The Ammophila digging her well late in the day leaves her work after
stopping the entrance with a stone, flits away from one flower to
another, goes into a new neighbourhood, and yet next day can return
with a caterpillar to the abode hollowed out the evening before,
notwithstanding her want of acquaintance with the locality—often new to
her; the Bembex, loaded with prey, alights with almost mathematical
precision on the threshold of a dwelling blocked by sand and rendered
uniform with the rest of the sandy surface. Where my sight and memory
are at fault, theirs have a certainty verging on infallibility. One
would say that the insect possessed something more subtle than mere
recollection—a kind of intuition of locality with which nothing in us
corresponds—in short, an indefinable faculty which I call memory for
lack of any other expression by which to designate it. The unknown
cannot be named. In order to throw if possible a little light on this
point in the psychology of animals I instituted a series of
experiments, which I will now describe.
The first had for its subject Cerceris tuberculata, which hunts the
Cleonus. About 10 a.m. I took a dozen females busy at the same bank and
of the same colony, either hollowing or storing burrows. Each was
enclosed in a twist of paper, and all were put into a box. About two
kilometres from the nests I freed my captives, first marking them with
a white dot in the middle of the thorax by means of a straw dipped in
an indelible colour, in order to recognise them later. They flew on
every side—some here, some there, but only a few paces, alighting on
blades of grass and passing their forelegs over their eyes for a
moment, as if dazzled by the bright sunshine to which they were
suddenly restored. Then they took flight—some earlier, some later; and
one and all took unhesitatingly a straight line south, i.e. in the
direction of their home. Five hours later I returned to the common
territory of the nests. Almost directly I saw two of my white-dotted
Cerceris working at their burrows. Soon a third came in, with a weevil
between her feet. A fourth soon followed—four out of twelve in a
quarter of an hour was enough for conviction; I judged it useless to
wait longer; what four could do, the others could, if indeed they had
not already done it, and one may very well suppose that the eight
absentees were out hunting, or perhaps had retired into the depths of
their burrows. Thus, carried to a distance of two kilometres, in a
direction and by a way which they could not possibly perceive from the
depths of their paper prison, the Cerceris—at all events part of
them—had returned home.
I do not know to what distance they go hunting; possibly they know the
country round for some two kilometres. In that case they would not have
been far enough away, and came home by their local knowledge. The
experiment had to be tried again at a greater distance, and from a
point which the Cerceris could not possibly know.
I therefore took nine females from the colony whence I had got them in
the morning; three of these had been already experimented upon. Again
they were conveyed in a dark box—each imprisoned in a paper twist. The
starting-place was to be the neighbouring town of Carpentras, about
three kilometres from the burrows. I meant to release them not amid
fields as before, but in a street in the midst of a populous quarter,
where the Cerceris, with their rustic habits, had assuredly never
penetrated. As the hour was late I put off the experiment, and my
captives spent the night in their prison cells.
The next morning, towards eight o’clock, I marked them with a double
white spot on the thorax to distinguish them from those of the evening
before, which had only one, and set them free successively in the
middle of the street. Each mounted vertically, as if to get as soon as
possible from between the houses and gain a wide horizon, then rising
above the roofs, instantly and energetically turned its flight south.
And it was from the south that I brought them into the town, and their
burrows are south. Nine times with my nine prisoners did I obtain this
striking result—that an insect quite beyond its bearings should not
hesitate a moment what direction to take to regain its nest.
Some hours later I too was at the burrows. I saw several of my
yesterday’s Cerceris with a white dot, but none of the last set free.
Had they been unable to find the way back? Were they out hunting, or in
their galleries recovering from the excitement of such a trial? I do
not know. The next morning I came again, and had the satisfaction of
finding at work, as active as if nothing extraordinary had happened to
them, five Cerceris with two white dots. Three kilometres of
distance,—the town with its houses, roofs, and smoky chimneys—all so
novel to my rustic Cerceris,—had proved no obstacle to their return
home.
