This distribution of the sexes agrees with what we have long known of the Hive-bee, who begins her laying with a long sequence of workers, or sterile females, and ends it with a long sequence of males.

The analogy continues down to the capacity of the cells and the quantities of provisions.

The real females, the Queen-bees, have wax cells incomparably more spacious than the cells of the males and receive a much larger amount of food.

Everything therefore demonstrates that we are here in the presence of a general rule.
OPTIONAL DETERMINATION OF THE SEXES.
But does this rule express the whole truth?
Is there nothing beyond a laying in two series?
Are the Osmiae, the Chalicodomae and the rest of them fatally bound by this distribution of the sexes into two distinct groups, the male group following upon the female group, without any mixing of the two?
Is the mother absolutely powerless to make a change in this arrangement, should circumstances require it?
The Three-pronged Osmia already shows us that the problem is far from being solved.

In the same bramble-stump, the two sexes occur very irregularly, as though at random.