It is not enough that animal industry should be able, to a certain
extent, to adapt itself to casual exigencies when choosing the site of
a nest; if the race is to thrive, something else is required, something
which hide-bound instinct is unable to provide. The Chaffinch, for
instance, introduces a great quantity of lichen into the outer layer of
his nest. This is his method of strengthening the edifice and making
a stout framework in which to place first the bottom mattress of moss,
fine straw and rootlets and then the soft bed of feathers, wool and
down. But, should the time-honoured lichen be lacking, will the bird
refrain from building its nest? Will it forgo the delight of hatching
its brood because it has not the wherewithal to settle its family in the
orthodox fashion?
No, the chaffinch is not perplexed by so small a matter; he is an expert
in materials, he understands botanical equivalents. In the absence of
the branches of the evernias, he picks the long beards of the usneas,
the wartlike rosettes of the parmelias, the membranes of the stictises
torn away in shreds; if he can find nothing better, he makes shift with
the bushy tufts of the cladonias. As a practical lichenologist, when one
species is rare or lacking in the neighbourhood, he is able to fall back
on others, varying greatly in shape, colour and texture. And, if the
impossible happened and lichen failed entirely, I credit the Chaffinch
with sufficient talent to be able to dispense with it and to build the
foundations of his nest with some coarse moss or other.
What the worker in lichens tells us the other weavers of textile
materials confirm. Each has his favourite flora, which hardly ever
varies when the plant is easily accessible and which can be supplemented
by plenty of others when it is not. The bird's botany would be worth
examining; it would be interesting to draw up the industrial herbal of
each species. In this connection, I will quote just one instance, so as
not to stray too far from the subject in hand.
The Red-backed Shrike (Lanius collurio), the commonest variety in my
district, is noteworthy because of his savage mania for forked gibbets,
the thorns in the hedgerows whereon he impales the voluminous contents
of his game-bag--little half-fledged birds, small Lizards, Grasshoppers,
caterpillars, Beetles--and leaves them to get high. To this passion for
the gallows, which has passed unnoticed by the country-folk, at least
in my part, he adds another, an innocent botanical passion, which is
so much in evidence that everybody, down to the youngest bird's-nester,
knows all about it. His nest, a massive structure, is made of hardly
any other materials than a greyish and very fluffy plant, which is
found everywhere among the corn. This is the Filago spathulata of the
botanists; and the bird also makes use, though less frequently, of the
Filago germanica, or common cotton-rose. Both are known in Provencal by
the name herbo dou tarnagas, or Shrike-herb. This popular designation
tells us plainly how faithful the bird is to its plant. To have struck
the agricultural labourer, a very indifferent observer, the Shrike's
choice of materials must be remarkably persistent.
Have we here a taste that is exclusive? Not in the least. Though
cotton-roses of all species are plentiful on level ground, they become
scarce and impossible to find on the parched hills. The bird, on its
side, is not given to journeys of exploration and takes what it finds to
suit it in the neighbourhood of its tree or hedge. But on arid ground,
the Micropus erectus, or upright micropus, abounds and is a satisfactory
substitute for the Filago so far as its tiny, cottony leaves and its
little fluffy balls of flowers are concerned. True, it is short and
does not lend itself well to weaver's work. A few long sprigs of another
cottony plant, the Helichrysum staechas, or wild everlasting, inserted
here and there, will give body to the structure. Thus does the Shrike
manage when hard up for his favourite materials: keeping to the same
botanical family, he is able to find and employ substitutes among the
fine cotton-clad stalks.
He is even able to leave the family of the Compositae and to go gleaning
more or less everywhere. Here is the result of my botanizings at the
expense of his nests. We must distinguish between two genera in the
Shrike's rough classification: the cottony plants and the smooth plants.
Among the first, my notes mention the following: Convolvulus cantabrica,
or flax-leaved bindweed; Lotus symmetricus, or bird's-foot trefoil;
Teucrium polium, or poly; and the flowery heads of the Phragmites
communis, or common reed. Among the second are these: Medicago lupulina,
or nonesuch; Trifolium repens, or white clover; Lathyrus pratensis, or
meadow lathyrus; Capsella bursa pastoris, or shepherd's purse; Vicia
peregrina, or broad-podded vetch; Convolvulus arvensis, or small
bindweed; Pterotheca nemausensis, a sort of hawkweed; and Poa pratensis,
or smooth-stalked meadow-grass. When it is downy, the plant forms almost
the whole nest, as is the case with the flax-leaved bindweed; when
smooth, it forms only the framework, destined to support a crumbling
mass of micropus, as is the case with the small bindweed. When making
this collection, which I am far from giving as the birds' complete
herbarium, I was struck by a wholly unexpected detail: of the various
plants, I found only the heads still in bud; moreover, all the sprigs,
though dry, possessed the green colouring of the growing plant, a sign
of swift desiccation in the sun. Save in a few cases, therefore, the
Shrike does not collect the dead and withered remains: it is from the
growing plants that he reaps his harvest, mowing them down with his beak
and leaving the sheaves to dry in the sun before using them. I caught
him one day hopping about and pecking at the twigs of a Biscayan
bindweed. He was getting in his hay, strewing the ground with it.
