I have just described the Bembex hovering, loaded with her prey, above
the nest, and descending with a vertical flight—very slow, and
accompanied by a plaintive hum. This cautious, hesitating mode of
arrival might suggest that the insect was examining from above in order
to find her door, and trying to recall the locality before alighting.
But I shall show that there is another motive. In ordinary conditions,
when nothing alarms her, she comes suddenly, without hovering or
plaintive hum or hesitation, and alights at her threshold, or close by.
So faithful is her memory that she has no need to search about. Let us
find out the cause of the hesitating arrival just described.
The insect hovers, descends slowly, mounts again, flies off and
returns, because serious danger threatens. That plaintive hum is a sign
of anxiety, and is never produced unless there is peril. But who is the
enemy? Is it I, sitting by and watching? Not in the least; I am quite
unimportant—a block unworthy of notice. The dreaded enemy—the foe who
must be avoided at any price—is on the ground, perfectly still upon the
sand, near the nest. It is a small Dipteron—nothing at all to look
at—of inoffensive aspect. This petty fly is the terror of the Bembex.
That bold assassin of Diptera, who so promptly twists the neck of
colossal gadflies, full fed on blood from an ox’s back, dares not enter
her home because she sees herself watched by another Dipteron—a mere
pigmy, which would scarce make one mouthful for her larva.
Why not pounce on it and get rid of it? The Bembex flies fast enough to
overtake it, and, small as it is, the larvæ would not disdain it, since
they eat all and every Diptera. Yet the Bembex flies in terror before
an enemy which one bite would hew in pieces. I really feel as though I
saw a cat wild with terror before a mouse. The ardent pursuer of
Diptera is driven away by a Dipteron, and that one of the smallest! I
bow before the facts without any hope of ever comprehending this
reversal of parts. To be able to get rid easily of a mortal enemy, who
is meditating the ruin of your family, and who might make a feast for
them—to be able, I say, to do this, and not to do it when the foe is
there, within reach, watching you, defying you,—is the height of folly
in an animal. Folly, however, is not rightly the word: let us rather
talk of the harmony of creatures, for since this wretched little
Dipteron has its small part to play in the great whole of things, the
Bembex must needs respect it and basely flee before it,—otherwise long
ago there would have been no more Dipteron of this species in the
world.
Let us trace the history of this parasite. Among Bembex nests there are
found, and that frequently, some which are occupied at the same time by
the larvæ of the Hymenoptera and by other larvæ—strangers to the family
and greedily sharing their food. These strangers are smaller than the
nursling of the Bembex—shaped like a tear, and of the colour of wine,
from the food paste which can be seen through their transparent bodies.
Their number varies from six to ten or more. They belong to a kind of
Dipteron, as may be perceived from their form and from the pupæ which
one afterwards finds in their place. The demonstration is completed by
bringing them up one’s self in a box, where, fed daily with flies, and
laid on sand, they turn into pupæ, whence issue the following year
little Diptera—Tachinids of the genus Miltogramma.
This is the Dipteron which, when lying in wait near the burrow, awakens
such alarm in the Bembex. Her terror is only too well founded. This is
what happens in the dwelling. Around the heap of food which the mother
wears herself out in providing in sufficient quantity, sit in company
with the legitimate nursling from six to ten hungry guests, who put
their sharp mouths into the general heap as unceremoniously as if they
were at home. Concord seems to reign at table. I have never seen the
legitimate larva take offence at the indiscretion of the strangers, nor
observed these attempt to trouble its repast. All keep themselves
together, and eat peaceably without annoying their neighbours.
So far all would be well, were it not that a grave difficulty arises.
However active may be the mother-nurse, it is clear that she cannot
meet such a consumption of food. She has to be incessantly on the wing
to feed one larva: what must happen if there are a dozen gluttons to
provide for? The result of this enormous increase of family can only be
want, or even famine, not for the larvæ of the Dipteron (which develop
more rapidly than that of the Bembex, profiting by the days when
abundance still reigns, their host being yet in early youth), but for
the latter, who reaches the moment of metamorphosis without being able
to make up for lost time. Besides, when the first guests become pupæ
and leave the table free to it, others come, as long as the mother
visits the nest, and complete its starvation.
