Bramble-Bees and Others Part 6

The nest of the Mason-bee of the Walls consists of a cl.u.s.ter of upright cells backing against one another. For the whole to take a spherical form, the height of the chambers must diminish from the centre of the dome to the circ.u.mference. Their elevation is the sine of the meridian arc starting from the plane of the pebble. Therefore, if they are to have any solidity, there must be large cells in the middle and small cells at the edges. And, as the work begins with the central chambers and ends with those on the circ.u.mference, the laying of the females, destined for the large cells, must precede that of the males, destined for the small cells. So the females come first and the males at the finish.

This is all very well when the mother herself founds the dwelling, when she lays the first rows of bricks. 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 the few vacant rooms, the large and the small alike, if the s.e.x 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 s.e.x 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 sh.e.l.ls and various hiding-places in walls, clay or wood. Their work is confined to repairs to the house, such as part.i.tions and covers. There are plenty of these retreats; and the insect would always find first-cla.s.s ones if it thought of going any distance to look for them. But the Osmia is a stay-at-home: she returns to her birth-place 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, s.p.a.cious 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, on two separate occasions, 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 gla.s.s 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 coc.o.o.ns of the two s.e.xes, on the thickness of the part.i.tions and the final lid, I shortened some of these to the exact dimensions required for two coc.o.o.ns only, of different s.e.xes.

Well, these short tubes, whether of gla.s.s 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 coc.o.o.ns. This order was invariable; what varied was the number of cells in the long tubes and the proportion between the two sorts of coc.o.o.ns, sometimes males predominating and sometimes females.

The experiment is of paramount importance; and it will perhaps make the result clearer if I quote one instance from among a mult.i.tude of similar cases. I give the preference to this particular instance because of the rather exceptional fertility of the laying. An Osmia marked on the thorax is watched, day by day, from the commencement to the end of her work. From the 1st to the 10th of May, she occupies a gla.s.s tube in which she lodges seven females followed by a male, which ends the series. From the 10th to the 17th of May, she colonizes a second tube, in which she lodges first three females and then three males. From the 17th to the 25th of May, a third tube, with three females and then two males. On the 26th of May, a fourth tube, which she abandons, probably because of its excessive width, after laying one female in it. Lastly, from the 26th to the 30th of May, a fifth tube, which she colonizes with two females and three males. Total: twenty-five Osmiae, including seventeen females and eight males. And it will not be superfluous to observe that these unfinished series do not in any way correspond with periods separated by intervals of rest. The laying is continuous, in so far as the variable condition of the atmosphere allows. As soon as one tube is full and closed, another is occupied by the Osmia without delay.

The tubes reduced to the exact length of two cells fulfilled my expectation in the great majority of cases: the lower cell was occupied by a female and the upper by a male. There were a few exceptions.

More discerning than I in her estimate of what was strictly necessary, better-versed in the economy of s.p.a.ce, the Osmia had found a way of lodging two females where I had only seen room for one female and a male.

This experiment speaks volumes. When confronted with tubes too small to receive all her family, she 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 s.e.xes 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 s.e.x of the egg according to the exigencies of s.p.a.ce.

And besides the exigencies of s.p.a.ce one might perhaps venture to add those connected with the earlier development of the males. These burst their coc.o.o.ns 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 coc.o.o.ns 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 sh.e.l.ls that are slower in hatching.

I experimented on Latreille's Osmia, using short and even very short stumps of reed. All that I had to do was to lay them just beside the nests of the Mason-bee of the Sheds, nests beloved by this particular Osmia. Old, disused hurdles supplied me with reeds inhabited from end to end by the Horned Osmia. In both cases I obtained the same results and the same conclusions as with the Three-horned Osmia.

I return to the latter, nidifying under my eyes in some old nests of the Mason-bee of the Walls, which I had placed within her reach, mixed up with the tubes. Outside my study, I had never yet seen the Three-horned Osmia adopt that domicile. This may be due to the fact that these nests are isolated one by one in the fields; and the Osmia, who loves to feel herself surrounded by her kin and to work in plenty of company, refuses them because of this isolation. But on my table, finding them close to the tubes in which the others are working, she adopts them without hesitation.

