ghtest chance of success: the Bee would
stubbornly refuse such a dwelling or would content herself with
entrusting only a very small portion of her eggs to it. On the other
hand, with narrow but short cavities, success, without being easy,
seems to me at least quite possible. Guided by these considerations,
I embarked upon the most arduous part of my problem: to obtain the
complete or almost complete permutation of one sex with the other;
to produce a laying consisting only of males by offering the mother a
series of lodgings suited only to males.

Let us in the first place consult the old nests of the Mason-bee of the
Shrubs. I have said that these mortar spheroids, pierced all over
with little cylindrical cavities, are adopted pretty eagerly by the
Three-horned Osmia, who colonizes them before my eyes with females in
the deep cells and males in the shallow cells. That is how things go
when the old nest remains in its natural state. With a grater, however,
I scrape the outside of another nest so as to reduce the depth of
the cavities to some ten millimetres. (About two-fifths of an
inch.--Translator's Note.) This leaves in each cell just room for one
cocoon, surmounted by the closing stopper. Of the fourteen cavities in
the nests, I leave two intact, measuring fifteen millimetres in depth.
(.585 inch.--Translator's Note.) Nothing could be more striking than the
result of this experiment, made in the first year of my home rearing.
The twelve cavities whose depth had been reduced all received males; the
two cavities left untouched received females.

A year passes and I repeat the experiment with a nest of fifteen cells;
but this time all the cells are reduced to the minimum depth with the
grater. Well, the fifteen cells, from first to last, are occupied by
males. It must be quite understood that, in each case, all the offspring
belonged to one mother, marked with her distinguishing spot and kept
in sight as long as her laying lasted. He would indeed be difficult to
please who refused to bow before the results of these two experiments.
If, however, he is not yet convinced, here is something to remove his
last doubts.

The Three-horned Osmia often settles her family in old shells,
especially those of the Common Snail (Helix aspersa), who is so common
under the stone-heaps and in the crevices of the little unmortared walls
that support our terraces. In this species, the spiral is wide open, so
that the Osmia, penetrating