In burrows invaded by numerous parasites the Bembex larva is
undoubtedly much smaller than one would expect from the heap of food
consumed, the remains of which encumber the cell. Limp, emaciated,—only
half or a third of its proper size,—it vainly tries to spin a cocoon,
the silk for which it has not got, and it perishes in a corner of the
cell, amid the pupæ of guests more fortunate than itself. Or its end
may be yet more tragic. Should provender fail, or the mother delay too
long in returning with food, the Diptera devour it. I ascertained this
black deed by bringing up the brood myself. All went well as long as
food was plentiful, but if through neglect, or on purpose, the daily
supply failed, next day or the day after I was sure to find the Diptera
larvæ greedily rending that of the Bembex. Thus, when the nest is
invaded by parasites, the legitimate larva is fated to perish either by
hunger or a violent death, and this it is which makes the sight of
Miltogramma prowling round the nest so odious to the Bembex.
The Bembex is not the only victim of these parasites: the burrows of
one and all of the mining Hymenoptera are invaded by Tachinids,
especially by the Miltogramma. Various observers—notably Lepeletier de
Saint Fargeau—have spoken of the manœuvres of these impudent Diptera;
but as far as I know none have perceived the very curious case of
parasitism at the expense of the Bembex—very curious, because the
conditions are quite different. Nests of other Fossors are stored
beforehand, and the Miltogramma drops an egg on the prey just as it is
being carried in. The provender stored and her egg laid, the
Hymenopteron closes up the cell where thenceforward live the legitimate
larva and the strangers, unvisited in their prison. Thus, the robbery
committed by the parasite is unknown to the mother, and must
consequently remain unpunished.
With the Bembex it is quite otherwise. The mother constantly returns
during the fortnight that she is bringing up the larva; she knows that
her offspring is living among numerous intruders, who appropriate the
greater part of the food; every time that she brings provender she
touches and feels at the bottom of her den these detestable guests,
who, far from contenting themselves with remains, seize what is best.
She must perceive, however small her powers of arithmetic may be, that
twelve are more than one; besides, she would discover this from the
disproportion between the consumption of food and her means of hunting,
and yet, instead of seizing these bold intruders and bundling them out,
she serenely tolerates them. Tolerates! Why, she feeds them and brings
them their rations, and perhaps feels as much tenderness for them as
for her own larva. It is a new version of the cuckoo story in yet more
singular circumstances. The theory that the cuckoo, almost as big as a
sparrowhawk and coloured like it, should look imposing enough to
introduce an egg unresisted into the nest of the weak hedge-sparrow,
and that the latter, overawed perhaps by the alarming look of her
toad-faced nursling, should accept and care for the stranger, has
something in its favour. But what shall we say of a sparrow which,
turning parasite, should go with splendid audacity and intrust her eggs
to the eyrie of a bird of prey—the nest of the sparrowhawk itself—the
sanguinary devourer of sparrows? What should we say of the bird of prey
who should accept the charge and bring up the brood tenderly? It is
precisely thus that the Bembex acts,—she, a captor of Diptera who yet
brings up other Diptera—a huntress who distributes food to a prey whose
last repast will be her own disembowelled offspring! I leave to
cleverer people the task of explaining these amazing relations.
