampered by her prey, which her
legs hold clasped to her body; and the miner is thus deprived of the
free use of her tools. And a still graver circumstance is this: brazen
parasites, veritable bandits in ambush, crouch here and there in the
neighbourhood of the burrow, spying on the mother Wasp as she makes her
laborious entrance, so that they may rush in and lay their egg on the
piece of game at the very moment when it is about to disappear down the
corridor. If they succeed, the Wasp’s nurseling, the son of the house,
will perish, starved by its gluttonous fellow-boarders.
The Bembex seems aware of these dangers and makes arrangements for her
entrance to be effected swiftly, without serious obstacles—in short,
for the sand blocking the door to yield to a mere push of her head,
aided by a brisk sweep of her front tarsi. With this object, the
material at the approaches to the home are subjected to a sort of
sifting. At leisure moments, under a kindly sun, when the larva has its
food and does not need her attentions, the mother rakes the ground in
front of her door; she removes little bits of wood, any extra-large
particles of gravel, any leaves that might get in the way and bar her
passage at the dangerous moment of her return. The Bembex whom we have
just seen so zealously employed was busy at this work of sifting: to
facilitate the access to her home, the materials of the corridor have
to be dug up, carefully sorted and rid of anything likely to obstruct
the road. Who indeed can tell whether, by that nimble eagerness, that
joyous activity, the insect is not expressing in its own way its
maternal satisfaction, its happiness in watching over the roof of the
cell to which the precious egg has been entrusted?
As the Wasp is confining herself to her duties outside the house,
without trying to penetrate into the sand, everything must be in order
inside and there is no hurry about anything. We should only wait in
vain: the insect would tell us nothing more for the time being. Let us
therefore examine the underground dwelling. If we scrape the dune
lightly with the blade of a knife at the point where the Bembex was
busiest, we soon discover the entrance-corridor, which, though blocked
for part of the way down, is nevertheless recognizable by the
distinctive appearance of the materials moved. This passage, which is
as wide as one’s finger and straight or winding, longer or shorter
according to the nature and the accidents of the ground, measures eight
to twelve inches. It leads to a single chamber, hollowed in the damp
sand, whose walls are not coated with any kind of mortar likely to
prevent a subsidence or to lend a polish to the rough surface. The
ceiling will do, if it can hold out while the larva is growing up; it
does not matter what falls in afterwards, when the larva is enclosed in
its stout cocoon, a sort of safe which we shall see it building. The
workmanship of the cell, therefore, is very rustic: the whole thing is
reduced to a rough excavation, of no definite shape, with a low roof
and space enough to contain two or three walnuts.
In this retreat lies a piece of game, one only, quite small and quite
insufficient for the greedy nurseling which it is meant to feed. It is
a golden-green Fly, a Green-bottle (Lucilia Cæsar), [33] who lives on
putrid flesh. The Fly served up as food is absolutely motionless. Is
she quite dead, or only paralysed? This question will be cleared up
later. For the moment we will note the presence, on the side of the
game, of a cylindrical egg, white, very slightly curved and a couple of
millimetres [34] long. It is the egg of the Bembex. As we expected from
the mother’s behaviour, there is nothing urgent indoors: the egg is
laid and provided with a first ration apportioned to the requirements
of the feeble grub which will hatch twenty-four hours hence. The Bembex
had no need to re-enter the underground passage for some time and was
confining herself to keeping a good look-out all round, or perhaps to
digging fresh burrows and continuing to lay her eggs, one by one, each
in a cell to itself.
