THE RETURN TO THE NEST
The Ammophila sinking her well at a late hour of the day leaves her
work, after closing the orifice with a stone lid, flits away from
flower to flower, goes to another part of the country, and yet next day
is able to come back with her caterpillar to the home excavated on the
day before, notwithstanding the unfamiliar locality, which is often
quite new to her. The Bembex, laden with game, alights with almost
mathematical precision on the threshold of her door, which is blocked
with sand and indistinguishable from the rest of the sandy expanse.
Where my sight and recollection are at fault, their eyes and their
memory possess a sureness that is very nearly infallible. One would
think that insects had something more subtle than mere remembrance, a
kind of intuition for places to which we have nothing similar, in
short, an indefinable faculty which I call memory, failing any other
expression to denote it. There can be no name for the unknown. In order
to throw if possible a little light on this detail of animal
psychology, I made a series of experiments which I will now describe.
[40]
The first has for its subject the Great Cerceris, who hunts
Cleonus-weevils. About ten o’clock in the morning I catch twelve
females, all belonging to the same colony and at work on the same bank,
busy digging burrows or victualling them. Each prisoner is placed
separately in a little paper bag and the whole lot put in a box. I walk
about a mile and a half from the site of the nests and then release my
Cerceres, first taking care, so that I may know them later, to mark
them with a white dot in the middle of the thorax, using a straw dipped
in indelible paint.
The Wasps fly only a few yards away, in every direction, one here,
another there; they settle on blades of grass, pass their fore-tarsi
over their eyes for a moment, as though dazzled by the bright sunshine
to which they have suddenly been restored; then they take flight, some
sooner, some later, and all, without hesitation, make straight for the
south, that is to say, for home. Five hours later I return to the
common site of the nests. I am hardly there when I see two of my
Cerceres with white dots working at the burrows; soon a third arrives
from the fields, with a Weevil between her legs; a fourth is not slow
in following. The recognition of four out of twelve in less than
fifteen minutes was enough to convince me. I thought it unnecessary to
wait any longer. What four could do the others would do, if they had
not already done it; and I was quite at liberty to presume that the
absent eight were out hunting or else hidden in their underground
galleries. Therefore, carried for a mile and a half in a direction and
by a road of which they could not have taken cognizance in their paper
prisons, the Cerceres, or at least some of them, had returned home.
I do not know how far the Cerceres’ hunting-grounds extend; and it is
possible that they know the country more or less over a radius of a
mile and a half. In that case, they would not have felt sufficiently
lost at the spot to which I moved them and they would have got home by
their acquired local knowledge. The experiment had to be repeated, at a
greater distance and from a starting-point which the Wasp could not be
suspected of knowing.
I therefore take nine female Cerceres from the same group of burrows
that supplied me in the morning. Three of them had just been subjected
to the previous test. They were again carried in a dark box, each
insect enclosed in its paper bag. The starting-point selected is the
nearest town, Carpentras, which lies at about two miles from the
burrow. I am to release my insects not among the fields, as on the
first occasion, but absolutely in the street, in the centre of a
crowded neighbourhood, where the Cerceres, with their rustic habits,
had certainly never penetrated. As the day is already far advanced, I
postpone the experiments; and my captives spend the night in their
prison-cells.
Next morning, at about eight, I mark them on the thorax with two white
spots, to distinguish them from yesterday’s lot, who were marked with
only one; and I set them free, one after the other, in the middle of
the street. Each Cerceris released first shoots straight up between the
two rows of houses, as though to escape as soon as possible from the
narrow street and gain the spacious horizons; then, rising above the
roofs, she at once darts away vigorously towards the south. And it was
from the south that I brought them; it is in the south that their
burrows are. Nine times, with nine prisoners, freed one after the
other, I had this striking instance of the way in which the insect
stranded far from home takes without hesitation the right direction for
returning to the nest.
I myself was at the burrows a few hours later. I saw several of
yesterday’s Cerceres, recognizing them by the one white spot on the
thorax; but I saw none of those whom I had just let loose. Had they not
been able to find their home again? Were they hunting? Or were they
hiding in their galleries to recover from the excitement of such a
trial? I do not know. Next day I paid a fresh visit; and this time I
had the satisfaction of finding at work, as active as though nothing
out of the way had happened, five of the Cerceres with two white spots
on the thorax. A journey of quite two miles, the town with its houses,
its roofs, its smoky chimneys, all things so new to these utter
rustics, had not prevented them from going back to the nest.
