An eighteenth-century philosopher, Condillac, describes an imaginary statue, organised like a man, but with none of a man’s senses. He then pictures the effect of endowing it with the five senses, one by one, and the first sense he gives it is that of smell. The statue, having no sense but smell, inhales the scent of a rose, and out of that single impression creates a whole world of ideas. In my youth I owed some happy moments to that statue. I seemed to see it come to life in that action of the nostrils, acquiring memory, concentration, judgment, and other mental qualities, even as still waters are aroused and rippled by the impact of a grain of sand. I recovered from my illusion under the teaching of my abler master the animal. The Capricorn taught me that the problem is more obscure than the Abbé Condillac led me to suppose.
When my winter supply of firewood is being prepared for me with wedge and mallet, the woodman selects, by my express orders, the oldest and most ravaged trunks in his stack. My tastes bring a smile to his lips; he wonders by what whimsy I prefer wood that is worm-eaten to sound wood, which burns so much better. I have my views on the subject, and the worthy man submits to them.
A fine oak-trunk, seamed with scars and gashed with wounds, contains many treasures for my studies. The mallet drives home, the wedges bite, the wood splits; and within, in the dry and hollow parts, are revealed groups of various insects who are capable of living through the cold season, and have here taken up their winter quarters. In the low-roofed galleries built by some Beetle the Osmia Bee has piled her cells one above the other. In the deserted chambers and vestibules Megachiles have arranged their leafy jars. In the live wood, filled with juicy sap, the larva of the Capricorn, the chief author of the oak’s undoing, has set up its home.
Truly they are strange creatures, these grubs: bits of intestines crawling about! In the middle of Autumn I find them of two different ages. The older are almost as thick as one’s finger; the others hardly attain the diameter of a pencil. I find, in addition, the pupa or nymph more or less fully coloured, and the perfect insect ready to leave the trunk when the hot weather comes again. Life inside the wood, therefore, lasts for three years.
How is this long period of solitude and captivity spent? In wandering lazily through the thickness of the oak, in making roads whose rubbish serves as food. The horse in the book of Job “swallows the ground” in a figure of speech: the Capricorn’s grub eats its way literally. With its carpenter’s-gouge—a strong black mandible, short and without notches, but scooped into a sharp-edged spoon—it digs the opening of its tunnel. From the piece cut out the grub extracts the scanty juices, while the refuse accumulates behind him in heaps. The path is devoured as it is made; it is blocked behind as it makes way ahead.
Since this harsh work is done with the two gouges, the two curved chisels of the mandibles, the Capricorn-grub requires much strength in the front part of its body, which therefore swells into a sort of pestle. The Buprestis-grub, that other industrious carpenter, adopts a similar form, and even exaggerates its pestle. The part that toils and carves hard wood requires to be robust; the rest of the body, which has but to follow after, continues slim. The essential thing is that the implement of the jaws should possess a solid support and powerful machinery. The Capricorn larva strengthens its chisels with a stout, black, horny armour that surrounds the mouth; yet, apart from its skull and its equipment of tools, this grub has a skin as fine as satin and as white as ivory. This dead white is caused by a thick layer of grease, which one would not expect a diet of wood to produce in the animal. True, it has nothing to do, at every hour of the day and night, but gnaw. The quantity of wood that passes into its stomach makes up for the lack of nourishing qualities.
The grub’s legs can hardly be called legs at all; they are mere suggestions of the legs the full-grown insect will have by and by. They are infinitesimal in size, and of no use whatever for walking. They do not even touch the supporting surface, being kept off it by the plumpness of the chest. The organs by means of which the animal progresses are something altogether different.
The grub of the Rose-chafer, with the aid of the hairs and pad-like projections upon its spine, manages to reverse the usual method of walking, and to wriggle along on its back. The grub of the Capricorn is even more ingenious: it moves at the same time on its back and its stomach. To take the place of its useless legs it has a walking apparatus almost like feet, which appear, contrary to every rule, on the surface of its back.