The evidence of the Shrike, confirmed by that of all the other
workers--weavers, basket-makers or woodcutters--whom we may care to call
as witnesses, shows us what a large part must be assigned to discernment
in the bird's choice of materials for its nest. Is the insect as highly
gifted? When it works with vegetable matter, is it exclusive in its
tastes? Does it know only one definite plant, its special province? Or
has it, for employment in its manufactures, a varied flora, in which its
discernment exercises a free choice? For answers to these questions we
may look, above all, to the Leaf-cutting Bees, the Megachiles. Reaumur
has told the story of their industry in detail; and I refer the reader
who wishes for further particulars to the master's Memoirs.
The man who knows how to use his eyes in his garden will observe, some
day or other, a number of curious holes in the leaves of his lilac- and
rose-trees, some of them round, some oval, as if idle but skilful
hands had been at work with the pinking-iron. In some places, there is
scarcely anything but the veins of the leaves left. The author of the
mischief is a grey-clad Bee, a Megachile. For scissors, she has her
mandibles; for compasses, producing now an oval and anon a circle, she
has her eye and the pivot of her body. The pieces cut out are made into
thimble-shaped wallets, destined to contain the honey and the egg:
the larger, oval pieces supply the floor and sides; the smaller, round
pieces are reserved for the lid. A row of these thimbles, placed one on
top of the other, up to a dozen or more, though often there are less:
that is, roughly, the structure of the Leaf-cutter's nest.
When taken out of the recess in which the mother has manufactured it,
the cylinder of cells seems to be an indivisible whole, a sort of tunnel
obtained by lining with leaves some gallery dug underground. The real
thing does not correspond with its appearance: under the least pressure
of the fingers, the cylinder breaks up into equal sections, which are so
many compartments independent of their neighbours as regards both floor
and lid. This spontaneous break up shows us how the work is done. The
method agrees with those adopted by the other Bees. Instead of a
general scabbard of leaves, afterwards subdivided into compartments by
transverse partitions, the Megachile constructs a string of separate
wallets, each of which is finished before the next is begun.
A structure of this sort needs a sheath to keep the pieces in place
while giving them the proper shape. The bag of leaves, in fact, as
turned out by the worker, lacks stability; its numerous pieces, not
glued together, but simply placed one after the other, come apart and
give way as soon as they lose the support of the tunnel that keeps them
united. Later, when it spins its cocoon, the larva infuses a little
of its fluid silk into the gaps and solders the pieces to one another,
especially the inner ones, so much so that the insecure bag in due
course becomes a solid casket whose component parts it is no longer
possible to separate entirely.
The protective sheath, which is also a framework, is not the work of
the mother. Like the great majority of the Osmiae, the Megachiles do not
understand the art of making themselves a home straight away: they
want a borrowed lodging, which may vary considerably in character.
The deserted galleries of the Anthophorae, the burrows of the fat
Earth-worms, the tunnels bored in the trunks of trees by the larva of
the Cerambyx-beetle (The Capricorn, the essay on which has not yet been
published in English.--Translator's Note.), the ruined dwellings of
the Mason-bee of the Pebbles, the Snail-shell nests of the Three-horned
Osmia, reed-stumps, when these are handy, and crevices in the walls
are all so many homes for the Leaf-cutters, who choose this or that
establishment according to the tastes of their particular genus.
For the sake of clearness, let us cease generalizing and direct our
attention to a definite species. I first selected the White-girdled
Leaf-cutter (Megachile albocincta, PEREZ), not on account of any
exceptional peculiarities, but solely because this is the Bee most
often mentioned in my notes. Her customary dwelling is the tunnel of an
Earth-worm opening on some clay bank. Whether perpendicular or slanting,
this tunnel runs down to an indefinite depth, where the climate would be
too damp for the Bee. Besides, when the time comes for the hatching of
the adult insect, its emergence would be fraught with peril if it had
to climb up from a deep pit through crumbling rubbish. The Leaf-cutter,
therefore, uses only the front portion of the Worm's gallery, two
decimetres at most. (7.8 inches.--Translator's Note.) What is to be done
with the rest of the tunnel? It is an ascending shaft, tempting to an
enemy; and some underground ravager might come this way and destroy the
nest by attacking the row of cells at the back.