When taken from his brood and carried to enormous distances, the Pigeon
returns promptly to the dovecote. If we wanted to work out a proportion
between the length of the journey and the size of the creature, how
greatly superior to the Pigeon would be the Cerceris, who finds her
burrow after being carried a distance of two miles! The bulk of the
insect is not a cubic centimetre, [41] whereas that of the Pigeon must
be quite a cubic decimetre, [42] if not more. The bird, being a
thousand times larger than the Wasp, ought therefore, in order to rival
her, to find the dovecote at a distance of two thousand miles, which is
thrice the greatest length of France from north to south. I do not know
that a Carrier-pigeon has ever performed such a feat. But power of
flight and, still less, lucidity of instinct are qualities that cannot
be measured by the yard. Comparative size cannot here be taken into
consideration; and we must just look upon the insect as a worthy rival
of the bird, without deciding which of the two has the advantage.
In returning to the dovecote and the burrow, when man has artificially
made them lose their bearings and carried them to great distances, in
unfamiliar directions and into regions which they have not yet visited,
are the Pigeon and the Cerceris guided by recollection? Is memory their
compass when, on reaching a certain height, whence they can, so to
speak, pick up the scent after a fashion, they dart with all their
power of wing towards the horizon where their nests are? Is it memory
that traces their road through the air, across regions which they are
seeing for the first time? Obviously not: there can be no recollection
of the unknown. The Wasp and the bird are unacquainted with the country
around; nothing can have told them the general direction in which they
were moved, for the journey was made in the darkness of a closed basket
or a box. Locality, relative position: everything is unknown to them;
and yet they find their way. They therefore have something better than
mere memory as a guide: they have a special faculty, a sort of
topographical sense of which we cannot possibly form an idea, having
nothing similar ourselves.
I will show by experiment how subtle and precise this faculty is within
its narrow province, and also how obtuse and dull it becomes when
driven to depart from the usual conditions in which it acts. This is
the invariable antithesis of instinct.
A Bembex, actively engaged in feeding her larva, leaves the burrow. She
will return presently with the produce of the chase. The entrance is
carefully stopped up with sand, which the insect has swept there
backwards before going away; there is nothing to distinguish it from
other points of the sandy surface; but this does not trouble the Wasp,
who finds her door with a skill which I have already emphasized. Let us
devise some insidious plot and change the conditions of the locality in
order to perplex the insect. I cover the entrance with a flat stone,
the size of my hand. The Wasp soon arrives. The great change effected
on her threshold during her absence appears to cause her not the
slightest hesitation; at least, the Bembex at once alights upon the
stone and tries, for an instant, to dig into it, not at random but at a
spot corresponding with the opening of the burrow. The hardness of the
obstacle soon dissuades her from her enterprise. She then runs about
the stone in every direction, goes all round it, slips underneath and
begins to dig in the exact direction of her dwelling.
The flat stone is not enough to mislead our wide-awake friend; we must
find something better. To cut things short, I do not allow the Bembex
to continue her excavations, which, I can see, will soon prove
successful; I drive her off with my handkerchief. The fairly long
absence of the frightened insect will give me time to prepare my snares
at leisure. What materials shall I employ now? In these improvised
experiments we must know how to turn everything to use. Not far off, on
the high-road, are the fresh droppings of some beast of burden. The
very thing! The droppings are collected, broken up, crumbled and then
spread in a layer at least an inch thick on the threshold of the burrow
and all around, covering about a quarter of a square yard. This
certainly is a house-front the like of which no Bembex ever knew. The
colouring, the nature of the materials, the stercoral effluvia all
combine to mystify the Wasp. Will she take all this—that expanse of
manure, that dung—for the front of her door? Why, yes: here she comes!
She inspects the unwonted condition of the place from above and settles
in the middle of the layer, just opposite the entrance. She digs, makes
a hole through the stringy mass and reaches the sand, where she at once
finds the orifice of the passage. I stop her and drive her away a
second time.
Is not the precision with which the Wasp alights just in front of her
door, though this be masked in a way so new to her, a proof that sight
and memory are not her only guide? What else can there be? Could it be
scent? It is very doubtful, for the emanations from the droppings have
not been able to baffle the insect’s perspicacity. Still, let us try a
different smell. I happen to have on me, as part of my entomological
luggage, a small phial of ether. I sweep away the sheet of manure and
replace it by a blanket of moss, not very thick, but spreading to a
considerable distance; and I pour the contents of my phial on it as
soon as I see the Bembex arrive. The ethereal fumes, at first too
strong, keep the Wasp away, but only for a moment. Then she alights on
the moss, which still exhales a very perceptible smell of ether, passes
through the obstacle and makes her way indoors. The ethereal effluvia
put her out no more than did the stercoral effluvia. Something surer
than scent tells her where her nest lies.
