The six or ten cells composing the group are certainly a solid dwelling, with their rustic covering of gravel, but the thickness of their walls and lids—two millimetres at most—hardly seems sufficient against rough weather. Set on its stone in the open air, quite unsheltered, the nest will undergo the heat of summer suns which will turn every cell into an oven; then will come the autumn rains which will slowly eat away the masonry, and then winter frosts which will crumble what the rain may have respected. However hard the cement may be, can it resist all these attacks, and if it can, will not the larvæ, sheltered by so thin a wall, suffer from over-heat in summer and too keen cold in winter? Without having gone through all these arguments, the bee acts wisely. When all the cells are completed she builds a thick cover over the whole group, which, being of a material impermeable to water and almost a non-conductor, is at once a defence against heat and cold and damp. This material is the usual mortar, made of earth and saliva, only with no small stones in it. The bee lays it on,—one pellet after another, one trowelful and then a second,—till there is a layer a centimetre thick over all the cells, which disappear entirely under it. The nest is now a rude dome, about as big as half an orange; one would take it for a clod of mud, half crushed by being flung against a stone where it had dried. Nothing outside betrays its contents—no suggestion of cells—none of labour. To the ordinary eye it is only a chance splash of mud. This general cover dries as rapidly as do our hydraulic cements, and the nest is almost as hard as a stone. A knife with a strong blade is needed to cut it. In its final shape the nest recalls in no degree the original work; one would suppose the elegant turrets adorned with pebble work, and the final dome, looking like a bit of mud, to be the work of two different species. But scratch away the cover of cement and we recognise the cells and their layers of tiny pebbles. Instead of building on a boulder yet unoccupied, Chalicodoma muraria likes to utilise old nests which have lasted through the year without notable injury. The mortared dome has remained much as it was at the beginning, so solid was the masonry; only it is pierced by a number of round holes corresponding to the chambers inhabited by the larvæ of the past generation. Such dwellings, only needing a little repair to put them in good condition, economise much time and toil; so Mason Bees seek them, and only undertake new constructions when old nests fail them.