On the middle part of its body, both above and below, there is a row of seven four-sided pads, which the grub can either expand or contract, making them stick out or lie flat at will. It is by means of these pads that it walks. When it wishes to move forwards it expands the hinder pads, those on the back as well as those on the stomach, and contracts its front pads. The swelling of the hind pads in the narrow gallery fills up the space, and gives the grub something to push against. At the same time the flattening of the front pads, by decreasing the size of the grub, allows it to slip forward and take half a step. Then, to complete the step, the hind-quarters must be brought up the same distance. With this object the front pads fill out and provide support, while those behind shrink and leave room for the grub to draw up its hind-quarters.
With the double support of its back and stomach, with alternate swellings and shrinkings, the animal easily advances or retreats along its gallery, a sort of mould which the contents fill without a gap. But if the pads grip only on one side progress becomes impossible. When placed on the smooth wood of my table the animal wriggles slowly; it lengthens and shortens without progressing by a hair’s breadth. Laid on the surface of a piece of split oak, a rough, uneven surface due to the gash made by the wedge, it twists and writhes, moves the front part of its body very slowly from left to right and right to left, lifts it a little, lowers it, and begins again. This is all it can do. The rudimentary legs remain inert and absolutely useless.
Though the Capricorn-grub possesses these useless legs, the germs of future limbs, there is no sign of the eyes with which the fully-developed insect will be richly gifted. The larva has not the least trace of any organs of sight. What would it do with sight, in the murky thickness of a tree-trunk? Hearing is likewise absent. In the untroubled silence of the oak’s inmost heart the sense of hearing would be superfluous. Where sounds are lacking, of what use is the faculty of discerning them?
To make the matter certain I carried out some experiments. If split lengthwise the grub’s abode becomes a half-tunnel, in which I can watch the occupant’s doings. When left alone it alternately works for a while, gnawing at its gallery, and rests for awhile, fixed by its pads to the two sides of the tunnel. I took advantage of these moments of rest to inquire into its power of hearing. The banging of hard bodies, the ring of metallic objects, the grating of a file upon a saw, were tried in vain. The animal remained impassive: not a wince, not a movement of the skin, no sign of awakened attention. I succeeded no better when I scratched the wood near it with a hard point, to imitate the sound of some other grub at work in its neighbourhood. The indifference to my noisy tricks could be no greater in a lifeless object. The animal is deaf.
Can it smell? Everything tells us that it cannot. Scent is of assistance in the search for food. But the Capricorn-grub need not go in quest of eatables. It feeds on its home; it lives on the wood that gives it shelter. Nevertheless I tested it. In a log of fresh cypress wood I made a groove of the same width as that of the natural galleries, and I placed the grub inside it. Cypress wood is strongly scented; it has the smell characteristic of most of the pine family. This resinous scent, so strange to a grub that lives always in oak, ought to vex it, to trouble it; and it should show its displeasure by some kind of commotion, some attempt to get away. It did nothing of the kind: once it had found the right position in the groove it went to the end, as far as it could go, and made no further movement. Then I set before it, in its usual channel, a piece of camphor. Again no effect. Camphor was followed by naphthaline. Still no result. I do not think I am going too far when I deny the creature a sense of smell.
Taste is there no doubt. But such taste! The food is without variety: oak, for three years at a stretch, and nothing else. What can the grub’s palate find to enjoy in this monotonous fare? The agreeable sensation of a fresh piece, oozing with sap; the uninteresting flavour of an over-dry piece. These, probably, are the only changes in the meal.
There remains the sense of touch, the universal passive sense common to all live flesh that quivers under the goad of pain. The Capricorn-grub, therefore, is limited to two senses, those of taste and touch, and both of these it possesses only in a very small degree. It is very little better off than Condillac’s statue. The imaginary being created by the philosopher had one sense only, that of smell, equal in delicacy to our own; the real being, the oak-eater has two, which are inferior even when put together to the one sense of the statue. The latter plainly perceived the scent of a rose, and clearly distinguished it from any other.
