Why this mixture in the series of cocoons of a Bee closely related to the Horned Osmia and the Three-horned Osmia, who stack theirs methodically by separate sexes in the hollow of a reed?
What the Bee of the brambles does cannot her kinswomen of the reeds do too?
Nothing, so far as I know, explains this fundamental difference in a physiological act of primary importance.
The three Bees belong to the same genus; they resemble one another in general outline, internal structure and habits; and, with this close similarity, we suddenly find a strange dissimilarity.
There is just one thing that might possibly arouse a suspicion of the cause of this irregularity in the Three-pronged Osmia's laying.

If I open a bramble-stump in the winter to examine the Osmia's nest, I find it impossible, in the vast majority of cases, to distinguish positively between a female and a male cocoon: the difference in size is so small.
The cells, moreover, have the same capacity: the diameter of the cylinder is the same throughout and the partitions are almost always the same distance apart.

If I open it in July, the victualling-period, it is impossible for me to distinguish between the provisions destined for the males and those destined for the females.

The measurement of the column of honey gives practically the same depth in all the cells.
We find an equal quantity of space and food for both sexes.
This result makes us foresee what a direct examination of the two sexes in the adult form tells us.

The male does not differ materially from the female in respect of size.