e
in the open air, I see her building now groups of cells as large as a
walnut, now domes emulating in size, shape and solidity those of her
rival, the Mason-bee of the Walls.

The stone support is the most frequent, though not the only one. I have
found nests, but sparsely inhabited it is true, on the trunks of trees,
in the seams of the rough bark of oaks. Among those whose support was
a living plant, I will mention two that stand out above all the others.
The first was built in the lobe of a torch-thistle as thick as my leg;
the second rested on a stalk of the opuntia, the Indian fig. Had the
fierce armour of these two stout cactuses attracted the attention of the
insect, which looked upon their tufts of spikes as furnishing a system
of defence for its nest? Perhaps so. In any case, the attempt was not
imitated; I never saw another installation of the kind. There is one
definite conclusion to be drawn from my two discoveries. Despite the
oddity of their structure, which is unparalleled among the local flora,
the two American importations did not compel the insect to go through an
apprenticeship of groping and hesitation. The one which found itself in
the presence of those novel growths, and which was perhaps the first of
its race to do so, took possession of their lobes and stalks just as it
would have done of a familiar site. From the start, the fleshy plants
from the New World suited it as well as the trunk of a native tree.

The Mason-bee of the Pebbles (Chalicodoma parietina) has none of this
elasticity in the choice of a site. In her case, the smooth stone of the
parched uplands is the almost invariable foundation of her structures.
Elsewhere, under a less clement sky, she prefers the support of a
wall, which protects the nest against the prolonged snows. Lastly, the
Mason-bee of the Shrubs (Chalicodoma rufescens, PEREZ) fixes her ball of
clay to a twig of any ligneous plant, from the thyme, the rock-rose and
the heath to the oak, the elm and the pine. The list of the sites that
suit her would almost form a complete catalogue of the ligneous flora.

The variety of places wherein the insect instals itself, so eloquent of
the part played by discernment in their selection, becomes still more
remarkable when it is accompanied by a corresponding variety in the
architecture of the cells. This is more particularly the case with
the Three-horned Osmia, who, as she uses clayey materials very easily
affected by the rain