A vain wish has often come to me in my dreams: to be able to think, for a few minutes, with the brain of my Dog, or to see the world with the eyes of a Gnat. How things would change in appearance! But they would change much more if understood only with the intellect of the grub. What has that incomplete creature learnt through its senses of touch and taste? Very little; almost nothing. It knows that the best bits of wood have a special kind of flavour, and that the sides of a passage, when not carefully smoothed, are painful to the skin. This is the limit of its wisdom. In comparison with this, the statue with the sensitive nostrils was a marvel of knowledge. It remembered, compared, judged, and reasoned. Can the Capricorn-grub remember? Can it reason? I described it a little time ago as a bit of intestine that crawls about. This description gives an answer to these questions. The grub has the sensations of a bit of intestine, no more and no less.
And this half-alive object, this nothing-at-all, is capable of marvellous foresight. It knows hardly anything of the present, but it sees very clearly into the future.
For three years on end the larva wanders about in the heart of the trunk. It goes up, goes down, turns to this side and that; it leaves one vein for another of better flavour, but without ever going too far from the inner depths, where the temperature is milder than near the surface, and greater safety reigns. But a day is at hand when the hermit must leave its safe retreat and face the perils of the outer world. Eating is not everything, after all; we have to get out of this.
But how? For the grub, before leaving the trunk, must turn into a long-horned Beetle. And though the grub, being well equipped with tools and muscular strength, finds no difficulty in boring through the wood and going where it pleases, it by no means follows that the coming Capricorn has the same powers. The Beetle’s short spell of life must be spent in the open air. Will it be able to clear itself a way of escape?
It is quite plain, at all events, that the Capricorn will be absolutely unable to make use of the tunnel bored by the grub. This tunnel is a very long and very irregular maze, blocked with great heaps of wormed wood. It grows constantly smaller and smaller as it approaches the starting-point, because the larva entered the trunk as slim as a tiny bit of straw, whereas to-day it is as thick as one’s finger. In its three years’ wanderings it always dug its gallery to fit the size of its body. Evidently the road of the larva cannot be the Capricorn’s way out. His overgrown antennæ, his long legs, his inflexible armour-plates would find the narrow, winding corridor impassable. The passage would have to be cleared of its wormed wood, and, moreover, greatly enlarged. It would be easier to attack the untouched timber and dig straight ahead. Is the insect capable of doing so? I determined to find out.
I made some cavities of suitable size in some oak logs that had been chopped in two, and in each of these cells I placed a Capricorn that had just been transformed from the grub. I then joined the two sides of the logs, fastening them together with wire. When June came I heard a sound of scraping inside the logs, and waited anxiously to see if the Capricorns would appear. They had hardly three-quarters of an inch to pierce. Yet not one came out. On opening the logs I found all my captives dead. A pinch of sawdust represented all they had done.
I had expected more from their sturdy mandibles. In spite of their boring-tools the hermits died for lack of skill. I tried enclosing some in reed-stumps, but even this comparatively easy work was too much for them. Some freed themselves, but others failed.
Notwithstanding his stalwart appearance the Capricorn cannot leave the tree-trunk by his own unaided efforts. The truth is that his way is prepared for him by the grub—that bit of intestine.
Some presentiment—to us an unfathomable mystery—causes the Capricorn-grub to leave its peaceful stronghold in the very heart of the oak and wriggle towards the outside, where its foe the Woodpecker is quite likely to gobble it up. At the risk of its life it stubbornly digs and gnaws to the very bark. It leaves only the thinnest film, the slenderest screen, between itself and the world at large. Sometimes, even, the rash one opens the doorway wide.
