The insect selects its stone-quarry in some well-trodden path, on some neighbouring road, at the driest, hardest spots.

With its mandibles, it scrapes together a small quantity of dust and saturates it with saliva until the whole becomes a regular hydraulic mortar which soon sets and is no longer susceptible to water.
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 of the plasterer, who rejects plaster injured by damp.

We shall see presently how the insects that build under shelter avoid this laborious macadam-scraping and give the preference to fresh earth already reduced to a paste by its own dampness.