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. (Ammophila hirsuta, who hunts the Grey Worm, the caterpillar of Noctua segetum, the Dart or Turnip Moth.—Translator's Note.) 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. (Marcus Vitruvius Pollio, the Roman architect and engineer.—Translator's Note.)
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. (Or Mason-bee.—Translator's Note.)
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 (.97 inch.—Translator's Note.), more or less, represent the diameter, and two centimetres the height. (.78 inch.—Translator's Note.) 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. (.118 inch.—Translator's Note.) 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 (.63 to .7 inch.—Translator's Note.); width: about 3 millimetres." (.12 inch.—Translator's Note.) A quarter of a century and more has elapsed since I jotted down this descriptive sketch; and to-day, at Sérignan, 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 (three species of Digger-wasps.—Translator's Note.) 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.
THEIR HABITS.
February has its sunny days, heralding spring, to which rude winter will reluctantly yield place. In snug corners, among the rocks, the great spurge of our district, the characias of the Greeks, the jusclo of the Provençals, begins to lift its drooping inflorescence and discreetly opens a few sombre flowers. Here the first midges of the year will come to slake their thirst. By the time that the tip of the stalks reaches the perpendicular, the worst of the cold weather will be over.
Another eager one, the almond-tree, risking the loss of its fruit, hastens to echo these preludes to the festival of the sun, preludes which are too often treacherous. A few days of soft skies and it becomes a glorious dome of white flowers, each twinkling with a roseate eye. The country, which still lacks green, seems dotted everywhere with white-satin pavilions. 'Twould be a callous heart indeed that could resist the magic of this awakening.
The insect nation is represented at these rites by a few of its more zealous members. There is first of all the Honey-bee, the sworn enemy of strikes, who profits by the least lull of winter to find out if some rosemary or other is not beginning to open somewhere near the hive. The droning of the busy swarms fills the flowery vault, while a snow of petals falls softly to the foot of the tree.
Together with the population of harvesters there mingles another, less numerous, of mere drinkers, whose nesting-time has not yet begun. This is the colony of the Osmiae, those exceedingly pretty solitary bees, with their copper-coloured skin and bright-red fleece. Two species have come hurrying up to take part in the joys of the almond-tree: first, the Horned Osmia, clad in black velvet on the head and breast, with red velvet on the abdomen; and, a little later, the Three-horned Osmia, whose livery must be red and red only. These are the first delegates despatched by the pollen-gleaners to ascertain the state of the season and attend the festival of the early blooms.
'Tis but a moment since they burst their cocoon, the winter abode: they have left their retreats in the crevices of the old walls; should the north wind blow and set the almond-tree shivering, they will hasten to return to them. Hail to you, O my dear Osmiae, who yearly, from the far end of the harmas, opposite snow-capped Ventoux (A mountain in the Provençal Alps, near Carpentras and Sérignan 6,271 feet.—Translator's Note.), bring me the first tidings of the awakening of the insect world! I am one of your friends; let us talk about you a little.
Most of the Osmiae of my region do not themselves prepare the dwelling destined for the laying. They want ready-made lodgings, such as the old cells and old galleries of Anthophorae and Chalicodomae. If these favourite haunts are lacking, then a hiding-place in the wall, a round hole in some bit of wood, the tube of a reed, the spiral of a dead Snail under a heap of stones are adopted, according to the tastes of the several species. The retreat selected is divided into chambers by partition-walls, after which the entrance to the dwelling receives a massive seal. That is the sum-total of the building done.
For this plasterer's rather than mason's work, the Horned and the Three-horned Osmia employ soft earth. This material is a sort of dried mud, which turns to pap on the addition of a drop of water. The two Osmiae limit themselves to gathering natural soaked earth, mud in short, which they allow to dry without any special preparation on their part; and so they need deep and well-sheltered retreats, into which the rain cannot penetrate, or the work would fall to pieces.
Latreille's Osmia uses different materials for her partitions and her doors. She chews the leaves of some mucilaginous plant, some mallow perhaps, and then prepares a sort of green putty with which she builds her partitions and finally closes the entrance to the dwelling. When she settles in the spacious cells of the Masked Anthophora (Anthophora personata, Illig.), the entrance to the gallery, which is wide enough to admit a man's finger, is closed with a voluminous plug of this vegetable paste. On the earthy banks, hardened by the sun, the home is then betrayed by the gaudy colour of the lid. It is as though the authorities had closed the door and affixed to it their great seals of green wax.
So far then as their building-materials are concerned, the Osmiae whom I have been able to observe are divided into two classes: one building compartments with mud, the other with a green-tinted vegetable putty. To the latter belongs Latreille's Osmia. The first section includes the Horned Osmia and the Three-horned Osmia, both so remarkable for the horny tubercles on their faces.
