A slender waist, a slim shape, an abdomen much compressed at the upper
part, and seemingly attached to the body by a mere thread, a black robe
with a red scarf on its under parts,—such is the description of these
Fossors; like Sphegidæ in form and colouring, but very different in
habits. The Sphegidæ hunt Orthoptera, crickets, ephippigers, and
grasshoppers, while the Ammophila chases caterpillars. This difference
of prey at once suggests new methods in the murderous tactics of
instinct.
Did not the name sound pleasant to the ear, I should be inclined to
quarrel with Ammophila, which means sand-lover, as being too exclusive
and often erroneous. The true lovers of sand—dry, powdery, and slippery
sand—are the Bembex, which prey on flies: but the caterpillar-hunters,
whose history I am about to tell, have no liking for pure, loose sand,
and even avoid it as being too subject to landslips which may be caused
by a mere trifle. Their vertical pits, which must remain open until the
cell is stored with food and an egg, require more solid materials if
they are not to be blocked prematurely. What they want is a light soil,
easy to mine, where the sandy element is cemented by a little clay and
lime. The edges of paths—slopes of thin grass exposed to the sun,—such
are the places they favour. In spring, from the first days of April,
one sees Ammophila hirsuta there; in September and October there are A.
sabulosa, A. argentata, and A. holosericea. I will make an abstract of
the notes furnished by these four species.
For all four the burrow is a vertical shaft, a kind of well, with at
most the dimension of a large goose quill, and about two inches deep.
At the bottom is a single cell, formed by a simple widening of the
shaft. To sum up, it is a poor dwelling, obtained at small expense, at
one sitting, affording no protection if the larva had not four wrappers
in its cocoon, like the Sphex. The Ammophila excavates alone,
deliberately, with no joyous ardour. As usual, the anterior tarsi do
duty as rakes and the mandibles as mining tools. If some grain of sand
offer too much resistance, you may hear rising from the bottom of the
well a kind of shrill grinding sound, produced by the vibration of the
wings and entire body as if to express the insect’s struggles.
Frequently the Hymenopteron comes up with a load of refuse in its jaws,
some bit of gravel which it drops as it flies some little way off, in
order not to block up the place. Some appear to merit special attention
by their form and size,—at least the Ammophila does not treat them like
the rest, for instead of carrying them away on the wing, she goes on
foot and drops them near the shaft. They are choice material—blocks
ready prepared to stop up the dwelling by and by.
This outside work is done with a self-contained air and great
diligence. High on its legs, its abdomen outstretched at the end of its
long petiole, it turns round and moves its whole body at once with the
geometrical stiffness of a line revolving on itself. If it has to throw
away to a distance the rubbish it decides to be only encumbrances, it
does this with little silent flights, often backwards, as if, having
come out of the shaft tail first, it thought to save time by not
turning round. Species with long-stalked bodies, like A. sabulosa and
A. argentata, are those that chiefly display this automaton-like
rigidity. Their abdomen, enlarged to a pear-shape at the end of a
thread, is very troublesome to manage; a sudden movement might injure
the fine stalk, and the insect has to walk with a kind of geometrical
precision, and if it flies, it goes backward to avoid tacking too
often. On the other hand, A. hirsuta, which has an abdomen with a short
petiole, works at its burrow with swift easy movements such as one
admires in most of the miners. It can move more freely, not being
embarrassed by its abdomen.
The dwelling is hollowed out. Later on, when the sun has passed from
the spot where the hole is bored, the Ammophila is sure to visit the
little heap of stones set aside during her burrowing, intent to choose
some bit which suits her. If she can find nothing that will do she
explores round about, and soon discovers what she wants—namely, a small
flat stone rather larger than the mouth of her well. She carries it off
in her mandibles, and for the time being closes the shaft with it. Next
day, when it is hot again, and when the sun bathes the slopes and
favours the chase, she will know perfectly well how to find her home
again, secured by the massive door, and she will return with a
paralysed caterpillar, seized by the nape of its neck and dragged
between its captor’s feet; she will lift the stone, which is just like
all the others near, and the secret of which is known only to her, will
carry down the prey, lay an egg, and then stop the burrow once for all
by sweeping into the shaft all the rubbish kept near at hand.
Several times I have seen this temporary closing of the hole by A.
sabulosa and A. argentata when the sun grew low and the late hour
obliged them to wait until the next day to go out hunting. When they
had put the seals on their dwellings I too waited for the morrow to
continue my observations, but first I made sure of the spot by taking
my bearings and sticking in some bits of wood in order to rediscover
the well when closed, and always, unless I came too early, if I let the
Hymenopteron profit by full sunshine, I found the burrow stored and
closed for good and all.
