Chapter XI. The Leucopses
Jean-Henri Fabre Mason-bees
Mason Bee cocoons
Let us visit (1) the nests of Chalicodoma muraria in July, detaching them from their pebbles with a sideward blow, as I explained when telling the story of the Anthrax. The Mason-bee's cocoons with two inhabitants, one devouring, the other in process of being devoured, are numerous enough to allow me to gather some dozens in the course of a morning, before the sun becomes unbearably hot. We will give a smart tap to the flints so as to loosen the clay domes, wrap these up in newspapers, fill our box and go home as fast as we can, for the air will soon be as fiery as the devil's kitchen.
Inspection, which is easier in the shade indoors, soon tells us that, though the devoured is always the wretched Mason-bee, the devourer belongs to two different species. In the one case, the cylindrical form, the creamy-white colouring and the little nipple constituting the head reveal to us the larva of the Anthrax, which does not concern us at present; in the other, the general structure and appearance betray the grub of some Hymenopteron. The Mason's second exterminator is, in fact, a Leucopsis (Leucopsis gigas, FAB.), a magnificent insect, stripped black and yellow, with an abdomen rounded at the end and hollowed out, as is also the back, into a groove to contain a long rapier, as slender as a horsehair, which the creature unsheathes and drives through the mortar right into the cell where it proposes to establish its egg. Before occupying ourselves with its capacities as an inoculator, let us learn how its larva lives in the invaded cell.
It is a hairless, legless, sightless grub, easily confused, by inexperienced eyes, with those of various honey-gathering Hymenoptera. Its more apparent characteristics consist of a colouring like that of rancid butter, a shiny and as it were oily skin and a segmentation accentuated by a series of marked swellings, so that, when looked at from the side, the back is very plainly indented. When at rest, the larva is like a bow bending round at one point. It is made up of thirteen segments, including the head. This head, which is very small compared with the rest of the body, displays no mouth-part under the lens; at most you see a faint red streak, which calls for the microscope. You then distinguish two delicate mandibles, very short and fashioned into a sharp point. A small round mouth, with a fine piercer on the right and left, is all that the powerful instrument reveals. As for my best single magnifying-glasses, they show me nothing at all. On the other hand, we can quite easily, without arming the eye with a lens, perceive the mouth-apparatus — and particularly the mandibles — of either a honey-eater, such as an Osmia, Chalicodoma or Megachile, or a game-eater, such as a Scolia, Ammophila or Bembex. All these possess stout pincers, capable of gripping, grinding and tearing. Then what is the purpose of the Leucopsis' invisible implements? His method of consuming will tell us.
Like his prototype, the Anthrax, the Leucopsis does not eat the Chalicodoma-grub, that is to say, he does not break it up into mouthfuls; he drains it without opening it and digging into its vitals. In him again we see exemplified that marvellous art which consists in feeding on the victim without killing it until the meal is over, so as always to have a portion of fresh meat. With its mouth assiduously applied to the unhappy creature's skin, the lethal grub fills itself and waxes fat, while the fostering larva collapses and shrivels, retaining just enough life, however, to resist decomposition. All that remains of the decanted corpse is the skin, which, when softened in water and blown out, swells into a balloon without the least escape of gas, thus proving the continuity of the integument. All the same, the apparently unpunctured bladder has lost its contents. It is a repetition of what the Anthrax has shown us, with this difference, that the Leucopsis seems not so well skilled in the delicate work of absorbing the victim. Instead of the clean white granule which is the sole residue when the Fly has finished her joint, the insect with the long probe has a plateful of leavings, not seldom soiled with the brownish tinge of food that has gone bad. It would seem that, towards the end, the act of consumption becomes more savage and does not disdain dead meat. I also notice that the Leucopsis is not able to get up from dinner or to sit down to it again as readily as the Anthrax. I have sometimes to tease him with the point of a hair-pencil in order to make him let go; and, once he has left the joint, he hesitates a little before putting his mouth to it again. His adhesion is not the mere result of a kiss like that of a cupping-glass; it can only be explained by hooks that need releasing.