The great reed of the south, Arundo donax, is often used, in the country, for making rough garden-shelters against the mistral or just for fences. These reeds, the ends of which are chopped off to make them all the same length, are planted perpendicularly in the earth. I have often explored them in the hope of finding Osmia-nests. My search has very seldom succeeded. The failure is easily explained. The partitions and the closing-plug of the Horned and of the Three-horned Osmia are made, as we have seen, of a sort of mud which water instantly reduces to pap. With the upright position of the reeds, the stopper of the opening would receive the rain and would become diluted; the ceilings of the storeys would fall in and the family would perish by drowning. Therefore the Osmia, who knew of these drawbacks before I did, refuses the reeds when they are placed perpendicularly.
The same reed is used for a second purpose. We make canisses of it, that is to say, hurdles, which, in spring, serve for the rearing of Silkworms and, in autumn, for the drying of figs. At the end of April and during May, which is the time when the Osmiae work, the canisses are indoors, in the Silkworm nurseries, where the Bee cannot take possession of them; in autumn, they are outside, exposing their layers of figs and peeled peaches to the sun; but by that time the Osmiae have long disappeared. If, however, during the spring, an old, disused hurdle is left out of doors, in a horizontal position, the Three-horned Osmia often takes possession of it and makes use of the two ends, where the reeds lie truncated and open.
There are other quarters that suit the Three-horned Osmia, who is not particular, it seems to me, and will make shift with any hiding-place, so long as it have the requisite conditions of diameter, solidity, sanitation and kindly darkness. The most original dwellings that I know her to occupy are disused Snail-shells, especially the house of the Common Snail (Helix aspersa). Let us go to the slope of the hills thick with olive-trees and inspect the little supporting-walls which are built of dry stones and face the south. In the crevices of this insecure masonry we shall reap a harvest of old Snail-shells, plugged with earth right up to the orifice. The family of the Three-horned Osmia is settled in the spiral of those shells, which is subdivided into chambers by mud partitions.
The Three-pronged Osmia (O. Tridentata, Duf. and Per.) alone creates a home of her own, digging herself a channel with her mandibles in dry bramble and sometimes in danewort.
The Osmia loves mystery. She wants a dark retreat, hidden from the eye. I would like, nevertheless, to watch her in the privacy of her home and to witness her work with the same facility as if she were nest-building in the open air. Perhaps there are some interesting characteristics to be picked up in the depths of her retreats. It remains to be seen whether my wish can be realized.
When studying the insect's mental capacity, especially its very retentive memory for places, I was led to ask myself whether it would not be possible to make a suitably-chosen Bee build in any place that I wished, even in my study. And I wanted, for an experiment of this sort, not an individual but a numerous colony. My preference lent towards the Three-horned Osmia, who is very plentiful in my neighbourhood, where, together with Latreille's Osmia, she frequents in particular the monstrous nests of the Chalicodoma of the Sheds. I therefore thought out a scheme for making the Three-horned Osmia accept my study as her settlement and build her nest in glass tubes, through which I could easily watch the progress. To these crystal galleries, which might well inspire a certain distrust, were to be added more natural retreats: reeds of every length and thickness and disused Chalicodoma-nests taken from among the biggest and the smallest. A scheme like this sounds mad. I admit it, while mentioning that perhaps none ever succeeded so well with me. We shall see as much presently.
My method is extremely simple. All I ask is that the birth of my insects, that is to say, their first seeing the light, their emerging from the cocoon, should take place on the spot where I propose to make them settle. Here there must be retreats of no matter what nature, but of a shape similar to that in which the Osmia delights. The first impressions of sight, which are the most long-lived of any, shall bring back my insects to the place of their birth. And not only will the Osmiae return, through the always open windows, but they will also nidify on the natal spot, if they find something like the necessary conditions.
And so, all through the winter, I collect Osmia-cocoons picked up in the nests of the Mason-bee of the Sheds; I go to Carpentras to glean a more plentiful supply in the nests of the Anthophora. I spread out my stock in a large open box on a table which receives a bright diffused light but not the direct rays of the sun. The table stands between two windows facing south and overlooking the garden. When the moment of hatching comes, those two windows will always remain open to give the swarm entire liberty to go in and out as it pleases. The glass tubes and reed-stumps are laid here and there, in fine disorder, close to the heaps of cocoons and all in a horizontal position, for the Osmia will have nothing to do with upright reeds. Although such a precaution is not indispensable, I take care to place some cocoons in each cylinder. The hatching of some of the Osmiae will therefore take place under cover of the galleries destined to be the building-yard later; and the site will be all the more deeply impressed on their memory. When I have made these comprehensive arrangements, there is nothing more to be done; and I wait patiently for the building-season to open.
