Chapter XI. The Eumenes
Jean-Henri Fabre The Wonders of Instinct
Mason wasp (Eumenes coarctatus)
A wasp-like garb of motley black and yellow; a slender and graceful figure; wings not spread out flat, when resting, but folded lengthwise in two; the abdomen a sort of chemist's retort, which swells into a gourd and is fastened to the thorax by a long neck, first distending into a pear, then shrinking to a thread; a leisurely and silent flight; lonely habits. There we have a summary sketch of the Eumenes. My part of the country possesses two species: the larger, Eumenes Amedei, Lep., measures nearly an inch in length; the other, Eumenes pomiformis, Fabr., is a reduction of the first to the scale of one-half. (I include three species promiscuously under this one name, that is to say, Eumenes pomiformis, Fabr., E. bipunctis, Sauss., and E. dubius, Sauss. As I did not distinguish between them in my first investigations, which date a very long time back, it is not possible for me to ascribe to each of them its respective nest. But their habits are the same, for which reason this confusion does not injuriously affect the order of ideas in the present chapter.--Author's Note.)
Similar in form and colouring, both possess a like talent for architecture; and this talent is expressed in a work of the highest perfection which charms the most untutored eye. Their dwelling is a masterpiece. The Eumenes follow the profession of arms, which is unfavourable to artistic effort; they stab a prey with their sting; they pillage and plunder. They are predatory Hymenoptera, victualling their grubs with caterpillars. It will be interesting to compare their habits with those of the operator on the Grey Worm (1) Though the quarry--caterpillars in either case--remain the same, perhaps instinct, which is liable to vary with the species, has fresh glimpses in store for us. Besides, the edifice built by the Eumenes in itself deserves inspection.
The Hunting Wasps whose story we have described in former volumes are wonderfully well versed in the art of wielding the lancet; they astound us with their surgical methods, which they seem to have learnt from some physiologist who allows nothing to escape him; but those skilful slayers have no merit as builders of dwelling-houses. What is their home, in point of fact? An underground passage, with a cell at the end of it; a gallery, an excavation, a shapeless cave. It is miner's work, navvy's work: vigorous sometimes, artistic never. They use the pick-axe for loosening, the crowbar for shifting, the rake for extracting the materials, but never the trowel for laying. Now in the Eumenes we see real masons, who build their houses bit by bit with stone and mortar and run them up in the open, either on the firm rock or on the shaky support of a bough. Hunting alternates with architecture; the insect is a Nimrod or a Vitruvius by turns (2).
And, first of all, what sites do these builders select for their homes? Should you pass some little garden-wall, facing south, in a sun-scorched corner, look at the stones that are not covered with plaster, look at them one by one, especially the largest; examine the masses of boulders, at no great height from the ground, where the fierce rays have heated them to the temperature of a Turkish bath; and, perhaps, if you seek long enough, you will light upon the structure of Eumenes Amedei. The insect is scarce and lives apart; a meeting is an event upon which we must not count with too great confidence. It is an African species and loves the heat that ripens the carob and the date. It haunts the sunniest spots and selects rocks or firm stones as a foundation for its nest. Sometimes also, but seldom, it copies the Chalicodoma of the Walls and builds upon an ordinary pebble (3).
Eumenes pomiformis is much more common and is comparatively indifferent to the nature of the foundation whereon she erects her cells. She builds on walls, on isolated stones, on the wood of the inner surface of half-closed shutters; or else she adopts an aerial base, the slender twig of a shrub, the withered sprig of a plant of some sort. Any form of support serves her purpose. Nor does she trouble about shelter. Less chilly than her African cousin, she does not shun the unprotected spaces exposed to every wind that blows.
When erected on a horizontal surface, where nothing interferes with it, the structure of Eumenes Amedei is a symmetrical cupola, a spherical skull-cap, with, at the top, a narrow passage just wide enough for the insect, and surmounted by a neatly funnelled neck. It suggests the round hut of the Eskimo or of the ancient Gael, with its central chimney. Two centimetres and a half (4), more or less, represent the diameter, and two centimetres the height (5). When the support is a perpendicular plane, the building still retains the domed shape, but the entrance- and exit-funnel opens at the side, upwards. The floor of this apartment calls for no labour: it is supplied direct by the bare stone.
