The Glow-Worm and Other Beetles Part 9

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 coa.r.s.e 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 pa.s.sage, 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 pa.s.sage curving gently towards the nearest outside point, the same barricade of shavings above the cell. Was the pa.s.sage 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 ma.s.s of packed sawdust.

[Footnote 2: .039 inch.--_Translator's Note_.]

To continue the subject would entail an excess of monotonous repet.i.tion. 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 pa.s.s 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 st.u.r.dy in appearance, be weaklings? I place nymphs of all the species that come to hand in gla.s.s tubes of the same diameter as the natal cell, lined with coa.r.s.e 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, succ.u.mb 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 part.i.tions.

[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-pa.s.sage 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 pa.s.s 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 enc.u.mbered 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 compa.s.ses. 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 pa.s.sages 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 a.s.sembled in a bundle down the middle, they diverge at the top and spread into a cl.u.s.ter of wide curves, each of which ends in one of the holes on the surface. It is a sheaf of pa.s.sages which has not the single head of a sheaf of corn, but shoots its innumerable sprouts. .h.i.ther 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 compa.s.ses the full s.p.a.ce 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 pa.s.sage, 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 pa.s.sage 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 pa.s.sage 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-pa.s.sage, 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 pa.s.sage is lacking because the labourer died on the road.

With this fact of the insect itself boring the exit pa.s.sage, the problem a.s.sumes 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 compa.s.ses, 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 fas.h.i.+on. 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 circ.u.mference 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 compa.s.ses. Well, logic agrees with reality: over lengths which sometimes exceed four inches, the track of the compa.s.ses is identical with that of the insect. The most p.r.o.nounced 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 pa.s.sed 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 pa.s.s 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 pa.s.s 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 pa.s.s through the eccentric starting-point, one, the plane which pa.s.ses 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 mult.i.tude 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 pa.s.ses 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 pa.s.ses 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 compa.s.s 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 compa.s.s? 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 pa.s.sages 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 Inst.i.tut de France received some packets of cartridges in which the bullets had been perforated by _Sirex juvencus_; a little later, at the Gren.o.ble a.r.s.enal, _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-compa.s.s, that is incontestable, both for the larvae preparing the pa.s.sage of deliverance and for the adult insect, the Sirex obliged to make that pa.s.sage 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 surpa.s.s 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 compa.s.s 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 pa.s.sages.

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-p.r.o.nged Osmia from the bramble-stems s.h.i.+fted 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 s.p.a.ce. 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."

CHAPTER IX THE DUNG-BEETLES OF THE PAMPAS

To travel the world, by land and sea, from pole to pole; to cross-question life, under every clime, in the infinite variety of its manifestations: that surely would be glorious luck for him that has eyes to see; and it formed the radiant dream of my young years, at the time when _Robinson Crusoe_ was my delight. These rosy illusions, rich in voyages, were soon succeeded by dull, stay-at-home reality. The jungles of India, the virgin forests of Brazil, the towering crests of the Andes, beloved by the Condor, were reduced, as a field for exploration, to a patch of pebbles enclosed within four walls.

Heaven forfend that I should complain! The gathering of ideas does not necessarily imply distant expeditions. Jean-Jacques Rousseau[1]

herborized with the bunch of chick-weed whereon he fed his Canary; Bernardin de Saint-Pierre[2] discovered a world on a strawberry-plant that grew by accident in a corner of his window; Xavier de Maistre,[3]

using an arm-chair by way of post-chaise, made one of the most famous of journeys around his room.

[Footnote 1: Jean-Jacques Rousseau (1712-1778), author of the _Confessions_, _La Nouvelle Heloise_, etc.--_Translator's Note_.]

[Footnote 2: Jacques Henri Bernardin de Saint-Pierre (1737-1814), author of _Paul et Virginie_, _La Chaumiere idienne_ and _Etudes de la nature_.--_Translator's Note_.]

[Footnote 3: Xavier de Maistre (1763-1852), best known for his _Voyage autour de ma chambre_ (1795).--_Translator's Note_.]

This manner of seeing country is within my means, always excepting the post-chaise, which is too difficult to drive through the bushes. I go the circuit of my enclosure over and over again, a hundred times, by short stages; I stop here and I stop there; patiently, I put questions and, at long intervals, I receive some sc.r.a.p of a reply.