To persuade the Osmia to nidify in a single tube long enough to receive the whole of her laying and at the same time narrow enough to leave her only just the possibility of admittance appears to me a project without the slightest 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 a 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.