Having chosen the site, the builder erects a circular fence about three millimetres thick (6). The materials consist of mortar and small stones. The insect selects its stone-quarry in some well-trodden path, on some neighbouring road, at the driest, hardest spots. With its mandibles, it scrapes together a small quantity of dust and saturates it with saliva until the whole becomes a regular hydraulic mortar which soon sets and is no longer susceptible to water. The Mason-bees have shown us a similar exploitation of the beaten paths and of the road-mender's macadam. All these open-air builders, all these erectors of monuments exposed to wind and weather require an exceedingly dry stone-dust; otherwise the material, already moistened with water, would not properly absorb the liquid that is to give it cohesion; and the edifice would soon be wrecked by the rains. They possess the sense of discrimination of the plasterer, who rejects plaster injured by damp. We shall see presently how the insects that build under shelter avoid this laborious macadam-scraping and give the preference to fresh earth already reduced to a paste by its own dampness. When common lime answers our purpose, we do not trouble about Roman cement. Now Eumenes Amedei requires a first-class cement, even better than that of the Chalicodoma of the Walls, for the work, when finished, does not receive the thick covering wherewith the Mason-bee protects her cluster of cells. And therefore the cupola-builder, as often as she can, uses the highway as her stone-pit.
With the mortar, flints are needed. These are bits of gravel of an almost unvarying size--that of a peppercorn--but of a shape and kind differing greatly, according to the places worked. Some are sharp-cornered, with facets determined by chance fractures; some are round, polished by friction under water. Some are of limestone, others of silicic matter. The favourite stones, when the neighbourhood of the nest permits, are little nodules of quartz, smooth and semitransparent. These are selected with minute care. The insect weighs them, so to say, measures them with the compass of its mandibles and does not accept them until after recognizing in them the requisite qualities of size and hardness.
A circular fence, we were saying, is begun on the bare rock. Before the mortar sets, which does not take long, the mason sticks a few stones into the soft mass, as the work advances. She dabs them half-way into the cement, so as to leave them jutting out to a large extent, without penetrating to the inside, where the wall must remain smooth for the sake of the larva's comfort. If necessary, a little plaster is added, to tone down the inner protuberances. The solidly embedded stonework alternates with the pure mortarwork, of which each fresh course receives its facing of tiny encrusted pebbles. As the edifice is raised, the builder slopes the construction a little towards the centre and fashions the curve which will give the spherical shape. We employ arched centrings to support the masonry of a dome while building: the Eumenes, more daring than we, erects her cupola without any scaffolding.
A round orifice is contrived at the summit; and, on this orifice, rises a funnelled mouthpiece built of pure cement. It might be the graceful neck of some Etruscan vase. When the cell is victualled and the egg laid, this mouthpiece is closed with a cement plug; and in this plug is set a little pebble, one alone, no more: the ritual never varies. This work of rustic architecture has naught to fear from the inclemency of the weather; it does not yield to the pressure of the fingers; it resists the knife that attempts to remove it without breaking it. Its nipple shape and the bits of gravel wherewith it bristles all over the outside remind one of certain cromlechs of olden time, of certain tumuli whose domes are strewn with Cyclopean stones.
Such is the appearance of the edifice when the cell stands alone; but the Hymenopteron nearly always fixes other domes against her first, to the number of five, six, or more. This shortens the labour by allowing her to use the same partition for two adjoining rooms. The original elegant symmetry is lost and the whole now forms a cluster which, at first sight, appears to be merely a clod of dry mud, sprinkled with tiny pebbles. But let us examine the shapeless mass more closely and we shall perceive the number of chambers composing the habitation with the funnelled mouths, each quite distinct and each furnished with its gravel stopper set in the cement.
