Bramble-Bees and Others Part 5

2a = 12 mm. (.468 inch.--Translator's Note.), 2b = 7 mm. (.273 inch.--Translator's Note.) in the males.

Once again, the ratio between 15 x 9 x 9 = 1215 and 12 x 7 x 7 = 588 lies between 2 to 1 and 3 to 1.

Besides the Bees who arrange their laying in a row, I have consulted others whose cells are grouped in a way that makes it possible to ascertain the relative order of the two s.e.xes, though not quite so precisely. One of these is the Mason-bee of the Walls. I need not describe again her dome-shaped nest, built on a pebble, which is now so well-known to us. (Cf. "The Mason-bees": chapter 1.--Translator's Note.)

Each mother chooses her stone and works on it in solitude. She is an ungracious landowner and guards her site jealously, driving away any Mason who even looks as though she might alight on it. The inhabitants of the same nest are therefore always brothers and sisters; they are the family of one mother.

Moreover, if the stone presents a large enough surface--a condition easily fulfilled--the Mason-bee has no reason to leave the support on which she began her laying and go in search of another whereon to deposit the rest of her eggs. She is too thrifty of her time and of her mortar to involve herself in such expenditure except for grave reasons.

Consequently, each nest, at least when it is new, when the Bee herself has laid the first foundations, contains the entire laying. It is a different thing when an old nest is restored and made into a place for depositing the eggs. I shall come back later to such houses.

A newly-built nest then, with rare exceptions, contains the entire laying of one female. Count the cells and we shall have the total list of the family. Their maximum number fluctuates round about fifteen.

The most luxuriant series will occasionally reach as many as eighteen, though these are very scarce.

When the surface of the stone is regular all around the site of the first cell, when the mason can add to her building with the same facility in every direction, it is obvious that the groups of cells, when finished, will have the oldest in the central portion and the more recent in the surrounding portion. Because of this juxtaposition of the cells, which serve partly as a wall to those which come next, it is possible to form some estimate of the chronological order of the cells in the Chalicodoma's nest and thus to discover the sequence of the two s.e.xes.

In winter, by which time the Bee has long been in the perfect state, I collect Chalicodoma-nests, removing them bodily from their support with a few smart sideward taps of the hammer on the pebbles. At the base of the mortar dome the cells are wide agape and display their contents. I take the coc.o.o.n from its box, open it and take note of the s.e.x of the insect enclosed.

I should probably be accused of exaggeration if I mentioned the total number of the nests which I have gathered and the cells which I have inspected by this method during the last six or seven years. I will content myself with saying that the harvest of a single morning sometimes consisted of as many as sixty nests of the Mason-bee. I had to have help in carrying home my spoils, even though the nests were removed from their stones on the spot.

From the enormous number of nests which I have examined, I am able to state that, when the cl.u.s.ter is regular, the female cells occupy the centre and the male cells the edges. Where the irregularity of the pebble has prevented an even distribution around the initial point, the same rule has been observed. A male cell is never surrounded on every side by female cells: either it occupies the edges of the nest, or else it adjoins, at least on some sides, other male cells, of which the last form part of the exterior of the cl.u.s.ter. As the surrounding cells are obviously of a later date than the inner cells, it follows that the Mason-bee acts like the Osmiae: she begins her laying with females and ends it with males, each of the s.e.xes forming a series of its own, independent of the other.

Some further circ.u.mstances add their testimony to that of the surrounded and surrounding cells. When the pebble projects sharply and forms a sort of dihedral angle, one of whose faces is more or less vertical and the other horizontal, this angle is a favourite site with the Mason, who thus finds greater stability for her edifice in the support given her by the double plane. These sites appear to me to be in great request with the Chalicodoma, considering the number of nests which I find thus doubly supported. In nests of this kind, all the cells, as usual, have their foundations fixed to the horizontal surface; but the first row, the row of cells first built, stands with its back against the vertical surface.

Well, these older cells, which occupy the actual edge of the dihedral angle, are always female, with the exception of those at either end of the row, which, as they belong to the outside, may be male cells. In front of this first row come others. The female cells occupy the middle portion and the male the ends. Finally, the last row, closing in the remainder, contains only male cells. The progress of the work is very visible here: the Mason has begun by attending to the central group of female cells, the first row of which occupies the dihedral angle, and has finished her task by distributing the male cells round the outside.

If the perpendicular face of the dihedral angle be high enough, it sometimes happens that a second row of cells is placed above the first row backing on to that plane; a third row occurs less often. The nest is then one of several storeys. The lower storeys, the older, contain only females; the upper, the more recent storey, contains none but males. It goes without saying that the surface layer, even of the lower storeys, can contain males without invalidating the rule, for this layer may always be looked upon as the Chalicodoma's last work.

Everything therefore contributes to show that, in the Mason-bee, the females take the lead in the order of primogeniture. Theirs is the central and best-protected part of the clay fortress; the outer part, that most exposed to the inclemencies of the weather and to accidents, is for the males.