My Osmiae leave their cocoons in the second half of April. Under the immediate rays of the sun, in well-sheltered nooks, the hatching would occur a month earlier, as we can see from the mixed population of the snowy almond-tree. The constant shade in my study has delayed the awakening, without, however, making any change in the nesting-period, which synchronizes with the flowering of the thyme. We now have, around my working-table, my books, my jars and my various appliances, a buzzing crowd that goes in and out of the windows at every moment. I enjoin the household henceforth not to touch a thing in the insects' laboratory, to do no more sweeping, no more dusting. They might disturb a swarm and make it think that my hospitality was not to be trusted. During four or five weeks I witness the work of a number of Osmiae which is much too large to allow my watching their individual operations. I content myself with a few, whom I mark with different-coloured spots to distinguish them; and I take no notice of the others, whose finished work will have my attention later.
The first to appear are the males. If the sun is bright, they flutter around the heap of tubes as if to take careful note of the locality; blows are exchanged and the rival swains indulge in mild skirmishing on the floor, then shake the dust off their wings. They fly assiduously from tube to tube, placing their heads in the orifices to see if some female will at last make up her mind to emerge.
One does, in point of fact. She is covered with dust and has the disordered toilet that is inseparable from the hard work of the deliverance. A lover has seen her, so has a second, likewise a third. All crowd round her. The lady responds to their advances by clashing her mandibles, which open and shut rapidly, several times in succession. The suitors forthwith fall back; and they also, no doubt to keep up their dignity, execute savage mandibular grimaces. Then the beauty retires into the arbour and her wooers resume their places on the threshold. A fresh appearance of the female, who repeats the play with her jaws; a fresh retreat of the males, who do the best they can to flourish their own pincers. The Osmiae have a strange way of declaring their passion: with that fearsome gnashing of their mandibles, the lovers look as though they meant to devour each other. It suggests the thumps affected by our yokels in their moments of gallantry.
The ingenuous idyll is soon over. The females, who grow more numerous from day to day, inspect the premises; they buzz outside the glass galleries and the reed dwellings; they go in, stay for a while, come out, go in again and then fly away briskly into the garden. They return, first one, then another. They halt outside, in the sun, or on the shutters fastened back against the wall; they hover in the window-recess, come inside, go to the reeds and give a glance at them, only to set off again and to return soon after. Thus do they learn to know their home, thus do they fix their birthplace in their memory. The village of our childhood is always a cherished spot, never to be effaced from our recollection. The Osmia's life endures for a month; and she acquires a lasting remembrance of her hamlet in a couple of days. 'Twas there that she was born; 'twas there that she loved; 'tis there that she will return. Dulces reminiscitur Argos.
(Now falling by another's wound, his eyes He casts to heaven, on Argos thinks and dies. —"Aeneid" Book 10, Dryden's translation.)
At last each has made her choice. The work of construction begins; and my expectations are fulfilled far beyond my wishes. The Osmiae build nests in all the retreats which I have placed at their disposal. And now, O my Osmiae, I leave you a free field!
The work begins with a thorough spring-cleaning of the home. Remnants of cocoons, dirt consisting of spoilt honey, bits of plaster from broken partitions, remains of dried Mollusc at the bottom of a shell: these and much other insanitary refuse must first of all disappear. Violently the Osmia tugs at the offending object and tears it out; and then off she goes in a desperate hurry, to dispose of it far away from the study. They are all alike, these ardent sweepers: in their excessive zeal, they fear lest they should block up the speck of dust which they might drop in front of the new house. The glass tubes, which I myself have rinsed under the tap, are not exempt from a scrupulous cleaning. The Osmia dusts them, brushes them thoroughly with her tarsi and then sweeps them out backwards. What does she pick up? Not a thing. It makes no difference: as a conscientious housewife, she gives the place a touch of the broom nevertheless.
Now for the provisions and the partition-walls. Here the order of the work changes according to the diameter of the cylinder. My glass tubes vary greatly in dimensions. The largest have an inner width of a dozen millimetres (Nearly half an inch.—Translator's Note.); the narrowest measure six or seven. (About a quarter of an inch.—Translator's Note.) In the latter, if the bottom suit her, the Osmia sets to work bringing pollen and honey. If the bottom do not suit her, if the sorghum-pith plug with which I have closed the rear-end of the tube be too irregular and badly-joined, the Bee coats it with a little mortar. When this small repair is made, the harvesting begins.
In the wider tubes, the work proceeds quite differently. At the moment when the Osmia disgorges her honey and especially at the moment when, with her hind-tarsi, she rubs the pollen-dust from her ventral brush, she needs a narrow aperture, just big enough to allow of her passage. I imagine that in a straitened gallery the rubbing of her whole body against the sides gives the harvester a support for her brushing-work. In a spacious cylinder this support fails her; and the Osmia starts with creating one for herself, which she does by narrowing the channel. Whether it be to facilitate the storing of the victuals or for any other reason, the fact remains that the Osmia housed in a wide tube begins with the partitioning.