The chambers presented by those old nests are more or less s.p.a.cious according to the thickness of the coat of mortar which the Chalicodoma has laid over the a.s.sembled chambers. To leave her cell, the Mason-bee has to perforate not only the plug, the lid built at the mouth of the cell, but also the thick plaster wherewith the dome is strengthened at the end of the work. The perforation results in a vestibule which gives access to the chamber itself. It is this vestibule which is sometimes longer and sometimes shorter, whereas the corresponding chamber is of almost constant dimensions, in the case of the same s.e.x, of course.

We will first consider the short vestibule, at the most large enough to receive the plug with which the Osmia will close up the lodging. There is then nothing at her disposal except the cell proper, a s.p.a.cious apartment in which one of the Osmia's females will find ample accommodation, for she is much smaller than the original occupant of the chamber, no matter the s.e.x; but there is not room for two coc.o.o.ns at a time, especially in view of the s.p.a.ce taken up by the intervening part.i.tion. Well, in those large, well-built chambers, formerly the homes of Chalicodomae, the Osmia settles females and none but females.

Let us now consider the long vestibule. Here, a part.i.tion is constructed, encroaching slightly on the cell proper, and the residence is divided into two unequal storeys, a large room below, housing a female, and a narrow cabin above, containing a male.

When the length of the vestibule permits, allowing for the s.p.a.ce required by the outer stopper, a third storey is built, smaller than the second; and another male is lodged in this cramped corner. In this way the old nest of the Mason-bee of the Pebbles is colonized, cell after cell, by a single mother.

The Osmia, as we see, is very frugal of the lodging that has fallen to her share; she makes the best possible use of it, giving to the females the s.p.a.cious chambers of the Mason-bee and to the males the narrow vestibules, subdivided into storeys when this is feasible. Economy of s.p.a.ce is the chief consideration, since her stay-at-home tastes do not allow her to indulge in distant quests. She has to employ the site which chance places at her disposal just as it is, now for a male and now for a female. Here we see displayed, more clearly than ever, her power of deciding the s.e.x of the egg, in order to adapt it judiciously to the conditions of the house-room available.

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. Their diameter is about seven 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 s.e.xes together, with a part.i.tion in the middle, the female occupying the lower and the male the upper storey. True, in such cases economy of s.p.a.ce is strained to the utmost, the apartments provided by the Mason-bee of the Shrubs being very small as it is, despite their entrance-halls. Lastly, the deeper cavities on the circ.u.mference are allotted to females and the shallower to males.

I will add that a single mother peoples each nest and also that she proceeds from cell to cell without troubling to ascertain the depth. She goes from the centre to the edges, from the edges to the centre, from a deep cavity to a shallow cavity and vice versa, which she would not do if the s.e.xes were to follow upon each other in a settled order. For greater certainty, I numbered the cells of one nest as each of them was closed. On opening them later, I was able to see that the s.e.xes were not subjected to a chronological arrangement. Females were succeeded by males and these by females without its being possible for me to make out any regular sequence. Only--and this is the essential point--the deep cavities were allotted to the females and the shallow ones to the 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. Exercising the very greatest care, I broke up some great lumps of earth removed from the banks inhabited by the Anthophora and sent to me from Carpentras by my dear friend and pupil M. Devillario. I examined them conscientiously in the quiet of my study. I found the Osmia's coc.o.o.ns arranged in short series, in very irregular pa.s.sages, the original work of which is due to the Anthophora. Touched up afterwards, made larger or smaller, lengthened or shortened, intersected with a network of crossings by the numerous generations that had succeeded one another in the same city, they formed an inextricable labyrinth.