The Chalicodoma of the Walls employs the same building methods as Eumenes Amedei: in the courses of cement she fixes, on the outside, small stones of minor bulk. Her work begins by being a turret of rustic art, not without a certain prettiness; then, when the cells are placed side by side, the whole construction degenerates into a lump governed apparently by no architectural rule. Moreover, the Mason-bee covers her mass of cells with a thick layer of cement, which conceals the original rockwork edifice. The Eumenes does not resort to this general coating: her building is too strong to need it; she leaves the pebbly facings uncovered, as well as the entrances to the cells. The two sorts of nests, although constructed of similar materials, are therefore easily distinguished.
The Eumenes' cupola is the work of an artist; and the artist would be sorry to cover his masterpiece with whitewash. I crave forgiveness for a suggestion which I advance with all the reserve befitting so delicate a subject. Would it not be possible for the cromlech-builder to take a pride in her work, to look upon it with some affection and to feel gratified by this evidence of her cleverness? Might there not be an insect science of aesthetics? I seem at least to catch a glimpse, in the Eumenes, of a propensity to beautify her work. The nest must be, before all, a solid habitation, an inviolable stronghold; but, should ornament intervene without jeopardizing the power of resistance, will the worker remain indifferent to it? Who would say?
Let us set forth the facts. The orifice at the top, if left as a mere hole, would suit the purpose quite as well as an elaborate door: the insect would lose nothing in regard to facilities for coming and going and would gain by shortening the labour. Yet we find, on the contrary, the mouth of an amphora, gracefully curved, worthy of a potter's wheel. A choice cement and careful work are necessary for the confection of its slender, funnelled shaft. Why this nice finish, if the builder be wholly absorbed in the solidity of her work?
Here is another detail: among the bits of gravel employed for the outer covering of the cupola, grains of quartz predominate. They are polished and translucent; they glitter slightly and please the eye. Why are these little pebbles preferred to chips of lime-stone, when both materials are found in equal abundance around the nest?
A yet more remarkable feature: we find pretty often, encrusted on the dome, a few tiny, empty snail-shells, bleached by the sun. The species usually selected by the Eumenes is one of the smaller Helices--Helix strigata--frequent on our parched slopes. I have seen nests where this Helix took the place of pebbles almost entirely. They were like boxes made of shells, the work of a patient hand.
A comparison offers here. Certain Australian birds, notably the Bower-birds, build themselves covered walks, or playhouses, with interwoven twigs, and decorate the two entrances to the portico by strewing the threshold with anything that they can find in the shape of glittering, polished, or bright-coloured objects. Every door-sill is a cabinet of curiosities where the collector gathers smooth pebbles, variegated shells, empty snail-shells, parrot's feathers, bones that have come to look like sticks of ivory. The odds and ends mislaid by man find a home in the bird's museum, where we see pipe-stems, metal buttons, strips of cotton stuff and stone axe-heads.
The collection at either entrance to the bower is large enough to fill half a bushel. As these objects are of no use to the bird, its only motive for accumulating them must be an art-lover's hobby. Our common Magpie has similar tastes: any shiny thing that he comes upon he picks up, hides and hoards.
Well, the Eumenes, who shares this passion for bright pebbles and empty snail-shells, is the Bower-bird of the insect world; but she is a more practical collector, knows how to combine the useful and the ornamental and employs her finds in the construction of her nest, which is both a fortress and a museum. When she finds nodules of translucent quartz, she rejects everything else: the building will be all the prettier for them. When she comes across a little white shell, she hastens to beautify her dome with it; should fortune smile and empty snail-shells abound, she encrusts the whole fabric with them, until it becomes the supreme expression of her artistic taste. Is this so? Or is it not so? Who shall decide?
The nest of Eumenes pomiformis is the size of an average cherry and constructed of pure mortar, without the least outward pebblework. Its shape is exactly similar to that which we have just described. When built upon a horizontal base of sufficient extent, it is a dome with a central neck, funnelled like the mouth of an urn. But when the foundation is reduced to a mere point, as on the twig of a shrub, the nest becomes a spherical capsule, always, of course, surmounted by a neck. It is then a miniature specimen of exotic pottery, a paunchy alcarraza. Its thickens is very slight, less than that of a sheet of paper; it crushes under the least effort of the fingers. The outside is not quite even. It displays wrinkles and seams, due to the different courses of mortar, or else knotty protuberances distributed almost concentrically.