Her division is made by a dab of clay placed at right angles to the axis of the cylinder, at a distance from the bottom determined by the ordinary length of a cell. The wad is not a complete round; it is more crescent-shaped, leaving a circular space between it and one side of the tube. Fresh layers are swiftly added to the dab of clay; and soon the tube is divided by a partition which has a circular opening at the side of it, a sort of dog-hole through which the Osmia will proceed to knead the Bee-bread. When the victualling is finished and the egg laid upon the heap, the whole is closed and the filled-up partition becomes the bottom of the next cell. Then the same method is repeated, that is to say, in front of the just completed ceiling a second partition is built, again with a side-passage, which is stouter, owing to its distance from the centre, and better able to withstand the numerous comings and goings of the housewife than a central orifice, deprived of the direct support of the wall, could hope to be. When this partition is ready, the provisioning of the second cell is effected; and so on until the wide cylinder is completely stocked.
The building of this preliminary party-wall, with a narrow, round dog-hole, for a chamber to which the victuals will not be brought until later is not restricted to the Three-horned Osmia; it is also frequently found in the case of the Horned Osmia and of Latreille's Osmia. Nothing could be prettier than the work of the last-named, who goes to the plants for her material and fashions a delicate sheet in which she cuts a graceful arch. The Chinaman partitions his house with paper screens; Latreille's Osmia divides hers with disks of thin green cardboard perforated with a serving-hatch which remains until the room is completely furnished. When we have no glass houses at our disposal, we can see these little architectural refinements in the reeds of the hurdles, if we open them at the right season.
By splitting the bramble-stumps in the course of July, we perceive also that the Three-pronged Osmia notwithstanding her narrow gallery, follows the same practice as Latreille's Osmia, with a difference. She does not build a party-wall, which the diameter of the cylinder would not permit; she confines herself to putting up a frail circular pad of green putty, as though to limit, before any attempt at harvesting, the space to be occupied by the Bee-bread, whose depth could not be calculated afterwards if the insect did not first mark out its confines.
If, in order to see the Osmia's nest as a whole, we split a reed lengthwise, taking care not to disturb its contents; or, better still, if we select for examination the string of cells built in a glass tube, we are forthwith struck by one detail, namely, the uneven distances between the partitions, which are placed almost at right angles to the axis of the cylinder. It is these distances which fix the size of the chambers, which, with a similar base, have different heights and consequently unequal holding-capacities. The bottom partitions, the oldest, are farther apart; those of the front part, near the orifice, are closer together. Moreover, the provisions are plentiful in the loftier cells, whereas they are niggardly and reduced to one-half or even one-third in the cells of lesser height. Let me say at once that the large cells are destined for the females and the small ones for the males.
DISTRIBUTION OF THE SEXES.
Does the insect which stores up provisions proportionate to the needs of the egg which it is about to lay know beforehand the sex of that egg? Or is the truth even more paradoxical? What we have to do is to turn this suspicion into a certainty demonstrated by experiment. And first let us find out how the sexes are arranged.
It is not possible to ascertain the chronological order of a laying, except by going to suitably-chosen species. Fortunately there are a few species in which we do not find this difficulty: these are the Bees who keep to one gallery and build their cells in storeys. Among the number are the different inhabitants of the bramble-stumps, notably the Three-pronged Osmiae, who form an excellent subject for observation, partly because they are of imposing size—bigger than any other bramble-dwellers in my neighbourhood—partly because they are so plentiful.
Let us briefly recall the Osmia's habits. Amid the tangle of a hedge, a bramble-stalk is selected, still standing, but a mere withered stump. In this the insect digs a more or less deep tunnel, an easy piece of work owing to the abundance of soft pith. Provisions are heaped up right at the bottom of the tunnel and an egg is laid on the surface of the food: that is the first-born of the family. At a height of some twelve millimetres (About half an inch.—Translator's Note.), a partition is fixed. This gives a second storey, which in its turn receives provisions and an egg, the second in order of primogeniture. And so it goes on, storey by storey, until the cylinder is full. Then the thick plug of the same green material of which the partitions are formed closes the home and keeps out marauders.
In this common cradle, the chronological order of births is perfectly clear. The first-born of the family is at the bottom of the series; the last-born is at the top, near the closed door. The others follow from bottom to top in the same order in which they followed in point of time. The laying is numbered automatically; each cocoon tells us its respective age by the place which it occupies.
