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