Taken from its brood and carried an enormous distance, the pigeon
returns promptly to its dovecote. If one were to consider the length of
journey in proportion to the size of the animal, how superior to the
pigeon is the Cerceris carried away three kilometres and returning to
its burrow! The size of the insect does not equal a cubic centimetre,
while that of the pigeon must quite equal the cube of a decimetre, if
it does not exceed it. The bird, a thousand times larger than the
Hymenopteron, ought, in order to rival it, to find its dovecote at a
distance of 3000 kilometres—thrice the greatest length of France from
north to south. I do not know if a carrier-pigeon has ever shown such
prowess, but wing-power and yet more lucidity of instinct cannot be
measured by yards. Nor can we here consider the question of size, and
one can only see in the insect a worthy rival to the bird without
deciding which has the advantage.
Are the two guided by memory when placed by man beyond their bearings
and carried to great distances—into regions with which they are
unacquainted and in unknown directions? Is memory as quick when, having
reached a certain height at which they can in some sort take their
bearings, they launch themselves with all their power of wing towards
that part of the horizon where are their nests? Is it memory which
traces their aerial way across regions seen for the first time?
Evidently not. It is not possible to recollect the unknown. The
Hymenopteron and the bird know nothing of their surroundings; nothing
can have taught them the general direction which they followed when
carried thither, for it was in the darkness of a closed box that the
journey was made. Locality, orientation,—all is unknown, and yet they
find their way. They have then as guide something better than simple
memory—a special faculty, a kind of topographic consciousness of which
we can form no idea, possessing nothing analogous to it.
I am now about to establish experimentally how subtle and precise is
this faculty in the narrow cycle where it is applied, and also how
limited and obtuse when it has to move out of habitual conditions. Such
is the invariable antithesis of instinct.
A Bembex, actively engaged in feeding her larva, has left her burrow.
She will return immediately with the product of the chase. The entrance
is carefully stopped with sand—swept backward by the insect before
departing. Nothing distinguishes it from the rest of the sandy surface.
But this offers no difficulty to the Hymenopteron, who finds her
doorway again with a sagacity which I have already described. Let us
plan some treachery; let us perplex her by altering the state of the
place. I cover the entrance with a flat stone as large as my hand. She
soon returns. The complete change made upon her threshold during her
absence does not seem to cause her the slightest hesitation; at all
events she alights immediately upon the stone, and tries for an instant
to hollow it, not at a chance spot, but exactly over the opening of her
burrow. Quickly turned aside from this attempt by the hardness of the
obstacle, she traverses the stone in every direction, goes round it,
slips underneath, and begins to dig in the precise direction of her
dwelling.
The flat stone is too trifling an obstacle to disconcert the clever
fly; let us find something better. I did not allow the Bembex to
continue her excavation, which I saw would soon prove successful, and
drove her far off with my handkerchief. The absence of the frightened
insect for a considerable time allowed me to prepare my snares
leisurely. What materials must now be employed? In these improvised
experiments one must know how to turn all things to profit. Not far off
on the high road is the fresh dropping of a beast of burden; here is
wood for our arrow. The dropping was collected, crushed, and spread in
a layer at least an inch thick on the threshold of the burrow and its
surroundings over more than a quarter of a yard. Assuredly here was
such a façade as never Bembex knew. Colour, the nature of the material,
the effluvium,—all combined to deceive the Hymenopteron. Can she take
this stretch of manure—this dung—for the front of the dwelling? She
does! Here she comes; studying from above the unusual condition of the
place, and settling in the middle of the layer, just opposite the
entrance, routing about, making a way through the fibrous mass, and
penetrating to the sand, she promptly discovers the mouth of the
passage. I stop and drive her away a second time.
Is not the precision with which the Bembex settles before her dwelling,
though masked in a way so novel, a proof that sight and memory are not
in such a case the only guides? What further can there be? Smell,
perhaps. That is very doubtful, for the emanations from the dung could
not baffle the perspicacity of the insect. Nevertheless, let us try
another odour. I happen to have with me, as part of my entomological
outfit, a little phial of ether. The covering of manure is swept off
and replaced by a cushion of moss, not very thick, but covering a wide
surface, on which I pour the contents of my phial the moment I see the
Bembex coming. The over-strong emanations keep her off, but only for an
instant. She alights on the moss, still reeking of ether, traverses the
obstacle and penetrates to her dwelling. The etherised effluvia did not
disturb her any more than did those of the manure; something surer than
smell tells where her nest is.
