Chapter VIII. The Leaf-Cutters
Leaf-Cutter Bee Megachile-willughbiella
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 (1), 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 (2). 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.'
Leaf-Cutter Megachile on a leaf
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.
1. The Capricorn, the essay on which has not yet been published in English.
2. 7.8 inches.