THE AMMOPHILÆ
A slender waist, a slim shape; an abdomen tapering very much at the
upper part and fastened to the body as though by a thread; black
raiment with a red sash across the belly: there you have a summary
description of these burrowers, who are akin to the Sphex in form and
colouring, but differ greatly from them in habits. The Sphex hunt
Orthoptera—Locusts, Grasshoppers, Crickets—while caterpillars are the
quarry of the Ammophilæ. This change of prey in itself suggests new
methods in the lethal tactics of instinct.
If the name did not sound so pleasant to the ear, I would willingly
quarrel with the term Ammophila, which means ‘sand-lover,’ as being too
exclusive and often erroneous. The real lovers of sand, of dry, dusty,
streaming sand, are the Bembex, who prey on Flies; but the
caterpillar-hunters, whose story I now propose to relate, have no
predilection for ordinary shifting sand, and even avoid it as being
liable to landslips on the slightest provocation. Their perpendicular
shaft, which has to remain open until the cell receives the provisions
and an egg, requires a firmer setting if it is not to be prematurely
blocked. What they want is a light soil, easily tunnelled, in which the
sandy element is cemented with a little clay and lime. Edges of paths,
sunny banks where the grass is rather bare: those are the favourite
spots. In spring, quite early in April, we see the Hairy Ammophila (A.
hirsuta) there; when September and October come, we find the Sandy
Ammophila (A. sabulosa), the Silvery Ammophila (A. argentata), and the
Silky Ammophila (A. holosericea). I will here condense the information
which I have gathered from the four species.
In the case of all four the burrow is a vertical shaft, a sort of well,
possessing at most the diameter of a thick goose-quill and a depth of
about two inches. At the bottom is the cell, which is always solitary
and consists of a mere widening of the entrance-shaft. It is, when all
is said, a poor lodging, obtained economically, in one day’s work; the
larva will find no protection there against the winter except from the
four wrappers of its cocoon, copied from that of the Sphex. The
Ammophila digs by herself, quietly, without hurrying, without any
joyous enthusiasm. As usual, the fore-tarsi serve as rakes and the
mandibles do duty as mining-tools. When some grain of sand offers too
much resistance to its removal, you hear rising from the bottom of the
well, as though to give voice to the insect’s efforts, a sort of shrill
grating sound produced by the quivering of the wings and of the whole
body. At frequent intervals the Wasp appears in the open with a load of
refuse in her teeth, some bit of gravel which she flies away with and
drops at a distance of a few inches, so as not to litter the place. Of
the grains extracted some appear to deserve special attention, owing to
their shape and size; at least, the Ammophila does not treat them as
she does the rest: instead of flying off and dropping them far from the
work-yard, she removes them on foot and lays them near the well. These
are picked materials, ready-made blocks of stone which will serve
presently for closing the dwelling.
This outside work is performed with measured movements and solemn
diligence. The insect stands high on its legs, with its abdomen
stretched at the end of its long pedicle, and turns round slowly,
pivoting its whole body stiffly, with the geometrical rigidity of a
line revolving on itself. If it wishes to fling to a distance the
rubbish which it thinks will be in the way, it does so in short silent
flights, often backwards, as though the Wasp, emerging from her well
head last, avoided turning, so as to save time. It is the species
carrying their abdomens on the longest stalks, such as the Sandy
Ammophila and the Silky Ammophila, which mainly display this
automaton-like rigidity in action. That belly swelling into a pear at
the end of a thread is in fact a very delicate thing to steer: a sudden
movement might warp the fine stalk. So we must walk with a sort of
geometrical rigour; if we have to fly, we will do so backwards, to
avoid tacking too often. On the other hand, the Hairy Ammophila, who
has a short abdominal pedicle, works at her burrow with the heedless,
nimble movements which we admire in most of the Digger-wasps. She has
more freedom of action, because her belly does not get in her way.
The home is dug. At a later hour in the day, or even merely when the
sun has left the place where the burrow has just been bored, the
Ammophila invariably visits the little heap of stones placed in reserve
during the excavating, with the object of choosing a bit to suit her.
