THE PROBLEM OF THE SIREX
The cherry-tree supports a small jet-black Capricorn, _Cerambyx
cerdo_, whose larval habits it was as well to study in order to learn
whether the instincts are modified when the form and the organization
remain identical. Has this pigmy of the family the same talents as the
giant, the ravager of the oak-tree? Does it work on the same
principles? The resemblance between the two, both in the larval state
and in that of the perfect insect, is complete; the denizen of the
cherry-tree is an exact replica, on a smaller scale, of the denizen of
the oak. If instinct is the inevitable consequence of the organism, we
ought to find in the two insects a strict similarity of habits; if
instinct is, on the other hand, a special aptitude favoured by the
organs, we must expect variations in the industry exercised. For the
second time the alternative is forced upon our attention: do the
implements govern the practice of the craft, or does the craft govern
the employment of the implements? Is instinct derived from the organ,
or is the organ instinct's servant? An old dead cherry-tree will
answer our question.
Beneath its ragged bark, which I lift in wide strips, swarms a
population of larvae all belonging to _Cerambyx cerdo_. There are big
larvae and little larvae; moreover, they are accompanied by nymphs.
These details tell us of three years of larval existence, a duration
of life frequent in the Longicorn series. If we hunt the thick of the
trunk, splitting it again and again, it does not show us a single grub
anywhere; the entire population is encamped between the bark and the
wood. Here we find an inextricable maze of winding galleries, crammed
with packed sawdust, crossing, recrossing, shrinking into little
alleys, expanding into wide spaces and cutting, on the one hand, into
the surface layer of the sap-wood and, on the other, into the thin
sheets of the inner bark. The position speaks for itself: the larva of
the little Capricorn has other tastes than its large kinsman's; for
three years it gnaws the outside of the trunk beneath the thin
covering of the bark, while the other seeks a deeper refuge and gnaws
the inside.
The dissimilarity is yet more marked in the preparations for the
nymphosis. Then the worm of the cherry-tree leaves the surface and
penetrates into the wood to a depth of about two inches, leaving
behind it a wide passage, which is hidden on the outside by a remnant
of bark that has been discreetly spared. This spacious vestibule is
the future insect's path of release; this screen of bark, easily
destroyed, is the curtain that masks the exit-door. In the heart of
the wood the larva finally scoops out the chamber destined for the
nymphosis. This is an egg-shaped recess an inch and a quarter to an
inch and three-quarters in length by two-fifths of an inch in
diameter. The walls are bare, that is to say, they are not lined with
the blanket of shredded fibres dear to the Capricorn of the Oak. The
entrance is blocked first by a plug of fibrous sawdust, then by a
chalky lid, similar, except in point of size, to that with which we
are already familiar. A thick layer of fine sawdust packed into the
concavity of the chalky lid, completes the barricade. Need I add that
the grub lies down and goes to sleep, for the nymphosis, with its head
against the door? Not one forgets to take this precaution.
The two Capricorns have, in short, the same system of closing their
cells. Note above all the lens-shaped stony lid. In each case we find
the same chemical composition, the same formation, like the cup of an
acorn. Dimensions apart, the two structures are identical. But no
other genus of Longicorn, so far as I am aware, practises this craft.
I will therefore complete the classic description of the
Cerambyx-beetles by adding one characteristic: they seal their
metamorphosis-chambers with a chalk slab.
The similarities of habit go no farther, despite the identity of
structure. There is even a very sharp contrast between the methods
pursued. The Capricorn of the Oak inhabits the deep layers of the
trunk; the Capricorn of the Cherry-tree inhabits the surface. In the
preparations for the transformation, the first ascends from the wood
to the bark, the second descends from the bark to the wood; the first
risks the perils of the outer world, the second shuns them and seeks a
retreat inside. The first hangs the walls of its chamber with velvet,
the second knows nothing of this luxury. Though the work is almost the
same in its results, it is at least carried out by contrary methods.
The tool, therefore, does not govern the trade. This is what the two
Cerambyx-beetles tell us.