Let us observe the tactics employed by the Tachinid, whose object is to
confide her egg to the nest of the miner. It is an invariable rule that
the fly should never penetrate into the burrow, even if left open and
the owner absent. The crafty parasite would take good care not to
entangle itself in a passage, where, having no possibility of flight,
it might pay dearly for its effrontery. The only moment for its
designs—a moment watched for with the greatest patience—is that when
the Hymenopteron enters the gallery, clasping her prey. At that
instant, brief as it is, when the Bembex or any other miner has half
her body within the entrance, and is about to disappear underground,
the Miltogramma arrives on the wing, perches on the prey slightly,
projecting beyond the hinder end of the Bembex, and while she is
delayed by the difficulties of entering, the Miltogramma, with
unparalleled promptitude, lays an egg on the prey, or two, or even
three eggs, successively. The hesitation of the Bembex, embarrassed by
her load, lasts but the twinkling of an eye; but that matters not—it is
long enough for the fly to accomplish its misdeed without being dragged
beyond the threshold. What must not be the suppleness of organs to
achieve this instantaneous laying of the egg! The Bembex disappears,
herself introducing the enemy, and the Tachinid goes and crouches in
the sun, close to the burrow, and meditates fresh crimes. If one would
make sure that the Dipteron’s eggs have really been deposited during
this rapid manœuvre, it suffices to open the burrow and follow the
Bembex to the bottom of her abode. The prey which one takes from her
bears underneath at least one egg—sometimes more, according to the
length of the delay at the entrance. These very minute eggs could only
belong to a parasite, and if any doubt remained, you can bring up the
brood in a box, and the result will be Diptera larvæ—later pupæ, and
finally Miltogramma.
The fly shows wonderful sagacity in the moment selected by it—the only
one which could permit of her carrying out her purpose with neither
peril nor vain efforts. The Bembex, half-way through the entrance,
cannot see her enemy audaciously perched on the hind quarters of the
prey, or, if she suspects the bandit’s presence, cannot drive it away,
having no freedom of movement in the strait passage, and in spite of
all precautions to facilitate speedy entrance, cannot always vanish
underground with the celerity required, so quick is the parasite. In
fact, this is the only propitious moment, since prudence forbids the
Dipteron to penetrate into the den, where other Diptera, far stronger
than itself, are served up as food for the larvæ. Outside, in the open
air, the difficulty is insurmountable, so great is the vigilance of the
Bembex. Let us give a moment to the arrival of the mother, when the
nest is being watched by the Miltogramma.
Some of these flies—more or fewer, generally three or four—have settled
on the sand and are quite motionless, all gazing at the burrow, the
entrance of which they know very well, carefully hid though it be.
Their dull-brown colour, their large crimson-red eyes, their intense
stillness, have often made me think of bandits who, dressed in a dark
material, with a red kerchief over their heads, are lying in wait to do
some evil deed. The Hymenopteron comes, loaded with prey. Had she no
anxieties she would alight straightway at her door. Instead, she hovers
at a certain height, descends slowly and circumspectly, hesitates, and
vibrates her wings, producing a plaintive hum denoting apprehension.
She must have seen the malefactors. They too have seen the Bembex. The
movement of their red heads shows that they are following her with
their eyes; every gaze is fixed on the coveted booty. Then come marches
and counter-marches of cunning versus prudence.
The Bembex drops straight down with an imperceptible flight, as if she
let herself sink gently, making a parachute of her wings. Now she is
hovering just above the ground; the flies take wing, placing themselves
one and all behind her,—some nearer, some farther,—in a geometrical
line. If she turns round to disconcert them, they turn too, with a
precision which keeps them all in the same straight line; if she
advances, so do they; if she draws back, they draw back too, measuring
their flight, now slow, now stationary, on that of the Bembex at the
head of the file. They do not attempt to fling themselves on the
desired object, their tactics being merely to hold themselves in
readiness in the position of rearguard, so as to avoid any hesitation
when the rapid final manœuvre shall come.
Sometimes, wearied out by their obstinate pursuit, the Bembex alights,
and the flies instantly settle on the sand, still behind her, and keep
quite still. She rises again, with a sharper hum—the sign no doubt of
increasing indignation; the flies follow her. One last means remains to
throw the tenacious Diptera off the track; the Bembex flies far
away—perhaps hoping to mislead the parasites by rapid evolutions over
the fields. But the crafty flies are not taken in; they let her go, and
settle down again on the sand round the burrow. When the Bembex returns
the same manœuvres begin again until the obstinacy of the parasites has
exhausted her prudence. At a moment when her vigilance fails, the flies
are instantly there. Whichever is at the most favourable point drops
upon the vanishing prey, and the thing is done—the egg is laid.