If there is nothing that satisfies her needs, she explores the
neighbourhood and soon discovers what she wants, a small flat stone
slightly larger in diameter than the mouth of her hole. She carries off
this slab in her mandibles and lays it, as a temporary door, over the
opening of the burrow. To-morrow, when the weather is once more hot and
the sun bathes the slopes and encourages hunting, the Wasp will know
quite well how to find her home, rendered inviolable by the massive
door; she will come back with a paralysed caterpillar, grasped by the
skin of its neck and dragged between its captor’s legs; she will lift
the slab, which nothing distinguishes from other little stones around
and which she alone is able to identify; she will let down the game to
the bottom of the well, lay her egg, and close the house for good by
sweeping into the perpendicular shaft all the rubbish which she has
kept in the vicinity.
Time after time the Sandy Ammophila and the Silvery Ammophila have
shown me this temporary closing of the hole when the sun begins to go
down and when the lateness of the hour compels the victualling to be
put off till the morrow. When the dwelling had been sealed up by the
Wasp, I too would postpone my observations till the next day, but only
after first making a map of the ground, choosing my lines and landmarks
and planting a few stalks as signposts to show me the way to the well
when it was filled. If I did not come back very early in the morning,
if I left the Wasp time to take advantage of the hours of bright
sunshine, I invariably found the burrow finally stocked with provisions
and closed.
This faithfulness of memory is striking. The Wasp, delayed in her task,
puts off the rest of her work to the next day. She does not spend the
evening, she does not spend the night in the home which she has just
dug: on the contrary, she leaves the premises altogether and goes away,
after concealing the entrance with a little stone. The locality is not
familiar to her; she knows it no better than any other spot, for the
Ammophilæ behave like the Languedocian Sphex and lodge their families
here or there, wherever they happen to roam. The Wasp was there by
chance; the soil suited her; she dug her burrow; and she now goes off.
Where to? Who can tell? Perhaps to the flowers not far away, where, by
the last gleams of daylight, she will sip a drop of sugary liquid at
the bottom of the cups, even as our miners, after toiling in their dark
galleries, fly for comfort to the bottle in the evening. She goes off,
to a less or greater distance, stopping at this bin and that in the
flowers’ cellar. The evening, the night, the morning slip by. Still,
she must return to the burrow and complete her task, she must return
after the marches and countermarches of the morning hunt and the
bewildering flight from flower to flower during the libations of the
evening before. That the Social Wasp should return to her nest and the
Social Bee to her hive does not surprise me at all: the hive and the
nest are permanent residences, the way to which becomes known by long
practice; but the Ammophila has no acquaintance with the locality which
could help her to return to her burrow after such a long absence. Her
tunnel is at a spot which she perhaps visited yesterday for the first
time and which she must find again to-morrow, when she is quite out of
her bearings and moreover hampered with a heavy load of game.
Nevertheless, this little feat of topographical memory is performed,
sometimes with a precision that left me astounded. The Wasp would walk
straight to her burrow as if she had long been using all the little
paths in the neighbourhood. At other times she would wander backwards
and forwards and renew her search over and over again.
If the quest is greatly prolonged, the prey, which is a troublesome
burden when you are in a hurry to find your home, is laid down in some
high place, on a cluster of thyme or a tuft of grass, where it will be
well in sight presently, when wanted. Thus eased, the Ammophila resumes
her active search. I made a pencil-sketch, as she moved about, of the
tracks followed. The result was a medley of tangled lines, with sudden
bends and turns, branches in and branches out, windings and repeated
intersections—in short, a regular labyrinth whose complicated maze was
an ocular demonstration of the perplexity of the lost one.
When the well has been found and the slab removed, the Wasp has to come
back to the caterpillar, which is not always done without some groping
about, in cases where her wanderings to and fro have been very
numerous. Though she left her prey easily visible, the Wasp appears to
foresee the difficulty of finding it again when the moment comes to
drag it home. At least, if the search is unduly prolonged, you see her
suddenly interrupt her exploration of the ground and return to her
caterpillar, which she feels and nibbles at for a moment, as though to
make sure that it is really her own game, her property. Then she
hurries back again to the field of search, which she leaves a second
time, if need be, and a third, in order to inspect the prey. I am not
at all sure that these repeated visits of the Wasp to the caterpillar
are not a means of refreshing her memory of the place where she left
it.
