This risk is avoided by the order which the Osmia adopts.
In my tubes, with their unusual arrangement, the mother might well find the dilemma perplexing: there is the narrowness of the space at her disposal and there is the emergence later on.

In the narrow tubes, the width is insufficient for the females; on the other hand, if she lodges males there, they are liable to perish, since they will be prevented from issuing at the proper moment.

This would perhaps explain the mother's hesitation and her obstinacy in settling females in some of my apparatus which looked as if they could suit none but males.
A suspicion occurs to me, a suspicion aroused by my attentive examination of the narrow tubes.

All, whatever the number of their inmates, are carefully plugged at the opening, just as separate tubes would be.

It might therefore be the case that the narrow gallery at the back was looked upon by the Osmia not as the prolongation of the large front gallery, but as an independent tube.