Having chosen the site, the builder erects a circular fence about an eighth of an inch thick. The materials consist of mortar and small stones. The insect selects its stone-quarry in some well-trodden path or on some neighbouring highroad, at the driest, hardest spots. With its mandibles, it scrapes together a small quantity of dust and soaks it with saliva until the whole becomes a regular hydraulic mortar which soon sets and is no longer susceptible to damp. The Mason-bees have shown us a similar exploitation of the beaten paths and of the road-mender’s macadam. All these open-air builders, all these erectors of monuments exposed to wind and weather require an exceedingly dry stone-dust; otherwise the material, already moistened with water, would not properly absorb the liquid that is to give it cohesion; and the edifice would soon be wrecked by the rains. They possess the sense of discrimination shown by the plasterer, who rejects plaster injured by the wet. We shall see presently how the insects that build under cover avoid this laborious macadam-scraping and give the preference to fresh earth already reduced to a paste by its own dampness. When common lime answers our purpose, we do not trouble about Roman cement. Now Eumenes Amadei requires a first-class cement, even superior to that of the Chalicodoma of the Walls, for the work, when finished, does not receive the thick outer casing wherewith the Mason-bee protects her cluster of cells. And therefore the cupola-builder, as often as she can, uses the highway as her stone-pit. With the mortar, bricks are needed. These are bits of gravel of an almost unvarying size—that of a pepper-corn—but of a shape and kind that differ greatly, according to the places worked. Some are sharp-cornered, with facets determined by chance fractures; some are round, polished by friction under water. Some are of limestone, others of flinty material. The favourite stones, when the neighbourhood of the nest permits, are smooth, semitransparent little lumps of quartz. These are selected with minute care. The insect weighs them, so to say, measures them with the compass of its mandibles and does not accept them until after making sure that they possess the requisite qualities of size and hardness.