Both Hymenoptera accumulate caterpillars in their coffers, whether domes or jars. Let us give an abstract of the bill of fare. These documents, for all their dryness, possess a value; they will enable whoso cares to interest himself in the Eumenes to perceive to what extent instinct varies the diet, according to the place and season. The food is plentiful, but lacks variety. It consists of tiny caterpillars, by which I mean the grubs of small Butterflies. We learn this from the structure, for we observe in the prey selected by either Hymenopteran the usual caterpillar organism. The body is composed of twelve segments, not including the head. The first three have true legs, the next two are legless, then come two segments with prolegs, two legless segments and, lastly, a terminal segment with prolegs. It is exactly the same structure which we saw in the Ammophila's Grey Worm.
My old notes give the following description of the caterpillars found in the nest of Eumenes Amedei: „a pale green or, less often, a yellowish body, covered with short white hairs; head wider than the front segment, dead-black and also bristling with hairs. Length: 16 to 18 millimetres (7); width: about 3 millimetres“ (8). A quarter of a century and more has elapsed since I jotted down this descriptive sketch; and to-day, at S?rignan, I find in the Eumenes' larder the same game which I noticed long ago at Carpentras. Time and distance have not altered the nature of the provisions.
The number of morsels served for the meal of each larva interests us more than the quality. In the cells of Eumenes Amedei, I find sometimes five caterpillars and sometimes ten, which means a difference of a hundred per cent in the quantity of the food, for the morsels are of exactly the same size in both cases. Why this unequal supply, which gives a double portion to one larva and a single portion to another? The diners have the same appetite: what one nurseling demands a second must demand, unless we have here a different menu, according to the sexes. In the perfect stage the males are smaller than the females, are hardly half as much in weight or volume. The amount of victuals, therefore, required to bring them to their final development may be reduced by one-half. In that case, the well-stocked cells belong to females; the others, more meagrely supplied, belong to males.
But the egg is laid when the provisions are stored; and this egg has a determined sex, though the most minute examination is not able to discover the differences which will decide the hatching of a female or a male. We are therefore needs driven to this strange conclusion: the mother knows beforehand the sex of the egg which she is about to lay; and this knowledge allows her to fill the larder according to the appetite of the future grub. What a strange world, so wholly different from ours! We fall back upon a special sense to explain the Ammophila's hunting; what can we fall back upon to account for this intuition of the future? Can the theory of chances play a part in the hazy problem? If nothing is logically arranged with a foreseen object, how is this clear vision of the invisible acquired?
The capsules of Eumenes pomiformis are literally crammed with game. It is true that the morsels are very small. My notes speak of fourteen green caterpillars in one cell and sixteen in a second cell. I have no other information about the integral diet of this Wasp, whom I have neglected somewhat, preferring to study her cousin, the builder of rockwork domes. As the two sexes differ in size, although to a lesser degree than in the case of Eumenes Amedei, I am inclined to think that those two well-filled cells belonged to females and that the males' cells must have a less sumptuous table. Not having seen for myself, I am content to set down this mere suspicion.
What I have seen and often seen is the pebbly nest, with the larva inside and the provisions partly consumed. To continue the rearing at home and follow my charge's progress from day to day was a business which I could not resist; besides, as far as I was able to see, it was easily managed. I had had some practice in this foster-father's trade; my association with the Bembex, the Ammophila, the Sphex (9) and many others had turned me into a passable insect-rearer. I was no novice in the art of dividing an old pen-box into compartments in which I laid a bed of sand and, on this bed, the larva and her provisions delicately removed from the maternal cell. Success was almost certain at each attempt: I used to watch the larvae at their meals, I saw my nurselings grow up and spin their cocoons. Relying upon the experience thus gained, I reckoned upon success in raising my Eumenes.
The results, however, in no way answered to my expectations. All my endeavours failed; and the larva allowed itself to die a piteous death without touching its provisions.