Sometimes these corridors did not communicate with any adjoining apartment; sometimes they gave access to the s.p.a.cious chamber of the Anthophora, which could be recognized, in spite of its age, by its oval shape and its coating of glazed stucco. In the latter case, the bottom cell, which once const.i.tuted, by itself, the chamber of the Anthophora, was always occupied by a female Osmia. Beyond it, in the narrow corridor, a male was lodged, not seldom two, or even three. Of course, clay part.i.tions, the work of the Osmia, separated the different inhabitants, each of whom had his own storey, his own closed cell.

When the accommodation consisted of no more than a simple cylinder, with no state-bedroom at the end of it--a bedroom always reserved for a female--the contents varied with the diameter of the cylinder. The series, of which the longest were series of four, included, with a wider diameter, first one or two females, then one or two males. It also happened, though rarely, that the series was reversed, that is to say, it began with males and ended with females. Lastly, there were a good many isolated coc.o.o.ns, of one s.e.x or the other. When the coc.o.o.n was alone and occupied the Anthophora's cell, it invariably belonged to a female.

I have observed the same thing in the nests of the Mason-bee of the Sheds, but not so easily. The series are shorter here, because the Mason-bee does not bore galleries but builds cell upon cell. The work of the whole swarm thus forms a stratum of cells that grows thicker from year to year. The corridors occupied by the Osmia are the holes which the Mason-bee dug in order to reach daylight from the deep layers.

In these short series, both s.e.xes are usually present; and, if the Mason-bee's chamber is at the end of the pa.s.sage, it is inhabited by a female Osmia.

We come back to what the short tubes and the old nests of the Mason-bee of the Pebbles have already taught us. The Osmia who, in tubes of sufficient length, divides her whole laying into a continuous sequence of females and a continuous sequence of males, now breaks it up into short series in which both s.e.xes are present. She adapts her sectional layings to the exigencies of a chance lodging; she always places a female in the sumptuous chamber which the Mason-bee or the Anthophora occupied originally.

Facts even more striking are supplied by the old nests of the Masked Anthophora (A. personata, ILLIG.), old nests which I have seen utilized by the Horned Osmia and the Three-horned Osmia at the same time. Less frequently, the same nests serve for Latreille's Osmia. Let us first describe the Masked Anthophora's nests.

In a steep bank of sandy clay, we find a set of round, wide-open holes.

There are generally only a few of them, each about half an inch in diameter. They are the entrance-doors leading to the Anthophora's abode, doors always left open, even after the building is finished. Each of them gives access to a short pa.s.sage, sometimes straight, sometimes winding, nearly horizontal, polished with minute care and varnished with a sort of white glaze. It looks as if it had received a thin coat of whitewash. On the inner surface of this pa.s.sage, in the thickness of the earthy bank, s.p.a.cious oval niches have been excavated, communicating with the corridor by means of a narrow bottle-neck, which is closed, when the work is done, with a substantial mortar stopper. The Anthophora polishes the outside of this stopper so well, smooths its surface so perfectly, bringing it to the same level as that of the pa.s.sage, is so careful to give it the white tint of the rest of the wall that, when the job is finished, it becomes absolutely impossible to distinguish the entrance-door corresponding with each cell.

The cell is an oval cavity dug in the earthy ma.s.s. The wall has the same polish, the same chalky whiteness as the general pa.s.sage. But the Anthophora does not limit herself to digging oval niches: to make her work more solid, she pours over the walls of the chamber a salivary liquid which not only whitens and varnishes but also penetrates to a depth of some millimetres into the sandy earth, which it turns into a hard cement. A similar precaution is taken with the pa.s.sage; and therefore the whole is a solid piece of work capable of remaining in excellent condition for years.

Moreover, thanks to the wall hardened by the salivary fluid, the structure can be removed from its matrix by chipping it carefully away.

We thus obtain, at least in fragments, a serpentine tube from which hangs a single or double row of oval nodules that look like large grapes drawn out lengthwise. Each of these nodules is a cell, the entrance to which, carefully hidden, opens into the tube or pa.s.sage. When she wishes to leave her cell, in the spring, the Anthophora destroys the mortar disk that closes the jar and thus reaches the general corridor, which is quite open to the outer air. The abandoned nest provides a series of pear-shaped cavities, of which the distended part is the old cell and the contracted part the exit-neck, rid of its stopper.

