f 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
inexpe