xx MASON BEES 287
surface of the nest, where the cells are hidden below
the old general mortar covering, new ones are built
as required. They are placed more or less horizontally,
one beside another, with no kind of order. Every
constructor builds as the fancy takes her, where and
as she wills ; only she must not interfere with her
neighbour's work, or rough treatment will soon call
her to order. The cells accumulate in chance
fashion in this workyard, where there is no general
plan whatever. Their form is that of a thimble
divided down the axis, and their enclosure is com-
pleted either by adjacent cells, or the surface of the
old nest. Outside they are rough, and look like
layers of knotted cords corresponding to the layers
of mortar. Inside the walls are level but not
smooth ; a cocoon will replace the absent polish.
As soon as a cell is built it is stored and walled
up, as we have seen with Chalicodoma muraria. This
work goes on through the whole of May. At
length all the eggs are laid, and the bees, without
any distinction as to what does or does not belong
to them, all set to work on a common shelter of the
colony—a thick bed of mortar, filling up spaces and
covering all the cells. In the end the nests look
like a large mass of dry mud—very irregular, arched,
thickest in the middle, the primitive kernel of the
establishment, thinnest at the edges, where there are
fewest cells, and very variable in extent, according
to the number of workers, and consequently to the
time when the nest was begun. Some are not much
larger than one's hand, while others will occupy the
greater part of the edge of a roof, and be measured
by square yards.