This is what happens in exceedingly complicated cases; but as a rule
the Wasp goes back quite easily to the well dug the day before on the
spot to which chance has taken her. The vagabond’s guide is her
topographical memory, whose marvellous feats I shall have to tell
later. As for me, in order to return next day to the well hidden under
the lid of the little flat stone, I dared not trust to my unaided
memory: I needed notes, sketches, lines of latitude and longitude,
landmarks—in short, all the minutiæ of geometry.
The temporary closing of the burrow with a flat stone, as practised by
the Sandy Ammophila and the Silvery Ammophila, is apparently unknown to
the other two species. At any rate, I never saw their homes protected
by a lid. Besides, this absence of a provisional door seems to be
obligatory upon the Hairy Ammophila. In fact, as far as I could see,
this species hunts its prey first and then digs its burrow near the
place of capture. In this way the storing of the provisions can be done
straight away; and there is no need to trouble about a lid. As for the
Silky Ammophila, I suspect that she has another reason for not
employing a temporary cover. Whereas the three others put only one
caterpillar in each burrow, she puts in as many as five, though much
smaller ones. Just as we ourselves neglect to shut a door through which
we are constantly passing, so perhaps the Silky Ammophila neglects the
precaution of placing a stone over a well down which she has to go at
least five times in a short space of time.
In the case of all four, the provisions of the larvæ consist of
caterpillars of Moths. The Silky Ammophila selects, though not
exclusively, those long, thin caterpillars which walk by looping and
unlooping their bodies. Their gait suggests a pair of compasses that
makes its way by opening and closing in turns. Hence they are known by
two expressive names: Loopers and Measuring-worms. [32] The same burrow
contains provisions varying greatly in colour, a proof that the
Ammophila hunts without distinction every species of Loopers, provided
that they be small, for the huntress herself is anything but large and
her grub cannot get through very much, in spite of the five pieces of
game set before her. If Loopers fail, the Wasp falls back on other
equally slender caterpillars. Curved into a hoop as the result of the
sting that paralysed them, the five pieces are stacked up in the cell:
the uppermost carries the egg for which the provisions are made.
The three other Ammophilæ give only one caterpillar to each larva. It
is true that here bulk makes up for number: the game selected is big,
plump, capable of amply satisfying the grub’s appetite. For instance, I
have taken from the mandibles of the Sandy Ammophila a caterpillar
weighing fifteen times as much as its captor: fifteen times, an
enormous figure when we consider the strength which the huntress must
expend in dragging game of this kind by the skin of the neck over the
countless obstacles on the road. No other Wasp, tried in the balance
with her prey, has shown me a like disproportion between spoiler and
booty.
The almost indefinite variety of colouring in the provisions which I
unearth from the burrows or see between the legs of the Ammophilæ also
proves that the three brigands have no preference and pounce upon the
first caterpillar which comes along, provided that it be of a suitable
size, neither too large nor too small, and that it belongs to the Moth
division. The commonest game consists of those grey-clad caterpillars
which penetrate a little way into the ground and devour the plant at
the junction of root and stem.
What governs the whole history of the Ammophilæ and more particularly
attracted my attention is the manner in which the insect overpowers its
prey and reduces it to the condition of helplessness which the safety
of the larva requires. The game hunted, the caterpillar, possesses a
very different structure from that of the victims which we have seen
immolated hitherto: Buprestes, Weevils, Locusts and Ephippigers. The
creature is composed of a series of similar rings or segments set end
to end. Three of these segments, the first three, carry the real legs,
which will become the legs of the future Moth; others have membranous
legs, or pro-legs, which are peculiar to the caterpillar and not
represented in the Moth; others, lastly, have no limbs at all. Each
segment has its nerve-nucleus, or ganglion, the seat of sensibility and
movement, so that the nervous system includes twelve distinct centres,
separated one from the other, without counting the ganglionic
neck-piece placed under the skull and comparable, in a manner of
speaking, with the brain.
