Osmia, the same sequence may occur, but less regularly.

This dual arrangement disappears when the place chosen for the nest is
not large enough to contain the entire laying. We then see broken
layings, beginning with females and ending with males.

The egg, as it issues from the ovary, has not yet a fixed sex. The
final impress that produces the sex is given at the moment of laying,
or a little before.

So as to be able to give each larva the amount of space and food that
suits it according as it is male or female, the mother can choose the
sex of the egg which she is about to lay. To meet the conditions of the
building, which is often the work of another or else a natural retreat
that admits of little or no alteration, she lays either a male egg or a
female egg AS SHE PLEASES. The distribution of the sexes depends upon
herself. Should circumstances require it, the order of the laying can
be reversed and begin with males; lastly, the entire laying can contain
only one sex.

The same privilege is possessed by the predatory Hymenoptera, the
Wasps, at least by those in whom the two sexes are of a different size
and consequently require an amount of nourishment that is larger in the
one case than in the other. The mother must know the sex of the egg
which she is going to lay; she must be able to choose the sex of that
egg so that each larva may obtain its proper portion of food.

Generally speaking, when the sexes are of different sizes, every insect
that collects food and prepares or selects a dwelling for its offspring
must be able to choose the sex of the egg in order to satisfy without
mistake the conditions imposed upon it.

The question remains how this optional assessment of the sexes is
effected. I know absolutely nothing about it. If I should ever learn
anything about this delicate point, I shall owe it to some happy chance
for which I must wait, or rather watch, patiently.

Then what explanation shall I give of the wonderful facts which I have
set forth? Why, none, absolutely none. I do not explain facts, I relate
them. Growing daily more sceptical of the interpretations suggested to
me and more hesitating as to those which I myself may have to suggest,
the more I observe and experiment, the more clearly I see rising out of
the black mists of possibility an enormous note of interrogation.

Dear insects, my study of you has sustained me and continues to sustain
me in my heaviest trials; I must take leave