The fidelity of memory shown here is striking. The insect, belated at
its work, puts off completing it until the morrow. It passes neither
evening nor night in the new-made abode, but departs after marking the
entrance with a small stone. The spot is no more familiar to it than
any other, for like Sphex occitanica the Ammophila lodges her family
here and there as she may chance to wander. The creature came here by
chance, like the soil, and dug the burrow, and now departs. Whither?
Who knows? Perhaps to the flowers near, to lick up by the last gleam of
day a drop of sugary liquid at the bottom of their cups, just as a
miner after labouring in his dark gallery seeks the consolation of his
bottle when evening comes. The Ammophila may be enticed farther and
farther by the inviting blossoms. Evening, night, and morning pass, and
now she must return to her burrow and complete her task,—return after
all her windings and wanderings in the chase that morning, and the
flight from flower to flower, and the libations of the previous
evening. That a wasp should return to the nest and a bee to the hive
does not surprise me; these are permanent abodes, and the ways back are
known by long practice, but the Ammophila, who has to return after so
long an absence, has no aid from acquaintance with the locality. Her
shaft is in a place which she visited yesterday, perhaps for the first
time, and must find again to-day when quite beyond her bearings, and,
moreover, when she is encumbered by heavy prey. Yet this exploit of
topographical memory is accomplished, and sometimes with a precision
which left me amazed. The insect made straight for the burrow as if
long used to every path in the neighbourhood; but at other times there
would be long visitation and repeated searches.
If the difficulty become serious, the prey, which is an embarrassing
load in a hurried exploration, is laid in some obvious place, on a tuft
of thyme or grass, where it can be easily seen when wanted. Freed from
this burden the Ammophila resumes an active search. As she hunted about
I have traced with a pencil the track made by her. The result was a
labyrinth of lines, with curves and sudden angles, now returning inward
and now branching outward—knots and meshes and repeated intersections—a
maze, showing how perplexed and astray was the insect.
The shaft found and the stone lifted, she must return to the prey, not
without some uncertainty when comings and goings have been too many.
Although it was left in a place obvious enough, the Ammophila often
seems at a loss when the time comes to drag it home; at least, if there
be a very long search for the burrow, one sees her suddenly stop and go
back to the caterpillar, feel it and give it a little bite, as if to
make sure that it is her very own game and property, hurrying back to
seek for the burrow, but returning a second time if needful, or even a
third, to visit her prey. I incline to believe that these repeated
visits are made to refresh her memory as to where she left it.
This is what happens in very complex cases, but generally the insect
returns without difficulty to the spot whither its vagrant life may
have led it. For guide it has that local memory whose marvellous feats
I shall later have occasion to relate. As for me, in order to return
next day to the burrow hidden under the little flat stone, I dared not
trust to my memory, but had to use notes, sketches, to take my
bearings, and stick in pegs—in short, a whole array of geometry.
The temporary closing of the burrow with a flat stone as practised by
A. sabulosa and A. holosericea appears unknown to the two other
species; at least I never saw their homes protected by a covering. This
is natural in the case of A. hirsuta, for, I believe, this species
hunts the prey first and then burrows near the place of capture. As
provender can therefore be at once stored it is useless to take any
trouble about a cover. As for A. holosericea, I suspect there is
another reason for not using any temporary door. While the two others
only put one caterpillar in each cell, she puts as many as five, but
much smaller ones. Just as we ourselves neglect to shut a door where
some one is constantly passing to and fro, perhaps this Ammophila
neglects to place a stone on a well which she will go down at least
five times within a short space of time. All four lay up caterpillars
of moths for their larvæ. A. holosericea chooses, though not
exclusively, those slender, long caterpillars known as Loopers. They
move as a compass might by opening and closing alternately, whence
their expressive French name of Measurers. The same burrow includes
provisions of varied colours—a proof that this Ammophila hunts all
kinds of Loopers so long as they are small, for she herself is but
feeble and the larva cannot eat much, in spite of the five heads of
game set before it. If Loopers fail, the Hymenopteron falls back on
other caterpillars equally small. Rolled up from the effect of the
sting which paralysed them, all five are heaped in the cell; the top
one bears the egg for which the provender is destined.
The three other Ammophilæ give but one caterpillar to each cell.