I ascribed my reverse to this, that and the other cause: perhaps I had injured the frail grub when demolishing the fortress; a splinter of masonry had bruised it when I forced open the hard dome with my knife; a too sudden exposure to the sun had surprised it when I withdrew it from the darkness of its cell; the open air might have dried up its moisture. I did the best I could to remedy all these probable reasons of failure. I went to work with every possible caution in breaking open the home; I cast the shadow of my body over the nest, to save the grub from sunstroke; I at once transferred larva and provisions into a glass tube and placed this tube in a box which I carried in my hand, to minimize the jolting on the journey. Nothing was of avail: the larva, when taken from its dwelling, always allowed itself to pine away.
For a long time I persisted in explaining my want of success by the difficulties attending the removal. Eumenes Amedei's cell is a strong casket which cannot be forced without sustaining a shock; and the demolition of a work of this kind entails such varied accidents that we are always liable to think that the worm has been bruised by the wreckage. As for carrying home the nest intact on its support, with a view to opening it with greater care than is permitted by a rough-and-ready operation in the fields, that is out of the question: the nest nearly always stands on an immovable rock or on some big stone forming part of a wall. If I failed in my attempts at rearing, it was because the larva had suffered when I was breaking up her house. The reason seemed a good one; and I let it go at that.
In the end, another idea occurred to me and made me doubt whether my rebuffs were always due to clumsy accidents. The Eumenes' cells are crammed with game: there are ten caterpillars in the cell of Eumenes Amedei and fifteen in that of Eumenes pomiformis. These caterpillars, stabbed no doubt, but in a manner unknown to me, are not entirely motionless. The mandibles seize upon what is presented to them, the body buckles and unbuckles, the hinder half lashes out briskly when stirred with the point of a needle. At what spot is the egg laid amid that swarming mass, where thirty mandibles can make a hole in it, where a hundred and twenty pairs of legs can tear it? When the victuals consist of a single head of game, these perils do not exist; and the egg is laid on the victim not at hazard, but upon a judiciously chosen spot. Thus, for instance, Ammophila hirsuta fixes hers, by one end, cross-wise, on the Grey Worm, on the side of the first prolegged segment. The eggs hang over the caterpillar's back, away from the legs, whose proximity might be dangerous. The worm, moreover, stung in the greater number of its nerve-centres, lies on one side, motionless and incapable of bodily contortions or said an jerks of its hinder segments. If the mandibles try to snap, if the legs give a kick or two, they find nothing in front of them: the Ammophila's egg is at the opposite side. The tiny grub is thus able, as soon as it hatches, to dig into the giant's belly in full security.
How different are the conditions in the Eumenes' cell. The caterpillars are imperfectly paralysed, perhaps because they have received but a single stab; they toss about when touched with a pin; they are bound to wriggle when bitten by the larva. If the egg is laid on one of them, the first morsel will, I admit, be consumed without danger, on condition that the point of attack be wisely chosen; but there remain others which are not deprived of every means of defence. Let a movement take place in the mass; and the egg, shifted from the upper layer, will tumble into a pitfall of legs and mandibles. The least thing is enough to jeopardize its existence; and this least thing has every chance of being brought about in the disordered heap of caterpillars. The egg, a tiny cylinder, transparent as crystal, is extremely delicate: a touch withers it, the least pressure crushes it.
No, its place is not in the mass of provisions, for the caterpillars, I repeat, are not sufficiently harmless. Their paralysis is incomplete, as is proved by their contortions when I irritate them and shown, on the other hand, by a very important fact. I have sometimes taken from Eumenes Amedei's cell a few heads of game half transformed into chrysalids. It is evident that the transformation was effected in the cell itself and, therefore, after the operation which the Wasp had performed upon them. Whereof does this operation consist? I cannot say precisely, never having seen the huntress at work. The sting most certainly has played its part; but where? And how often? This is what we do not know. What we are able to declare is that the torpor is not very deep, inasmuch as the patient sometimes retains enough vitality to shed its skin and become a chrysalid. Everything thus tends to make us ask by what stratagem the egg is shielded from danger.
