Does the insect know beforehand the sex of the egg which it is about to
lay? When examining the stock of food in the cells just now, we began
to suspect that it does, for each little heap of provisions is carefully
proportioned to the needs at one time of a male and at another of a
female. 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. Digging up the burrows of
Cerceris-, Bembex- or Philanthus-wasps will never tell us that this grub
has taken precedence of that in point of time nor enable us to decide
whether one cocoon in a colony belongs to the same family as another. To
compile a register of births is absolutely impossible here. 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, formed of bramble saw-dust and of a green paste obtained by
masticating particles of the leaves of some plant that has not yet
been identified. 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 a 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.
To know the sexes, we must wait for the month of June. But it would be
unwise to postpone our investigations until that period. Osmia-nests are
not so common that we can hope to pick one up each time that we go out
with that object; besides, if we wait for the hatching-period before
examining the brambles, it may happen that the order has been disturbed
through some insects' having tried to make their escape as soon as
possible after bursting their cocoons; it may happen that the male
Osmiae, who are more forward than the females, are already gone. I
therefore set to work a long time beforehand and devote my leisure in
winter to these investigations.
The bramble-sticks are split and the cocoons taken out one by one and
methodically transferred to glass tubes, of approximately the same
diameter as the native cylinder. These cocoons are arranged one on
top of the other in exactly the same order that they occupied in the
bramble; they are separated from one another by a cotton plug, an
insuperable obstacle to the future insect. There is thus no fear that
the contents of the cells may become mixed or transposed; and I am saved
the trouble of keeping a laborious watch. Each insect can hatch at its
own time, in my presence or not: I am sure of always finding it in
its place, in its proper order, held fast fore and aft by the cotton
barrier. A cork or sorghum-pith partition would not fulfil the same
purpose: the insect would perforate it and the register of births would
be muddled by changes of position. Any reader wishing to undertake
similar investigations will excuse these practical details, which may
facilitate his work.
We do not often come upon complete series, comprising the whole laying,
from the first-born to the youngest. As a rule, we find part of a
laying, in which the number of cocoons varies greatly, sometimes falling
as low as two, or even one. The mother has not deemed it advisable to
confide her whole family to a single bramble-stump; in order to make the
exit less toilsome, or else for reasons which escape me, she has left
the first home and elected to make a second home, perhaps a third or
more.
We also find series with breaks in them. Sometimes, in cells distributed
at random, the egg has not developed and the provisions have remained
untouched, but mildewed; sometimes, the larva has died before spinning
its cocoon, or after spinning it. Lastly, there are parasites, such
as the Unarmed Zonitis (Zonitis mutica, one of the
Oil-beetles.--Translator's Note.) and the Spotted Sapyga (A
Digger-wasp.--Translator's Note.), who interrupt the series by
substituting themselves for the original occupant. All these disturbing
factors make it necessary to examine a large number of nests of the
Three-pronged Osmia, if we would obtain a definite result.
I have been studying the bramble-dwellers for seven or eight years and I
could not say how many strings of cocoons have passed through my hands.
During a recent winter, in view particularly of the distribution of the
sexes, I collected some forty of this Osmia's nests, transferred their
contents into glass tubes and made a careful summary of the sexes.
I give some of my results. The figures start in their order from the
bottom of the tunnel dug in the bramble and proceed upwards to the
orifice. The figure 1 therefore denotes the first-born of the series,
the oldest in date; the highest figure denotes the last-born. The letter
M, placed under the corresponding figure, represents the male and the
letter F the female sex.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 F F M F M F M M F F F F M F M
This is the longest series that I have ever been able to procure. It is
also complete, inasmuch as it comprises the entire laying of the Osmia.
My statement requires explaining, otherwise it would seem impossible to
know whether a mother whose acts one has not watched, nay more, whom
one has never seen, has or has not finished laying her eggs. The
bramble-stump under consideration leaves a free space of nearly four
inches above the continuous string of cocoons. Beyond it, at the actual
orifice, is the terminal stopper, the thick plug which closes the
entrance to the gallery. In this empty portion of the tunnel there is
ample accommodation for numerous cocoons. The fact that the mother has
not made use of it proves that her ovaries were exhausted; for it is
exceedingly unlikely that she has abandoned first-rate lodgings to
go laboriously digging a new gallery elsewhere and there continue her
laying.
You may say that, if the unoccupied space marks the end of the laying,
nothing tells us that the beginning is actually at the bottom of the
cul-de-sac, at the other end of the tunnel. You may also say that the
laying is done in shifts, separated by intervals of rest. The space left
empty in the channel would mean that one of these shifts was finished
and not that there were no more eggs ripe for hatching. In answer
to these very plausible explanations, I will say that, the sum of my
observations--and they have been extremely numerous--is that the total
number of eggs laid not only by the Osmiae but by a host of other Bees
fluctuates round about fifteen.