These pear-shaped hollows form splendid lodgings, impregnable strongholds, in which the Osmiae find a safe and commodious retreat for their families. The Horned Osmia and the Three-horned Osmia establish themselves there at the same time. Although it is a little too large for her, Latrielle's Osmia also appears very well satisfied with it.

I have examined some forty of the superb cells utilized by each of the first two. The great majority are divided into two storeys by means of a transversal part.i.tion. The lower storey includes the larger portion of the Anthophora's cell; the upper storey includes the rest of the cell and a little of the bottle-neck that surmounts it. The two-roomed dwelling is closed, in the pa.s.sage, by a shapeless, bulky ma.s.s of dried mud. What a clumsy artist the Osmia is, compared with the Anthophora!

Against the exquisite work of the Anthophora, part.i.tion and plug strike a note as hideously incongruous as a lump of dirt on polished marble.

The two apartments thus obtained are of a very unequal capacity, which at once strikes the observer. I measured them with my five-millimetre tube. On an average, the bottom one is represented by a column of sand 50 millimetres deep (1.95 inches.--Translator's Note.) and the top one by a column of 15 millimetres (.585 inch.--Translator's Note.). The holding-capacity of the one is therefore about three times as large as that of the other. The coc.o.o.ns enclosed present the same disparity. The bottom one is big, the top one small. Lastly, the lower one belongs to a female Osmia and the upper to a male Osmia.

Occasionally the length of the bottle-neck allows of a fresh arrangement and the cavity is divided into three storeys. The bottom one, which is always the most s.p.a.cious, contains a female; the two above, both smaller than the first and one smaller than the other, contain males.

Let us keep to the first case, which is always the most frequent. The Osmia is in the presence of one of these pear-shaped hollows. It is a find that must be employed to the best advantage: a prize of this sort is rare and falls only to fortune's favourites. To lodge two females in it at once is impossible; there is not sufficient room. To lodge two males in it would be undue generosity to a s.e.x that is ent.i.tled to but the smallest consideration. Besides, the two s.e.xes must be represented in almost equal numbers. The Osmia decides upon one female, whose portion shall be the better room, the lower one, which is larger, better-protected and more nicely polished, and one male, whose portion shall be the upper storey, a cramped attic, uneven and rugged in the part which encroaches on the bottle-neck. This decision is proved by numerous undeniable facts. Both Osmiae therefore can choose the s.e.x of the egg about to be laid, seeing that they are now breaking up the laying into groups of two, a female and a male, as required by the conditions of the lodging.

I have only once found Latreille's Osmia established in the nest of the Masked Anthophora. She had occupied but a small number of cells, because the others were not free, being inhabited by the Anthophora. The cells in question were divided into three storeys by part.i.tions of green mortar; the lower storey was occupied by a female, the two others by males, with smaller coc.o.o.ns.

I came to an even more remarkable example. Two Anthidia of my district, A. septemdentatum, LATR., and A. bellicosum, LEP., adopt as the home of their offspring the empty sh.e.l.ls of different snails: Helix aspersa, H.

algira, H. nemoralis, H. caespitum. The first-named, the Common Snail, is the most often used, under the stone-heaps and in the crevices of old walls. Both Anthidia colonize only the second whorl of the spiral. The central part is too small and remains unoccupied. Even so with the front whorl, the largest, which is left completely empty, so much so that, on looking through the opening, it is impossible to tell whether the sh.e.l.l does or does not contain the Bee's nest. We have to break this last whorl if we would perceive the curious nest tucked away in the spiral.