We are here very far removed from the nerve-centralization of the
Weevils and the Buprestes, which lends itself so well to general
paralysis by a single prick of the sting; we are also a long way from
the thoracic ganglia which the Sphex smites, one after the other, to
suppress all movement in her Crickets. Instead of a solitary
centralized point or of three nerve-nuclei, the caterpillar has twelve,
separated from one another by the distance between one segment and the
next and arranged like a string of beads on the ventral surface, along
the median line of the body. Moreover, as is the general rule in the
lower animals, where the same organ is repeated a great number of times
and loses power by its diffusion, these different nerve-centres are
largely independent of one another: each of them exercises its
influence over its particular segment; and its functions are only very
gradually affected by the derangement of the adjoining segments. One of
the caterpillar’s rings can lose its power of moving and feeling and
the remainder will nevertheless remain capable of both for a
considerable time. These facts are enough to show the great interest
attaching to the methods of slaughter which the Wasp adopts with her
prey.
But, while the interest is great, the difficulty of observation is not
small. The solitary habits of the Ammophilæ, their distribution one by
one over wide areas, the fact that one almost always comes across them
merely by chance: all this makes it hardly possible to carry out
premeditated experiments with them, anymore than with the Languedocian
Sphex. You have to be on the look-out a long time for an opportunity,
to wait for it with untiring patience, and to know how to profit by it
at the very moment when at last it presents itself, a moment when you
were not thinking of it. I watched for that opportunity for years and
years; then one day it suddenly appeared before my eyes, offering a
facility of examination and a clearness of detail that compensated me
for my long waiting.
At the beginning of my investigations I was twice enabled to witness
the murder of the caterpillar, and I saw, as far as the swiftness of
the operation permitted, the Wasp’s sting applied once and for all to
either the fifth or the sixth segment of the victim. To confirm this
result, I thought of ascertaining which ring had been stabbed on
caterpillars which I had not seen sacrificed, but which I had taken
from their captors while they were being dragged to the burrow. It was
no use employing a magnifying-glass, for no magnifying-glass enables
one to discover the least trace of a wound upon the victim. The method
adopted is the following: when the caterpillar is quite still, I try
each segment with the point of a fine needle and thus measure the
amount of sensibility by the more or less manifest signs of pain in the
insect. When the needle pricks the fifth segment or the sixth, even
piercing it right through, the caterpillar does not stir. But if you
prick even slightly a second segment, behind or in front of that
insensible segment, the caterpillar wriggles and struggles with a
violence which increases in proportion to the distance of the point
attacked from the original segment. At the hinder end in particular,
the least touch provokes wild contortions. There was only one sting,
therefore, and it was administered to the fifth or sixth ring.
What peculiarity then do these two segments possess that one or other
of them should be the target of the assassin’s weapon? None whatever in
their organization; but their position is another matter. Leaving the
Silky Ammophila’s Measuring-worms on one side, I find that the prey of
the others is organized as follows, the head being counted as the first
segment: three pairs of real legs on the second, third and fourth
rings; four pairs of membranous legs on the seventh, eighth, ninth and
tenth rings; lastly, a final pair of membranous legs on the thirteenth
and last ring, making in all eight pairs of legs, of which the first
seven form two vigorous groups, one of three, the other of four pairs.
These two groups are separated by two legless segments, which are
precisely the fifth and sixth.
Now, in order to deprive the caterpillar of its means of escape, to
render it motionless, will the Wasp drive her sting into each of the
eight rings provided with locomotory organs? Above all, will she take
this superfluity of precaution when the prey is quite weak and small?
Certainly not: a single stab will be enough; but it will be given at a
central point, whence the torpor produced by the tiny drop of poison
can spread gradually, with the least possible delay, to the segments
furnished with legs. There is no doubt about the segment to be picked
out for this single inoculation: it must be the fifth or the sixth,
which separate the two groups of locomotory rings. The point indicated
by rational inferences is therefore also the point adopted by instinct.
Lastly, let us add that the Ammophila’s egg is invariably laid on the
ring that has been rendered insensible. Here and here alone the young
larva can bite without provoking dangerous contortions; where a
needle-prick has no effect, the grub’s bite will have no effect either.
The grub will thus remain motionless until the nurseling has gained
strength and can forge ahead without running a risk.
