What stimulus does the insect obey when it employs the reserve powers that slumber in its race? Of what use are its industrial variations? The Osmia will yield us her secret with no great difficulty. Let us examine her work in a cylindrical habitation. I have described in full detail, in the foregoing pages, the structure of her nests when the dwelling adopted is a reed-stump or any other cylinder; and I will content myself here with recapitulating the essential features of that nest-building.
We must first distinguish three classes of reeds according to their diameter: the small, the medium-sized and the large. I call small those whose narrow width just allows the Osmia to go about her household duties without discomfort. She must be able to turn where she stands in order to brush her abdomen and rub off its load of pollen, after disgorging the honey in the centre of the heap of flour already collected. If the width of the tube does not admit of this operation, if the insect is obliged to go out and then come in again backwards in order to place itself in a favourable posture for the discharge of the pollen, then the reed is too narrow and the Osmia is rather reluctant to accept it. The middle-sized reeds and a fortiori the large ones leave the victualler entire liberty of action; but the former do not exceed the width of a cell, a width agreeing with the bulk of the future cocoon, whereas the latter, with their excessive diameter, require more than one chamber on the same floor.
When free to choose, the Osmia settles by preference in the small reeds. Here, the work of building is reduced to its simplest expression and consists in dividing the tube by means of earthen partitions into a straight row of cells. Against the partition forming the back wall of the preceding cell the mother places first a heap of honey and pollen; next, when the portion is seen to be enough, she lays an egg in the centre of it. Then and then only she resumes her plasterer's work and marks out the length of the new cell with a mud partition. This partition in its turn serves as the rear-wall of another chamber, which is first victualled and then closed; and so on until the cylinder is sufficiently colonized and receives a thick terminal stopper at its orifice. In a word, the chief characteristic of this method of nest-building, the roughest of all, is that the partition in front is not undertaken so long as the victualling is still incomplete, or, in other words, that the provisions and the egg are deposited before the Bee sets to work on the partition.
At first sight, this latter detail hardly deserves attention: is it not right to fill the pot before we put a lid on? The Osmia who owns a medium-sized reed is not at all of this opinion; and other plasterers share her views, as we shall see when we watch the Odynerus building her nest (1). Here we have an excellent illustration of one of those latent powers held in reserve for exceptional occasions and suddenly brought into play, although often very far removed from the insect's regular methods. If the reed, without being of inordinate width from the point of view of the cocoon, is nevertheless too spacious to afford the Bee a suitable purchase against the wall at the moment when she is disgorging honey and brushing off her load of pollen; the Osmia altogether changes the order of her work; she sets up the partition first and then does the victualling.
All round the inside of the tube she places a ring of mud, which, as the result of her constant visits to the mortar, ends by becoming a complete diaphragm minus an orifice at the side, a sort of round dog-hole, just large enough for the insect to pass through. When the cell is thus marked out and almost wholly closed, the Osmia attends to the storing of her provisions and the laying of her eggs. Steadying herself against the margin of the hole at one time with her fore-legs and at another with her hind-legs, she is able to empty her crop and to brush her abdomen; by pressing against it, she obtains a foothold for her little efforts in these various operations. When the tube was narrow, the outer wall supplied this foothold and the earthen partition was postponed until the heap of provisions was completed and surmounted by the egg; but in the present case the passage is too wide and would leave the insect floundering helplessly in space, so the partition with its serving-hatch takes precedence of the victuals. This method is a little more expensive than the other, first in materials, because of the diameter of the reed, and secondly in time, if only because of the dog-hole, a delicate piece of mortar-work which is too soft at first and cannot be used until it has dried and become harder. Therefore the Osmia, who is sparing of her time and strength, accepts medium-sized reeds only when there are no small ones available.
The large tubes she will use only in grave emergencies and I am unable to state exactly what these exceptional circumstances are. Perhaps she decides to make use of those roomy dwellings when the eggs have to be laid at once and there is no other shelter in the neighbourhood. While my cylinder-hives gave me plenty of well-filled reeds of the first and second class, they provided me with but half-a-dozen at most of the third, notwithstanding my precaution to furnish the apparatus with a varied assortment.