Let us vary the testimony of the Longicorns. I am not selecting; I am
recording it in the order of my discoveries. The Shagreen Saperda (_S.
carcharias_) lives in the black poplar; the Scalary Saperda (_S.
scalaris_) lives in the cherry-tree. In both we find the same
organization and the same implements, as is fitting in two
closely-related species. The Saperda of the Poplar adopts the method
of the Capricorn of the Oak in its general features. It inhabits the
interior of the trunk. On the approach of the transformation, it makes
an exit-gallery, the door of which is open or else masked by a remnant
of bark. Then, retracing its steps, it blocks the passage with a
barricade of coarse packed shavings; and, at a depth of about eight
inches, not far from the heart of the tree, it hollows out a cavity
for the nymphosis without any particular upholstering. The defensive
system is limited to the long column of shavings. To deliver itself,
the insect will only have to push the heap of woody rubbish back, in
so many lots; the path will open in front of it ready-made. If some
screen of bark hide the gallery from the outside, its mandibles will
easily dispose of that: it is soft and not very thick.
The Scalary Saperda imitates the habits of its messmate, the Capricorn
of the Cherry-tree. Its larva lives between the wood and the bark. To
undergo its transformation, it goes down instead of coming up. In the
sap-wood, parallel with the surface of the trunk, under a layer of
wood barely a twenty-fifth of an inch in thickness, it makes a
cylindrical cell, rounded at the ends and roughly padded with ligneous
fibres. A solid plug of shavings barricades the entrance, which is not
preceded by any vestibule. Here the work of deliverance is the
simplest. The Saperda has only to clear the door of his chamber to
find beneath his mandibles the little bit of bark that remains to be
pierced. As you see, we once more have to do with two specialists,
each working in his own manner with the same tools.
The Buprestes, as zealous as the Longicorns in the destruction of
trees, whether sound or ailing, tell us the same tale as the Cerambyx-
and Saperda-beetles. The Bronze Buprestis (_B. aenea_) is an inmate of
the black poplar. Her larva gnaws the interior of the trunk. For the
nymphosis it installs itself near the surface in a flattened, oval
cell, which is prolonged at the back by the wandering-gallery, firmly
packed with wormed wood, and in front by a short, slightly curved
vestibule. A layer of wood not a twenty-fifth of an inch thick is left
intact at the end of the vestibule. There is no other defensive
precaution; no barricade, no heap of shavings. In order to come out,
the insect has only to pierce an insignificant sheet of wood and then
the bark.
The Nine-spotted Buprestis (_Ptosima novemmaculata_) behaves in the
apricot-tree precisely as the Bronze Buprestis does in the poplar. Its
larva bores the inside of the trunk with very low-ceilinged galleries,
usually parallel with the axis; then, at a distance of an inch and a
quarter or an inch and a half from the surface, it suddenly makes a
sharp turn and proceeds in the direction of the bark. It tunnels
straight ahead, taking the shortest road, instead of advancing by
irregular windings as at first. Moreover, a sensitive intuition of
coming events inspires its chisel to alter the plan of work. The
perfect insect is a cylinder; the grub, wide in the thorax but slender
elsewhere, is a strap, a ribbon. The first, with its unyielding
cuirass, needs a cylindrical passage; the second needs a very low
tunnel, with a roof that will give a purchase to the ambulatory
nipples of the back. The larva therefore changes its manner of boring
utterly: yesterday, the gallery, suited to a wandering life in the
thickness of the wood, was a wide burrow with a very low ceiling,
almost a slot; to-day the passage is cylindrical: a gimlet could not
bore it more accurately. This sudden change in the system of
road-making on behalf of the coming insect once more suggests for our
meditation the eminent degree of foresight possessed by a bit of an
intestine.
The cylindrical exit-way passes through the strata of wood along the
shortest line, almost normally, after a slight bend which connects the
vertical with the horizontal, a curve with a radius large enough to
allow the stiff Buprestis to tack about without difficulty. It ends in
a blind-alley, less than a twelfth of an inch from the surface of the
wood. The eating away of the untouched sheet of wood and of the bark
is all the labour that the grub leaves the insect to perform. Having
made these preparations, the larva withdraws, strengthening the wooden
screen, however, with a layer of fine sawdust; it reaches the end of
the round gallery, which is prolonged by the completely choked flat
gallery; and here, scorning a special chamber or any upholstery, it
goes to sleep for the nymphosis, with its head towards the exit.