The danger is foreseen. Before fashioning her first honey-bag, the
Bee blocks the passage with a strong barricade composed of the only
materials used in the Leaf-cutter's guild. Fragments of leaves are
piled up in no particular order, but in sufficient quantities to make
a serious obstacle. It is not unusual to find in the leafy rampart some
dozens of pieces rolled into screws and fitting into one another like
a stack of cylindrical wafers. For this work of fortification, artistic
refinement seems superfluous; at any rate, the pieces of leaves are for
the most part irregular. You can see that the insect has cut them out
hurriedly, unmethodically and on a different pattern from that of the
pieces intended for the cells.
I am struck with another detail in the barricade. Its constituents
are taken from stout, thick, strong-veined leaves. I recognize young
vine-leaves, pale-coloured and velvety; the leaves of the whitish
rock-rose (Cistus albidus), lined with a hairy felt; those of the
holm-oak, selected among the young and bristly ones; those of the
hawthorn, smooth but tough; those of the cultivated reed, the only one
of the Monocotyledones exploited, as far as I know, by the Megachiles.
In the construction of cells, on the other hand, I see smooth leaves
predominating, notably those of the wild briar and of the common acacia,
the robinia. It would appear, therefore, that the insect distinguishes
between two kinds of materials, without being an absolute purist and
sternly excluding any sort of blending. The very much indented leaves,
whose projections can be completely removed with a dexterous snip of
the scissors, generally furnish the various layers of the barricade; the
little robinia-leaves, with their fine texture and their unbroken edges,
are better suited to the more delicate work of the cells.
A rampart at the back of the Earth-worm's shaft is a wise precaution and
the Leaf-cutter deserves all credit for it; only it is a pity for the
Megachiles' reputation that this protective barrier often protects
nothing at all. Here we see, under a new guise, that aberration of
instinct of which I gave some examples in an earlier chapter. My notes
contain memoranda of various galleries crammed with pieces of leaves
right up to the orifice, which is on a level with the ground, and
entirely devoid of cells, even of an unfinished one. These were
ridiculous fortifications, of no use whatever; and yet the Bee treated
the matter with the utmost seriousness and took infinite pains over her
futile task. One of these uselessly barricaded galleries furnished me
with some hundred pieces of leaves arranged like a stack of wafers;
another gave me as many as a hundred and fifty. For the defence of a
tenanted nest, two dozen and even fewer are ample. Then what was the
object of the Leaf-cutter's ridiculous pile?
I wish I could believe that, seeing that the place was dangerous, she
made her heap bigger so that the rampart might be in proportion to
the danger. Then, perhaps, at the moment of starting on the cells, she
disappeared, the victim of an accident, blown out of her course by
a gust of wind. But this line of defence is not admissible in the
Megachile's case. The proof is palpable: the galleries aforesaid are
barricaded up to the level of the ground; there is no room, absolutely
none, to lodge even a single egg. What was her object, I ask again, when
she persisted in obstinately piling up her wafers? Has she really an
object?
I do not hesitate to say no. And my answer is based upon what the Osmiae
taught me. I have described above how the Three-horned Osmia, towards
the end of her life, when her ovaries are depleted, expends on useless
operations such energy as remains to her. Born a worker, she is bored by
the inactivity of retirement; her leisure requires an occupation. Having
nothing better to do, she sets up partitions; she divides a tunnel
into cells that will remain empty; she closes with a thick plug reeds
containing nothing. Thus is the modicum of strength of her decline
exhausted in vain labours. The other Builder-bees behave likewise. I see
Anthidia laboriously provide numerous bales of cotton to stop galleries
wherein never an egg was laid; I see Mason-bees build and then
religiously close cells that will remain unvictualled and uncolonized.
The long and useless barricades then belong to the last hours of the
Megachile's life, when the eggs are all laid; the mother, whose ovaries
are exhausted, persists in building. Her instinct is to cut out and heap
up pieces of leaves; obeying this impulse, she cuts out and heaps up
even when the supreme reason for this labour ceases. The eggs are no
longer there, but some strength remains; and that strength is expended
as the safety of the species demanded in the beginning. The wheels of
action go on turning in the absence of the motives for action; they
continue their movement as though by a sort of acquired velocity. What
clearer proof can we hope to find of the unconsciousness of the animal
stimulated by instinct?
Let us return to the Leaf-cutter's work under normal conditions.
Immediately after a protective barrier comes the row of cells, which
vary considerably in number, like those of the Osmia in her reed.