The Osmia's repugnance to big cylinders is quite justified. The work in fact is longer and more costly when the tubes are wide. An inspection of a nest constructed under these conditions is enough to convince us. It now consists not of a string of chambers obtained by simple transverse partitions, but of a confused heap of clumsy, many-sided compartments, standing back to back, with a tendency to group themselves in storeys without succeeding in doing so, because any regular arrangement would mean that the ceilings possessed a span which it is not in the builder's power to achieve. The edifice is not a geometrical masterpiece and it is even less satisfactory from the point of view of economy. In the previous constructions, the sides of the reed supplied the greater part of the walls and the work was limited to one partition for each cell. Here, except at the actual periphery, where the tube itself supplies a foundation, everything has to be obtained by sheer building: the floor, the ceiling, the walls of the many-sided compartment are one and all made of mortar. The structure is almost as costly in materials as that of the Chalicodoma or the Pelopaeus.
It must be pretty difficult, too, when one thinks of its irregularity. Fitting as best she can the projecting angles of the new cell into the recessed corners of the cell already built, the Osmia runs up walls more or less curved, upright or slanting, which intersect one another at various points, so that each compartment requires a new and complicated plan of construction, which is very different from the circular-partition style of architecture, with its row of parallel dividing-disks. Moreover, in this composite arrangement, the size of the recesses left available by the earlier work to some extent decides the assessment of the sexes, for, according to the dimensions of those recesses, the walls erected take in now a larger space, the home of a female, and now a smaller space, the home of a male. Roomy quarters therefore have a double drawback for the Osmia: they greatly increase the outlay in materials; and also they establish in the lower layers, among the females, males who, because of their earlier hatching, would be much better placed near the mouth of the nest. I am convinced of it: if the Osmia refuses big reeds and accepts them only in the last resort, when there are no others, it is because she objects to additional labour and to the mixture of the sexes.
The Snail-shell, then, is but an indifferent home for her, which she is quite ready to abandon should a better offer. Its expanding cavity represents an average between the favourite small cylinder and the unpopular large cylinder, which is accepted only when there is no other obtainable. The first whorls of the spiral are too narrow to be of use to the Osmia, but the middle ones have the right diameter for cocoons arranged in single file. Here things happen as in a first-class reed, for the helical curve in no way affects the method of structure employed for a rectilinear series of cells. Circular partitions are erected at the required distances, with or without a serving-hatch, according to the diameter. These mark out the first cells, one after the other, which are reserved solely for the females. Then comes the last whorl, which is much too wide for a single row of cells; and here we once more find, exactly as in a wide reed, a costly profusion of masonry, an irregular arrangement of the cells and a mixture of the sexes.
Having said so much, let us go back to the Osmia of the quarries. Why, when I offer them simultaneously Snail-shells and reeds of a suitable size, do the old frequenters of the shells prefer the reeds, which in all probability have never before been utilized by their race? Most of them scorn the ancestral dwelling and enthusiastically accept my reeds. Some, it is true, take up their quarters in the Snail-shell; but even among these a goodly number refuse my new shells and return to their birth-place, the old Snail-shell, in order to utilize the family property, without much labour, at the cost of a few repairs. Whence, I ask, comes this general preference for the cylinder, never used hitherto? The answer can be only this: of two lodgings at her disposal the Osmia selects the one that provides a comfortable home at a minimum outlay. She economizes her strength when restoring an old nest; she economizes it when replacing the Snail-shell by the reed.
Can animal industry, like our own, obey the law of economy, the sovran law that governs our industrial machine even as it governs, at least to all appearances, the sublime machine of the universe? Let us go deeper into the question and bring other workers into evidence, those especially who, better equipped perhaps and at any rate better fitted for hard work, attack the difficulties of their trade boldly and look down upon alien establishments with scorn. Of this number are the Chalicodomae, the Mason-bees proper.
The Mason-bee of the Pebbles does not make up her mind to build a brand-new dome unless there be a dearth of old and not quite dilapidated nests. The mothers, sisters apparently and heirs-at-law to the domain, dispute fiercely for the ancestral abode. The first who, by sheer brute force, takes possession of the dome, perches upon it and, for long hours, watches events while polishing her wings. If some claimant puts in an appearance, forthwith the other turns her out with a volley of blows. In this way the old nests are employed so long as they have not become uninhabitable hovels.
Without being equally jealous of the maternal inheritance, the Mason-bee of the Sheds eagerly uses the cells whence her generation issued. The work in the huge city under the eaves begins thus: the old cells, of which, by the way, the good-natured owner yields a portion to Latreille's Osmia and to the Three-horned Osmia alike, are first made clean and wholesome and cleared of broken plaster and then provisioned and shut. When all the accessible chambers are occupied, the actual building begins with a new stratum of cells upon the former edifice, which becomes more and more massive from year to year.