This peculiarity of beginning the provisioning with a single head of
small game is not confined to the Rostrate Bembex. All the other
species do the same thing. If we open the cell of any Bembex shortly
after the egg is laid, we shall always find the tiny cylinder glued to
the side of a Fly, who constitutes the entire provision; moreover, this
initial ration is invariably small, as though the mother went in search
of the tenderest mouthfuls for the feeble nurseling. Besides, another
reason, the abiding freshness of the food, might easily prompt her to
make this choice. We will look into that later. This first portion,
always a scanty one, varies greatly in nature, according to the
frequency of this or that kind of game in the neighbourhood of the
nest. It is sometimes a Green-bottle, sometimes a Stomoxys, or some
small Eristalis, sometimes a dainty Bee-fly clad in black velvet; but
the most usual dish is a slim-bellied Sphærophoria.
This general fact, to which there is no exception, of the victualling
of the egg with a single Fly, a ration infinitely too small for a larva
blessed with a voracious appetite, at once puts us on the track of the
most remarkable habit of the Bembex. Wasps whose larvæ live on prey
heap up in each cell the number of victims necessary for the rearing of
the grub; they lay the egg on one of the bodies and close the dwelling,
which they do not enter again. From that moment the larva hatches and
develops alone, having before it from the very beginning the whole
stock of provisions which it is to consume. The Bembex form an
exception to this rule. The cell is first stocked with a single head of
game, always small in size, and the egg is laid on it. When that is
done, the mother leaves the burrow, which closes of itself; besides,
before going away, the insect is careful to rake over the outside, so
as to smooth the surface and hide the entrance from any eye but her
own.
Two or three days elapse; the egg hatches and the little larva eats up
the choice ration served to it. Meanwhile the mother remains in the
neighbourhood and you see her sometimes feeding herself by sipping the
sugary exudations of the field eringo, sometimes settling happily on
the burning sand, no doubt watching the outside of the house. Every now
and again she sifts the sand at the entrance; then she flies away and
disappears, perhaps to dig other cells elsewhere and to stock them in
the same way. But, however long she may stay away, she never forgets
the young larva so scantily provided for; the instinct of a mother
tells her the hour when the grub has finished its food and is calling
for fresh nourishment. She therefore returns to the nest, of which she
is wonderfully capable of discovering the invisible entrance; she goes
down into the earth, this time carrying a bulkier piece of game. After
depositing her prey, she again leaves the house and waits outside till
the moment arrives to serve a third course. This moment is not slow in
coming, for the larva devours its food with a lusty appetite. Again the
mother appears with fresh provisions.
During nearly a fortnight, while the larva is growing up, the meals
thus follow in succession, one by one, as needed, and coming closer
together as the nurseling waxes bigger. Towards the end of the
fortnight it takes all the mother’s activity to satisfy the appetite of
the glutton, who crawls heavily along with his great lumbering belly,
amid the scorned leavings: rejected wings and legs and horny abdominal
segments. You see her at every moment returning with a recent capture,
at every moment setting out again upon the chase. In short, the Bembex
brings up her family from day to day, without storing up provisions in
advance, just as the bird does, which feeds its nestlings from hand to
mouth. Of the many proofs that are evidence of this method of
upbringing, a very singular method for a Wasp who feeds her offspring
on prey, I have already mentioned the presence of the egg in a cell
containing no provisions but one small Fly, never more. And here is
another one, which can be verified at any time.
Let us look into the burrow of a Wasp who stocks her grubs’ provisions
in advance: if we select the moment when the insect is going in with
its prey, we shall find in the cell a certain number of victims, the
commencement of a larder, but never at that time a grub, nor even an
egg, for this is not laid until the provisions are quite complete. When
the egg is laid, the cell is closed and the mother does not return to
it. It is therefore only in burrows where the mother’s visits are no
longer necessary that we can find larvæ side by side with larger or
smaller stocks of food. On the other hand, let us inspect the home of a
Bembex at the moment when she is entering with the fruits of her
hunting. We are certain of finding in the cell a larva, big or little
as the case may be, among remnants of provisions already consumed. The
portion which the mother is now bringing is therefore intended to
prolong a meal which has already lasted several days and which is to
continue for some time further with the produce of future hunting
expeditions. Should we be fortunate enough to make this search towards
the end of the larva’s infancy—an advantage which I have enjoyed as
often as I wished to—we shall find, on a copious heap of remnants, a
large and portly grub, to which the mother is still bringing fresh
victuals. The Bembex does not cease her catering and does not leave the
cell for good until the larva, distended by a purply paste, refuses its
food and lies down, stuffed to repletion, on the jumble of legs and
wings of the game which it has devoured.