The males' cells do not differ from the females' only by being placed at the outside of the cl.u.s.ter; they differ also in their capacity, which is much smaller. To estimate the respective capacities of the two sorts of cells, I go to work as follows: I fill the empty cell with very fine sand and pour this sand back into a gla.s.s tube measuring 5 millimetres (.195 inch.--Translator's Note.) in diameter. From the height of the column of sand we can estimate the comparative capacity of the two kinds of cells. I will take one at random among my numerous examples of cells thus gauged.

It comprises thirteen cells and occupies a dihedral angle. The female cells give me the following figures, in millimetres, as the height of the columns of sand:

40, 44, 43, 48, 48, 46, 47 (1.56, 1.71, 1.67, 1.87, 1.87, 1.79, 1.83 inches.--Translator's Note.),

averaging 45. (1.75 inches.--Translator's Note.)

The male cells give me:

32, 35, 28, 30, 30, 31 (1.24, 1.36, 1.09, 1.17, 1.17, 1.21 inches.--Translator's Note.),

averaging 31. (1.21 inches.--Translator's Note.)

The ratio of the capacity of the cells for the two s.e.xes is therefore roughly a ratio of 4 to 3. The actual contents of the cell being proportionate to its capacity, the above ratio must also be more or less the ratio of provisions and sizes between females and males. These figures will a.s.sist us presently to tell whether an old cell, occupied for a second or third time, belonged originally to a female or a male.

The Chalicodoma of the Sheds cannot give us any information on this matter. She builds under the same eaves, in excessively populous colonies; and it is impossible to follow the labours of any single Mason, whose cells, distributed here and there, are soon covered up with the work of her neighbours. All is muddle and confusion in the individual output of the swarming throng.

I have not watched the work of the Chalicodoma of the Shrubs with close enough attention to be able to state definitely that this Bee is a solitary builder. Her nest is a ball of clay hanging from a bough.

Sometimes, this nest is the size of a large walnut and then appears to be the work of one alone; sometimes, it is the size of a man's fist, in which case I have no doubt that it is the work of several. Those bulky nests, comprising more than fifty cells, can tell us nothing exact, as a number of workers must certainly have collaborated to produce them.

The walnut-sized nests are more trustworthy, for everything seems to indicate that they were built by a single Bee. Here females are found in the centre of the group and males at the circ.u.mference, in somewhat smaller cells, thus repeating what the Mason-bee of the Pebbles has told us.

One clear and simple rule stands out from this collection of facts.

Apart from the strange exception of the Three-p.r.o.nged Osmia, who mixes the s.e.xes 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 s.e.xes 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 a.n.a.logy continues down to the capacity of the cells and the quant.i.ties of provisions. The real females, the Queen-bees, have wax cells incomparably more s.p.a.cious 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.

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 s.e.xes 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 circ.u.mstances require it?

The Three-p.r.o.nged Osmia already shows us that the problem is far from being solved. In the same bramble-stump, the two s.e.xes occur very irregularly, as though at random. Why this mixture in the series of coc.o.o.ns of a Bee closely related to the Horned Osmia and the Three-horned Osmia, who stack theirs methodically by separate s.e.xes 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, can explain this 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-p.r.o.nged 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 coc.o.o.n: 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 part.i.tions 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 quant.i.ty of s.p.a.ce and food for both s.e.xes.

This result makes us foresee what a direct examination of the two s.e.xes 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 coc.o.o.ns. In the Mason-bee of the Walls there is also a difference in size, though less p.r.o.nounced.

The Three-p.r.o.nged Osmia has not therefore to trouble about adjusting the dimensions of the dwelling and the quant.i.ty of the food to the s.e.x 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 s.e.xes 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 s.e.xes, have to be careful about the twofold consideration of board and lodging. And that, I think, is why they begin with s.p.a.cious cells and generous rations for the homes of the females and end with narrow, scantily-provisioned cells, the homes of the males.

With this sequence, sharply defined for the two s.e.xes, there is less fear of mistakes which might give to one what belongs to another. If this is not the explanation of the facts, I see no other.

The more I thought about this curious question, the more probable it appeared to me that the irregular series of the Three-p.r.o.nged Osmia and the regular series of the other Osmiae, of the Chalicodomae 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.

CHAPTER 4. THE MOTHER DECIDES THE s.e.x OF THE EGG.

I will begin with the Mason-bee of the Pebbles. (This is the same insect as the Mason-bee of the Walls. Cf. "The Mason-bees": pa.s.sim.--Translator's Note.) The old nests are often used, when they are in good enough repair. Early in the season the mothers quarrel fiercely over them; and, when one of the Bees has taken possession of the coveted dome, she drives any stranger away from it. The old house is far from being a ruin, only it is perforated with as many holes as it once had occupants. The work of restoration is no great matter. The heap of earth due to the destruction of the lid by the outgoing tenant is taken out of the cell and flung away at a distance, atom by atom. The remnants of the coc.o.o.n are also thrown away, but not always, for the delicate silken wrapper sometimes adheres closely to the masonry.