The antennæ have been often suggested as the seat of a special sense to
guide insects. I have already shown how the suppression of these organs
appears to offer no obstacle to the researches of the Hymenoptera. Let
us try once more in wider conditions. The Bembex is caught, its antennæ
amputated to the roots, and is then released. Stung by the pain—wild
with terror at being held between my fingers—the insect flies off
swifter than an arrow. I had to wait a whole hour, uncertain as to its
return. However, it came, and with its invariable precision alighted
quite close to its doorway, whose look I had changed for the fourth
time, having covered the site with a large mosaic of pebbles the size
of a nut. My work, which, compared to the Bembex, surpassed what for us
are the Megalithic monuments of Brittany, or the lines of Menhirs at
Carnac, was powerless to deceive the mutilated insect. Though deprived
of antennæ it found the entrance in the midst of my mosaic as easily as
would have done an insect under other conditions. This time I let the
faithful mother go home in peace.
The site transformed four times over, the outworks of the abode changed
in colour, scent, and material, the pain of a double wound,—all failed
to disconcert the Hymenopteron or even to make her doubtful as to the
precise locality of her doorway. I had exhausted my stratagems, and
understood less than ever how the insect, if it have no special guide
in some faculty unknown to us, can find its way when sight and smell
are baffled by the artifices of which I have spoken. Some days later an
experience gave me the opportunity to take up the problem from a new
point of view. The Bembex burrow had to be bared in its whole extent,
without quite destroying it, to which operation its shallowness and
almost horizontal direction, and the light soil in which it was made,
lent themselves readily. The sand was gradually scraped off with the
blade of a knife, and thus, deprived of roof from end to end, the
underground abode became a semi-canal or conduit, straight or curved,
some eight inches long, open where was the entrance, and ending in a
cul-de-sac where lay the larva amid its food.
The dwelling was uncovered in full sunshine; how would the mother
behave on her return? Let us consider the question scientifically. The
observer may be greatly embarrassed: what I have already seen leads me
to expect it. The mother’s impulse is to bring food to her larva, but
to reach this larva she must first find the door. Grub and entrance are
the points which appear to deserve being separately examined; therefore
I take away grub and food, and the end of the passage is cleared. There
is nothing more to do but arm one’s self with patience.
At last the Bembex arrives and makes straight for her absent door, only
the threshold of which remains. There for a good hour did I see her
dig, sweep the surface, send the sand flying, and persist, not in
making a new gallery, but in seeking the loose sand barrier which
should yield to the mere pressure of her head and let her pass easily.
Instead of loose materials she finds firm soil not yet disturbed.
Warned by this resistance she limits her efforts to exploring the
surface, always close to where the door should be, only allowing
herself to deviate a few inches. She returns to sound and sweep places
already sounded and swept some twenty times, unable to leave her narrow
circle, so obstinately convinced is she that the door must be there and
nowhere else. With a straw I pushed her gently and repeatedly to
another point. She would have none of it, and came back at once to
where the door ought to have been. Now and then the gallery, turned
into a semi-canal, appeared to attract her attention, but very faintly.
She would go a few steps along it, still raking, and then return to the
entrance. Two or three times I saw her go the whole length of the
gallery and reach the cul-de-sac where the larva should be, do a little
careless raking, and hurry back where the entrance used to be, and
continue searching with a patience which exhausted mine. More than an
hour had passed, and still she sought on the site whence the door had
disappeared.
What would happen in the presence of the larva? That was the second
part of the question. To continue the experiment with the same Bembex
would not have offered sufficient guarantee, as the creature, rendered
more obstinate by her vain search, seemed possessed by a fixed idea,
and this would have interfered with the facts which I wanted to prove.
