, requires, like the Pelopaeus, a dry shelter for
her cells, a shelter which she finds ready-made and uses just as it is,
after a few touches by way of sweeping and cleansing. The homes which I
see her adopt are especially the shells of Snails that have died under
the stone-heaps and in the low, unmortared walls which support the
cultivated earth of the hills in shelves or terraces. The use of
Snail-shells is accompanied by the no less active use of the old cells
of both the Mason-bee of the Sheds and of certain Anthophorae (A.
pilipes, A. parietina and A. personata).

We must not forget the reed, which is highly appreciated when--a rare
find--it appears under the requisite conditions. In its natural state,
the plant with the mighty hollow cylinders is of no possible use to the
Osmia, who knows nothing of the art of perforating a woody wall. The
gallery of an internode has to be wide open before the insect can
take possession of it. Also, the clean-cut stump must be horizontal,
otherwise the rain would soften the fragile edifice of clay and soon lay
it low; also, the stump must not be lying on the ground and must be kept
at some distance from the dampness of the soil. We see therefore that,
without the intervention of man, involuntary in the vast majority of
cases and deliberate only on the experimenter's part, the Osmia would
hardly ever find a reed-stump suited to the installation of her family.
It is to her a casual acquisition, a home unknown to her race before
men took it into their heads to cut reeds and make them into hurdles for
drying figs in the sun.

How did the work of man's pruning-knife bring about the abandonment of
the natural lodging? How was the spiral staircase of the Snail-shell
replaced by the cylindrical gallery of the reed? Was the change from one
kind of house to another effected by gradual transitions, by attempts
made, abandoned, resumed, becoming more and more definite in their
results as generation succeeded generation? Or did the Osmia, finding
the cut reed that answered her requirements, instal herself there
straightway, scorning her ancient dwelling, the Snail-shell? These
questions called for a reply; and they have received one. Let us
describe how things happened.

Near Serignan are some great quarries of coarse limestone,
characteristic of the miocene formation of the Rhone valley. These
have been worked for many generations. The ancient public buildings of
Orange, notably the coloss