The victualling of the renovated cell is now begun. Next comes the laying; and lastly the orifice is sealed with a mortar plug. A second cell is utilized in the same way, followed by a third and so on, one after the other, as long as any remain unoccupied and the mother's ovaries are not exhausted. Finally, the dome receives, mainly over the apertures already plugged, a coat of plaster which makes the nest look like new. If she has not finished her laying, the mother goes in search of other old nests to complete it. Perhaps she does not decide to found a new establishment except when she can find no second-hand dwellings, which mean a great economy of time and labour. In short, among the countless number of nests which I have collected, I find many more ancient than recent ones.

How shall we distinguish one from the other? The outward aspect tells you nothing, owing to the great care taken by the Mason to restore the surface of the old dwelling equal to new. To resist the rigours of the winter, this surface must be impregnable. The mother knows that and therefore repairs the dome. Inside, it is another matter: the old nest stands revealed at once. There are cells whose provisions, at least a year old, are intact, but dried up or musty, because the egg has never developed. There are others containing a dead larva, reduced by time to a blackened, curled-up cylinder. There are some whence the perfect insect was never able to issue: the Chalicodoma wore herself out in trying to pierce the ceiling of her chamber; her strength failed her and she perished in the attempt. Others again and very many are occupied by ravagers, Leucopses (Cf. "The Mason-bees": chapter 11.--Translator's Note.) and Anthrax-flies, who will come out a good deal later, in July.

Altogether, the house is far from having every room vacant; there are nearly always a considerable number occupied either by parasites that were still in the egg-stage at the time when the Mason-bee was at work or by damaged provisions, dried grubs or Chalicodomae in the perfect state who have died without being able to effect their deliverance.

Should all the rooms be available, a rare occurrence, there still remains a method of distinguis.h.i.+ng between an ancient nest and a recent one. The coc.o.o.n, as I have said, adheres pretty closely to the walls; and the mother does not always take away this remnant, either because she is unable to do so, or because she considers the removal unnecessary. Thus the base of the new coc.o.o.n is set in the bottom of the old coc.o.o.n. This double wrapper points very clearly to two generations, two separate years. I have even found as many as three coc.o.o.ns fitting one into another at their bases. Consequently, the nests of the Mason-bee of the Pebbles are able to do duty for three years, if not more. Eventually they become utter ruins, abandoned to the Spiders and to various smaller Bees or Wasps, who take up their quarters in the crumbling rooms.

As we see, an old nest is hardly ever capable of containing the Mason-bee's entire laying, which calls for some fifteen apartments. The number of rooms at her disposal is most unequal, but always very small.

It is saying much when there are enough to receive about half the laying. Four or five cells, sometimes two or even one: that is what the Mason usually finds in a nest that is not her own work. This large reduction is explained when we remember the numerous parasites that live upon the unfortunate Bee.

Now, how are the s.e.xes distributed in those layings which are necessarily broken up between one old nest and another? They are distributed in such a way as utterly to upset the idea of an invariable succession first of females and then of males, the idea which occurs to us on examining the new nests. If this rule were a constant one, we should be bound to find in the old domes at one time only females, at another only males, according as the laying was at its first or at its second stage. The simultaneous presence of the two s.e.xes would then correspond with the transition period between one stage and the next and should be very unusual. On the contrary, it is very common; and, however few cells there may be, we always find both females and males in the old nests, on the sole condition that the compartments have the regulation holding-capacity, a large capacity for the females, a lesser for the males, as we have seen.

The old male cells can be recognized by their position on the outer edges and by their capacity, measuring on an average the same as a column of sand 31 millimetres high in a gla.s.s tube 5 millimetres wide.

(1.21 x.195 inches.--Translator's Note.) These cells contain males of the second or third generation and none but males. In the old female cells, those in the middle, whose capacity is measured by a similar column of sand 45 millimetres high (1.75 inches.--Translator's Note.), are females and none but females.

This presence of both s.e.xes at a time, even when there are but two cells free, one s.p.a.cious and the other small, proves in the plainest fas.h.i.+on 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 s.p.a.ce 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 s.e.x of the egg 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 s.e.xes 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 s.p.a.ce in the actual nest which she happens to be occupying.

Just now, in the new nest, we saw the Mason-bee arranging her total laying into series first of females and next of males; and here she is, mistress of an old nest of which she has not the power to alter the arrangement, breaking up her laying into sections comprising both s.e.xes just as required by the conditions imposed upon her. She therefore decides the s.e.x 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 s.e.x for which those chambers were originally built; and this happens however small the number of chambers to be filled.

When the nest is new, I think I see a reason why the Mason-bee should seriate her laying into females and then males. Her nest is a half-sphere. That of the Mason-bee of the Shrubs is very nearly a sphere. Of all shapes, the spherical shape is the strongest. Now these two nests require an exceptional power of resistance. Without protection of any kind, they have to brave the weather, one on its pebble, the other on its bough. Their spherical configuration is therefore very practical.