Strings of about a dozen are rare; the most frequent consist of five or
six. No less subject to variation is the number of pieces joined to make
a cell: pieces of two kinds, some, the oval ones, forming the honey-pot;
others, the round ones, serving as a lid. I count, on an average, eight
to ten pieces of the first kind. Though all cut on the pattern of an
ellipse, they are not equal in dimensions and come under two categories.
The larger, outside ones are each of them almost a third of the
circumference and overlap one another slightly. Their lower end bends
into a concave curve to form the bottom of the bag. Those inside, which
are considerably smaller, increase the thickness of the sides and fill
up the gaps left by the first.
The Leaf-cutter therefore is able to use her scissors according to the
task before her: first, the large pieces, which help the work forward,
but leave empty spaces; next, the small pieces, which fit into the
defective portions. The bottom of the cell particularly comes in for
after-touches. As the natural curve of the larger pieces is not enough
to provide a cup without cracks in it, the Bee does not fail to improve
the work with two or three small oval pieces applied to the imperfect
joins.
Another advantage results from the snippets of unequal size. The three
or four outer pieces, which are the first placed in position, being
the longest of all, project beyond the mouth, whereas the next, being
shorter, do not come quite up to it. A brim is thus obtained, a ledge
on which the round disks of the lid rest and are prevented from touching
the honey when the Bee presses them into a concave cover. In other
words, at the mouth the circumference comprises only one row of leaves;
lower down it takes two or three, thus restricting the diameter and
securing an hermetic closing.
The cover of the pot consists solely of round pieces, very nearly alike
and more or less numerous. Sometimes I find only two, sometimes I count
as many as ten, closely stacked. At times, the diameter of these pieces
is of an almost mathematical precision, so much so that the edges of the
disk rest upon the ledge. No better result would be obtained had they
been cut out with the aid of compasses. At times, again, the piece
projects slightly beyond the mouth, so that, to enter, it has to be
pressed down and curved cupwise. There is no variation in the diameter
of the first pieces placed in position, those nearest to the honey.
They are all of the same size and thus form a flat cover which does not
encroach on the cell and will not afterwards interfere with the larva,
as a convex ceiling would. The subsequent disks, when the pile is
numerous, are a little larger; they only fit the mouth by yielding to
pressure and becoming concave. The Bee seems to make a point of this
concavity, for it serves as a mould to receive the curved bottom of the
next cell.
When the row of cells is finished, the task still remains of blocking up
the entrance to the gallery with a safety-stopper similar to the earthen
plug with which the Osmia closes her reeds. The Bee then returns to the
free and easy use of the scissors which we noticed at the beginning when
she was fencing off the back part of the Earth-worm's too deep burrow;
she cuts out of the foliage irregular pieces of different shapes and
sizes and often retaining their original deeply-indented margins; and
with all these pieces, very few of which fit at all closely the orifice
to be blocked, she succeeds in making an inviolable door, thanks to the
huge number of layers.
Let us leave the Leaf-cutter to finish depositing her eggs in other
galleries, which will be colonized in the same manner, and consider for
a moment her skill as a cutter. Her edifices consist of a multitude of
fragments belonging to three categories: oval pieces for the sides
of the cells; round pieces for the lids; and irregular pieces for the
barricades at the front and back. The last present no difficulty: the
Bee obtains them by removing from the leaf some projecting portion,
as it stands, a serrate lobe which, owing to its notches, shortens the
insect's task and lends itself better to scissor-work. So far, there
is nothing to deserve attention: it is unskilled labour, in which an
inexperienced apprentice might excel.
With the oval pieces, it becomes another matter. What model has the
Megachile when cutting her neat ellipses out of the delicate material
for her wallets, the robinia-leaves? What mental pattern guides her
scissors? What system of measurement tells her the dimensions? One would
like to picture the insect as a living pair of compasses, capable of
tracing an elliptic curve by a certain natural inflexion of its body,
even as our arm traces a circle by swinging from the shoulder. A
blind mechanism, the mere outcome of its organization, would alone be
responsible for its geometry. This explanation would tempt me if the
large oval pieces were not accompanied by much smaller ones, also oval,
which are used to fill the empty spaces. A pair of compasses which
changes its radius of its own accord and alters the curve according to
the plan before it appears to me an instrument somewhat difficult to
believe in. There must be something better than that. The circular
pieces of the lid suggest it to us.