I find numbers of specimens of a black Buprestis (_B. octoguttata_) in
the old stumps of pine-trees left standing in the ground, hard outside
but soft within, where the wood is as pliable as tinder. In this
yielding substance, which has a resinous aroma, the larvae spend their
life. For the metamorphosis they leave the unctuous regions of the
centre and penetrate the hard wood, where they hollow out oval
recesses, slightly flattened, measuring from twenty-five to thirty
millimetres[1] in length. The major axis of these cells is always
vertical. They are continued by a wide exit-path, sometimes straight,
sometimes slightly curved, according as the tree is to be quitted
through the section above or through the side. The exit-channel is
nearly always bored completely; the window by which the insect escapes
opens directly upon the outside world. At most, in a few rare
instances, the grub leaves the Buprestis the trouble of piercing a
leaf of wood so thin as to be translucent. But, if easy paths are
necessary to the insect, protective ramparts are no less needed for
the safety of the nymphosis; and the larva plugs the liberating
channel with a fine paste of masticated wood, very different from the
ordinary sawdust. A layer of the same paste divides the bottom of the
chamber from the low-ceilinged gallery, the work of the grub's active
life. Lastly, the magnifying-glass reveals upon the walls of the cell
a tapestry of woody fibres, very finely divided, standing erect and
closely shorn, so as to make a sort of velvet pile. This quilted
lining, of which the Cerambyx of the Oak showed us the first example,
is, it seems to me, pretty often employed by the wood-eaters,
Buprestes as well as Longicorns.
[Footnote 1: .975 to 1.17 inch.--_Translator's Note_.]
After these migrants, which travel from the centre of the tree to the
surface, we will mention some others which from the surface plunge
into the interior. A small Buprestis who ravages the cherry-trees,
_Anthaxia nitidula_, passes his larval existence between the wood and
the bark. When the time comes for changing its shape, the pigmy
concerns itself, like the others, with future and present needs. To
assist the perfect insect, the grub first gnaws the under side of the
bark, leaving a thin screen of cuticle untouched, and then sinks in
the wood a perpendicular well, blocked with unresisting sawdust. That
is on behalf of the future: the frail Buprestis will be able to leave
without hindrance. The bottom of the well, better wrought than the
rest and ceiled with the aid of an adhesive fluid which holds the fine
sawdust of the stopper in place, is a thing of the present; it is the
nymphosis-chamber.
A second Buprestis, _Chrysobothrys chrysostigma_, likewise an
exploiter of the cherry-tree, between the wood and the bark, although
more vigorous, expends less labour on its preparations. Its chamber,
with modestly varnished walls, is merely an expanded extension of the
ordinary gallery. The grub, disinclined for persistent labour, does
not bore the wood. It confines itself to hollowing a slanting dug-out
in the bark, without touching the surface layer, through which the
insect will have to gnaw its own way.
Thus each species displays special methods, tricks of the trade which
cannot be explained merely by reference to its tools. As these minute
details have consequences of some importance, I do not hesitate to
multiply them: they all help to throw light upon the subject which we
are investigating. Let us once more see what the Longicorns are able
to tell us.
An inhabitant of old pine-stumps, _Criocephalus ferus_ makes an
exit-gallery which yawns widely on the outside world, opening either
on the section of the stump or on the sides. The road is barricaded
about two inches down with a long plug of coarse shavings. Next comes
the nymph's cylindrical, compressed apartment, which is padded with
woody fibres. It is continued underneath by the labyrinth of the
larva, the burrow crammed full of digested wood. Note also the
complete boring of the liberating passage, including the bark when
there is any.
I find _Stromatium strepens_ in ilex-logs which have been stripped of
their bark. There is the same method of deliverance, the same passage
curving gently towards the nearest outside point, the same barricade
of shavings above the cell. Was the passage also carried through the
bark? The stripped logs leave me ignorant as to this detail.