A number of eggs bordering on fifteen represents the entire family of an Osmia, and my observations enable me to state that the distribution of the sexes is not governed by any rule. All that I can say in general is that the complete series begins with females and nearly always ends with males. The incomplete series—those which the insect has laid in various places—can teach us nothing in this respect, for they are only fragments starting we know not whence; and it is impossible to tell whether they should be ascribed to the beginning, to the end, or to an intermediate period of the laying. To sum up: in the laying of the Three-pronged Osmia, no order governs the succession of the sexes; only, the series has a marked tendency to begin with females and to finish with males.
The mother occupies herself at the start with the stronger sex, the more necessary, the better-gifted, the female sex, to which she devotes the first flush of her laying and the fullness of her vigour; later, when she is perhaps already at the end of her strength, she bestows what remains of her maternal solicitude upon the weaker sex, the less-gifted, almost negligible male sex. There are, however, other species where this law becomes absolute, constant and regular.
In order to go more deeply into this curious question I installed some hives of a new kind on the sunniest walls of my enclosure. They consisted of stumps of the great reed of the south, open at one end, closed at the other by the natural knot and gathered into a sort of enormous pan-pipe, such as Polyphemus might have employed. The invitation was accepted: Osmiae came in fairly large numbers, to benefit by the queer installation.
Three Osmiae especially (O. Tricornis, Latr., O. cornuta, Latr., O. Latreillii, Spin.) gave me splendid results, with reed-stumps arranged either against the wall of my garden, as I have just said, or near their customary abode, the huge nests of the Mason-bee of the Sheds. One of them, the Three-horned Osmia, did better still: as I have described, she built her nests in my study, as plentifully as I could wish.
We will consult this last, who has furnished me with documents beyond my fondest hopes, and begin by asking her of how many eggs her average laying consists. Of the whole heap of colonized tubes in my study, or else out of doors, in the hurdle-reeds and the pan-pipe appliances, the best-filled contains fifteen cells, with a free space above the series, a space showing that the laying is ended, for, if the mother had any more eggs available, she would have lodged them in the room which she leaves unoccupied. This string of fifteen appears to be rare; it was the only one that I found. My attempts at indoor rearing, pursued during two years with glass tubes or reeds, taught me that the Three-horned Osmia is not much addicted to long series. As though to decrease the difficulties of the coming deliverance, she prefers short galleries, in which only a part of the laying is stacked. We must then follow the same mother in her migration from one dwelling to the next if we would obtain a complete census of her family. A spot of colour, dropped on the Bee's thorax with a paint-brush while she is absorbed in closing up the mouth of the tunnel, enables us to recognize the Osmia in her various homes.
In this way, the swarm that resided in my study furnished me, in the first year, with an average of twelve cells. Next year, the summer appeared to be more favourable and the average became rather higher, reaching fifteen. The most numerous laying performed under my eyes, not in a tube, but in a succession of Snail-shells, reached the figure of twenty-six. On the other hand, layings of between eight and ten are not uncommon. Lastly, taking all my records together, the result is that the family of the Osmia fluctuates roundabout fifteen in number.
I have already spoken of the great differences in size apparent in the cells of one and the same series. The partitions, at first widely spaced, draw gradually nearer to one another as they come closer to the aperture, which implies roomy cells at the back and narrow cells in front. The contents of these compartments are no less uneven between one portion and another of the string. Without any exception known to me, the large cells, those with which the series starts, have more abundant provisions than the straitened cells with which the series ends. The heap of honey and pollen in the first is twice or even thrice as large as that in the second. In the last cells, the most recent in date, the victuals are but a pinch of pollen, so niggardly in amount that we wonder what will become of the larva with that meagre ration.
One would think that the Osmia, when nearing the end of the laying, attaches no importance to her last-born, to whom she doles out space and food so sparingly. The first-born receive the benefit of her early enthusiasm: theirs is the well-spread table, theirs the spacious apartments. The work has begun to pall by the time that the last eggs are laid; and the last-comers have to put up with a scurvy portion of food and a tiny corner.
The difference shows itself in another way after the cocoons are spun. The large cells, those at the back, receive the bulky cocoons; the small ones, those in front, have cocoons only half or a third as big. Before opening them and ascertaining the sex of the Osmia inside, let us wait for the transformation into the perfect insect, which will take place towards the end of summer. If impatience get the better of us, we can open them at the end of July or in August. The insect is then in the nymphal stage; and it is easy, under this form, to distinguish the two sexes by the length of the antennae, which are larger in the males, and by the glassy protuberances on the forehead, the sign of the future armour of the females. Well, the small cocoons, those in the narrow front cells, with their scanty store of provisions, all belong to males; the big cocoons, those in the spacious and well-stocked cells at the back, all belong to females.
The conclusion is definite: the laying of the Three-horned Osmia consists of two distinct groups, first a group of females and then a group of males.