I now see the use of the microscopic mandibles. Those two delicate spikes are incapable of chewing anything, but they may very well serve to pierce the epidermis with an aperture smaller than that made by the finest needle; and it is through this puncture that the Leucopsis sucks the juices of his prey. They are instruments made to perforate the bag of fat which slowly, without suffering any internal injury, is emptied through an opening repeated here and there. The Anthrax' cupping-glass is here replaced by piercers of exceeding sharpness and so short that they cannot hurt anything beyond the skin. Thus do we see in operation, with a different sort of implements, that wise system which keeps the provisions fresh for the consumer.
It is hardly necessary to say, to those who have read the story of the Anthrax, that this kind of feeding would be impossible with a victim whose tissues possessed their final hardness. The Mason-bee's grub is therefore emptied by the Leucopsis' larva while it is in a semifluid state and deep in the torpor of the nymphosis. The last fortnight in July and the first fortnight in August are the best times to witness the repast, which I have seen going on for twelve and fourteen days. Later, we find nothing in the Mason-bee's cocoon except the Leucopsis' larva, gloriously fat, and, by its side, a sort of thin, rancid rasher, the remains of the deceased wet-nurse. Things then remain as they are until the hot part of the following summer or at least until the end of June.
Then appears the nymph, which teaches us nothing striking; and at last the perfect insect, whose hatching may be delayed until August. Its exit from the Mason's fortress has no likeness to the strange method employed by the Anthrax. Endowed with stout mandibles, the perfect insect splits the ceiling of its abode by itself without much difficulty. At the time of its deliverance, the Mason-bees, who work in May, have long disappeared. The nests on the pebbles are all closed, the provisioning is finished, the larvae are sleeping in their yellow cocoons. As the old nests are utilized by the Mason so long as they are not too much dilapidated, the dome which has just been vacated by the Leucopsis, now more than a year old, has its other cells occupied by the Bee's children. There is here, without seeking farther, a fat living for the Leucopsis' offspring which she well knows how to turn to profit. It depends but on herself to make the house in which she was born into the residence of her family. Besides, if she has a fancy for distant exploration, clay domes abound in the harmas. The inoculation of the eggs through the walls will begin shortly. Before witnessing this curious performance, let us examine the needle that is to effect it.
The insect's abdomen is hollowed, at the top, into a furrow that runs up to the base of the thorax; the end, which is broader and rounded, has a narrow slit, which seems to divide this region into two. The whole thing suggests a pulley with a fine groove. When at rest, the inoculating-needle or ovipositor remains packed in the slit and the furrow. The delicate instrument thus almost completely encircles the abdomen. Underneath, on the median line, we see a long, dark-brown scale, pointed, keel-shaped, fixed by its base to the first abdominal segment, with its sides prolonged into membranous wings which are fastened tightly to the insect's flanks. Its function is to protect the underlying region, a soft-walled region in which the probe has its source. It is a cuirass, a lid which protects the delicate motor-machinery during periods of inactivity but swings from back to front and lifts when the implement has to be unsheathed and used.
We will now remove this lid with the scissors, so as to have the whole apparatus before our eyes, and then raise the ovipositor with the point of a needle. The part that runs along the back comes loose without the slightest difficulty, but the part embedded in the groove at the end of the abdomen offers a resistance that warns us of a complication which we did not notice at first. The tool, in fact, consists of three pieces, a central piece, or inoculating-filament, and two side-pieces, which together constitute a scabbard. The two latter are more substantial, are hollowed out like the sides of a groove and, when uniting, form a complete groove in which the filament is sheathed. This bivalvular scabbard adheres loosely to the dorsal part; but, farther on, at the tip of the abdomen and under the belly, it can no longer be detached, as its valves are welded to the abdominal wall. Here, therefore, we find, between the two joined protecting parts, a simple trench in which the filament lies covered up. As for this filament, it is easily extracted from its sheath and released down to its base, under the shield formed by the scale.
Seen under the magnifying-glass, it is a round, stiff, horny thread, midway in thickness between a human hair and a horse-hair. Its tip is a little rough, pointed and bevelled to some length down. The microscope becomes necessary if we would see its real structure, which is much less simple than it at first appears. We perceive that the bevelled end-part consists of a series of truncated cones, fitting one into the other, with their wide base slightly projecting. This arrangement produces a sort of file, a sort of rasp with very much blunted teeth. When pressed on the slide, the thread divides into four pieces of unequal length. The two longer end in the toothed bevel. They come together in a very narrow groove, which receives the two other, rather shorter pieces. These both end in a point, which, however, is not toothed and does not project as far as the final rasp. They also unite to form a groove, which fits into the groove of the other two, the whole constituting a complete channel or duct. Moreover, the two shorter pieces, considered together, can move, lengthwise, in the groove that receives them; they can also move one over the other, always lengthwise, so much so that, on the slide of the microscope, their terminal points are seldom situated on the same level.
