r in August. The insect is then in
the nymphal stage; and it is easy, under this form, to distinguish the
two sexes by the length of the antennae, which are larger in the males,
and by the glassy protuberances on the forehead, the sign of the future
armour of the females. Well, the small cocoons, those in the narrow
front cells, with their scanty store of provisions, all belong to
males; the big cocoons, those in the spacious and well-stocked cells at
the back, all belong to females.

The conclusion is definite: the laying of the Three-horned Osmia
consists of two distinct groups, first a group of females and then a
group of males.

With my pan-pipe apparatus displayed on the walls of my enclosure and
with old hurdle-reeds left lying flat out of doors, I obtained the
Horned Osmia in fair quantities. I persuaded Latreille's Osmia to build
her nest in reeds, which she did with a zeal which I was far from
expecting. All that I had to do was to lay some reed-stumps
horizontally within her reach, in the immediate neighbourhood of her
usual haunts, namely, the nests of the Mason-bee of the Sheds. Lastly,
I succeeded without difficulty in making her build her nests in the
privacy of my study, with glass tubes for a house. The result surpassed
my hopes.

With both these Osmiae, the division of the gallery is the same as with
the Three-horned Osmia. At the back are large cells with plentiful
provisions and widely-spaced partitions; in front, small cells, with
scanty provisions and partitions close together. Also, the larger cells
supplied me with big cocoons and females; the smaller cells gave me
little cocoons and males. The conclusion therefore is exactly the same
in the case of all three Osmiae.

These conclusions, as my notes show, apply likewise, in every respect,
to the various species of Mason-bees; and one clear and simple rule
stands out from this collection of facts. Apart from the strange
exception of the Three-pronged Osmia, who mixes the sexes without any
order, the Bees whom I studied and probably a crowd of others produce
first a continuous series of females and then a continuous series of
males, the latter with less provisions and smaller cells. 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 q