If, by the mere flexion inherent in her structure, the Leaf-cutter
succeeds in cutting out ovals, how does she succeed in cutting out
rounds? Can we admit the presence of other wheels in the machinery for
the new pattern, so different in shape and size? Besides, the real point
of the difficulty does not lie there. These rounds, for the most part,
fit the mouth of the jar with almost exact precision. When the cell
is finished, the Bee flies hundreds of yards away to make the lid. She
arrives at the leaf from which the disk is to be cut. What picture, what
recollection has she of the pot to be covered? Why, none at all: she has
never seen it; she does her work underground, in utter darkness! At the
utmost, she can have the indications of touch: not actual indications,
of course, for the pot is not there, but past indications, useless in
a work of precision. And yet the disk to be cut out must have a fixed
diameter: if it were too large, it would not go in; if too small, it
would close badly, it would slip down on the honey and suffocate the
egg. How shall it be given its correct dimensions without a pattern? The
Bee does not hesitate for a moment. She cuts out her disk with the same
celerity which she would display in detaching any shapeless lobe that
might do for a stopper; and that disk, without further measurement, is
of the right size to fit the pot. Let whoso will explain this geometry,
which in my opinion is inexplicable, even when we allow for memory
begotten of touch and sight.
One winter evening, as we were sitting round the fire, whose cheerful
blaze unloosed our tongues, I put the problem of the Leaf-cutter to my
family:
'Among your kitchen-utensils,' I said, 'you have a pot in daily use;
but it has lost its lid, which was knocked over and broken by the Tomcat
playing among the shelves. To-morrow is market-day and one of you will
be going to Orange to buy the week's provisions. Would she undertake,
without a measure of any kind, with the sole aid of memory, which we
would allow her to refresh before starting by a careful examination of
the object, to bring back exactly what the pot wants, a lid neither too
large nor too small, in short the same size as the top?'
It was admitted with one accord that nobody would accept such a
commission without taking a measure with her, or at least a bit of
string giving the width. Our memory for sizes is not accurate enough.
She would come back from the town with something that 'might do'; and it
would be the merest chance if this turned out to be the right size.
Well, the Leaf-cutter is even less well-off than ourselves. She has no
mental picture of her pot, because she has never seen it; she is not
able to pick and choose in the crockery-dealer's heap, which acts as
something of a guide to our memory by comparison; she must, without
hesitation, far away from her home, cut out a disk that fits the top of
her jar. What is impossible to us is child's-play to her. Where we could
not do without a measure of some kind, a bit of string, a pattern or
a scrap of paper with figures upon it, the little Bee needs nothing at
all. In housekeeping matters she is cleverer than we are.
One objection was raised. Was it not possible that the Bee, when at work
on the shrub, should first cut a round piece of an approximate diameter,
larger than that of the neck of the jar, and that afterwards, on
returning home, she should gnaw away the superfluous part until the lid
exactly fitted the pot? These alterations made with the model in front
of her would explain everything.
That is perfectly true; but are there any alterations? To begin with, it
seems to me hardly possible that the insect can go back to the cutting
once the piece is detached from the leaf: it lacks the necessary support
to gnaw the flimsy disk with any precision. A tailor would spoil his
cloth if he had not the support of a table when cutting out the pieces
for a coat. The Megachile's scissors, so difficult to wield on anything
not firmly held, would do equally bad work.
Besides, I have better evidence than this for my refusal to believe in
the existence of alterations when the Bee has the cell in front of her.
The lid is composed of a pile of disks whose number sometimes reaches
half a score. Now the bottom part of all these disks is the under
surface of the leaf, which is paler and more strongly veined; the top
part is the upper surface, which is smooth and greener. In other words,
the insect places them in the position which they occupy when gathered.
Let me explain. In order to cut out a piece, the Bee stands on the
upper surface of the leaf. The piece detached is held in the feet and
is therefore laid with its top surface against the insect's chest at the
moment of departure. There is no possibility of its being turned over on
the journey. Consequently, the piece is laid as the Bee has just picked
it, with the lower surface towards the inside of the cell and the upper
surface towards the outside. If alterations were necessary to reduce the
lid to the diameter of the pot, the disk would be bound to get turned
over: the piece, manipulated, set upright, turned round, tried this way
and that, would, when finally laid in position, have its top or bottom
surface inside just as it happened to come. But this is exactly what
does not take place. Therefore, as the order of stacking never changes,
the disks are cut, from the first clip of the scissors, with their
proper dimensions. The insect excels us in practical geometry. I look
upon the Leaf-cutter's pot and lid as an addition to the many other
marvels of instinct that cannot be explained by mechanics; I submit it
to the consideration of science; and I pass on.
The Silky Leaf-cutter (Megachile sericans, FONSCOL.; M. Dufourii, LEP.)
makes her nests in the disused galleries of the Anthophorae. I know
her to occupy another dwelling which is more elegant and affords a more
roomy installation: I mean the old dwelling of the fat Capricorn, the
denizen of the oaks. The metamorphosis is effected in a spacious chamber
lined with soft felt. When the long-horned Beetle reaches the adult
stage, he releases himself and emerges from the tree by following a
vestibule which the larva's powerful tools have prepared beforehand.