If with our scissors we cut a piece of the inoculating-thread from the living insect and examine the section under the magnifying-glass, we shall see the inner groove lengthen out and project beyond the outer groove and then go in again in turn, while from the wound there oozes a tiny albimunous drop, doubtless proceeding from the liquid that gives the egg the singular appendage to which we shall come presently. By means of these longitudinal movements of the inner trench inside the outer trench and of the sliding, one over the other, of the two portions of the former, the egg can be despatched to the end of the ovipositor notwithstanding the absence of any muscular contraction, which is impossible in a horny conduit.
We have only to press the upper surface of the abdomen to see it disjoint itself from the first segment, as though the insect had been cut almost in two at that point. A wide gap or hiatus appears between the first and second rings; and, under a thin membrane, the base of the ovipositor bulges out, bent back into a stout hook. Here the filament passes through the insect from end to end and emerges underneath. Its issue is therefore near the base of the abdomen, instead of at the tip, as usual. This curious arrangement has the effect of shortening the lever-arm of the ovipositor and bringing the starting-point of the filament nearer to the fulcrum, namely, the legs of the insect, and of thus assisting the difficult task of inoculation by making the most of the effort expended.
To sum up, the ovipositor when at rest goes round the abdomen. Starting at the base, on the lower surface, it runs round the belly from front to back and then returns from back to front on the upper surface, where it ends at almost the same level as its starting-point. Its length is 14 millim?tres (2). This fixes the limit of the depth which the probe is able to reach in the Mason-bee's nests.
One last word on the Leucopsis' weapon. In the dying insect, beheaded, stripped of legs and wings, with a pin stuck through its body, the sides of the fissure containing the inoculating-thread quiver violently, as if the belly were going to open, divide in two along the median line and then reunite its two halves. The thread itself gives convulsive tremblings; it comes out of its scabbard, goes back and slips out again. It is as though the laying-implement could not persuade itself to die before accomplishing its mission. The insect's supreme aim is the egg; and, so long as the least spark of life remains, it makes dying efforts to lay.
Leucopsis gigas exploits the nests of the Mason-bee of the Pebbles and the Mason-bee of the Sheds with equal zest. To observe the insertion of the egg at my ease and to watch the operator at work over and over again, I gave the preference to the last-named Mason, whose nests, removed from the neighbouring roofs by my orders, have hung for some years in the arch of my basement. These clay hives fastened to tiles supply me with fresh records each summer. I am much indebted to them in the matter of the Leucopsis' life-history.
By way of comparison with what took place under my roof, I used to observe the same scenes on the pebbles of the surrounding wastelands. My excursions, alas, did not all reward my zeal, which zeal was not without merit in the merciless sunshine; but still, at rare intervals, I succeeded in seeing some Leucopsis digging her probe into the mortar dome. Lying flat on the ground, from the beginning to the end of the operation, which sometimes lasted for hours, I closely watched the insect in its every movement, while my Dog, weary of being out of doors in that scorching heat, would discreetly retire from the fray and, with his tail between his legs and his tongue hanging out, go home and stretch himself at full length on the cool tiles of the hall. How wise he was to scorn this pebble-gazing! I would come in half-roasted, as brown as a berry, to find my friend Bull wedged into a corner, his back to the wall, sprawling on all fours, while, with heaving sides, he panted forth the last sprays of steam from his overheated interior. Yes, he was much better-advised to return as fast as he could to the shade of the house. Why does man want to know things? Why is he not indifferent to them, with the lofty philosophy of the animals? What interest can anything have for us that does not fill our stomachs? What is the use of learning? What is the use of truth, when profit is all that matters? Why am I — the descendant, so they tell me, of some tertiary Baboon — afflicted with the passion for knowledge from which Bull, my friend and companion, is exempt? Why…oh, where have I got to? I was going in, wasn't I, with a splitting headache? Quick, let us get back to our subject!
