AN UNKNOWN SENSE
I have described the Ammophila’s hunting tactics in detail. The facts
which I ascertained seem to me so rich in results that, even if the
harmas laboratory supplied me with nothing more, I should think myself
indemnified by this one observation. The surgical methods adopted by
the Wasp with the object of paralysing the Grey Worm are the highest
manifestation in the realm of instinct that I have hitherto met. This
inborn science is eminently calculated to give us food for thought.
What a subtle logician, what an unerring operator is that unconscious
physiologist, the Ammophila!
He who would witness these marvels for himself can hardly count on what
a country walk may happen to show him; besides, if the lucky
opportunity did present itself, he would not have time to profit by it.
An observation, which I kept up for five hours on end, without even
then managing to complete the experiment and obtain the proofs which I
anticipated, is one that, to be properly conducted, should be made at
leisure in one’s own garden. I owe my success, therefore, to my rustic
laboratory. I make a present of the secret to whosoever would continue
those magnificent studies: the harvest is inexhaustible; there will be
sheaves for all.
When we follow the Ammophila’s hunting in the due sequence of her
actions, the first question that suggests itself is this: how does the
Wasp go to work to recognize the spot beneath which the Grey Worm lies?
There is nothing outside, nothing, at least, perceptible to the eye, to
indicate the caterpillar’s hiding-place. The soil that conceals the
quarry may be grassy or bare, flinty or earthy, smooth or seamed with
little cracks. These varieties of appearance are matters of
indifference to the huntress, who prospects every spot without showing
preference for one more than another. At no place where the Wasp stops
and digs with some persistency do I see anything particular, in spite
of all my attention; and yet there must be a Grey Worm there, as I have
but now convinced myself, five times in succession, by lending a
helping hand to the insect, which was at first discouraged by a task
out of proportion to its strength. Sight, therefore, is certainly out
of the question here.
What sense, then? That of touch? Let us inquire. Everything tells us
that the organs of search are the antennæ. With their tips, bent like a
bow and quivering with a continual vibration, the insect tests the
ground, giving a number of little taps. When some crack shows, the
restless threads enter and sound it; when some grass-tuft spreads its
tangled root-stock along the ground, the quivering of the antennæ
redoubles as they grope among its knots and angles. Their tips are
applied for an instant to the spot explored, moulding themselves, so to
speak, upon it. They suggest two tactile filaments, two long fingers of
incomparable mobility, which gather information by feeling. But the
sense of touch can play no part in revealing what is underground: the
thing to be felt is the Grey Worm; and the worm is lying snug in its
burrow, at a depth of some inches below the surface.
We thereupon turn our thoughts to the faculty of scent. Insects, there
is no denying, possess the sense of smell, often very highly developed.
The Necrophori, [46] the Silphæ, [47] the Histers, [48] the Dermestes
[49] hasten from every side to the spot where lies a little corpse of
which the ground is to be purged. Guided by scent, these grave-diggers
hurry towards the dead Mole.
But, while the presence of the olfactory sense in insects is
indisputable, we still ask ourselves where it is seated. Many declare
that the seat is in the antennæ. Let us admit this, though it is
difficult to understand how a rod consisting of horny segments, jointed
end to end, can fulfil the office of a nostril which is so very
differently constructed. The organization of one apparatus having
naught in common with the other, can the impressions received by both
be of the same nature? When tools are dissimilar, do their functions
remain alike?
Besides, there are grave objections in the case of our Wasp. Smell is a
passive rather than an active sense; it does not, like touch,
anticipate the impression: it receives it; it does not inquire after
the scented effluvium: it accepts it when it comes. Now the Ammophila’s
antennæ are always moving: they investigate, they anticipate the
impression. The impression of what? If it were really an impression of
smell, repose would serve them better than a perpetual quivering.
But there is more to be said: the olfactory sense goes for nothing when
there is no smell. Now I have tested the Grey Worm for myself; I have
given it to young nostrils to sniff, nostrils much more sensitive than
mine: not one of us has perceived the faintest trace of smell in the
caterpillar. When the Dog, famed for his scent, becomes aware of the
truffle underground, he is guided by the tuber’s savour, which is
highly appreciable by ourselves, even through the thickness of the
soil. I admit that the Dog has a more subtle sense of smell than we
have: it is exercised at greater distances, it receives more vivid and
lasting impressions; nevertheless, it is impressed by odorous effluvia
which becomes perceptible to our own nostrils under the proper
conditions of proximity.