With my pan-pipe apparatus displayed on the walls of my enclosure and with old hurdle-reeds left lying flat out of doors, I obtained the Horned Osmia in fair quantities. I persuaded Latreille's Osmia to build her nest in reeds, which she did with a zeal which I was far from expecting. All that I had to do was to lay some reed-stumps horizontally within her reach, in the immediate neighbourhood of her usual haunts, namely, the nests of the Mason-bee of the Sheds. Lastly, I succeeded without difficulty in making her build her nests in the privacy of my study, with glass tubes for a house. The result surpassed my hopes.
With both these Osmiae, the division of the gallery is the same as with the Three-horned Osmia. At the back are large cells with plentiful provisions and widely-spaced partitions; in front, small cells, with scanty provisions and partitions close together. Also, the larger cells supplied me with big cocoons and females; the smaller cells gave me little cocoons and males. The conclusion therefore is exactly the same in the case of all three Osmiae.
These conclusions, as my notes show, apply likewise, in every respect, to the various species of Mason-bees; and one clear and simple rule stands out from this collection of facts. Apart from the strange exception of the Three-pronged Osmia, who mixes the sexes without any order, the Bees whom I studied and probably a crowd of others produce first a continuous series of females and then a continuous series of males, the latter with less provisions and smaller cells. This distribution of the sexes agrees with what we have long known of the Hive-bee, who begins her laying with a long sequence of workers, or sterile females, and ends it with a long sequence of males. The analogy continues down to the capacity of the cells and the quantities of provisions. The real females, the Queen-bees, have wax cells incomparably more spacious than the cells of the males and receive a much larger amount of food. Everything therefore demonstrates that we are here in the presence of a general rule.
OPTIONAL DETERMINATION OF THE SEXES.
But does this rule express the whole truth? Is there nothing beyond a laying in two series? Are the Osmiae, the Chalicodomae and the rest of them fatally bound by this distribution of the sexes into two distinct groups, the male group following upon the female group, without any mixing of the two? Is the mother absolutely powerless to make a change in this arrangement, should circumstances require it?
The Three-pronged Osmia already shows us that the problem is far from being solved. In the same bramble-stump, the two sexes occur very irregularly, as though at random. Why this mixture in the series of cocoons of a Bee closely related to the Horned Osmia and the Three-horned Osmia, who stack theirs methodically by separate sexes in the hollow of a reed? What the Bee of the brambles does cannot her kinswomen of the reeds do too? Nothing, so far as I know, explains this fundamental difference in a physiological act of primary importance. The three Bees belong to the same genus; they resemble one another in general outline, internal structure and habits; and, with this close similarity, we suddenly find a strange dissimilarity.
There is just one thing that might possibly arouse a suspicion of the cause of this irregularity in the Three-pronged Osmia's laying. If I open a bramble-stump in the winter to examine the Osmia's nest, I find it impossible, in the vast majority of cases, to distinguish positively between a female and a male cocoon: the difference in size is so small. The cells, moreover, have the same capacity: the diameter of the cylinder is the same throughout and the partitions are almost always the same distance apart. If I open it in July, the victualling-period, it is impossible for me to distinguish between the provisions destined for the males and those destined for the females. The measurement of the column of honey gives practically the same depth in all the cells. We find an equal quantity of space and food for both sexes.
This result makes us foresee what a direct examination of the two sexes in the adult form tells us. The male does not differ materially from the female in respect of size. If he is a trifle smaller, it is scarcely noticeable, whereas, in the Horned Osmia and the Three-horned Osmia, the male is only half or a third the size of the female, as we have seen from the respective bulk of their cocoons. In the Mason-bee of the Walls there is also a difference in size, though less pronounced.
The Three-pronged Osmia has not therefore to trouble about adjusting the dimensions of the dwelling and the quantity of the food to the sex of the egg which she is about to lay; the measure is the same from one end of the series to the other. It does not matter if the sexes alternate without order: one and all will find what they need, whatever their position in the row. The two other Osmiae, with their great disparity in size between the two sexes, have to be careful about the twofold consideration of board and lodging.
The more I thought about this curious question, the more probable it appeared to me that the irregular series of the Three-pronged Osmia and the regular series of the other Osmiae and of the Bees in general were all traceable to a common law. It seemed to me that the arrangement in a succession first of females and then of males did not account for everything. There must be something more. And I was right: that arrangement in series is only a tiny fraction of the reality, which is remarkable in a very different way. This is what I am going to prove by experiment.
The succession first of females and then of males is not, in fact, invariable. Thus, the Chalicodoma, whose nests serve for two or three generations, ALWAYS lays male eggs in the old male cells, which can be recognized by their lesser capacity, and female eggs in the old female cells of more spacious dimensions.