It was in the first week of July that I saw the inoculation begin on my Chalicodoma sicula nests. The parasite is at her task in the hottest part of the day, close on three o'clock in the afternoon; and work goes on almost to the end of the month, decreasing gradually in activity. I count as many as twelve Leucopses at a time on the most thickly-populated pair of tiles. The insect slowly and awkwardly explores the nests. It feels the surface with its antennae, which are bent at a right angle after the first joint. Then, motionless, with lowered head, it seems to meditate and to debate within itself on the fitness of the spot. Is it here or somewhere else that the coveted larva lies? There is nothing outside, absolutely nothing, to tell us. It is a stony expanse, bumpy but yet very uniform in appearance, for the cells have disappeared under a layer of plaster, a work of public interest to which the whole swarm devotes its last days. If I myself, with my long experience, had to decide upon the suitable point, even if I were at liberty to make use of a lens for examining the mortar grain by grain and to auscultate the surface in order to gather information from the sound emitted, I should decline the job, persuaded in advance that I should fail nine times out of ten and only succeed by chance.
Where my discernment, aided by reason and my optical contrivances, fails, the insect, guided by the wands of its antennae, never blunders. Its choice is made. See it unsheathing its long instrument. The probe points normally towards the surface and occupies nearly the central spot between the two middle-legs. A wide dislocation appears on the back, between the first and second segments of the abdomen; and the base of the instrument swells like a bladder through this opening; while the point strives to penetrate the hard clay. The amount of energy expended is shown by the way in which the bladder quivers. At every moment we expect to see the frail membrane burst with the violence of the effort. But it does not give way; and the wire goes deeper and deeper.
Raising itself high on its legs, to give free play to its apparatus, the insect remains motionless, the only sign of its arduous labours being a slight vibration. I see some perforators who have finished operating in a quarter of an hour. These are the quickest at the business. They have been lucky enough to come across a wall which is less thick and less hard than usual. I see others who spend as many as three hours on a single operation, three long hours of patient watching for me, in my anxiety to follow the whole performance to the end, three long hours of immobility for the insect, which is even more anxious to make sure of board and lodging for its egg. But then is it not a task of the utmost difficulty to introduce a hair into the thickness of a stone? To us, with all the dexterity of our fingers, it would be impossible; to the insect, which simply pushes with its belly, it is just hard work.
Notwithstanding the resistance of the substance traversed, the Leucopsis perseveres, certain of succeeding; and she does succeed, although I am still unable to understand her success. The material through which the probe has to penetrate is not a porous substance; it is homogeneous and compact, like our hardened cement. In vain do I direct my attention to the exact point where the instrument is at work; I see no fissure, no opening that can facilitate access. A miner's drill penetrates the rock only by pulverizing it. This method is not admissible here; the extreme delicacy of the implement is opposed to it. The frail stem requires, so it seems to me, a ready-made way, a crevice through which it can slip; but this crevice I have never been able to discover. What about a dissolving fluid which would soften the mortar under the point of the ovipositor? No, for I see not a trace of humidity around the point where the thread is at work. I fall back upon a fissure, a lack of continuity somewhere, although my examination fails to discover any on the Mason-bee's nest. I was better served in another case. Leucopsis dorsigera, FAB., settles her eggs on the larva of the Diadem Anthidium, who sometimes makes her nest in reed-stumps. I have repeatedly seen her insert her auger through a slight rupture in the side of the reed. As the wall was different, wood in the latter case and mortar in the former, perhaps it will be best to look upon the matter as a mystery.
My sedulous attendance, during the best part of July, in front of the tiles hanging from the walls of the arch, allowed me to reckon the inoculations. Each time that the insect, on finishing the operation, removed its probe, I marked in pencil the exact point at which the instrument was withdrawn; and I wrote down the date beside it. This information was to be utilized when the Leucopsis finished her labours.
When the perforators are gone, I proceed with my examination of the nests, covered with my hieroglyphics, the pencilled notes. One result, one which I fully expected, compensates me straightway for all my weary waitings. Under each spot marked in black, under each spot whence I saw the ovipositor withdrawn, I always find a cell, with not a single exception. And yet there are intervals of solid stone between the cells: the partition-walls alone would account for some. Moreover, the compartments, which are very irregularly disposed by a swarm of toilers who all work in their own sweet way, have great irregular cavities between them, which end by being filled up with the general plastering of the nest. The result of this arrangement is that the massive portions cover almost the same space as the hollow portions. There is nothing outside to show whether the underlying regions are full or empty. It is quite impossible for me to decide if, by digging straight down, I shall come to a hollow cell or to a solid wall.