This is the Capricorn’s way out. The insect has but to file the screen a little with his mandibles, to bump against it with his forehead, in order to bring it down. He will even have nothing at all to do when the doorway is open, as often happens. The unskilled carpenter, burdened with his extravagant head-dress, will come out from the darkness through this opening when the summer heat arrives.
As soon as the grub has attended to the important business of making a doorway into the world, it begins to busy itself with its transformation into a Beetle. First, it requires space for the purpose. So it retreats some distance down its gallery, and in the side of the passage digs itself a transformation-chamber more sumptuously furnished and barricaded than any I have ever seen. It is a roomy hollow with curved walls, three to four inches in length and wider than it is high. The width of the cell gives the insect a certain degree of freedom of movement when the time comes for forcing the barricade, which is more than a close-fitting case would do.
The barricade—a door which the larva builds as a protection from danger—is twofold, and often threefold. Outside, it is a stack of woody refuse, of particles of chopped timber; inside, a mineral lid, a concave cover, all in one piece, of a chalky white. Pretty often, but not always, there is added to these two layers an inner casing of shavings.
Behind this threefold door the larva makes its arrangements for its transformation. The sides of the chamber are scraped, thus providing a sort of down formed of ravelled woody fibres, broken into tiny shreds. This velvety stuff is fixed on the wall, in a thick coating, as fast as it is made. The chamber is thus padded throughout with a fine swan’s-down, a delicate precaution taken by the rough grub out of kindness for the tender creature it will become when it has cast its skin.
Let us now go back to the most curious part of the furnishing, the cover or inner door of the entrance. It is like an oval skull-cap, white and hard as chalk, smooth within and rough without, with some resemblance to an acorn-cup. The rough knots show that the material is supplied in small, pasty mouthfuls, which become solid outside in little lumps. The animal does not remove them, because it is unable to get at them; but the inside surface is polished, being within the grub’s reach. This singular lid is as hard and brittle as a flake of limestone. It is, as a matter of fact, composed solely of carbonate of lime, and a sort of cement which gives consistency to the chalky paste.
I am convinced that this stony deposit comes from a particular part of the grub’s stomach, called the chylific ventricle. The chalk is kept separate from the food, and is held in reserve until the right time comes to discharge it. This freestone factory causes me no astonishment. It serves for various chemical works in different grubs when undergoing transformation. Certain Oil-beetles keep refuse in it, and several kinds of Wasps use it to manufacture the shellac with which they varnish the silk of their cocoons.
When the exit way is prepared, and the cell upholstered in velvet and closed with a threefold barricade, the industrious grub has finished its task. It lays aside its tools, sheds its skin, and becomes a pupa—weakness personified, in the swaddling-clothes of a cocoon. The head is always turned towards the door. This is a trifling detail in appearance; but in reality it is everything. To lie this way or that in the long cell is a matter of great indifference to the grub, which is very supple, turning easily in its narrow lodging and adopting whatever position it pleases. The coming Capricorn will not enjoy the same privileges. Stiffly encased in his horny armour, he will not be able to turn from end to end; he will not even be capable of bending, if some sudden curve should make the passage difficult. He must, without fail, find the door in front of him, or he will perish in the transformation-room. If the grub should forget this little matter, and lie down to sleep with its head at the back of the cell, the Capricorn would be infallibly lost. His cradle would become a hopeless dungeon.
But there is no fear of this danger. The “bit of intestine” knows too much about the future to neglect the formality of keeping its head at the door. At the end of spring the Capricorn, now in possession of his full strength, dreams of the joys of the sun, of the festivals of light. He wants to get out.
What does he find before him? First, a heap of filings easily dispersed with his claws; next, a stone lid which he need not even break into fragments, for it comes undone in one piece. It is removed from its frame with a few pushes of the forehead, a few tugs of the claws. In fact, I find the lid intact on the threshold of the abandoned cell. Last comes a second mass of woody remnants as easy to scatter as the first. The road is now free: the Capricorn has but to follow the wide vestibule, which will lead him, without any possibility of mistake, to the outer exit. Should the doorway not be open, all that he has to do is to gnaw through a thin screen, an easy task. Behold him outside, his long antennæ quivering with excitement.