When the deserted cabin, owing to its position, remains wholesome and
there is no sign of any running from its walls, no brown stuff smelling
of the tan-yard, it is soon visited by the Silky Megachile, who finds in
it the most sumptuous of the apartments inhabited by the Leaf-cutters.
It combines every condition of comfort: perfect safety, an even
temperature, freedom from damp, ample room; and so the mother who is
fortunate enough to become the possessor of such a lodging uses it
entirely, vestibule and drawing-room alike. Accommodation is found for
all her family of eggs; at least, I have nowhere seen nests as populous
as here.
One of them provides me with seventeen cells, the highest number
appearing in my census of the Megachile clan. Most of them are lodged in
the nymphal chamber of the Capricorn; and, as the spacious recess is too
wide for a single row, the cells are arranged in three parallel series.
The remainder, in a single string, occupy the vestibule, which is
completed and filled up by the terminal barricade. In the materials
employed, hawthorn-and paliurus-leaves predominate. The pieces, both
in the cells and in the barrier, vary in size. It is true that the
hawthorn-leaves, with their deep indentations, do not lend themselves to
the cutting of neat oval pieces. The insect seems to have detached each
morsel without troubling overmuch about the shape of the piece, so long
as it was big enough. Nor has it been very particular about arranging
the pieces according to the nature of the leaf: after a few bits of
paliurus come bits of vine and hawthorn; and these again are followed by
bits of bramble and paliurus. The Bee has collected her pieces anyhow,
taking a bit here and there, just as her fancy dictated. Nevertheless,
paliurus is the commonest, perhaps for economical reasons.
I notice, in fact, that the leaves of this shrub, instead of being
used piecemeal, are employed whole, when they do not exceed the proper
dimensions. Their oval form and their moderate size suit the insect's
requirements; and there is therefore no necessity to cut them into
pieces. The leaf-stalk is clipped with the scissors and, without more
ado, the Megachile retires the richer by a first-rate bit of material.
Split up into their component parts, two cells give me altogether
eighty-three pieces of leaves, whereof eighteen are smaller than the
others and of a round shape. The last-named come from the lids. If they
average forty-two each, the seventeen cells of the nest represent seven
hundred and fourteen pieces. These are not all: the nest ends, in the
Capricorn's vestibule, with a stout barricade in which I count three
hundred and fifty pieces. The total therefore amounts to one thousand
and sixty-four. All those journeys and all that work with the scissors
to furnish the deserted chamber of the Cerambyx! If I did not know the
Leaf-cutter's solitary and jealous disposition, I should attribute the
huge structure to the collaboration of several mothers; but there is
no question of communism in this case. One dauntless creature and one
alone, one solitary, inveterate worker, has produced the whole of
the prodigious mass. If work is the best way to enjoy life, this one
certainly has not been bored during the few weeks of her existence.
I gladly award her the most honourable of eulogies, that due to the
industrious; and I also compliment her on her talent for closing the
honey-pots. The pieces stacked into lids are round and have nothing
to suggest those of which the cells and the final barricade are made.
Excepting the first, those nearest the honey, they are perhaps cut a
little less neatly than the disks of the White-girdled Leaf-cutter; no
matter: they stop the jar perfectly, especially when there are some ten
of them one above the other. When cutting them, the Bee was as sure of
her scissors as a dressmaker guided by a pattern laid on the stuff; and
yet she was cutting without a model, without having in front of her the
mouth to be closed. To enlarge on this interesting subject would mean to
repeat oneself. All the Leaf-cutters have the same talent for making the
lids of their pots.
A less mysterious question than this geometrical problem is that of the
materials. Does each species of Megachile keep to a single plant, or
has it a definite botanical domain wherein to exercise its liberty of
choice? The little that I have already said is enough to make us suspect
that the insect is not restricted to one plant; and this is confirmed
by an examination of the separate cells, piece by piece, when we find a
variety which we were far from imagining at first. Here is the flora
of the Megachiles in my neighbourhood, a very incomplete flora and
doubtless capable of considerable amplification by future researches.
The Silky Leaf-cutter gathers the materials for her pots, her lids and
her barricades from the following plants: paliurus, hawthorn, vine,
wild briar, bramble, holm-oak, amelanchier, terebinthus, sage-leaved
rock-rose. The first three supply the greater part of the leaf-work; the
last three are represented only by rare fragments.
The Hare-footed Leaf-cutter (Megachile lagopoda, LIN.) which I see very
busy in my enclosure, though she only collects her materials there,
exploits the lilac and the rose-tree by preference. From time to time,
I see her also cutting bits out of the robinia, the quince-tree and the
cherry-tree. In the open country, I have found her building with the
leaves of the vine alone.
