s of space.

And besides the exigencies of space one might perhaps venture to add
those connected with the earlier development of the males. These burst
their cocoons a couple of weeks or more before the females; they are the
first 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 experimented on Latreille's Osmia, using short and even very short
stumps of reed. All that I had to do was to lay them just beside the
nests of the Mason-bee of the Sheds, nests beloved by this particular
Osmia. Old, disused hurdles supplied me with reeds inhabited from end to
end by the Horned Osmia. In both cases I obtained the same results and
the same conclusions as with the Three-horned Osmia.

I return to the latter, nidifying under my eyes in some old nests of the
Mason-bee of the Walls, which I had placed within her reach, mixed up
with the tubes. Outside my study, I had never yet seen the Three-horned
Osmia adopt that domicile. This may be due to the fact that these nests
are isolated one by one in the fields; and the Osmia, who loves to feel
herself surrounded by her kin and to work in plenty of company, refuses
them because of this isolation. But on my table, finding them close
to the tubes in which the others are working, she adopts them without
hesitation.

The chambers presented by those old nests are more or less spacious
according to the thickness of the coat of mortar which the Chalicodoma
has laid over the assembled chambers. To leave her cell, the Mason-bee
has to perforate not only the plug, the lid built at the mouth of the
cell, but also the thick plaster wherewith the dome is strengthened at
the end of the work. The perforation results in a vestibule which gives
access to the chamber itself. It is this vestibule which is sometimes
longer and sometimes shorter, whereas the corresponding chamber is of
almost constant dimensions, in the case of the same sex, of course.

We will first consider the short vestibule, at the most large enough to
receive the plug with which the Osmia will close up the lodging. There
is th