I required a new subject, concerned solely with the impulses of the
actual moment. An opportunity soon came. The burrow was uncovered, as I
have just said; but I did not touch the contents; larva and food were
left in their places,—all was in order inside, the roof only was
wanting. Well, with this open dwelling, whose every detail the eye
could embrace,—vestibule, gallery, cell at the far end, with the grub
and its heap of provender,—this dwelling turned into a roofless gallery
at the end of which the larva was moving restlessly, under the hot sun,
its mother continued the manœuvres already described. She alighted just
where the entrance had been, and there it was that she hunted about and
swept the sand—there that she always returned after some hasty attempt
elsewhere in a circuit of a few inches. No exploration of the
gallery—no anxiety for the distressed larva; though the grub, whose
delicate skin has just exchanged the gentle moisture of a cave for
burning sunshine, is writhing on its heap of chewed Diptera, the mother
takes no notice of it. For her it is no more than any one of the
objects strewn on the sand,—a little pebble, a clod, a scrap of dried
mud,—nothing more. It is undeserving of attention. This tender,
faithful mother, who wears herself out in efforts to reach her
nursling’s cradle, cares nothing just now but for her entrance door—the
door she is used to. That which goes to her maternal heart is the
longing to find the well-known passage. Yet the way is open; nothing
holds her back, and under her eyes wriggles the grub, the final object
of her anxiety. With one spring she would be at the side of the unhappy
larva who so needs help. Why does she not rush to her beloved nursling?
She could dig a new habitation and get it swiftly underground. But
no—she persists in seeking a way which no longer exists, while her son
is grilled under her eyes. I was boundlessly surprised by this obtuse
maternity, since maternity is the most powerful and most fertile in
resource of all feelings which move the animal. Hardly could I have
believed my eyes but for endless experiments on the Cerceris and
Philanthidæ, as well as on Bembecidæ of different species. Stranger
still, the mother, after long hesitation, at length entered the
unroofed passage—all that was left of the corridor. She advanced, drew
back, and gave a few careless sweeps without stopping. Guided by vague
recollections, and perhaps by the smell of venison exhaled from the
heap of Diptera, she came occasionally as far as the end of the
gallery, the very spot where lay the larva. Mother and son had met. At
this moment of reunion after long anxiety, were there earnest
solicitude, sign of tenderness, or of maternal joy? Whoever thinks so
has only to repeat my experiment to convince himself of the contrary.
The Bembex did not recognise her larva at all; it was a worthless
thing, in her way,—nothing but an embarrassment. She walked over it and
trampled it unheeding, as she hurried backwards and forwards. If she
wanted to dig at the bottom of the cell, she rudely kicked it behind
her,—pushed, upset, expelled it, as she might have treated a large bit
of gravel which got in her way while at work. Thus maltreated, the
larva bethought itself of defence. I have seen it seize her by one
tarsus with no more ceremony than she would have shown in biting the
foot of a Dipteron caught by her. The struggle was sharp, but at last
the fierce mandibles let go, and the mother flew wildly away with her
sharpest hum. This unnatural scene of the son biting the mother, and
perhaps even trying to eat her, is unusual, and brought about by
circumstances which the observer is not always able to conjure up. What
one can always witness is the profound indifference of the Hymenopteron
for its offspring, and the brutal disdain with which that inconvenient
heap, the grub, is treated. Once she has raked out the far end of the
passage, which is done in a moment, the Bembex returns to her favourite
point, the threshold, to resume her useless researches. As for the
grub, it continues to struggle and wriggle wherever the maternal kicks
may have landed it. It will perish unaided by its mother, who could not
recognise it because she was unable to find the passage she was used
to. If we return to-morrow, we shall find it in the gallery,
half-broiled by the sun, and already a prey to the flies—once its own
prey.
Such is the connection in acts of instinct; one leading to the next in
an order that the most serious circumstances have no power to alter.
After all, what was the Bembex seeking? Her larva, evidently. But to
reach this larva she had to enter the burrow, and to enter the burrow
she had to find the door, and the mother persists in seeking this door
while the gallery lay open with provender and larva all before her. The
ruined abode, the endangered family, were for the moment unimportant;
all she could think of was the familiar passage reached through loose
sand. Let all go—habitation and inhabitant—if this passage be not
found! Her actions are like a series of echoes, awaking one another in
a fixed order, the following one only sounding when the preceding has
sounded. Not because there was any obstacle; the burrow was all open,
but for want of the usual entrance the first action could not take
place. That decides everything; the first echo is mute, and so all the
rest are silent. What a gulf between intelligence and instinct! Through
the ruins of the shattered dwelling a mother guided by intelligence
rushes straight to her son; guided by instinct she stops obstinately
where once was the door.