Besides, when we consider that the active life of these insects lasts
hardly a month; when we remember that this period of activity is
disturbed by dark, rainy or very windy days, during which all work is
suspended; when lastly we ascertain, as I have done ad nauseam in the
case of the Three-horned Osmia, the time required for building and
victualling a cell, it becomes obvious that the total laying must be
kept within narrow bounds and that the mother has no time to lose if she
wishes to get fifteen cells satisfactorily built in three or four weeks
interrupted by compulsory rests. I shall give some facts later which
will dispel your doubts, if any remain.
I assume, therefore, that a number of eggs bordering on fifteen
represents the entire family of an Osmia, as it does of many other Bees.
Let us consult some other complete series. Here are two:
1 2 3 4 5 6 7 8 9 10 11 12 13 F F M F M F M F F F F M F F M F F F M F F
M F M
In both cases, the laying is taken as complete, for the same reasons as
above.
We will end with some series that appear to me incomplete, in view of
the small number of cells and the absence of any free space above the
pile of cocoons:
1 2 3 4 5 6 7 8 M M F M M M M M M M F M F M M M F M F F M M M M M F M F
F F F M M M F M
These examples are more than sufficient. It is quite evident that the
distribution of the sexes is not governed by any rule. All that I can
say on consulting the whole of my notes, which contain a good many
instances of complete layings--most of them, unfortunately, spoilt
through gaps caused by parasites, the death of the larva, the failure of
the egg to hatch and other accidents--all that I can say in general is
that the complete series begins with females and nearly always ends with
males. The incomplete series 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 brambles, in my district, harbour two other Osmiae, both of much
smaller size: O. detrita, PEREZ, and O. parvula, DUF. The first is very
common, the second very rare; and until now I have found only one of
her nests, placed above a nest of O. detrita, in the same bramble. Here,
instead of the lack of order in the distribution of the sexes which we
find with O. tridentata, we have an order remarkable for consistency
and simplicity. I have before me the list of the series of O. detrita
collected last winter. Here are some of them:
1. A series of twelve: seven females, beginning with the bottom of the
tunnel, and then five males.
2. A series of nine: three females first, then six males.
3. A series of eight: five females followed by three males.
4. A series of eight: seven females followed by one male.
5. A series of eight: one female followed by seven males.
6. A series of seven: six females followed by one male.
The first series might very well be complete. The second and fifth
appear to be the end of layings, of which the beginning has taken place
elsewhere, in another bramble-stump. The males predominate and finish
off the series. Nos. 3, 4 and 6, on the other hand, look like the
beginnings of layings: the females predominate and are at the head of
the series. Even if these interpretations should be open to doubt, one
result at least is certain: with O. detrita, the laying is divided into
two groups, with no intermingling of the sexes; the first group laid
yields nothing but females, the second, or more recent, yields nothing
but males.
What was only a sort of attempt with the Three-pronged Osmia--who, it is
true, begins with females and ends with males, but muddles up the order
and mixes the two sexes anyhow between the extreme points--becomes a
regular law with her kinswoman. 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.
O. parvula, of whom I unfortunately possess but one series, repeats
what the previous witness has just shown us. This series, one of nine
cocoons, comprises five females followed by four males, without any
mixing of the sexes.
Next to these disgorgers of honey and gleaners of pollen-dust, it would
be well to consult other Hymenoptera, Wasps who devote themselves to the
chase and pile their cells one after the other, in a row, showing
the relative age of the cocoons. The brambles house several of these:
Solenius vagus, who stores up Flies; Psen atratus, who provides her
grubs with a heap of Plant-lice; Trypoxylon figulus, who feeds them with
Spiders.
Solenius vagus digs her gallery in a bramble-stick that is lopped short,
but still fresh and green. The house of this Fly-huntress, therefore,
suffers from damp, as the sap enters, especially on the lower floors.
This seems to me rather insanitary. To avoid the humidity, or for other
reasons which escape me, the Solenius does not dig very far into her
bramble-stump and consequently can stack but a small number of cells in
it. A series of five cocoons gives me first four females and then one
male; another series, also of five, contains first three females, with
two males following. These are the most complete that I have for the
moment.
I reckoned on the Black Psen, or Psen atratus, whose series are pretty
long; it is a pity that they are nearly always greatly interfered with
by a parasite called Ephialtes mediator. (Cf. "The Life of the Fly":
chapter 2.--Translator's Note.) I obtained only three series free
from gaps: one of eight cocoons, comprising only females; one of six,
likewise consisting wholly of females; lastly, one of eight, formed
exclusively of males. These instances seem to show that the Psen
arranges her laying in a succession of females and a succession of
males; but they tell us nothing of the relative order of the two series.