The Silvery Leaf-cutter (Megachile argentata, FAB.), another of my
guests, shares the taste of the aforesaid for the lilac and the rose,
but her domain includes in addition the pomegranate-tree, the bramble,
the vine, the common dogwood and the cornelian cherry.
The White-girdled Leaf-cutter likes the robinia, to which she adds, in
lavish proportions, the vine, the rose and the hawthorn and sometimes,
in moderation, the reed and the whitish-leaved rock-rose.
The Black-tipped Leaf-cutter (Megachile apicalis, SPIN.) has for her
abode the cells of the Mason-bee of the Pebbles and the ruined nests of
the Osmiae and Anthidia in the Snail-shells. I have not known her to use
any other materials than the wild briar and the hawthorn.
Incomplete though it be, this list tells us that the Megachiles do not
have exclusive botanical tastes. Each species manages extremely well
with several plants differing greatly in appearance. The first condition
to be fulfilled by the shrub exploited is that it be near the
nest. Frugal of her time, the Leaf-cutter declines to go on distant
expeditions. Whenever I come upon a recent Megachile-nest, I am not long
in finding in the neighbourhood, without much searching, the tree or
shrub from which the Bee has cut her pieces.
Another main condition is a fine and supple texture, especially for the
first disks used in the lid and for the pieces which form the lining of
the wallet. The rest, less carefully executed, allows of coarser
stuff; but even then the piece must be flexible and lend itself to the
cylindrical configuration of the tunnel. The leaves of the rock-roses,
thick and roughly fluted, fulfil this condition unsatisfactorily, for
which reason I see them occurring only at very rare intervals. The
insect has gathered pieces of them by mistake and, not finding them good
to use, has ceased to visit the unprofitable shrub. Stiffer still, the
leaf of the holm-oak in its full maturity is never employed: the Silky
Leaf-cutter uses it only in the young state and then in moderation; she
can get her velvety pieces better from the vine. In the lilac-bushes so
zealously exploited before my eyes by the Hare-footed Leaf-cutter occur
a medley of different shrubs which, from their size and the lustre of
their leaves, should apparently suit that sturdy pinker. They are the
shrubby hare's-ear, the honeysuckle, the prickly butcher's-broom, the
box. What magnificent disks ought to come from the hare's-ear and the
honeysuckle! One could get an excellent piece, without further labour,
by merely cutting the leaf-stalk of the box, as Megachile sericans does
with her paliurus. The lilac-lover disdains them absolutely. For
what reason? I fancy that she finds them too stiff. Would she think
differently if the lilac-bush were not there? Perhaps so.
In short, apart from the questions of texture and proximity to the
nest, the Megachile's choice, it seems to me, must depend upon whether a
particular shrub is plentiful or not. This would explain the lavish use
of the vine, an object of widespread cultivation, and of the hawthorn
and the wild briar, which form part of all our hedges. As these are to
be found everywhere, the fact that the different Leaf-cutters make use
of them is no reflection upon a host of equivalents varying according to
the locality.
If we had to believe what people tell us about the effects of heredity,
which is said to hand down from generation to generation, ever more
firmly established, the individual habits of those who come before, the
Megachiles of these parts, experienced in the local flora by the long
training of the centuries, but complete novices in the presence of
plants which their race encounters for the first time, ought to refuse
as unusual and suspicious any exotic leaves, especially when they have
at hand plenty of the leaves made familiar by hereditary custom. The
question was deserving of separate study.
Two subjects of my observations, the Hare-footed and the Silvery
Leaf-cutter, both of them inmates of my open-air laboratory, gave me a
definite answer. Knowing the points frequented by the two Megachiles,
I planted in their work-yard, overgrown with briar and lilac, two
outlandish plants which seemed to me to fulfil the required conditions
of suppleness of texture, namely, the ailantus, a native of Japan, and
the Virginian physostegia. Events justified the selection: both Bees
exploited the foreign flora with the same assiduity as the local
flora, passing from the lilac to the ailantus, from the briar to the
physostegia, leaving the one, going back to the other, without drawing
distinctions between the known and the unknown. Inveterate habit could
not have given greater certainty, greater ease to their scissors, though
this was their first experience of such a material.
The Silvery Leaf-cutter lent herself to an even more conclusive test. As
she readily makes her nest in the reeds of my apparatus, I was able,
up to a certain point, to create a landscape for her and select its
vegetation myself. I therefore moved the reed-hive to a part of the
enclosure stocked chiefly with rosemary, whose scanty foliage is not
adapted for the Bee's work, and near the apparatus I arranged an exotic
shrubbery in pots, including notably the smooth lopezia, from Mexico,
and the long-fruited capsicum, an Indian annual. Finding close at hand
the wherewithal to build her nest, the Leaf-cutter went no further
afield. The lopezia suited her especially, so much so that almost the
whole nest was composed of it. The rest had been gathered from the
capsicum.