_Clytus tropicus_, a sapper of the cherry-tree, _C. arietis_ and _C.
arvicola_, sappers of the hawthorn, have a cylindrical exit-gallery,
with a sharp turn to it. The gallery is masked on the outside by a
remnant of bark or wood, hardly a millimetre thick,[2] and widens, not
far from the surface, into a nymphosis-chamber, which is divided from
the burrow by a mass of packed sawdust.
[Footnote 2: .039 inch.--_Translator's Note_.]
To continue the subject would entail an excess of monotonous
repetition. The general law stands out very clearly from these few
data: the wood-eating grubs of the Longicorns and Buprestes prepare
the path of deliverance for the perfect insect, which will have merely
in one case to pass a barricade of shavings or wormed wood, or in
another to pierce a slight thickness of wood or bark. Thanks to a
curious reversal of its usual attributes, youth is here the season of
energy, of strong tools, of stubborn work; adult age is the season of
leisure, of industrial ignorance, of idle diversions, without trade or
profession. The infant has its paradise in the arms of its mother, its
providence; here the infant, the grub, is the providence of the
mother. With its patient tooth, which neither the perils of the
outside world nor the difficult task of boring through hard wood are
able to deter, it clears a way for her to the supreme delights of the
sun. The youngster prepares an easy life for the adult.
Can these armour-wearers, so sturdy in appearance, be weaklings? I
place nymphs of all the species that come to hand in glass tubes of
the same diameter as the natal cell, lined with coarse paper, which
will provide a good purchase for the boring. The obstacle to be
pierced varies: a cork a centimetre thick;[3] a plug of poplar, very
much softened by decay; a circular disk of sound wood. Most of my
captives easily pierce the cork and the soft wood; these represent to
them the barricade to be overthrown, the bark curtain to be
perforated. A few, however, succumb before the front to be attacked;
and all perish, after fruitless attempts, before the disk of hard
wood. Thus perished the strongest of them all, the Great Capricorn, in
my artificial oak-wood cells and even in my reed-stumps closed with
their natural partitions.
[Footnote 3: .39 inch.--_Translator's Note_.]
They have not the strength, or rather the patient art; and the larva,
more highly gifted, works for them. It gnaws with indomitable
perseverance, an essential to success even for the strong; it digs
with amazing foresight. It knows the future shape of the adult,
whether round or oval, and bores the exit-passage accordingly, making
it cylindrical in one case and elliptical in the other. It knows that
the adult is very impatient to reach the light; and it leads her
thither by the shortest way. In its wandering life in the heart of the
tree, it loves low-roofed, winding tunnels, just big enough to pass
through, or widening into stations when it strikes a vein with a
better flavour; now, it makes a short, straight, roomy corridor,
leading with a sharp bend to the outside world. It had plenty of time
during its capricious wanderings; the adult has none to spare: his
days are numbered; he must get out as quickly as he can. Hence the
shortest road and as little encumbered by obstacles as is consistent
with safety. The grub knows that the too sudden junction of the
horizontal and the vertical part would stop the stiff, inflexible
insect and bends it towards the outside with a gentle curve. This
elbow changing the direction occurs whenever the larva ascends from
the depths; it is very short when the nymphosis-chamber is next to the
surface, but continues for some length when the chamber is well inside
the trunk. In this case, the path traced by the grub has so regular a
curve that you feel inclined to subject the work to geometrical
measurement.
For want of sufficient data, I should have left this elbow in the
shadow of a note of interrogation, had I had at my disposal only the
emergence-galleries of the Longicorns and Buprestes, which are too
short to lend themselves to trustworthy examination with the
compasses. A lucky find provided me with the factors required. This
was the trunk of a dead poplar, riddled, to a height of several yards,
with an infinite number of round holes the diameter of a pencil. The
precious pole, still standing, is uprooted with due respect, in view
of my designs, and carried into my study, where it is sawn into
longitudinal sections planed smooth.
The wood, while retaining its structure, has been greatly softened by
the presence of the mycelium of a mushroom, the agaric of the poplar.