But the insect makes no mistake: the excavations under my pencil-marks bear witness to that; it always directs its apparatus towards the hollow of a cell. How is it apprised whether the part below is empty or full? Its organs of information are undoubtedly the antennae, which feel the ground. They are two fingers of unparalleled delicacy, which pry into the basement by tapping on the part above it. Then what do those puzzling organs perceive? A smell? Not at all; I always had my doubts of that and now I am certain of the contrary, after what I shall describe in a moment. Do they perceive a sound? Are we to treat them as a superior kind of microphone, capable of collecting the infinitesimal echoes of what is full and the reverberations of what is empty? It is an attractive idea, but unfortunately the antennae play their part equally well on a host of occasions when there are no vaults to reverberate. We know nothing and are perhaps destined never to know anything of the real value of the antennal sense, to which we have nothing analogous; but, though it is impossible for us to say what it does perceive, we are at least able to recognize to some extent what it does not perceive and, in particular, to deny it the faculty of smell.
As a matter of fact, I notice, with extreme surprise, that the great majority of the cells visited by the Leucopsis' probe do not contain the one thing which the insect is seeking, namely, the young larva of the Mason-bee enclosed in its cocoon. Their contents consist of the refuse so often met with in old Chalicodoma-nests: liquid honey left unemployed, because the egg has perished; spoilt provisions, sometimes mildewed, or sometimes a tarry mass; a dead larva, stiffened into a brown cylinder; the shrivelled corpse of a perfect insect, which lacked the strength to effect its deliverance; dust and rubbish which has come from the exit-window afterwards closed up by the outer coating of plaster. The odoriferous effluvia that can emanate from these relics certainly possess very diverse characters. A sense of smell with any subtlety at all would not be deceived by this stuff, sour, ‚high,‘ musty or tarry as the case may be; each compartment, according to its contents, has a special aroma, which we might or might not be able to perceive; and this aroma most certainly bears no resemblance to that which we may assume the much-desired fresh larva to possess. If nevertheless the Leucopsis does not distinguish between these various cells and drives the probe into all of them indifferently, is this not an evident proof that smell is no guide whatever to her in her search? Other considerations, when I was treating of the Hairy Ammophila, enabled me to assert that the antennae have no olfactory powers. To-day, the frequent mistakes of the Leucopsis, whose antennae are nevertheless constantly exploring the surface, make this conclusion absolutely certain.
The perforator of clay nests has, so it seems to me, delivered us from an old physiological fallacy. She would deserve studying, if for no other result than this; but her interest is far from being exhausted. Let us look at her from another point of view, whose full importance will not be apparent until the end; let us speak of something which I was very far from suspecting when I was so assiduously watching the nests of my Mason-bees.
The same cell can receive the Leucopsis' probe a number of times, at intervals of several days. I have said how I used to mark in black the exact place at which the laying-implement had entered and how I wrote the date of the operation beside it. Well, at many of these already visited spots, concerning which I possessed the most authentic documents, I saw the insect return a second, a third and even a fourth time, either on the same day or some while after, and drive its inoculating-thread in again, at precisely the same place, as though nothing had happened. Was it the same individual repeating her operation in a cell which she had visited before but forgotten, or different individuals coming one after the other to lay an egg in a compartment thought to be unoccupied? I cannot say, having neglected to mark the operators, for fear of disturbing them.
As there is nothing, except the mark of my pencil, a mark devoid of meaning to the insect, to indicate that the auger has already been at work there, it may easily happen that the same operator, finding under her feet a spot already exploited by herself but effaced from her memory, repeats the thrust of her tool in a compartment which she believes herself to be discovering for the first time. However retentive its memory for places may be, we cannot admit that the insect remembers for weeks on end, as well as point by point, the topography of a nest covering a surface of some square yards. Its recollections, if it have any, serve it badly; the outward appearance gives it no information; and its drill enters wherever it may happen to discover a cell, at points that have already perhaps been pierced several times over.
It may also happen — and this appears to me the most frequent case — that one exploiter of a cell is succeeded by a second, a third, a fourth and others still, all fired with the newcomer's zeal because their predecessors have left no trace of their passage. In one way or another, the same cell is exposed to manifold layings, though its contents, the Chalicodoma-grub, be only the bare ration of a single Leucopsis-grub.