There is ample evidence that the Bembex is conscious of danger, and
knows how disastrous for the future of her nest is the presence of the
hated fly; her long efforts to throw the parasites off her track, her
hesitation and flights, leave not a doubt on the subject. How is it
then, I ask myself once more, that the enemy of Diptera should allow
herself to be annoyed by another Dipteron—a tiny robber, incapable of
the least resistance, which, if she chose, she could destroy instantly?
Why, when once free from the prey which hampers her, does she not
pounce on these ill-doers? What is needed to exterminate the evil brood
around her burrow? Merely a battle which would take but a few instants.
But the harmony of those laws which govern the preservation of species
will not have it so, and the Bembex will always allow herself to be
harassed without ever learning from the famous “struggle for life” the
radical means of extermination. I have seen some which, pressed too
closely, let fall their prey and flew off wildly, but without any
hostile demonstration, although dropping their game left them full
liberty of action. The prey, so ardently desired a moment earlier by
the Tachinidæ, lay on the ground at the mercy of them all, and not one
cared about it. It had no value for the flies, whose larvæ need the
shelter of a burrow. It was valueless also to the Bembex, who came
back, felt it for an instant and left it disdainfully. The little break
in her custody of it had rendered her suspicious of it.
Let us end this chapter by the history of the larva. Its monotonous
life offers nothing remarkable during the two weeks while it eats and
grows. Then comes making a cocoon. The slight development of
silk-producing organs does not allow of a dwelling of pure silk, like
those of the Ammophila and Sphegidæ—made of several wrappers which
protect the larva, and later the nymph, from damp in the ill-protected,
shallow burrow during autumn rains and winter snows. Yet this Bembex
burrow is in worse conditions than those of the Sphex, being made at a
depth of only a few inches in very permeable soil. To fashion a
sufficient shelter the larva supplements by its industry the small
amount of silk at its disposal. With grains of sand artistically put
together and connected by silky matter, it constructs a most solid
cocoon—impenetrable to damp.
Three general methods are employed by fossorial Hymenoptera to
construct the dwelling in which metamorphosis is to take place. Some
hollow burrows at a great depth under a shelter, and then the cocoon
consists of a single wrapper, so thin as to be transparent. Such is the
case with Philanthidæ and Cerceris. Others are content with a shallow
burrow in open ground; but in that case they have silk enough for
manifold wrappings of the cocoon, as with Sphegidæ, Ammophila, and
Scolia; or if the quantity be insufficient, they use agglutinated
sand—as, for instance, the Bembex and Palarus. One might take a Bembex
cocoon for a solid kernel, so compact and resistant is it. The form is
cylindrical—one end rounded, the other pointed. The length is about two
centimetres. Outside it is slightly wrinkled and coarse, but within the
walls are smoothed by a fine varnish.
Rearing at home has enabled me to follow every detail of the
construction of this curious piece of architecture—a real strong box
which can brave all the severity of the weather. First of all the larva
pushes away the remains of its feast into a corner of the cell, or the
compartment arranged for it in a box with paper partitions. Having
cleared a space, it affixes to the walls of its abode threads of a
beautiful white silk, forming a spidery web which keeps the heap of
food-remains at a distance, and serves as scaffolding for the work to
come.
This work consists of a hammock, suspended far from anything that can
defile it, in the centre of threads stretched from wall to wall. Fine,
beautiful white silk is the only material used. The shape is that of a
sack open at one end, with a wide circular orifice, closed at the other
and ending in a point; a fisherman’s basket gives a very fair idea of
it. Then the edges of the aperture are permanently kept apart by
numerous threads fastened to the neighbouring walls. The tissue of the
bag is extremely fine, allowing all that the grub does to be seen.