This presence of both sexes at a time, even when there are but two cells free, one spacious and the other small, proves in the plainest fashion that the regular distribution observed in the complete nests of recent production is here replaced by an irregular distribution, harmonizing with the number and holding-capacity of the chambers to be stocked. The Mason-bee has before her, let me suppose, only five vacant cells: two larger and three smaller. The total space at her disposal would do for about a third of the laying. Well, in the two large cells, she puts females; in the three small cells she puts males.
As we find the same sort of thing in all the old nests, we must needs admit that the mother knows the sex of the eggs which she is going to lay, because that egg is placed in a cell of the proper capacity. We can go further, and admit that the mother alters the order of succession of the sexes at her pleasure, because her layings, between one old nest and another, are broken up into small groups of males and females according to the exigencies of space in the actual nest which she happens to be occupying.
Here then is the Chalicodoma, when mistress of an old nest of which she has not the power to alter the arrangement, breaking up her laying into sections comprising both sexes just as required by the conditions imposed upon her. She therefore decides the sex of the egg at will, for, without this prerogative, she could not, in the chambers of the nest which she owes to chance, deposit unerringly the sex for which those chambers were originally built; and this happens however small the number of chambers to be filled.
When the mother herself founds the dwelling, when she lays the first rows of bricks, the females come first and the males at the finish. But, when she is in the presence of an old nest, of which she is quite unable to alter the general arrangement, how is she to make use of a few vacant rooms, the large and small alike, if the sex of the egg be already irrevocably fixed? She can only do so by abandoning the arrangement in two consecutive rows and accommodating her laying to the varied exigencies of the home. Either she finds it impossible to make an economical use of the old nest, a theory refuted by the evidence, or else she determines at will the sex of the egg which she is about to lay.
The Osmiae themselves will furnish the most conclusive evidence on the latter point. We have seen that these Bees are not generally miners, who themselves dig out the foundation of their cells. They make use of the old structures of others, or else of natural retreats, such as hollow stems, the spirals of empty shells and various hiding-places in walls, clay or wood. Their work is confined to repairs to the house, such as partitions and covers. There are plenty of these retreats; and the insects would always find first-class ones if it thought of going any distance to look for them. But the Osmia is a stay-at-home: she returns to her birthplace and clings to it with a patience extremely difficult to exhaust. It is here, in this little familiar corner, that she prefers to settle her progeny. But then the apartments are few in number and of all shapes and sizes. There are long and short ones, spacious ones and narrow. Short of expatriating herself, a Spartan course, she has to use them all, from first to last, for she has no choice. Guided by these considerations, I embarked on the experiments which I will now describe.
I have said how my study became a populous hive, in which the Three-horned Osmia built her nests in the various appliances which I had prepared for her. Among these appliances, tubes, either of glass or reed, predominated. There were tubes of all lengths and widths. In the long tubes, entire or almost entire layings, with a series of females followed by a series of males, were deposited. As I have already referred to this result, I will not discuss it again. The short tubes were sufficiently varied in length to lodge one or other portion of the total laying. Basing my calculations on the respective lengths of the cocoons of the two sexes, on the thickness of the partitions and the final lid, I shortened some of these to the exact dimensions required for two cocoons only, of different sexes.
Well, these short tubes, whether of glass or reed, were seized upon as eagerly as the long tubes. Moreover, they yielded this splendid result: their contents, only a part of the total laying, always began with female and ended with male cocoons. This order was invariable; what varied was the number of cells in the long tubes and the proportion between the two sorts of cocoons, sometimes males predominating and sometimes females.
When confronted with tubes too small to receive all her family, the Osmia is in the same plight as the Mason-bee in the presence of an old nest. She thereupon acts exactly as the Chalicodoma does. She breaks up her laying, divides it into series as short as the room at her disposal demands; and each series begins with females and ends with males. This breaking up, on the one hand, into sections in all of which both sexes are represented and the division, on the other hand, of the entire laying into just two groups, one female, the other male, when the length of the tube permits, surely provide us with ample evidence of the insect's power to regulate the sex of the egg according to the exigencies of space.
And besides the exigencies of space one might perhaps venture to add those connected with the earlier development of the males. These burst their cocoons a couple of weeks or more before the females; they are the first who hasten to the sweets of the almond-tree. In order to release themselves and emerge into the glad sunlight without disturbing the string of cocoons wherein their sisters are still sleeping, they must occupy the upper end of the row; and this, no doubt, is the reason that makes the Osmia end each of her broken layings with males. Being next to the door, these impatient ones will leave the home without upsetting the shells that are slower in hatching.