Each time that the mother enters the burrow on returning from the
chase, she brings but a single Fly. If it were possible, by counting
the remnants contained in a cell whose occupant is full-grown, to tell
the number of victims supplied to the larva, we should know how often
at the least the Wasp visited her burrow after laying the egg.
Unfortunately, these broken victuals, chewed and chewed again at
moments of scarcity, are for the most part unrecognizable. But, if we
open a cell with a less forward nurseling, the provisions lend
themselves to examination, some of them being still whole or nearly
whole, while others, more numerous, are represented by fragments in a
state of preservation that enables them to be identified. Incomplete
though it be, the list obtained under these conditions is surprising
and shows what activity the Wasp must display to satisfy the needs of
such a table. I will set forth one of the bills of fare which I have
observed.
At the end of September, around the larva of a Jules’ Bembex (Bembex
Julii), [35] which has reached almost a third of the size which it will
finally attain, I find the following heads of game: six Echinomyia
rubescens (two whole and four in pieces); four Syrphus corollæ (two
complete, the other two broken up); three Gonia atra (all three
untouched: one of them had that moment been brought along by the
mother, which led to my discovering the burrow); two Pollenia rufescens
(one untouched, the other partly eaten); one Bombylius (reduced to
pulp); two Echinomyia intermedia (in bits); and two Pollenia floralis
(likewise in bits): twenty pieces in all. This certainly makes a both
plentiful and varied bill of fare; but, as the larva was only a third
of its ultimate size, the complete menu might easily number as many as
sixty items.
It is not at all difficult to verify this sumptuous figure: I will
myself take the place of the Bembex in her maternal functions and
supply the larva with food till it is ready to burst. I move the cell
into a little cardboard box which I furnish with a layer of sand. I
place the larva on this bed, with all due consideration for its
delicate skin. Around it, without omitting a single fragment, I arrange
the provisions with which it was supplied. Then I go home, still
holding the box in my hand, to avoid any shaking which might turn the
house upside down and endanger my charge during a walk of several
miles. Any one who had met me on the dusty Nîmes Road, dropping with
fatigue and religiously carrying in my hand, as the sole fruit of my
laborious trip, an ugly grub battening on a heap of Flies, would
certainly have smiled at my simplicity.
The journey was effected without damage: when I reached home, the larva
was placidly eating its Flies as though nothing had happened. On the
third day of captivity the provisions taken from the burrow were
finished; the grub was rummaging with its pointed mouth among the heap
of remains without finding anything to suit it; the dry particles taken
hold of, all horny, juiceless bits, were rejected with disgust. The
moment has come for me to continue the food supply. The first Flies
within reach shall form my prisoner’s diet. I kill them by pressing
them in my fingers, but without crushing them. The first ration
consists of three Eristalis tenax and one Sarcophaga. [36] This is all
gobbled up in twenty-four hours. Next day I provide two Eristales, or
Drone-flies, and four House-flies. It was enough for the day, but left
nothing over. I went on like this for eight days, giving the grub a
larger portion every morning. On the ninth day the larva refused all
food and began to spin its cocoon. The full record of this eight days’
feast amounts to sixty-two pieces, composed mainly of Drone-flies and
House-flies, which, added to the twenty items found whole or in pieces
in the cell, brings up the total to eighty-two.
