The diameter is about 7 millimetres (.273 inch .-- Translator's Note.); their depth at the centre of the heap is 23 millimetres (.897 inch .-- Translator's Note.) and at the edge averages 14 millimetres.

(.546 inch .-- Translator's Note.) The deep central cells receive only the females of the Osmia; sometimes even the two sexes together, with a partition in the middle, the female occupying the lower and the male the upper storey.

Lastly, the deeper cavities on the circumference are allotted to females and the shallower to males.
We know that the Three-horned Osmia prefers to haunt the habitations of the Bees who nidify in populous colonies, such as the Mason-bee of the Sheds and the Hairy-footed Anthophora, in whose nests I have noted similar facts.
Thus the sex of the egg is optional.

The choice rests with the mother, who is guided by considerations of space and, according to the accommodation at her disposal, which is frequently fortuitous and incapable of modification, places a female in this cell and a male in that, so that both may have a dwelling of a size suited to their unequal development.

This is the unimpeachable evidence of the numerous and varied facts which I have set forth.