I had offered at the same time to the Osmiae in my study some old nests of the Mason-bee of the Shrubs, which are clay spheroids with cylindrical cavities in them. These cavities are formed, as in the old nests of the Mason-bee of the Pebbles, of the cell properly so-called and of the exit-way which the perfect insect cut through the outer coating at the time of its deliverance. The diameter is about 7 millimetres (.273 inch.—Translator's Note.); their depth at the centre of the heap is 23 millimetres (.897 inch.—Translator's Note.) and at the edge averages 14 millimetres. (.546 inch.—Translator's Note.)
The deep central cells receive only the females of the Osmia; sometimes even the two sexes together, with a partition in the middle, the female occupying the lower and the male the upper storey. Lastly, the deeper cavities on the circumference are allotted to females and the shallower to males.
We know that the Three-horned Osmia prefers to haunt the habitations of the Bees who nidify in populous colonies, such as the Mason-bee of the Sheds and the Hairy-footed Anthophora, in whose nests I have noted similar facts.
Thus the sex of the egg is optional. The choice rests with the mother, who is guided by considerations of space and, according to the accommodation at her disposal, which is frequently fortuitous and incapable of modification, places a female in this cell and a male in that, so that both may have a dwelling of a size suited to their unequal development. This is the unimpeachable evidence of the numerous and varied facts which I have set forth. People unfamiliar with insect anatomy—the public for whom I write—would probably give the following explanation of this marvellous prerogative of the Bee: the mother has at her disposal a certain number of eggs, some of which are irrevocably female and the others irrevocably male: she is able to pick out of either group the one which she wants at the actual moment; and her choice is decided by the holding capacity of the cell that has to be stocked. Everything would then be limited to a judicious selection from the heap of eggs.
Should this idea occur to him, the reader must hasten to reject it. Nothing could be more false, as the most casual reference to anatomy will show. The female reproductive apparatus of the Hymenoptera consists generally of six ovarian tubes, something like glove-fingers, divided into bunches of three and ending in a common canal, the oviduct, which carries the eggs outside. Each of these glove-fingers is fairly wide at the base, but tapers sharply towards the tip, which is closed. It contains, arranged in a row, one after the other, like beads on a string, a certain number of eggs, five or six for instance, of which the lower ones are more or less developed, the middle ones halfway towards maturity, and the upper ones very rudimentary. Every stage of evolution is here represented, distributed regularly from bottom to top, from the verge of maturity to the vague outlines of the embryo. The sheath clasps its string of ovules so closely that any inversion of the order is impossible. Besides, an inversion would result in a gross absurdity: the replacing of a riper egg by another in an earlier stage of development.
Therefore, in each ovarian tube, in each glove-finger, the emergence of the eggs occurs according to the order governing their arrangement in the common sheath; and any other sequence is absolutely impossible. Moreover, at the nesting-period, the six ovarian sheaths, one by one and each in its turn, have at their base an egg which in a very short time swells enormously. Some hours or even a day before the laying, that egg by itself represents or even exceeds in bulk the whole of the ovigerous apparatus. This is the egg which is on the point of being laid. It is about to descend into the oviduct, in its proper order, at its proper time; and the mother has no power to make another take its place. It is this egg, necessarily this egg and no other, that will presently be laid upon the provisions, whether these be a mess of honey or a live prey; it alone is ripe, it alone lies at the entrance to the oviduct; none of the others, since they are farther back in the row and not at the right stage of development, can be substituted at this crisis. Its birth is inevitable.
What will it yield, a male or a female? No lodging has been prepared, no food collected for it; and yet both food and lodging have to be in keeping with the sex that will proceed from it. And here is a much more puzzling condition: the sex of that egg, whose advent is predestined, has to correspond with the space which the mother happens to have found for a cell. There is therefore no room for hesitation, strange though the statement may appear: the egg, as it descends from its ovarian tube, has no determined sex. It is perhaps during the few hours of its rapid development at the base of its ovarian sheath, it is perhaps on its passage through the oviduct that it receives, at the mother's pleasure, the final impress that will produce, to match the cradle which it has to fill, either a female or a male.
PERMUTATIONS OF SEX.
Thereupon the following question presents itself. Let us admit that, when the normal conditions remain, a laying would have yielded m females and n males. Then, if my conclusions are correct, it must be in the mother's power, when the conditions are different, to take from the m group and increase the n group to the same extent; it must be possible for her laying to be represented as m - 1, m - 2, m - 3, etc. females and by n + 1, n + 2, n + 3, etc. males, the sum of m + n remaining constant, but one of the sexes being partly permuted into the other. The ultimate conclusion even cannot be disregarded: we must admit a set of eggs represented by m - m, or zero, females and of n + m males, one of the sexes being completely replaced by the other. Conversely, it must be possible for the feminine series to be augmented from the masculine series to the extent of absorbing it entirely. It was to solve this question and some others connected with it that I undertook, for the second time, to rear the Three-horned Osmia in my study.