The inside is decayed. The outer layers, to a depth of over four
inches, are in good condition, save for the innumerable curved
passages that cut through them. In a section involving the whole
diameter of the trunk, the galleries of the late occupant produce a
pleasing effect, of which a sheaf of corn gives us a pretty faithful
image. Almost straight, parallel with one another and assembled in a
bundle down the middle, they diverge at the top and spread into a
cluster of wide curves, each of which ends in one of the holes on the
surface. It is a sheaf of passages which has not the single head of a
sheaf of corn, but shoots its innumerable sprouts hither and thither,
at all heights.
I am enraptured by this magnificent specimen. The curves, of which I
uncover a layer at every stroke of the plane, far exceed my
requirements; they are strikingly regular; they afford the compasses
the full space needed for accurate measurement.
Before calling in geometry, let us, if possible, name the creator of
these beautiful curves. The inhabitants of the poplar have
disappeared, perhaps long ago, as is proved by the mycelium of the
agaric; the insect would not gnaw and bore its way through timber all
permeated with the felt-like growth of the cryptogam. A few weaklings,
however, have died without being able to escape. I find their remains
swathed in mycelium. The agaric has preserved them from destruction by
wrapping them in tight cerements. Under these mummy-bandages, I
recognise a Saw-fly, _Sirex augur_, KLUG., in the state of the perfect
insect. And--this is an important detail--all these adult remains,
without a single exception, occupy spots which have no means of
communication with the outside. I find them sometimes in a
partly-constructed curved passage, beyond which the wood remains
intact, sometimes at the end of the straight central gallery, choked
with sawdust, which is not continued in front. These remains, with no
thoroughfare before them, tell us plainly that the Sirex adopts for
its exit methods not employed by the Buprestes and the Longicorns.
The larva does not prepare the path of deliverance; it is left for the
perfect insect to open itself a passage through the wood. What I have
before my eyes tells me more or less plainly the sequence of events.
The larva, whose presence is proved by galleries blocked with packed
sawdust, do not leave the centre of the trunk, a quieter retreat, less
subject to the vicissitudes of the climate. Metamorphosis is effected
at the junction of the straight gallery and the curved passage which
is not yet made. When strength comes, the perfect insect tunnels ahead
for a distance of more than four inches and opens up the exit-passage,
which I find choked, not with compact sawdust, but with loose powdery
rubbish. The dead insects which I strip of their mycelium-shrouds are
weaklings whose strength deserted them mid-way. The rest of the
passage is lacking because the labourer died on the road.
With this fact of the insect itself boring the exit passage, the
problem assumes a more troublesome form. If the larva, rich in leisure
and satisfied with its sojourn in the interior of the trunk,
simplifies the coming emergence by shortening the road, what must not
the adult do, who has so short a time to live and who is in so great a
hurry to leave the hateful darkness? He above any other should be a
judge of short cuts. To go from the murky heart of the tree to the
sun-steeped bark, why does he not follow a straight line? It is the
shortest way.
Yes, for the compasses, but not perhaps for the sapper. The length
traversed is not the only factor of the work accomplished, of the
total activity expended. We must take into account the resistance
overcome, a resistance which varies according to the depth of the more
or less hard strata and according to the method of attacking the woody
fibres, which are either broken across or divided lengthwise. Under
these conditions, whose value remains to be determined, can there be a
curve involving a minimum of mechanical labour in cutting through the
wood?
I was already trying to discover how the resistance may vary according
to depth and direction; I was working out my differentials and my
minimum integrals, when a very simple idea overturned my slippery
scaffolding. The calculation of variations has nothing to do with the
matter. The animal is not the moving body of the mathematicians, the
particle of matter guided in its trajectory solely by the motive
forces and the resistance of the medium traversed; it bears within
itself conditions which control the others. The adult insect does not
even enjoy the larva's privileges; it cannot bend freely in all
directions. Under its harness it is almost a stiff cylinder. To
simplify the explanation, we may liken the insect to a section of an
inflexible straight line.