I will allow the Ammophila, if you like, a scent as delicate as that of
the Dog, more delicate even; but still a smell is needed; and I ask
myself how that which is inodorous at the very entrance to our nostrils
can be odoriferous to an insect through the intervening obstacle of the
ground. The senses, if they have the same functions, have the same
excitants, from man to the Infusoria. No animal, so far as I know, can
see clearly in what to us is absolute darkness. True, it may be said
that, in the zoological progression, perception, always fundamentally
the same, has varying degrees of power: this species is capable of more
and that species of less; what is perceptible to one is imperceptible
to another. This is perfectly right; and yet the insect, generally
considered, does not appear to possess exceptional keenness of scent:
the effluvia that attract it are perceived without a sense of smell of
unusual delicacy. When Dermestes, Silphæ and Histers pour into the
chalice of a carrion-scented arum lily, never to come out again; when
swarms of Flies buzz around a dead Dog’s blue and swollen belly, the
whole neighbourhood reeks with the stench. It hardly requires a scent
of exquisite accuracy on the insect’s part to discover putrid meat and
rotten cheese. Wherever we see its hordes gather, with scent for their
undoubted guide, we ourselves are cognizant of a smell.
There remains hearing. This is another sense about which entomologists
are not adequately informed. Where is its seat? In the antennæ, we are
told. Those fine, quivering stalks would seem fairly well suited to be
put in motion under the impulse of sound. In that case the Ammophila,
exploring the region with her antennæ, would be warned of the presence
of the Grey Worm by a slight noise coming up from the ground, the noise
of the mandibles nibbling a root, the noise of the caterpillar
wriggling its hind-quarters. What a faint sound and how difficult to
transmit through the spongy cushion of the earth!
It is less than faint, it is non-existent. The Grey Worm is nocturnal
in its habits. By day it skulks in its lair and does not stir. It does
not nibble either; at least, the Grey Worms which I unearthed upon the
Wasp’s indications were nibbling nothing, for the very simple reason
that they had nothing to nibble. They were completely motionless and
therefore silent in a layer of earth devoid of roots. The sense of
hearing must be rejected with that of smell.
The question recurs, more abstruse than ever. How does the Ammophila go
to work to recognize the spot beneath which the Grey Worm lies? The
antennæ are, beyond a doubt, the organs that guide her. They do not, in
this case, act as olfactory instruments, unless we admit that their dry
and tough surface, which has none of the delicate structure required
for the ordinary sense of smell, is nevertheless capable of perceiving
scents that are non-existent to us. This would be equivalent to
admitting that coarse tools tend to perfection of work. Nor do they act
as instruments of hearing, for there is no sound to be discerned. What
then is their function? I do not know and I despair of ever knowing.
Inclined as we are—and it could not well be otherwise—to judge all
things by our standard, the only one in any way known to us, we
attribute to animals our own means of perception and do not dream that
they might easily possess others of which it is impossible for us to
have an exact idea because there is nothing like them in ourselves. Are
we quite certain that they are not equipped, in very varying degrees,
for the purpose of sensations as foreign to ourselves as the sensation
of colours would be if we were blind? Has matter no secrets left for
us? Are we so very sure that it is revealed to the living being only by
light, sound, taste, smell and touch? Physics and chemistry, young
though they be, already declare to us that the dark unknown contains an
enormous harvest, in comparison with which our scientific sheaf is the
merest penury. A new sense, perhaps that which dwells in the
grotesquely exaggerated nose of the Rhinolophus, [50] perhaps that
which dwells in the antennæ of the Ammophila, would open to our search
a world which our physical structure no doubt condemns us to leave for
ever unexplored. Cannot certain properties of matter, which have no
perceptible action upon us, find a receptive echo in animals, which are
differently equipped?
When Spallanzani, [51] after blinding some Bats, released them in a
room converted into a maze by means of cords stretched in every
direction and of heaped-up brambles, how were those animals able to
find their way about, to fly quickly, to move to and fro, from end to
end of the room, without hitting the interposed articles? What sense
analogous to any of ours guided them? Would some one tell me and, above
all, make me understand? I should also like to understand how the
Ammophila infallibly finds her caterpillar’s burrow with the aid of her
antennæ. It is not a case of the sense of smell: we should have to
presume it to possess an unparalleled delicacy, while recognizing that
it is exercised by an organ in which no provision seems made for the
perception of smells.