What have we learnt from him? Nothing from him, but much from his grub. This grub, so poor in organs of sensation, gives us much to think about. It knows that the coming Beetle will not be able to cut himself a road through the oak, and it therefore opens one for him at its own risk and peril. It knows that the Capricorn, in his stiff armour, will never be able to turn round and make for the opening of the cell; and it takes care to fall into its sleep of transformation with its head towards the door. It knows how soft the pupa’s flesh will be, and it upholsters the bedroom with velvet. It knows that the enemy is likely to break in during the slow work of the transformation, and so, to make a protection against attack, it stores lime inside its stomach. It knows the future with a clear vision, or, to be accurate, it behaves as if it knew the future.
What makes it act in this way? It is certainly not taught by the experiences of its senses. What does it know of the outside world? I repeat—as much as a bit of intestine can know. And this senseless creature astounds us! I regret that the philosopher Condillac, instead of creating a statue that could smell a rose, did not gift it with an instinct. How soon he would have seen that the animals—including man—have powers quite apart from the senses; inspirations that are born with them, and are not the result of learning.
This curious life and this marvellous foresight are not confined to one kind of grub. Besides the Capricorn of the Oak there is the Capricorn of the Cherry-tree. In appearance the latter is an exact copy of the former, on a much smaller scale; but the little Capricorn has different tastes from its large kinsman’s. If we search the heart of the cherry-tree it does not show us a single grub anywhere: the entire population lives between the bark and the wood. This habit is only varied when transformation is at hand. Then the grub of the cherry-tree leaves the surface, and scoops out a cavity at a depth of about two inches. Here the walls are bare: they are not lined with the velvety fibres dear to the Capricorn of the Oak. The entrance is blocked, however, by sawdust, and a chalky lid similar to the other except in point of size. Need I add that the grub lies down and goes to sleep with his head against the door? Not one forgets to take this precaution.
There is also a Saperda of the Poplar and a Saperda of the Cherry-tree. They have the same organisation and the same tools; but the former follows the methods of the Capricorn of the Oak, while the latter imitates the Capricorn of the Cherry-tree.
The poplar-tree is also inhabited by the Bronze Buprestis, which takes no defensive measures before going to sleep. It makes no barricade, no heap of shavings. And in the apricot-tree the Nine-spotted Buprestis behaves in the same way. In this case the grub is inspired by its intuitions to alter its plan of work to suit the coming Beetle. The perfect insect is a cylinder; the grub is a strap, a ribbon. The former, which wears unyielding armour, needs a cylindrical passage; the latter needs a very low tunnel, with a roof that it can reach with the pads on its back. The grub therefore changes its manner of boring: 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 roadmaking on behalf of the coming insect once more shows us the foresight of this “bit of intestine.”
I could tell you of many other wood-eaters. Their tools are the same; yet each species displays special methods, tricks of the trade that have nothing to do with the tools. These grubs, then, like so many insects, show us that instinct is not made by the tools, so to speak, but that the same tools may be used in various ways.
To continue the subject would be monotonous. The general rule stands out very clearly from these facts: the wood-eating grubs prepare the path of deliverance for the perfect insect, which will merely have to pass a barricade of shavings or pierce a screen of bark. By a curious reversal of the usual state of things, infancy is here the season of energy, of strong tools, of stubborn work; mature age is the season of leisure, of industrial ignorance, of idle diversions, without trade or profession. The providence of the human infant is the mother; here the baby grub is the mother’s providence. With its patient tooth, which neither the peril of the outside world nor the difficult task of boring through hard wood is able to discourage, it clears away for her to the supreme delights of the sun.