Things had been in this state since the previous evening, when I heard
the larva scratching in the box. On opening, I found my captive busy
scratching the cardboard walls with the tips of its mandibles, its body
half out of the bag. Already it had made considerable progress, and a
heap of little fragments were piled before the opening of the hammock,
to be utilised later. For lack of other materials it would doubtless
have used these scrapings for its constructions, but I thought it
better to provide according to its tastes and give it sand. Never did
Bembex larva build with such sumptuous material. I poured out for my
prisoner sand for drying writing,—sand well sprinkled with gilded
grains of mica,—before the opening of the bag, which was in a
horizontal position, suitable to the work which would follow. The
larva, half out of its hammock, chose its sand almost grain by grain,
routing in the heap with its mandibles, and, if one too bulky presented
itself, it was seized and cast aside. The sand being sorted, the larva
introduced a certain quantity with its mouth into the silken fabric,
then retired into its sack and began spreading the materials in a
uniform layer on the inner surface, then glued together various grains
and inlaid them in the fabric, with silk for cement. The outer surface
was constructed more slowly. These grains were carried singly and fixed
on with silk gum.
This first deposit of sand only concerns the anterior part of the
cocoon—that half which ends in the opening. Before turning round to
work at the back part, the larva renews its store of materials and
takes certain precautions, so as not to be embarrassed in its masonry.
The sand heaped before the entrance might slip inside and hinder the
builder in so narrow a space. The grub foresees this, and glues some
grains together, making a coarse curtain of sand, which stops up the
orifice, imperfectly indeed, but enough for the purpose. These
precautions taken, the grub labours at the back part of the cocoon.
From time to time it turns round to get fresh materials from outside,
tearing away a corner of the protecting curtain, and through this
window grasping the materials needed. The cocoon is still
incomplete—wide open at the upper end and without the spherical cap
needed to close it. For this final bit of work the grub provides itself
abundantly with sand, and then pushes away the heap before the
entrance. A silken cap is now woven and fitted close to the mouth of
this primitive basket. On this silken foundation are deposited, one by
one, the sand grains kept in the interior and cemented with
silk-spittle. This lid completed, the larva has only to give the last
finish to the interior of the dwelling and glaze the walls with
varnish, to protect its tender skin from the roughness of the sand.
The hammock of pure silk and the cap which later closes it are
evidently only scaffolding intended to support the masonry of sand and
to give it a regular curve. One might compare them to the constructions
used by builders when making an arch or vault. The work being
completed, the silken support disappears, partly lost in the masonry,
and partly destroyed by contact with rough earth, and no trace remains
of the ingenious method employed to put together a construction
perfectly regular, yet made of a material so little coherent as is
sand. The spherical cap which closes the original basket is a separate
work, adjusted to the main body of the cocoon. However well the two
pieces are fitted and soldered, the solidity is not such as the larva
would obtain had it built the whole dwelling continuously. Thus, on the
circumference of the cover there is a circular line less capable of
resistance, but this is not a fault of construction. On the contrary it
is a fresh perfection. The insect would experience grave difficulty in
issuing from its strong box, so thick are the walls, did not the line
of junction, weaker than the rest, apparently save much effort, as it
is usually along this line that the cover is detached when the perfect
Bembex emerges.
I have called the cocoon a strong box. It is indeed a solid article,
both from its shape and the nature of its materials. Landslips or
falling sand cannot alter its form, since the strongest pressure of
one’s fingers cannot always crush it. Thus it matters little to the
larva if the ceiling of its burrow, dug in loose soil, should sooner or
later fall in, and it need not fear, even should a passing foot press
down the thin covering of sand; it runs no risks when once enclosed in
its stout shelter. Nor does damp endanger it. I have immersed Bembex
cocoons for a fortnight in water without finding any trace of damp
inside them. Ah! why cannot we have such waterproof for our dwellings?
To sum up: the cocoon, of graceful oval shape, appears rather the
product of patient art than the work of a grub. For any one not behind
the scenes, the cocoons which I saw in process of construction with the
sand from my inkstand might well have been precious articles of some
unknown industry—great beads starred with golden dots on a ground of
lapis lazuli, destined for the necklace of some Polynesian belle.