What a number of other incomprehensible things do we not ascribe to the
insect’s sense of smell! We are satisfied with a word: the explanation
is ready-found, without laborious search. But, if we care to consider
the matter thoroughly, if we compare the requisite array of facts, then
the cliff of the unknown rises abruptly, not to be climbed by the path
which we insist on following. Let us then change our path and admit
that animals may have other means of information than our own. Our
senses do not represent the sum total of the methods whereby an animal
communicates with that which is not itself: there are others not
capable of comparison, however remote, with those which we possess.
If the act of the Ammophila were an isolated fact, I should not have
lingered over it as I have done; but I propose to speak of others
stranger still, which will carry conviction to the most exacting mind.
After relating them, therefore, I shall return to the subject of
special senses, irreducible senses, unknown to us.
For the moment, let us go back to the Grey Worm, which it would be as
well for us to know in a less casual fashion. I have four of them, dug
up with the knife at the spots indicated by the Ammophila. My intention
was to substitute them, by turns, for the doomed victim, so as to see
the Wasp’s operation repeated. When my plan failed, I placed the worms
in a glass jar, with a layer of earth and a lettuce-stalk above them.
By day, my captives remained buried in the earth; at night, they came
up to the surface, where I caught them gnawing at the salad from below.
In August, they dug deep down, not to come up again, and fashioned
themselves a cocoon apiece of earth, very rough on the outer surface,
oval in shape and the size of a small pigeon’s egg. The moth appeared
at the end of the same month. I recognized the Dart or Turnip Moth
(Noctua segetum, Hübn.).
The Hairy Ammophila, therefore, feeds her grubs on the caterpillars of
Noctuæ; and her choice falls exclusively on the species that live
underground. These caterpillars, commonly known as Grey Worms, because
of their drab garb, are a most formidable scourge to agricultural
crops, as well as to garden produce. Curled in their burrows by day,
they climb to the surface at night and gnaw the base or collar of the
herbaceous plants. Everything suits them: ornamental plants and edible
plants alike. Flower-beds, market-gardens, fields are laid waste
without distinction. When a seedling withers without apparent cause,
draw it to you gently; and the dying plant will come up, but maimed,
severed from its root. The Grey Worm has passed that way in the night;
its greedy mandibles have performed the deadly amputation. Its havoc
rivals that wrought by the White Worm, the grub of the Cockchafer. When
it swarms in a beet-country, the damage amounts to millions. This is
the terrible enemy against which the Ammophila comes to our aid.
I point out and urgently recommend to agriculturalists this valuable
auxiliary, so zealous in her search of the Grey Worm in spring, so
skilful in discovering its hiding-place. An Ammophila in a garden may
mean the saving of a lettuce-bed, the snatching of a balsam-border from
danger. But there is need here for recommendations. None would dream of
destroying the pretty Wasp that goes fluttering nimbly from one path to
the other, that visits this corner of the garden, then that, then the
next, then the one over there; none dreams either—and none,
unfortunately, can dream—of assisting her to multiply.
In the immense majority of cases the insect evades our influence: to
exterminate it, if it be harmful, to propagate it, if it be useful, are
impracticable undertakings for us. By a singular contrast of strength
and weakness, man cuts through the neck of continents to join two seas,
he pierces the Alps, he weighs the sun; and yet he cannot prevent a
wretched maggot from enjoying his cherries before he himself does, nor
an odious Louse from destroying his vines! The Titan is vanquished by
the pigmy.
Now we have here, in this insect-world, an auxiliary of high merit, the
supreme foe of our grievous foe the Grey Worm. Can we do anything to
stock our fields and gardens with it at will? We cannot; for the first
condition of multiplying the Ammophila would be to multiply the Grey
Worm, the only food of her family of grubs. I do not speak of the
insurmountable difficulties which this breeding would present. We have
not to do with the Bee, who is faithful to her hive, because of her
social habits; still less with the stupid Silkworm, perched on its
mulberry-leaf, or its clumsy Moth, who for a moment flutters her wings,
pairs, lays her eggs and dies: we have to do with an insect that is
capricious in its wanderings, swift of flight and independent in its
ways.
Besides, the first condition shatters all our hopes. Would we have the
helpful Ammophila? Then we must resign ourselves to accepting the Grey
Worm. We move in a vicious circle: to produce good we must invoke the
aid of evil. The hostile band brings the friendly troop to our fields;
but the second cannot live without the first; and the two show an even
balance in numbers. If the Grey Worm abound, the Ammophila finds
copious provender for her grubs and her race prospers; if the Grey Worm
be rare, the Ammophila’s offspring decrease and disappear. This balance
between prosperity and decadence is the immutable law that governs the
proportions between devourers and devoured.