This stratagem I longed to discover; I would not be put off by the scarcity of nests, by the irksomeness of the searches, by the risk of sunstroke, by the time taken up, by the vain breaking open of unsuitable cells; I meant to see and I saw. Here is my method: with the point of a knife and a pair of nippers, I make a side opening, a window, beneath the dome of Eumenes Amedei and Eumenes pomiformis. I work with the greatest care, so as not to injure the recluse. Formerly I attacked the cupola from the top, now I attack it from the side. I stop when the breach is large enough to allow me to see the state of things within.
What is this state of things? I pause to give the reader time to reflect and to think out for himself a means of safety that will protect the egg and afterwards the grub in the perilous conditions which I have set forth. Seek, think and contrive, such of you as have inventive minds. Have you guessed it? Do you give it up? I may as well tell you.
The egg is not laid upon the provisions; it is hung from the top of the cupola by a thread which vies with that of a Spider's web for slenderness. The dainty cylinder quivers and swings to and fro at the least breath; it reminds me of the famous pendulum suspended from the dome of the Pantheon to prove the rotation of the earth. The victuals are heaped up underneath.
Second act of this wondrous spectacle. In order to witness it, we must open a window in cell upon cell until fortune deigns to smile upon us. The larva is hatched and already fairly large. Like the egg, it hangs perpendicularly, by the rear, from the ceiling; but the suspensory cord has gained considerably in length and consists of the original thread eked out by a sort of ribbon. The grub is at dinner: head downwards, it is digging into the limp belly of one of the caterpillars. I touch up the game that is still intact with a straw. The caterpillars grow restless. The grub forthwith retires from the fray. And how? Marvel is added to marvels: what I took for a flat cord, for a ribbon, at the lower end of the suspensory thread, is a sheath, a scabbard, a sort of ascending gallery wherein the larva crawls backwards and makes its way up. The cast shell of the egg, retaining its cylindrical form and perhaps lengthened by a special operation on the part of the new-born grub, forms this safety-channel. At the least sign of danger in the heap of caterpillars, the larva retreats into its sheath and climbs back to the ceiling, where the swarming rabble cannot reach it. When peace is restored, it slides down its case and returns to table, with its head over the viands and its rear upturned and ready to withdraw in case of need.
Third and last act. Strength has come; the larva is brawny enough not to dread the movements of the caterpillars' bodies. Besides, the caterpillars, mortified by fasting and weakened by a prolonged torpor, become more and more incapable of defence. The perils of the tender babe are succeeded by the security of the lusty stripling; and the grub, henceforth scorning its sheathed lift, lets itself drop upon the game that remains. And thus the banquet ends in normal fashion.
That is what I saw in the nests of both species of the Eumenes and that is what I showed to friends who were even more surprised than I by these ingenious tactics. The egg hanging from the ceiling, at a distance from the provisions, has naught to fear from the caterpillars, which flounder about below. The new-hatched larva, whose suspensory cord is lengthened by the sheath of the egg, reaches the game and takes a first cautious bite at it. If there be danger, it climbs back to the ceiling by retreating inside the scabbard. This explains the failure of my earlier attempts. Not knowing of the safety-thread, so slender and so easily broken, I gathered at one time the egg, at another the young larva, after my inroads at the top had caused them to fall into the middle of the live victuals. Neither of them was able to thrive when brought into direct contact with the dangerous game.
If any one of my readers, to whom I appealed just now, has thought out something better than the Eumenes' invention, I beg that he will let me know: there is a curious parallel to be drawn between the inspirations of reason and the inspirations of instinct.
1. Ammophila hirsuta, who hunts the Grey Worm, the caterpillar of Noctua segetum, the Dart or Turnip Moth.
2. Marcus Vitruvius Pollio, the Roman architect and engineer.
3. Or Mason-bee.
4. .97 inch.
5. .78 inch.
6. .118 inch.
7. .63 to .7 inch.
8. .12 inch.
9. three species of Digger-wasps.