These reiterated borings are not at all rare: I noted a score of them on my tiles; and, in the case of some cells, the operation was repeated before my eyes as often as four times. Nothing tells us that this number was not exceeded in my absence. The little that I observed prevents me from fixing any limit. And now a momentous question arises: is the egg really laid each time that the probe enters a cell? I can see not the slightest excuse for supposing the contrary. The ovipositor, because of its horny nature, can have but a very dull sense of touch. The insect is apprised of the contents of the cell only by the end of that long horse-hair, a not very trustworthy witness, I should imagine. The absence of resistance tells it that it has reached an empty space; and this is probably the only information that the insensible implement can supply. The drill boring through the rock cannot tell the miner anything about the contents of the cavern which it has entered; and the case must be the same with the rigid filament of the Leucopses.
Now that the thread has reached its goal, what does the cell contain? Mildewed honey, dust and rubbish, a shrivelled larva, or a larva in good condition? Above all, does it already contain an egg? This last question calls for a definite answer, but as a matter of fact it is impossible for the insect to learn anything from a horse-hair on that most delicate matter, the presence or absence of an egg, a mere atom of a thing, in that vast apartment. Even admitting some sense of touch at the end of the drill, one insuperable difficulty would always remain: that of finding the exact spot where the tiny speck lies in those spacious and mysterious regions. I go so far as to believe that the ovipositor tells the insect nothing, or at any rate very little, of the inside of the cell, whether propitious or not to the development of the germ. Perhaps each thrust of the instrument, provided that it meets with no resistance from solid matter, lays the egg, to whose lot there falls at one time good, wholesome food, at another mere refuse.
These anomalies call for more conclusive proofs than the rough deductions drawn from the nature of the horny ovipositor. We must ascertain in a direct fashion whether the cell into which the auger has been driven several times over actually contains several occupants in addition to the larva of the Mason-bee. When the Leucopses had finished their borings, I waited a few days longer so as to give the young grubs time to develop a little, which would make my examination easier. I then moved the tiles to the table in my study, in order to investigate their secrets with the most scrupulous care. And here such a disappointment as I have rarely known awaited me. The cells which I had seen, actually seen, with my own eyes, pierced by the probe two or three or even four times, contained but one Leucopsis-grub, one alone, eating away at its Chalicodoma. Others, which had also been repeatedly probed, contained spoilt remnants, but never a Leucopsis. O holy patience, give me the courage to begin again! Dispel the darkness and deliver me from doubt!
I begin again. The Leucopsis-grub is familiar to me; I can recognize it, without the possibility of a mistake, in the nests of both the Chalicodoma of the Pebbles and the Chalicodoma of the Sheds. All through the winter, I rush about, getting my nests from the roofs of old sheds and the pebbles of the waste-lands; I stuff my pockets with them, fill my box, load Favier's knapsack; I collect enough to litter all the tables in my study; and, when it is too cold out of doors, when the biting mistral blows, I tear open the fine silk of the cocoons to discover the inhabitant. Most of them contain the Mason in the perfect state; others give me the larva of the Anthrax; others — very numerous, these — give me the larva of the Leucopsis. And this last is alone, always alone, invariably alone. The whole thing is utterly incomprehensible when one knows, as I know, how many times the probe entered those cells.
My perplexity only increases when, on the return of summer, I witness for the second time the Leucopsis' repeated operations on the same cells and for the second time find a single larva in the compartments which have been bored several times over. Shall I then be forced to accept that the auger is able to recognize the cells already containing an egg and that it thenceforth refrains from laying there? Must I admit an extraordinary sense of touch in that bit of horse-hair, or even better, a sort of divination which declares where the egg lies without having to touch it? But I am raving! There is certainly something that escapes me; and the obscurity of the problem is simply due to my incomplete information. O patience, supreme virtue of the observer, come to my aid once more! I must begin all over again for the third time.