It is possible that I did not rear my larva with the wholesome
frugality and the wise economy which the mother would have shown; there
was perhaps some waste in the daily provisions served all at one time
and left entirely to the grub’s discretion. In some respects I feel
inclined to believe that things do not happen just like that in the
maternal cell, for my notes contain such details as the following. In
the alluvial sands of the Durance I discover a burrow which the Wasp
(Bembex oculata) has just entered with a Sarcophaga agricola. Inside I
find a larva, numerous fragments and a few whole Flies, namely, four
Sphærophoria scripta, one Onesia viarum and two Sarcophaga agricola,
including the one which the Bembex has just brought along before my
eyes. Now it is worthy of remark that half of this game, namely, the
Sphærophoriæ, is right at the end of the cell, under the larva’s very
teeth, whereas the other half is still in the passage, on the threshold
of the cell, and therefore beyond the reach of the grub, which is
unable to change its position. It seems to me then that, when game is
plentiful, the mother lays her captures on the threshold of the cell
for the time and forms a reserve on which she draws as and when
necessary, especially on rainy days when all labour is at a standstill.
Thus practised with economy, the distribution of food would save a
waste which I was not able to prevent with my larva, treated I dare say
too sumptuously. I therefore lower the figure obtained and reduce it to
some sixty pieces, of middling size, between that of the House-fly and
of the Eristalis tenax. This would about represent the number of Flies
supplied by the mother to the larva when the prey is of a moderate
size, as is the case with all the Bembex of my district except the
Rostrate Bembex (B. rostrata) and the Two-pronged Bembex (B.
bidentata), who have a preference for Gad-flies. With them, the number
of victims would be from one to two dozen, according to the size of the
Fly, which varies greatly in the different species of Gad-flies.
To avoid reopening this question of the nature of the provisions, I
will here give a list of the Flies observed in the burrows of the six
species of Bembex that form the subject of this essay.
1. Bembex olivacea, Rossi. I only once saw this species, at Cavaillon,
feeding on Green-bottles. The five other species are common in the
Avignon neighbourhood.
2. Bembex oculata, Jur. The Fly carrying the egg is most often a
Sphærophoria, especially S. scripta; sometimes it is a Geron gibbosus.
The later provisions include Stomoxys calcitrans, Pollenia ruficollis,
P. rudis, Pipiza nigripes, Syrphus corollæ, Onesia viarum, Calliphora
vomitoria, [37] Echinomyia intermedia, Sarcophaga agricola and Musca
domestica. [38] The usual fare consists of Stomoxys calcitrans, of
which I have many a time found fifty or sixty in a single burrow.
3. Bembex tarsata, Lat. This one also lays her egg on Sphærophoria
scripta. She next hunts: Anthrax flava, Bombylius nitidulus, Eristalis
æneus, E. sepulchralis, Merodon spinipes, Syrphus corollæ, Helophilus
trivittatus and Zodion notatum. Her favourite game consists of
Bombylii, or Bee-flies, and Anthrax-flies. [39]
4. Bembex Julii (sp. nov.). The egg is laid on a Sphærophoria or on a
Pollenia floralis. The provisions are a hotchpotch of Syrphus corollæ,
Echinomyia rubescens, E. intermedia, Gonia atra, Pollenia floralis, P.
ruficollis, Clytia pellucens, Lucilia Cæsar, Dexia rustica and
Bombylius.
5. Bembex rostrata, Fab. This is preeminently a consumer of Gad-flies.
She lays her egg on a Syrphus corollæ or a Lucilia Cæsar, after which
she feeds her larva exclusively on big game belonging to the various
species of the genus Tabanus.
6. Bembex bidentata, V. L. Another ardent huntress of Gad-flies. I have
never seen her pursue other game and I do not know on what Fly the egg
is laid.
This great variety of provisions shows that the Bembex have no
exclusive tastes and fall upon any species of Flies, indifferently,
which the hazards of the chase place within their reach. They seem
nevertheless to entertain a few preferences. Thus one species feeds
more particularly on Bee-flies, a second on Stomoxys-flies, a third and
a fourth on Gad-flies.