The antennæ have often been suggested as the seat of a special sense
able to guide insects. I have already shown how the amputation of those
organs seems in no way to impede the Wasp’s investigations. Let us try
once more, under more complicated conditions. I seize the Bembex, cut
off her antennæ at the roots, and at once release her. Goaded by pain,
maddened at having been imprisoned in my fingers, the insect darts off
faster than an arrow. I have to wait for a good hour, very uncertain as
to whether it will come back. The Wasp arrives however and, with her
unvarying precision, alights quite close to her door, whose appearance
I have changed for the fourth time. The site of the nest is now covered
with a spreading mosaic of pebbles the size of a walnut. My work,
which, as regards the Bembex, surpasses what the megalithic monuments
of Brittany or the rows of menhirs at Carnac are to us, is powerless to
deceive the mutilated insect. Though deprived of her antennæ, the Wasp
finds her entrance in the middle of my mosaic as easily as the same
insect, supplied with those organs, would have done under other
conditions. This time I let the faithful mother go indoors in peace.
Four successive alterations in the site; changes in the colour, the
smell, the materials of the outside of the home; lastly, the pain of a
double wound: all had failed to baffle the Wasp or even to make her
waver as to the precise locality of her door. I had come to the end of
my stratagems and understood less than ever how the insect, if it
possess no special guide in some faculty unknown to us, can find its
way when sight and scent are baffled by the artifices which I have
mentioned.
A few days later, a lucky experiment reopened the question and allowed
me to study it under another aspect. In this case we uncover the
Bembex’ burrow all the way along, without changing its appearance too
much, an operation made easier by the shallowness of the burrow, its
almost horizontal direction, and the lack of consistency of the soil in
which it is dug. With this object we scrape the sand away gradually
with a knife. Thus deprived of its roof from end to end, the
underground dwelling becomes an open trench, a conduit, straight or
curved, some eight inches long, open at the spot where the
entrance-door used to be and finishing in a blind alley at the other
end, where the larva lies amid its victuals.
Here is the home uncovered, in the bright light, under the sun’s rays.
How will the mother behave on her return? Let us consider the question
in detail, according to scientific precepts: it is a perplexing
position for the observer, as my recent experiences make me suspect.
Here is the problem: the mother on arriving has the feeding of her
larva as her object in view; but to reach this larva she must first
find the door. The grub and the entrance-door: those are the two
aspects of the question that appear to me to merit separate
consideration. I therefore take away the grub, together with the
provisions, and the end of the passage becomes a clear space. After
making these preparations there is nothing to do but exercise patience.
The Wasp arrives at last and goes straight to where its door ought to
be, that door of which naught but the threshold remains. Here, for more
than an hour, I see her digging on the surface, sweeping, making the
sand fly, and persisting, not in scooping out a new gallery, but in
looking for that loose door which ought easily to give way before a
mere push of the head and let the insect through. Instead of yielding
materials, she finds firm soil, not yet disturbed. Warned by this
resistance, she confines herself to exploring the surface, always in
close proximity to the spot where the entrance should be. A few inches
on either side is all that she allows herself. The places which she has
already tested and swept twenty times over she returns to test and
sweep again, unable to bring herself to leave her narrow radius, so
obstinate is her conviction that the door must be here and not
elsewhere. Several times in succession I push her gently with a straw
to some other point. She will not be put off: she returns straightway
to the place where her door once stood. At rare intervals the gallery,
now an open trench, seems to attract her attention, though very
faintly. The Bembex takes a few steps towards it, still raking, and
then goes back to the entrance. Twice or thrice I see her run the whole
length of the conduit and reach the blind alley, the abode of her grub;
here she gives a few careless strokes of the rake and hurries back to
the spot where the entrance used to be, continuing her quest there with
a persistency that ends by wearying mine. More than an hour has passed
and the stubborn Wasp is still pursuing her search on the site of the
vanished doorway.
What will happen when the larva is present? This is the next aspect of
the question. To continue the experiment with the same Bembex would not
have given me the positive evidence which I wanted, for the insect,
rendered more obstinate by its vain quest, seemed to me now obsessed by
a fixed idea, which would certainly have obscured the facts which I
wished to ascertain. I needed a fresh subject, one not over-excited and
solely concerned with the impulses of the first moment. An opportunity
soon presented itself.