From the Spider-huntress, Trypoxylon figulus, I learnt nothing
decisive. She appeared to me to rove about from one bramble to the next,
utilizing galleries which she has not dug herself. Not troubling to be
economical with a lodging which it has cost her nothing to acquire, she
carelessly builds a few partitions at very unequal heights, stuffs
three or four compartments with Spiders and passes on to another
bramble-stump, with no reason, so far as I know, for abandoning the
first. Her cells, therefore, occur in series that are too short to give
us any useful information.
This is all that the bramble-dwellers have to tell us; I have enumerated
the list of the principal ones in my district. We will now look
into some other Bees who arrange their cocoons in single files: the
Megachiles (Cf. Chapter 8 of the present volume.--Translator's Note.),
who cut disks out of leaves and fashion the disks into thimble-shaped
receptacles; the Anthidia (Cf. Chapters 9 and 10 of the present
volume.--Translator's Note.), who weave their honey-wallets out
of cotton-wool and arrange their cells one after the other in some
cylindrical gallery. In most cases, the home is the produce of neither
the one nor the other. A tunnel in the upright, earthy banks, the old
work of some Anthophora, is the usual dwelling. There is no great depth
to these retreats; and all my searches, zealously prosecuted during a
number of winters, procured me only series containing a small number of
cocoons, four or five at most, often one alone. And, what is quite as
serious, nearly all these series are spoilt by parasites and allow me to
draw no well-founded deductions.
I remembered finding, at rare intervals, nests of both the Anthidium and
the Megachile in the hollows of cut reeds. I thereupon 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, Anthidia and Megachiles came in
fairly large numbers, especially the first, to benefit by the queer
installation.
In this way I obtained some magnificent series of Anthidia and
Megachiles, running up to a dozen. There was a melancholy side to
this success. All my series, with not one exception, were ravaged by
parasites. Those of the Megachile (M. sericans, FONSCOL), who fashions
her goblets with robinia-, holm-, and terebinth-leaves, were inhabited
by Coelioxys octodentata (A Parasitic Bee.--Translator's Note.); those
of the Anthidium (A. florentinum, LATR.) were occupied by a Leucopsis.
Both kinds were swarming with a colony of pigmy parasites whose name I
have not yet been able to discover. In short, my pan-pipe hives, though
very useful to me from other points of view, taught me nothing about the
order of the sexes among the Leaf-cutters and the cotton-weavers.
I was more fortunate with three Osmiae (O. tricornis, LATR., O. cornuta,
LATR., and O. Latreillii, SPIN.), all of whom gave me splendid results,
with reed-stumps arranged either against the walls 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, using reeds, glass tubes and other retreats
of my selecting for her galleries.
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 round about 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 a 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 gets 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.
Before dismissing the Osmiae, let us devote a moment to their cocoons, a
comparison of which, in the matter of bulk, will furnish us with fairly
accurate evidence as to the relative size of the two sexes, for the
thing contained, the perfect insect, is evidently proportionate to the
silken wrapper in which it is enclosed. These cocoons are oval-shaped
and may be regarded as ellipsoids formed by a revolution around the
major axis. The volume of one of these solids is expressed in the
following formula:
4 / 3 x pi x a x (b squared),
in which 2a is the major axis and 2b the minor axis.
Now, the average dimensions of the cocoons of the Three-horned Osmia are
as follows:
2a = 13 mm. (.507 inch.--Translator's Note.), 2b = 7 mm. (.273
inch.--Translator's Note.) in the females;
2a = 9 mm. (.351 inch.--Translator's Note.), 2b = 5 mm. (.195
inch.--Translator's Note.) in the males.
The ratio therefore between 13 x 7 x 7 = 637 and 9 x 5 x 5 = 225 will be
more or less the ratio between the sizes of the two sexes. This ratio
is somewhere between 2 to 1 and 3 to 1. The females therefore are two or
three times larger than the males, a proportion already suggested by a
comparison of the mass of provisions, estimated simply by the eye.
The Horned Osmia gives us the following average dimensions:
2a = 15 mm. (.585 inch.--Translator's Note.), 2b = 9 mm. (.351
inch.--Translator's Note.) in the females;
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 sexes, 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
sexes.
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 cocoon from its box, open it and take note of the sex 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 cluster 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 cluster. 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 sexes forming a series of its own,
independent of the other.
Some further circumstances 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 cluster; 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 glass 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 sexes 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 assist 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 circumference, 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-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.
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, 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-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. And that, I think, is why they begin
with spacious 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 sexes, 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-pronged 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.