In my later researches, as the number of my observations increased, I
began to entertain doubts, not as to the conclusions which I had
formed, but as to their general application. That feeble Loopers and
other small caterpillars are rendered harmless by a single thrust,
especially when the sting strikes the favourable spot described, is a
thing quite probable in itself and one which can also be proved either
by direct observation or by testing the insect’s sensibility with a
needle. But the Sandy Ammophila and especially the Hairy Ammophila
capture enormous victims, whose weight, as I have said, is fifteen
times that of the kidnapper. Will this giant prey be treated in the
same manner as the frail Measuring-worm? Will one dagger-thrust be
sufficient to subdue the monster and render it incapable of doing harm?
Will the horrid Grey Worm, lashing the walls of the cell with its
powerful tail, not endanger either the egg or the little grub? We dare
not picture the encounter, in the narrow cell of the burrow, between
those two—the feeble, new-hatched creature and that dragony thing still
possessing freedom in its movements to twist and untwist its tortuous
coils.
My suspicions were confirmed by an examination of the caterpillar from
the point of view of sensibility. Whereas the small game of the Silky
Ammophila and the Silvery Ammophila struggle violently if the needle
touches them elsewhere than in the ring stung by the Wasp, the big
caterpillars of the Sandy Ammophila and especially of the Hairy
Ammophila remain motionless, no matter which segment we prick. With
them there are no contortions, no sudden twists of the hinder parts;
the steel point produces no sign of a remnant of sensibility beyond a
faint quivering of the skin. The power of moving and feeling is
therefore almost wholly abolished, as it needs must be if the grub is
to feed in safety on this monstrous prey. Before placing it in the
burrow, the Wasp has turned it into an inert though still living mass.
I have been permitted to watch the Ammophila operating with her scalpel
on the sturdy caterpillar, and never did the intuitive science of
instinct show me anything more exciting. With a friend—soon, alas, to
be snatched from me by death!—I was coming back from the plateau of Les
Angles to lay snares for the Sacred Beetle and put his skill to the
test, when we caught sight of a Hairy Ammophila very busily employed at
the foot of a tuft of thyme. We at once lay down on the ground, close
to where she was working. Our presence did not frighten the Wasp; in
fact, she came and settled on my sleeve for a moment, decided that her
two visitors were harmless, since they did not move, and returned to
her tuft of thyme. As an old stager, I knew what that daring
familiarity meant: the Wasp’s attention was occupied with a serious
business. We would wait and see.
The Ammophila scratched the ground at the foot of the plant, at the
junction of root and stem, pulled up slender grass rootlets and poked
her head under the little clods which she had lifted. She ran hurriedly
this way and that around the thyme, inspecting every crevice that could
give access to what lay below. She was not digging herself a home but
hunting some game hidden underground; this was evident from her
behaviour, which resembled that of a Dog trying to dig a Rabbit out of
his hole. Presently, excited by what was happening overhead and
close-pressed by the Ammophila, a big Grey Worm made up his mind to
leave his lair and come up to the light of day. That settled him; the
huntress was on the spot at once, gripping him by the skin of his neck
and holding tight in spite of his contortions. Perched on the monster’s
back, the Wasp bent her abdomen and deliberately, without hurrying,
like a surgeon thoroughly acquainted with his patient’s anatomy, drove
her lancet into the ventral surface of each of the victim’s segments,
from the first to the last. Not a ring was left without receiving a
stab; all, whether with legs or without, were dealt with in order, from
front to back.
That is what I saw with all the leisure and ease that an observation
needs in order to be above reproach. The Wasp acts with a precision
that would make science turn green with envy; she knows what man hardly
ever knows; she knows her victim’s complex nervous system and reserves
her successive dagger-thrusts for the successive ganglia of her
caterpillar. I said, she knows; what I should say is, she behaves as
though she knew. Her act is simple inspiration. Animals obey their
compelling instinct, without realizing what they do. But whence comes
that sublime inspiration? Can theories of atavism, of natural
selection, of the struggle for life interpret it reasonably? To me and
my friend, this was and remained one of the most eloquent revelations
of the unutterable logic that rules the world and guides the ignorant
by the laws of its inspiration. Stirred to our innermost being by this
flash of truth, both of us felt tears of undefinable emotion spring to
our eyes.