d, 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. If he is a trifle smaller, it is scarcely
noticeable, whereas, in the Horned Osmia and the Three-horned Osmia,
the male is only half or a third the size of the female, as we have seen
from the respective bulk of their cocoons. In the Mason-bee of the Walls
there is also a difference in size, though less pronounced.

The Three-pronged Osmia has not therefore to trouble about adjusting the
dimensions of the dwelling and the quantity of the food to the sex of
the egg which she is about to lay; the measure is the same from one end
of the series to the other. It does not matter if the sexes alternate
without order: one and all will find what they need, whatever their
position in the row. The two other Osmiae, with their great disparity
in size between the two sexes, have to be careful about the twofold
consideration of board and lodging. And that, I think, is why they begin
with spacious cells and generous rations for the homes of the females
and end with narrow, scantily-provisioned cells, the homes of the males.
With this sequence, sharply defined for the two sexes, there is less
fear of mistakes which might give to one what belongs to another. If
this is not the explanation of the facts, I see no other.

The more I thought about this curious question, the more probable it
appeared to me that the irregular series of the Three-pronged Osmia and
the regular series o