I uncover the burrow from end to end as I have just explained, but
without touching the contents: I leave the larva in its place, I
respect the provisions; everything in the house is in order; there is
nothing lacking but the roof. Well, in front of this open dwelling, of
which the eye freely takes in every detail: entrance-hall, gallery,
cell at the back with the grub and its heap of Flies; in front of this
dwelling now a trench, at the end of which the larva wriggles under the
blistering rays of the sun, the mother behaves exactly as her
predecessor did. She alights at the point where the entrance used to
be. It is here that she does her digging and sweeping; and it is here
that she always returns after hurried visits elsewhere, within a radius
of a few inches. There is no exploration of the tunnel, no anxiety
about the tortured larva. The grub, whose delicate epidermis has just
passed from the cool moisture of an underground cave to the fierce
blaze of an untempered sun, is writhing on its heap of chewed Flies;
the mother does not give it a thought. To her it is no more than any
other object lying on the sand: a little pebble, a pellet of earth, a
scrap of dry mud, nothing more. It is unworthy of attention. This
tender and faithful mother, who wears herself out in trying to reach
her nurseling’s cradle, is wanting at the moment her entrance-door, the
usual door and nothing but that door. What stirs her maternal heart is
her yearning for the well-known passage. And yet the way is open: there
is nothing to stop the mother; and the grub, the ultimate object of her
anxiety, is tossing restlessly before her eyes. One bound would bring
her to the side of the poor thing clamouring for assistance. Why does
she not rush to her beloved nurseling? She could dig it a new dwelling
and swiftly place it in safety underground. But no; the mother persists
in seeking a passage that no longer exists, while her child is grilling
in the sun before her eyes. My surprise is intense in the presence of
this short-sighted mother, though the sense of motherhood is the most
powerful and resourceful of all the feelings that stir the animal
creation. I should hardly believe the evidence of my eyes but for
experiments endlessly repeated with Cerceres and Philanthi as well as
with Bembex of different species.
Here is something more remarkable still: the mother, after prolonged
hesitation, at last enters the roofless trench, all that remains of the
original corridor. She goes forward, draws back, goes forward again,
giving a few careless sweeps, here and there, without stopping. Guided
by vague recollections and perhaps also by the smell of game emitted by
the heap of Flies, she occasionally reaches the end of the gallery, the
very spot at which the larva lies. Mother and son are now together. At
this moment of meeting after long suffering, have we a display of eager
solicitude, exuberant affection, any signs whatever of maternal joy? If
you think so, you need only repeat my experiments to persuade yourself
to the contrary. The Bembex does not recognize her larva at all; it is
to her a worthless thing, something in her way, a nuisance. She walks
over the grub, treads on it ruthlessly, as she hurries to and fro. When
she wants to try and dig at the bottom of the cell, she thrusts it back
with a brutal kick; she shoves it on one side, topples it over, flings
it out as unceremoniously as if it were a big bit of gravel that
hindered her in her work. Thus knocked about, the grub thinks of
defending itself. I have seen it seize its mother by the tarsus with no
more ceremony than it shows when it bites off the leg of its prey, the
Fly. The struggle was hotly contested; but at last the fierce mandibles
let go and the mother vanished in terror, making a shrill whimpering
noise with her wings. This unnatural sight of the son biting his mother
and perhaps even trying to eat her is uncommon and is brought about by
circumstances which the observer has not at his command; but what can
always be witnessed is the Wasp’s profound indifference towards her
offspring and the brutal contempt with which she treats that irksome
lump of rubbish, the grub. Once she has raked out the end of the
passage, which is the work of a moment, the Bembex returns to her
favourite spot, the threshold, where she resumes her useless search. As
for the grub, it continues to writhe and wriggle wherever its mother
has kicked it. It will die without the mother’s coming to its
assistance, for she fails to recognize it because she was unable to
find the customary passage. Go back to-morrow and you shall see it
lying in its trench, half baked by the sun and already a prey to the
very Flies that were once its prey.
Such is the concatenation of instinctive actions, linked one to the
other in an order which the gravest circumstances are powerless to
disturb. What, after all, is the Bembex looking for? Her larva,
obviously. But, to get at that larva, she must enter the burrow; and,
to enter that burrow, she must first of all find the door. And it is in
the search for this door that the mother persists, despite the
wide-open gallery, despite the provisions, despite the grub, all
exposed to view. At the moment she cares not that her house is in ruins
and her family in danger; what she wants above all things is the
familiar passage, the passage through the loose sand. Perish
everything, dwelling and inmate, if this passage be not found! Her
actions are like a series of echoes each awakening the next in a
settled order, which allows none to sound until the previous one has
sounded. The first action could not be performed, not because of an
obstacle, for the house is wide open, but for want of the usual
entrance. That is enough: the subsequent actions shall not be
performed; the first echo was dumb and all the rest are silent. What a
gulf separates intelligence and instinct! Through the ruins of the
demolished dwelling, a mother guided by intelligence hurries straight
to her son; guided by instinct, she comes to a stubborn halt on the
site of her old door.