Let us consider the former case first. After selecting her pebble, the Mason-bee of the Walls arrives with a little ball of mortar in her mandibles and lays it in a circular pad on the surface of the stone. The fore-legs and above all the mandibles, which are the mason's chief tools, work the material, which is kept plastic by the salivary fluid as this is gradually disgorged. In order to consolidate the clay, angular bits of gravel, the size of a lentil, are inserted separately, but only on the outside, in the as yet soft mass. This is the foundation of the structure. Fresh layers follow, until the cell has attained the desired height of two or three centimetres. (Three-quarters of an inch to one inch.—Translator's Note.)

Man's masonry is formed of stones laid one above the other and cemented together with lime. The Chalicodoma's work can bear comparison with ours. To economise labour and mortar, the Bee employs coarse materials, big pieces of gravel, which to her represent hewn stones. She chooses them carefully one by one, picks out the hardest bits, generally with corners which, fitting one into the other, give mutual support and contribute to the solidity of the whole. Layers of mortar, sparingly applied, hold them together. The outside of the cell thus assumes the appearance of a piece of rustic architecture, in which the stones project with their natural irregularities; but the inside, which requires a more even surface in order not to hurt the larva's tender skin, is covered with a coat of pure mortar. This inner whitewash, however, is put on without any attempt at art, indeed one might say that it is ladled on in great splashes; and the grub takes care, after finishing its mess of honey, to make itself a cocoon and hang the rude walls of its abode with silk. On the other hand, the Anthophorae and the Halicti, two species of Wild Bees whose grubs weave no cocoon, delicately glaze the inside of their earthen cells and give them the gloss of polished ivory.

The structure, whose axis is nearly always vertical and whose orifice faces upwards so as not to let the honey escape, varies a little in shape according to the supporting base. When set on a horizontal surface, it rises like a little oval tower; when fixed against an upright or slanting surface, it resembles the half of a thimble divided from top to bottom. In this case, the support itself, the pebble, completes the outer wall.

When the cell is finished, the Bee at once sets to work to victual it. The flowers round about, especially those of the yellow broom (Genista scoparia), which in May deck the pebbly borders of the mountain streams with gold, supply her with sugary liquid and pollen. She comes with her crop swollen with honey and her belly yellowed underneath with pollen dust. She dives head first into the cell; and for a few moments you see some spasmodic jerks which show that she is disgorging the honey-syrup. After emptying her crop, she comes out of the cell, only to go in again at once, but this time backwards. The Bee now brushes the lower side of her abdomen with her two hind-legs and rids herself of her load of pollen. Once more she comes out and once more goes in head first. It is a question of stirring the materials, with her mandibles for a spoon, and making the whole into a homogeneous mixture. This mixing-operation is not repeated after every journey: it takes place only at long intervals, when a considerable quantity of material has been accumulated.

The victualling is complete when the cell is half full. An egg must now be laid on the top of the paste and the house must be closed. All this is done without delay. The cover consists of a lid of pure mortar, which the Bee builds by degrees, working from the circumference to the centre. Two days at most appeared to me to be enough for everything, provided that no bad weather—rain or merely clouds—came to interrupt the labour. Then a second cell is built, backing on the first and provisioned in the same manner. A third, a fourth, and so on follow, each supplied with honey and an egg and closed before the foundations of the next are laid. Each task begun is continued until it is quite finished; the Bee never commences a new cell until the four processes needed for the construction of its predecessor are completed: the building, the victualling, the laying of the egg and the closing of the cell.

As the Mason-bee of the Walls always works by herself on the pebble which she has chosen and even shows herself very jealous of her site when her neighbours alight upon it, the number of cells set back to back upon one pebble is not large, usually varying between six and ten. Do some eight grubs represent the Bee's whole family? Or does she afterwards go and establish a more numerous progeny on other boulders? The surface of the same stone is spacious enough to provide a support for further cells if the number of eggs called for them; the Bee could build there very comfortably, without hunting for another site, without leaving the pebble to which she is attached by habit and long acquaintance. It seems to me therefore, exceedingly probable that the family is a small one and that it is all installed on the one stone, at any rate when the Mason-bee is building a new home.

The six to ten cells composing the cluster are certainly a solid dwelling, with their rustic gravel covering; but the thickness of their walls and lids, two millimetres (.078 inch—Translator's Note.) at most, seems hardly sufficient to protect the grubs against the inclemencies of the weather. Set on its pebble in the open air, without any sort of shelter, the nest will have to undergo the heat of summer, which will turn each cell into a stifling furnace, followed by the autumn rains, which will slowly wear away the stonework, and by the winter frosts, which will crumble what the rains have respected. However hard the cement may be, can it possibly resist all these agents of destruction? And, even if it does resist, will not the grubs, sheltered by too thin a wall, have to suffer from excess of heat in summer and of cold in winter?

Without arguing all this out, the Bee nevertheless acts wisely. When all the cells are finished, she builds a thick cover over the group, formed of a material, impermeable to water and a bad conductor of heat, which acts as a protection at the same time against damp, heat and cold. This material is the usual mortar, made of earth mixed with saliva, but on this occasion with no small stones in it. The Bee applies it pellet by pellet, trowelful by trowelful, to the depth of a centimetre (.39 inch—Translator's Note.) over the cluster of cells, which disappear entirely under the clay covering. When this is done, the nest has the shape of a rough dome, equal in size to half an orange. One would take it for a round lump of mud which had been thrown and half crushed against a stone and had then dried where it was. Nothing outside betrays the contents, no semblance of cells, no semblance of work. To the inexperienced eye, it is a chance splash of mud and nothing more.

This outer covering dries as quickly as do our hydraulic cements; and the nest is now almost as hard as a stone. It takes a knife with a strong blade to break open the edifice. And I would add, in conclusion, that, under its final form, the nest in no way recalls the original work, so much so that one would imagine the cells of the start, those elegant turrets covered with stucco-work, and the dome of the finish, looking like a mere lump of mud, to be the product of two different species. But scrape away the crust of cement and we shall easily recognize the cells below and their layers of tiny pebbles.

Instead of building a brand-new nest, on a hitherto unoccupied boulder, the Mason-bee of the Walls is always glad to make use of the old nests which have lasted through the year without suffering any damage worth mentioning. The mortar dome has remained very much what it was at the beginning, thanks to the solidity of the masonry, only it is perforated with a number of round holes, corresponding with