Let us return to the Sirex, reduced by abstraction to its axis. The
metamorphosis is effected not far from the centre of the trunk. The
insect lies lengthwise in the tree with its head up, very rarely with
its head down. It must reach the outside as quickly as possible. The
section of an inflexible straight line that represents it nibbles away
a little wood in front of it and obtains a shallow cavity wide enough
to allow of a very slight turn towards the outside. An infinitesimal
advance is made; a second follows, the result of a similar cavity and
a similar turn in the same direction. In short, each change of
position is accompanied by the tiny deviation permitted by the slight
excess of width of the hole; and this deviation invariably points the
same way. Imagine a magnetic needle swung out of its position and
tending to return to it while moving with a uniform speed through a
resisting medium in which a sheath of a diameter slightly greater than
the needle's opens bit by bit. The Sirex behaves more or less in the
same fashion. His magnetic pole is the light outside. He makes for
that direction by imperceptible deviations as his tooth digs.
The problem of the Sirex is now solved. The trajectory is composed of
equal elements, with an invariable angle between them; it is the curve
whose tangents, divided by infinitely small distances, retain the same
inclination between each one and the next; the curve, in a word, with
a constant angle of contingence. This characteristic betrays the
circumference of the circle.
It remains to discover whether the facts confirm the logical argument.
I take accurate tracings of a score of galleries, selecting those
whose length best lends itself to the test of the compasses. Well,
logic agrees with reality: over lengths which sometimes exceed four
inches, the track of the compasses is identical with that of the
insect. The most pronounced deviations do not exceed the small
variations which we must reasonably expect in a problem of a physical
nature, a problem incompatible with the absolute accuracy of abstract
truths.
The Sirex' exit-gallery then is a wide arc of a circle whose lower
extremity is connected with the corridor of the larva and whose upper
extremity is prolonged in a straight line which ends at the surface
with a perpendicular or slightly oblique incidence. The wide
connecting arc enables the insect to tack about. When, starting from a
position parallel with the axis of the tree, the Sirex has passed
gradually to a transversal position, he completes his course in a
straight line, which is the shortest road.
Does the trajectory imply the minimum of work? Yes, under the
conditions of the insect's existence. If the larva had taken the
precaution to place itself in a different direction when preparing for
the nymphosis, to turn its head towards the nearest point of the bark
instead of turning it lengthwise with the trunk, obviously the adult
would escape more easily: he would merely have to gnaw straight in
front of him in order to pass through the minimum thickness. But
reasons of convenience whereof the grub is the sole judge, reasons
dictated perhaps by weight, cause the vertical to precede the
horizontal position. In order to pass from the former to the latter,
the insect veers round by describing the arc of a circle. When this
turn has been effected, the distance is completed in a straight line.
Let us consider the Sirex at his starting-point. His stiffness of
necessity compels him to turn gradually. Here the insect can do
nothing of its own initiative; everything is mechanically determined.
But, being free to pivot on its axis and to attack the wood on either
side of the sheath, it has the option of attempting this reversal in a
host of different ways, by a series of connected arcs, not in the same
plane. Nothing prevents it from describing winding curves by revolving
upon itself: spirals, loops constantly changing their direction, in
fact, the complicated route of a creature that has lost its way. It
might wander in a tortuous maze, making fresh attempts here, there and
everywhere, groping for ever so long without succeeding.
But it does not grope and it succeeds very well. Its gallery is still
contained within one plane, the first condition of the minimum of
labour. Moreover, of the different vertical planes that can pass
through the eccentric starting-point, one, the plane which passes
through the axis of the tree, corresponds on the one side with the
minimum of resistance to be overcome and on the other with the
maximum. Nothing prevents the Sirex from tracing his path in any one
of the multitude of planes on which the path would possess an
intermediate value between the shortest and the longest. The insect
refuses them all and constantly adopts the one which passes through
the axis, choosing, of course, the side that entails the shortest
path. In brief, the Sirex' gallery is contained in a plane pointing
towards the axis of the tree and the starting-point; and of the two
portions of this plane the channel passes through the less extensive.
Under the conditions, therefore, imposed upon him by his stiffness the
hermit of the poplar-tree releases himself with the minimum of
mechanical labour.