The Mason-bee of the Shrubs, with her spherical nests hardly larger than walnuts, puzzled me at first. Does she use the old buildings or does she abandon them for good? To-day perplexity makes way for certainty: she uses them very readily. I have several times surprised her lodging her family in the empty rooms of a nest where she was doubtless born herself. Like her kinswoman of the Pebbles, she returns to the native dwelling and fights for its possession. Also, like the dome-builder, she is an anchorite and prefers to cultivate the lean inheritance alone. Sometimes, however, the nest is of exceptional size and harbours a crowd of occupants, who live in peace, each attending to her business, as in the colossal hives in the sheds. Should the colony be at all numerous and the estate descend to two or three generations in succession, with a fresh layer of masonry each year, the normal walnut-sized nest becomes a ball as large as a man's two fists. I have gathered on a pine-tree a nest of the Mason-bee of the Shrubs that weighed a kilogram (2) and was the size of a child's head. A twig hardly thicker than a straw served as its support. The casual sight of that lump swinging over the spot on which I had sat down made me think of the mishap that befell Garo (3). If such nests were plentiful in the trees, any one seeking the shade would run a serious risk of having his head smashed.
After the Masons, the Carpenters. Among the guild of wood-workers, the most powerful is the Carpenter-bee (4), a very large Bee of formidable appearance, clad in black velvet with violet-coloured wings. The mother gives her larvae as a dwelling a cylindrical gallery which she digs in rotten wood. Useless timber lying exposed to the air, vine-poles, large logs of fire-wood seasoning out of doors, heaped up in front of the farmhouse porch, stumps of trees, vine-stocks and big branches of all kinds are her favourite building-yards. A solitary and industrious worker, she bores, bit by bit, circular passages the width of one's thumb, as clear-cut as though they were made with an auger. A heap of saw-dust accumulates on the ground and bears witness to the severity of the task. Usually, the same aperture is the entrance to two or three parallel corridors. With several galleries there is accommodation for the entire laying, though each gallery is quite short; and the Bee thus avoids those long series which always create difficulties when the moment of hatching arrives. The laggards and the insects eager to emerge are less likely to get in each other's way.
After obtaining the dwelling, the Carpenter-bee behaves like the Osmia who is in possession of a reed. Provisions are collected, the egg is laid and the chamber is walled in front with a saw-dust partition. The work is pursued in this way until the two or three passages composing the house are completely stocked. Heaping up provisions and erecting partitions are an invariable feature of the Xylocopa's programme; no circumstance can release the mother from the duty of providing for the future of her family, in the matter both of ready-prepared food and of separate compartments for the rearing of each larva. It is only in the boring of the galleries, the most laborious part of the work, that economy can occasionally be exercised by a piece of luck. Well, is the powerful Carpenter, all unheeding of fatigue, able to take advantage of such fortunate occasions? Does she know how to make use of houses which she has not tunnelled herself? Why, yes: a free lodging suits her just as much as it does the various Mason-bees. She knows as well as they the economic advantages of an old nest that is still in good condition: she settles down, as far as possible, in her predecessors' galleries, after freshening up the sides with a superficial scraping. And she does better still. She readily accepts lodgings which have never known a drill, no matter whose. The stout reeds used in the trellis-work that supports the vines are valuable discoveries, providing as they do sumptuous galleries free of cost. No preliminary work or next to none is required with these. Indeed, the insect does not even trouble to make a side-opening, which would enable it to occupy the cavity contained within two nodes; it prefers the opening at the end cut by man's pruning-knife. If the next partition be too near to give a chamber of sufficient length, the Xylocopa destroys it, which is easy work, not to be compared with the labour of cutting an entrance through the side. In this way, a spacious gallery, following on the short vestibule made by the pruning-knife, is obtained with the least possible expenditure of energy.
Guided by what was happening on the trellises, I offered the black Bee the hospitality of my reed-hives. From the very beginning, the insect gladly welcomed my advances; each spring, I see it inspect my rows of cylinders, pick out the best ones and instal itself there. Its work, reduced to a minimum by my intervention, is limited to the partitions, the materials for which are obtained by scraping the inner sides of the reed.