Another recruit, whose co-operation I had in no way engineered, came
spontaneously to offer me her evidence. This was the Feeble Leaf-cutter
(Megachile imbecilla, GERST.). Nearly a quarter of a century ago, I saw
her, all through the month of July, cutting out her rounds and ellipses
at the expense of the petals of the Pelargonium zonale, the common
geranium. Her perseverance devastated--there is no other word for
it--my modest array of pots. Hardly was a blossom out, when the
ardent Megachiles came and scalloped it into crescents. The colour was
indifferent to her: red, white or pink, all the petals underwent
the disastrous operation. A few captures, ancient relics of my
collecting-boxes by this time, indemnified me for the pillage. I have
not seen this unpleasant Bee since. With what does she build when there
are no geranium-flowers handy? I do not know; but the fact remains that
the fragile tailoress used to attack the foreign flower, a fairly
recent acquisition from the Cape, as though all her race had never done
anything else.
These details leave us with one obvious conclusion, which is contrary to
our original ideas, based on the unvarying character of insect industry.
In constructing their jars, the Leaf-cutters, each following the taste
peculiar to her species, do not make use of this or that plant to
the exclusion of the others; they have no definite flora, no domain
faithfully transmitted by heredity. Their pieces of leaves vary
according to the surrounding vegetation; they vary in different layers
of the same cell. Everything suits them, exotic or native, rare or
common, provided that the bit cut out be easy to employ. It is not the
general aspect of the shrub, with its fragile or bushy branches, its
large or small, green or grey, dull or glossy leaves, that guides
the insect: such advanced botanical knowledge does not enter into the
question at all. In the thicket chosen as a pinking-establishment, the
Megachile sees but one thing: leaves useful for her work. The Shrike,
with his passion for plants with long, woolly sprigs, knows where
to find nicely-wadded substitutes when his favourite growth, the
cotton-rose, is lacking; the Megachile has much wider resources:
indifferent to the plant itself, she looks only into the foliage. If she
finds leaves of the proper size, of a dry texture capable of defying the
damp and of a suppleness favourable to cylindrical curving, that is
all she asks; and the rest does not matter. She has therefore an almost
unlimited field for her labour.
These sudden and wholly unprovoked changes give cause for reflection.
When my geranium-flowers were devastated, how had the obtrusive Bee,
untroubled by the profound dissimilarity between the petals, snow-white
here, bright scarlet there, how had she learnt her trade? Nothing tells
us that she herself was not for the first time exploiting the plant from
the Cape; and, if she really did have predecessors, the habit had not
had time to become inveterate, considering the modern importation of the
geranium. Where again did the Silvery Megachile, for whom I created an
exotic shrubbery, make the acquaintance of the lopezia, which comes from
Mexico? She certainly is making a first start. Never did her village or
mine possess a stalk of that chilly denizen of our hot-houses. She is
making a first start; and behold her straightway a graduate, versed in
the art of carving unfamiliar foliage.
People often talk of the long apprenticeships served by instinct, of its
gradual acquirements, of its talents, the laborious work of the ages.
The Megachiles affirm the exact opposite. They tell me that the animal,
though invariable in the essence of its art, is capable of innovation
in the details; but at the same time they assure me that any such
innovation is sudden and not gradual. Nothing prepares the innovations,
nothing improves them or hands them down; otherwise a selection would
long ago have been made amid the diversity of foliage; and the
shrub recognized as the most serviceable, especially when it is also
plentiful, would alone supply all the building-materials needed. If
heredity transmitted industrial discoveries, a Megachile who thought of
cutting her disks out of pomegranate-leaves and found them satisfactory
ought to have instilled a liking for similar materials into her
descendants; and we should this day find Leaf-cutters faithful to the
pomegranate-leaves, workers who remained exclusive in their choice of
the raw material. The facts refute these theories.
People also say:
'Grant us a variation, however small, in the insect's industry; and
that variation, accentuated more and more, will produce a new race and
finally a fixed species.'
This trifling variation is the fulcrum for which Archimedes clamoured in
order to lift the world with his system of levers. The Megachiles
offer us one and a very great one: the indefinite variation of their
materials. What will the theorists' levers lift with this fulcrum? Why,
nothing at all! Whether they cut the delicate petals of the geranium or
the tough leaves of the lilac-bushes, the Leaf-cutters are and will
be what they were. This is what we learn from the persistence of each
species in its structural details, despite the great variety of the
foliage employed.