True—size makes up for this; the game selected is corpulent, plump,
amply sufficing the grub’s appetite. For instance, I have taken out of
the mandibles of A. holosericea a caterpillar fifteen times her own
weight—fifteen times!—an enormous sum if you consider what an
expenditure of strength it implies to drag such game by the nape of its
neck over the endless difficulties of the ground. No other Hymenopteron
tried in the scales with its prey has shown me a like disproportion
between spoiler and capture. The almost endless variety of colouring in
the provender exhumed from the burrows or recognised in the grasp of
the various species also proves that the three have no preference, but
seize the first caterpillar met with, provided it be neither too large
nor too small, and belongs to the moths. The commonest prey are those
gray caterpillars which infest the plant at the junction of a root and
stem just below the soil.
That which governs the whole history of the Ammophila, and more
especially attracted my attention, was the way in which the insect
masters its prey and plunges it into the harmless state required for
the safety of the larva. The prey, a caterpillar, is very differently
organised from the victims which we have hitherto seen
sacrificed—Buprestids, Weevils, Grasshoppers, and Ephippigers. It is
composed of a series of segments or rings set end to end, the three
first bearing the true feet which will be those of the future
butterfly; others bear membranous or false feet special to the
caterpillar and not represented in the butterfly; others again are
without limbs. Each ring has its ganglion, the source of feeling and
movement, so that the nerve system comprehends twelve distinct centres
well separated from each other, without counting the œsophageal
ganglion placed under the skull, and which may be compared to the
brain.
We are here a long way from the nerve centralisation of the Weevil and
Buprestis that lends itself so readily to general paralysis by a single
stab; very far too from the thoracic ganglia which the Sphex wounds
successively to put a stop to the movements of her crickets. Instead of
a single centralised point—instead of three nerve centres—the
caterpillar has twelve, separated one from another by the length of a
segment and arranged in a ventral chain along the median line of the
body. Moreover, as is the rule among lower animals, where the same
organ is very often repeated and loses power by diffusion, these
various nervous centres are largely independent of each other, each
animating its own segment, and are but slightly disturbed by disorder
in neighbouring ones. Let one segment lose motion and feeling, yet
those uninjured will none the less remain long capable of both. These
facts suffice to show the high interest attaching to the murderous
proceedings of the Hymenopteron with regard to its prey.
But if the interest be great, the difficulty of observation is not
small. The solitary habits of the Ammophila,—their being scattered
singly over wide spaces, and their being almost always met with by mere
chance,—almost forbid, as in the case of Sphex occitanica, any
experiment being prepared beforehand. Long must a chance be watched for
and awaited with unalterable patience, and one must know how instantly
to profit by it when at last it comes just when least expected. I have
waited for such a chance for years and years, and then, all at once, I
got the opportunity with a facility for observation and clearness of
detail which made up for the long waiting.
At the beginning of my observations I succeeded twice in watching the
murder of the caterpillar, and saw, as far as the rapidity of the
operation allowed, that the sting of the Hymenopteron struck once for
all at the fifth or sixth segment of the victim. To confirm this I
bethought myself of making sure which ring was stabbed by examining
caterpillars which I had not seen sacrificed, but had carried off from
their captors while they were being dragged to the burrow; but it was
vain to use a microscope,—no microscope can show any trace of such a
wound. This was the plan adopted. The caterpillar being quite still, I
tried each segment with the point of a fine needle, measuring the
amount of sensibility by the greater or less pain given. Should the
needle entirely transpierce the fifth segment or the sixth, there is no
movement. But prick even slightly one in front or behind, the
caterpillar struggles with a violence proportioned to the distance from
the poisoned segment. Especially does the least touch on the hinder
ones produce frantic contorsions. So there was but one stab, and it was
given in the fifth or sixth segment.
What special reason is there that one or other of these two should be
the spot chosen by the assassin? None in their organisation, but their
position is another thing. Omitting the Loopers of Ammophila
holosericea, I find that the prey of the others has the following
organisation, counting the head as the first segment:—Three pairs of
true feet on rings two, three, and four; four pairs of membranous feet
on rings seven, eight, nine, and ten, and a last similar pair set on
the thirteenth and final ring; in all eight pairs of feet, the seven
first making two marked groups—one of three, the other of four pairs.
These two groups are divided by two segments without feet, which are
the fifth and sixth.