Until now, my investigations have been made some time after the laying, at a period when the larva is at least fairly developed. Who knows? Something perhaps happens, at the very commencement of infancy, that may mislead me afterwards. I must apply to the egg itself if I would learn the secret which the grub will not reveal. I therefore resume my observations in the first fortnight of July, when the Leucopses are beginning to visit busily both Mason-bee's nests. The pebbles in the waste-lands supply me with plenty of buildings of the Chalicodoma of the Walls; the byres scattered here and there in the fields give me, under their dilapidated roofs, in fragments broken off with the chisel, the edifices of the Chalicodoma of the Sheds. I am anxious not to complete the destruction of my home hives, already so sorely tried by my experiments; they have taught me much and can teach me more. Alien colonies, picked up more or less everywhere, provide me with my booty. With my lens in one hand and my forceps in the other, I go through my collection on the same day, with the prudence and care which only the laboratory-table permits. The results at first fall far short of my expectations. I see nothing that I have not seen before. I make fresh expeditions, after a few days' interval; I bring back fresh loads of lumps of mortar, until at last fortune smiles upon me.
Reason was not at fault. Each thrust means the laying of an egg when the probe reaches the cell. Here is a cocoon of the Mason-bee of the Pebbles with an egg side by side with the Chalicodoma-grub. But what a curious egg! Never have my eyes beheld the like; and then is it really the egg of the Leucopsis? Great was my apprehension. But I breathed again when I found, a couple of weeks later, that the egg had become the larva with which I was familiar. Those cocoons with a single egg are as numerous as I can wish; they exceed my wishes: my little glass receptacles are too few to hold them.
And here are others, more precious ones still, with manifold layings. I find plenty with two eggs; I find some with three or four; the best-colonised offer me as many as five. And, to crown my delight, the joy of the seeker to whom success comes at the last moment, when he is on the verge of despair, here again, duly furnished with an egg, is a sterile cocoon, that is to say, one containing only a shrivelled and decaying larva. All my suspicions are confirmed, down to the most inconsequent: the egg housed with a mass of putrefaction.
The nests of the Mason-bee of the Walls are the more regular in structure and are easier to examine, because their base is wide open once it is separated from the supporting pebble; and it was these which supplied me with by far the greater part of my information. Those of the Mason-bee of the Sheds have to be chipped away with a hammer before one can inspect their cells, which are heaped up anyhow; and they do not lend themselves anything like so well to delicate investigations, as they suffer both from the shock and the ill-treatment.
And now the thing is done: it remains certain that the Leucopsis' laying is exposed to very exceptional dangers. She can entrust the egg to sterile cells, without provisions fit to use; she can establish several in the same cell, though this cell contains nourishment for one only. Whether they proceed from a single individual returning several times, by inadvertence, to the same place, or are the work of different individuals unaware of the previous borings, those multiple layings are very frequent, almost as much so as the normal layings. The largest which I have noticed consisted of five eggs, but we have no authority for looking upon this number as an outside limit. Who could say, when the perforators are numerous, to what lengths this accumulation can go? I will set forth on some future occasion how the ration of one egg remains in reality the ration of one egg, despite the multiplicity of banqueters.
I will end by describing the egg, which is a white, opaque object, shaped like a much-elongated oval. One of the ends is lengthened out into a neck or pedicle, which is as long as the egg proper. This neck is somewhat wrinkled, sinuous and as a rule considerably curved. The whole thing is not at all unlike certain gourds with an elongated paunch and a snake-like neck. The total length, pedicle and all, is about 3 millimetres. (About one-eighth of an inch. — Translator's Note. (3) It is needless to say, after recognizing the grub's manner of feeding, that this egg is not laid inside the fostering larva. Yet, before I knew the habits of the Leucopsis, I would readily have believed that every Hymenopteron armed with a long probe inserts her eggs into the victim's sides, as the Ichneumon-flies do to the Caterpillars. I mention this for the benefit of any who may be under the same erroneous impression.
The Leucopsis' egg is not even laid upon the Mason-bee's larva; it is hung by its bent pedicle to the fibrous wall of the cocoon. When I go to work very delicately, so as not to disturb the arrangement in knocking the nest off its support, and then extract and open the cocoon, I see the egg swinging from the silken vault. But it takes very little to make it fall. And so, most often, even though it be merely the effect of the shock sustained when the nest is removed from its pebble, I find the egg detached from its suspension-point and lying beside the larva, to which it never adheres in any circumstances. The Leucopsis' probe does not penetrate beyond the cocoon traversed; and the egg remains fastened to the ceiling, in the crook of some silky thread, by means of its hooked pedicle.
1. This chapter should be read in conjunction with the essays entitled „The Anthrax“ and „Larval Dimorphism“, forming chapters 2 and 4 of „The Life of the Fly.“
2. .546 inch.