e rule has been observed. A male cell is never surrounded on every
side by female cells: either it occupies the edges of the nest, or else
it adjoins, at least on some sides, other male cells, of which the last
form part of the exterior of the cluster. As the surrounding cells are
obviously of a later date than the inner cells, it follows that the
Mason-bee acts like the Osmiae: she begins her laying with females
and ends it with males, each of the sexes forming a series of its own,
independent of the other.

Some further circumstances add their testimony to that of the surrounded
and surrounding cells. When the pebble projects sharply and forms a sort
of dihedral angle, one of whose faces is more or less vertical and the
other horizontal, this angle is a favourite site with the Mason, who
thus finds greater stability for her edifice in the support given her by
the double plane. These sites appear to me to be in great request with
the Chalicodoma, considering the number of nests which I find thus
doubly supported. In nests of this kind, all the cells, as usual, have
their foundations fixed to the horizontal surface; but the first row,
the row of cells first built, stands with its back against the vertical
surface.

Well, these older cells, which occupy the actual edge of the dihedral
angle, are always female, with the exception of those at either end of
the row, which, as they belong to the outside, may be male cells. In
front of this first row come others. The female cells occupy the middle
portion and the male the ends. Finally, the last row, closing in the
remainder, contains only male cells. The progress of the work is very
visible here: the Mason has begun by attending to the central group of
female cells, the first row of which occupies the dihedral angle, and
has finished her task by distributing the male cells round the outside.

If the perpendicular face of the dihedral angle be high enough, it
sometimes happens that a second row of cells is placed above the first
row backing on to that plane; a third row occurs less often. The nest is
then one of several storeys. The lower storeys, the older, contain only
females; the upper, the more recent storey, contains none but males. It
goes without saying that the surface layer, even of the lower storeys,
can contain males without invalidating the rule, for this layer may
always be looked upon as the Chalicodoma's last work.

Everything therefore contributes to show