As first-rate joiners, next to the Carpenter-bees come the Lithurgi, of whom my district possesses two species: L. cornutus, FAB., and L. chrysurus, BOY. By what aberration of nomenclature was the name of Lithurgus, a worker in stone, given to insects which work solely in wood? I have caught the first, the stronger of the two, digging galleries in a large block of oak that served as an arch for a stable-door; I have always found the second, who is more widely distributed, settling in dead wood mulberry, cherry, almond, poplar that was still standing. Her work is exactly the same as the Xylocopa's, on a smaller scale. A single entrance-hole gives access to three or four parallel galleries, assembled in a serried group; and these galleries are subdivided into cells by means of saw-dust partitions. Following the example of the big Carpenter-bee, Lithurgus chrysurus knows how to avoid the laborious work of boring, when occasion offers: I find her cocoons lodged almost as often in old dormitories as in new ones. She too has the tendency to economize her strength by turning the work of her predecessors to account. I do not despair of seeing her adopt the reed if, one day, when I possess a large enough colony, I decide to try this experiment on her. I will say nothing about L. cornutus, whom I only once surprised at her carpentering.
The Anthophorae, those children of the precipitous earthy banks, show the same thrifty spirit as the other members of the mining corporation. Three species, A. parietina, A. personata and A. pilipes, dig long corridors leading to the cells, which are scattered here and there and one by one. These passages remain open at all seasons of the year. When spring comes, the new colony uses them just as they are, provided that they are well preserved in the clayey mass baked by the sun; it increases their length if necessary, runs out a few more branches, but does not decide to start boring in new ground until the old city, which, with its many labyrinths, resembles some monstrous sponge, is too much undermined for safety. The oval niches, the cells that open on those corridors, are also profitably employed. The Anthophora restores their entrance, which has been destroyed by the insect's recent emergence; she smooths their walls with a fresh coat of whitewash, after which the lodging is fit to receive the heap of honey and the egg. When the old cells, insufficient in number and moreover partly inhabited by diverse intruders, are all occupied, the boring of new cells begins, in the extended sections of the galleries, and the rest of the eggs are housed. In this way, the swarm is settled at a minimum of expense.
To conclude this brief account, let us change the zoological setting and, as we have already spoken of the Sparrow, see what he can do as a builder. The simplest form of his nest is the great round ball of straw, dead leaves and feathers, in the fork of a few branches. It is costly in material, but can be set up anywhere, when the hole in the wall or the shelter of a tile are lacking. What reasons induced him to give up the spherical edifice? To all seeming, the same reasons that led the Osmia to abandon the Snail-shell's spiral, which requires a fatiguing expenditure of clay, in favour of the economical cylinder of the reed. By making his home in a hole in the wall, the Sparrow escapes the greater part of his work. Here, the dome that serves as a protection from the rain and the thick walls that offer resistance to the wind both become superfluous. A mere mattress is sufficient; the cavity in the wall provides the rest. The saving is great; and the Sparrow appreciates it quite as much as the Osmia.
This does not mean that the primitive art has disappeared, lost through neglect; it remains an ineffaceable characteristic of the species, ever ready to declare itself should circumstances demand it. The generations of to-day are as much endowed with it as the generations of yore; without apprenticeship, without the example of others, they have within themselves, in the potential state, the industrial aptitude of their ancestors. If aroused by the stimulus of necessity, this aptitude will pass suddenly from inaction to action. When, therefore, the Sparrow still from time to time indulges in spherical building, this is not progress on his part, as is sometimes contended; it is, on the contrary, a retrogression, a return to the ancient customs, so prodigal of labour. He is behaving like the Osmia who, in default of a reed, makes shift with a Snail-shell, which is more difficult to utilize but easier to find. The cylinder and the hole in the wall stand for progress; the spiral of the Snail-shell and the ball-shaped nest represent the starting-point.
I have, I think, sufficiently illustrated the inference which is borne out by the whole mass of analogous facts. Animal industry manifests a tendency to achieve the essential with a minimum of expenditure; after its own fashion, the insect bears witness to the economy of energy. On the one hand, instinct imposes upon it a craft that is unchangeable in its fundamental features; on the other hand, it is left a certain latitude in the details, so as to take advantage of favourable circumstances and attain the object aimed at with the least possible expenditure of time, materials and work, the three elements of mechanical labour. The problem in higher geometry solved by the Hive-bee is only a particular case true, a magnificent case, of this general law of economy which seems to govern the whole animal world. The wax cells, with their maximum capacity as against a minimum wall-space, are the equivalent, with the superaddition of a marvellous scientific skill, of the Osmia's compartments in which the stonework is reduced to a minimum through the selection of a reed. The artificer in mud and the artificer in wax obey the same tendency: they economize. Do they know what they are doing? Who would venture to suggest it in the case of the Bee grappling with her transcendental problem? The others, pursuing their rustic art, are no wiser. With all of them, there is no calculation, no premeditation, but simply blind obedience to the law of general harmony.