The Glow-Worm and Other Beetles

by Jean-Henri Fabre

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.

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.

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.