arvae takes place without any
order; secondly, the exodus proceeds regularly from summit to base, but
only in consequence of the insect's inability to move forward so long
as the upper cells are not vacated. We have here not an exceptional
evolution, in the inverse ratio to age, but the simple impossibility of
emerging otherwise. Should a chance occur of going out before its turn,
the insect does not fail to seize it, as we can see by the lateral
movements which send the impatient ones a few ranks ahead and even
release the more favoured altogether. The only remarkable thing that
I perceive is the scrupulous respect shown to the as yet unopened
neighbouring cocoon. However eager to come out, the Osmia is most
careful not to touch it with his mandibles: it is taboo. He will
demolish the partition, he will gnaw the side-wall fiercely, even though
there be nothing left but wood, he will reduce everything around him to
dust; but touch a cocoon that obstructs his way? Never! He will not make
himself an outlet by breaking up his sisters' cradles.

It may happen that the Osmia's patience is in vain and that the
barricade that blocks the way never disappears at all. Sometimes, the
egg in a cell does not mature; and the unconsumed provisions dry up and
become a compact, sticky, mildewed plug, through which the occupants
of the floors below could never clear themselves a passage. Sometimes,
again, a grub dies in its cocoon; and the cradle of the deceased, now
turned into a coffin, forms an everlasting obstacle. How shall the
insect cope with such grave circumstances?

Among the many bramble-stumps which I have collected, some few have
presented a remarkable peculiarity. In addition to the orifice at the
top, they had at the side one and sometimes two round apertures that
looked as though they had been punched out with an instrument. On
opening these stalks, which were old, deserted nests, I discovered the
cause of these very exceptional windows. Above each of them was a cell
full of mouldy honey. The egg had perished and the provisions remained
untouched: hence the impossibility of getting out by the ordinary road.
Walled in by the unsurmountable obstacle, the Osmia on the floor below
had contrived an outlet through the side of the shaft; and those in the
lower storeys had benefited by this ingenious innovation. The usual
door being inaccessible, a side-window had been opened by means of the
insect's jaws. The cocoons, torn, but