Now, to deprive the caterpillar of means of escape, and to render it
motionless, will the Hymenopteron dart its sting into each of the eight
rings provided with feet? Especially will it do so when the prey is
small and weak? Certainly not: a single stab will suffice if given in a
central spot, whence the torpor produced by the venomous droplet can
spread gradually with as little delay as possible into the midst of
those segments which bear feet. There can be no doubt which to choose
for this single inoculation; it must be the fifth or sixth, which
separate the two groups of locomotive rings. The point indicated by
rational deduction is also the one adopted by instinct. Finally, let us
add that the egg of the Ammophila is invariably laid on the paralysed
ring. There, and there alone, can the young larva bite without inducing
dangerous contorsions; where a needle prick has no effect, the bite of
a grub will have none either, and the prey will remain immovable until
the nursling has gained strength and can bite farther on without
danger.
With further researches doubts assailed me, not as to my deductions,
but as to how widely I might extend them. That many feeble Loopers and
other small caterpillars are disabled by a single stab, especially when
struck at so favourable a point as the one just named, is very probable
in itself, and, moreover, is shown both by direct observation and by
experiments on their sensibility with the point of a needle. But
Ammophila sabulosa and hirsuta catch huge prey, whose weight, as
already said, is fifteen times that of the captor. Can such giant prey
be treated like a poor Looper? Can a single stab subdue the monster and
render it incapable of harm? If the fearsome gray worm strike the cell
walls with its strong body, will it not endanger the egg or the little
larva? One dares not imagine a tête-à-tête in the small cell at the
bottom of the burrow between the frail, newly-hatched creature and this
kind of dragon:—still able to coil and uncoil its lithe folds.
My suspicions were heightened by examination as to the sensitiveness of
the caterpillar. While the small game of Ammophila holosericea and
hirsuta struggle violently if pricked elsewhere than in the part
stabbed, the large caterpillars of A. sabulosa, and above all of A.
hirsuta, remain motionless, no matter which segment be stimulated. They
show no contortions or sudden twisting of the body, the steel point
only producing as a sign faint shudderings of the skin. As the safety
of a larva provided with such huge prey requires, motion and feeling
are almost quite destroyed. Before introducing it into the burrow, the
Hymenopteron turns it into a mass—inert indeed, yet not dead.
I have been able to watch the Ammophila use her instrument on the
robust caterpillar, and never did the infused science of instinct show
me anything more striking. With a friend—alas! soon after snatched from
me by death—I was returning from the tableland of Les Angles after
preparing snares to put the cleverness of Scarabæus sacer to the proof,
when we caught sight of an Ammophila hirsuta very busy at the foot of a
tuft of thyme. We instantly lay down very close by. Our presence noways
alarmed the insect, which alighted for a moment on my sleeve, decided
that since her visitors did not move they must be harmless, and
returned to her tuft of thyme. Well used to the ways of Ammophila, I
knew what this audacious tameness meant—she was occupied by some
serious affair. We would wait and see. The Ammophila scratched in the
ground round the collar of the plant, pulling up thin little grass
roots, and poked her head under the tiny clods which she raised up, ran
hurriedly, now here, now there, round the thyme, visiting every crack
which gave access under it; yet she was not digging a burrow, but
hunting something hidden underground, as was shown by manœuvres like
those of a dog trying to get a rabbit out of its hole. And presently,
disturbed by what was going on overhead and closely tracked by the
Ammophila, a big gray worm made up his mind to quit his abode and come
up to daylight. It is all over with him; the hunter is instantly on the
spot, gripping the nape of his neck and holding on in spite of his
contortions. Settled on the monster’s back the Ammophila bends her
abdomen, and methodically, deliberately—like a surgeon thoroughly
familiar with the anatomy of his subject—plunges a lancet into the
ventral surface of every segment, from the first to the last. Not one
ring is omitted; with or without feet each is stabbed in due order from
the front to the back.
This is what I saw with all the leisure and ease required for an
irreproachable observation. The Hymenopteron acts with a precision of
which science might be jealous; it knows what man but rarely knows; it
is acquainted with the complex nervous system of its victim, and keeps
repeated stabs for those with numerous ganglia. I said “It knows; is
acquainted”: what I ought to say is, “It acts as if it did.” What it
does is suggested to it; the creature obeys, impelled by instinct,
without reasoning on what it does. But whence comes this sublime
instinct? Can theories of atavism, of selection, of the struggle for
life, interpret it reasonably? For my friend and myself it was and is
one of the most eloquent revelations of the ineffable logic which rules
the world and guides the unconscious by the laws which it inspires.
Stirred to the heart by this flash of truth, both of us felt a tear of
emotion rise to our eyes.