We then find first a transversal part.i.tion, formed of tiny bits of gravel cemented by a putty made from resin, which is collected in fresh drops from the oxycedrus and the Aleppo pine. Beyond this is a stout barricade made up of rubbish of all kinds: bits of gravel, sc.r.a.ps of earth, juniper-needles, the catkins of the conifers, small sh.e.l.ls, dried excretions of Snails. Next come a part.i.tion of pure resin, a large coc.o.o.n in a roomy chamber, a second part.i.tion of pure resin and, lastly, a smaller coc.o.o.n in a narrow chamber. The inequality of the two cells is the necessary consequence of the shape of the sh.e.l.l, whose inner s.p.a.ce gains rapidly in width as the spiral gets nearer to the orifice. Thus, by the mere general arrangement of the home and without any work on the Bee's part beyond some slender part.i.tions, a large room is marked out in front and a much smaller room at the back.

By a very remarkable exception, which I have mentioned casually elsewhere, the males of the genus Anthidium are generally larger than the females; and this is the case with the two species in particular that divide the Snail's spiral with resin part.i.tions. I collected some dozens of nests of both species. In at least half the cases, the two s.e.xes were present together; the female, the smaller, occupied the front cell and the male, the bigger, the back cell. Other cells, which were smaller or too much obstructed at the back by the dried-up remains of the Mollusc, contained only one cell, occupied at one time by a female and at another by a male. A few, lastly, had both cells inhabited now by two males and now by two females. The most frequent arrangement was the simultaneous presence of both s.e.xes, with the female in front and the male behind. The Anthidia who make resin-dough and live in Snail-sh.e.l.ls can therefore alternate the s.e.xes regularly to meet the exigencies of the spiral dwelling-house.

One more thing and I have done. My apparatus of reeds, fixed against the walls of the garden, supplied me with a remarkable nest of the Horned Osmia. The nest is established in a bit of reed 11 millimetres wide inside. (.429 inch--Translator's Note.) It comprises thirteen cells and occupies only half the cylinder, although the orifice is plugged with the usual stopper. The laying therefore seems here to be complete.

Well, this laying is arranged in a most singular fashion. There is first, at a suitable distance from the bottom or the node of the reed, a transversal part.i.tion, perpendicular to the axis of the tube. This marks off a cell of unusual size, in which a female is lodged. After that, in view of the excessive width of the tunnel, which is too great for a series in single file, the Osmia appears to alter her mind. She therefore builds a part.i.tion perpendicular to the transversal part.i.tion which she has just constructed and thus divides the second storey into two rooms, a larger room, in which she lodges a female, and a smaller, in which she lodges a male. She next builds a second transversal part.i.tion and a second longitudinal part.i.tion perpendicular to it. These once more give two unequal chambers, stocked likewise, the large one with a female, the smaller one with a male.

From this third storey onwards, the Osmia abandons geometrical accuracy; the architect seems to be a little out in her reckoning. The transversal part.i.tions become more and more slanting and the work grows irregular, but always with a sprinkling of large chambers for the females and small chambers for the males. Three females and two males are housed in this way, the s.e.xes alternating.

By the time that the base of the eleventh cell is reached, the transversal part.i.tion is once more almost perpendicular to the axis.

Here what happened at the bottom is repeated. There is no longitudinal part.i.tion; and the s.p.a.cious cell, covering the whole diameter of the cylinder, receives a female. The edifice ends with two transversal part.i.tions and one longitudinal part.i.tion, which mark out, on the same level, chambers twelve and thirteen, both of which contain males.

There is nothing more curious than this mixing of the two s.e.xes, when we know with what precision the Osmia separates them in a linear series, where the narrow width of the cylinder demands that the cells shall be set singly, one above the other. Here, the Bee is making use of a tube whose diameter is not suited to her work; she is constructing a complex and difficult edifice, which perhaps would not possess the necessary solidity if the ceilings were too broad. The Osmia therefore supports these ceilings with longitudinal part.i.tions; and the unequal chambers resulting from the introduction of these part.i.tions receive females at one time and males at another, according to their capacity.

CHAPTER 5. PERMUTATIONS OF s.e.x.

The s.e.x of the egg is optional. The choice rests with the mother, who is guided by considerations of s.p.a.ce 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.