st who hasten to the sweets of the almond-tree. In order to
release themselves and emerge into the glad sunlight without disturbing
the string of cocoons wherein their sisters are still sleeping, they
must occupy the upper end of the row; and this, no doubt, is the reason
that makes the Osmia end each of her broken layings with males. Being
next to the door, these impatient ones will leave the home without
upsetting the shells that are slower in hatching.

I had offered at the same time to the Osmiae in my study some old nests
of the Mason-bee of the Shrubs, which are clay spheroids with
cylindrical cavities in them. These cavities are formed, as in the old
nests of the Mason-bee of the Pebbles, of the cell properly so-called
and of the exit-way which the perfect insect cut through the outer
coating at the time of its deliverance. The diameter is about 7
millimetres (.273 inch.--Translator's Note.); their depth at the centre
of the heap is 23 millimetres (.897 inch.--Translator's Note.) and at
the edge averages 14 millimetres. (.546 inch.--Translator's Note.)

The deep central cells receive only the females of the Osmia; sometimes
even the two sexes together, with a partition in the middle, the female
occupying the lower and the male the upper storey. Lastly, the deeper
cavities on the circumference are allotted to females and the shallower
to males.

We know that the Three-horned Osmia prefers to haunt the habitations of
the Bees who nidify in populous colonies, such as the Mason-bee of the
Sheds and the Hairy-footed Anthophora, in whose nests I have noted
similar facts.

Thus the sex of the egg is optional. The choice rests with the mother,
who is guided by considerations of space and, according to the
accommodation at her disposal, which is frequently fortuitous and
incapable of modification, places a female in this cell and a male in
that, so that both may have a dwelling of a size suited to their
unequal development. This is the unimpeachable evidence of the numerous
and varied facts which I have set forth. People unfamiliar with insect
anatomy--the public for whom I write--would probably give the following
explanation of this marvellous prerogative of the Bee: the mother has
at her disposal a certain number of eggs, some of which are irrevocably
female and the others irrevocably male: she is able to pick out of
either group the one which she wants at the actual moment; and her
choice is decided by the holding