The miner guides himself by the compass in the unknown depths
underground, the sailor does the same in the unknown ocean solitudes.
How does the wood-eating insect guide itself in the thickness of a
tree-trunk? Has it a compass? One would almost say that it had, so
successfully does it keep to the quickest road. Its goal is the light.
To reach this goal, it suddenly chooses the economical plane
trajectory, after spending its larval leisure in roaming tortuous
passages full of irregular curves; it bends it in an arc which allows
it to turn about; and, with its head held plumb with the adjacent
surface, it goes straight ahead by the nearest way.
The most extraordinary obstacles are powerless to turn it aside from
its plane and its curve, so imperative is its guiding force. It will
gnaw metal, if need be, rather than turn its back upon the light,
which it feels to be close at hand. The entomological records place
this incredible fact beyond a doubt. At the time of the Crimean War,
the Institut de France received some packets of cartridges in which
the bullets had been perforated by _Sirex juvencus_; a little later,
at the Grenoble Arsenal, _S. gigas_ carved himself a similar exit. The
larva was in the wood of the cartridge-boxes; and the adult insect,
faithful to its direction of escape, had bored through the lead
because the nearest daylight was behind that obstacle.
There is an exit-compass, that is incontestable, both for the larvae
preparing the passage of deliverance and for the adult insect, the
Sirex obliged to make that passage for himself. What is it? Here the
problem becomes surrounded with a darkness which is perhaps
impenetrable; we are not well enough equipped with means of receiving
impressions even to imagine the causes which guide the creature. There
is, in certain events, another world of the senses in which our organs
perceive nothing, a world which is closed to us. The eye of the camera
sees the invisible and photographs the image of the ultra-violet rays;
the tympanum of the microphone hears what to us is silence. A
scientific toy, a chemical contrivance surpass us in sensibility.
Would it be rash to attribute similar faculties to the delicate
organization of the insect, even with regard to agencies unknown to
our science, because they do not fall within the domain of our senses?
To this question there is no positive reply; we have suspicions and
nothing more. Let us at least dispel a few false notions that might
occur to us.
Does the wood guide the insect, adult or larva, by its structure?
Gnawed across the grain, it must produce a certain impression; gnawed
lengthwise, it must produce a different impression. Is there not
something here to guide the sapper? No, for in the stump of a tree
left standing the emergence takes place, according to the proximity of
the light, sometimes by way of the horizontal section, by means of a
rectilinear path running along the grain, and sometimes by way of the
side, by means of a curved road cutting across the grain.
Is the compass a chemical influence, or electrical, or calorific, or
what not? No, for in an upright trunk the emergence is effected as
often by the north face, which is always in the shade, as by the south
face, which receives the sun all day long. The exit-door opens in the
side which is nearest, without any other condition. Can it be the
temperature? Not that either, for the shady side, though cooler, is
utilized as often as the side facing the sun.
Can it be sound? Not so. The sound of what, in the silence of
solitude? And are the noises of the outside world propagated through
half an inch of wood in such a way as to make differences perceptible?
Can it be weight? No again, for the trunk of the poplar shows us more
than one Sirex travelling upside down, with his head towards the
ground, without any change in the direction of the curved passages.
What then is the guide? I have no idea. It is not the first time that
this obscure question has been put to me. When studying the emergence
of the Three-pronged Osmia from the bramble-stems shifted from their
natural position by my wiles, I recognized the uncertainty in which
the evidence of physical science leaves us; and, in the impossibility
of finding any other reply, I suggested a special sense, the sense of
open space. Instructed by the Sirex, the Buprestes, the Longicorns, I
am once again compelled to make the same suggestion. It is not that I
care for the expression: the unknown cannot be named in any language.
It means that the hermits in the dark know how to find the light by
the shortest road; it is the confessions of an ignorance which no
honest observer will blush to share. Now that the evolutionists'
interpretations of instinct have been recognized as worthless, we all
come to that stimulating maxim of Anaxagoras', which laconically sums
up the result of my researches:
"[Greek: Nous panta diekosmese]. Mind orders all things."