are the quickest at the business. They have
been lucky enough to come across a wall which is less thick and less
hard than usual. I see others who spend as many as three hours on a
single operation, three long hours of patient watching for me, in my
anxiety to follow the whole performance to the end, three long hours of
immobility for the insect, which is even more anxious to make sure of
board and lodging for its egg. But then is it not a task of the utmost
difficulty to introduce a hair into the thickness of a stone? To us,
with all the dexterity of our fingers, it would be impossible; to the
insect, which simply pushes with its belly, it is just hard work.

Notwithstanding the resistance of the substance traversed, the Leucopsis
perseveres, certain of succeeding; and she does succeed, although I am
still unable to understand her success. The material through which the
probe has to penetrate is not a porous substance; it is homogeneous and
compact, like our hardened cement. In vain do I direct my attention to
the exact point where the instrument is at work; I see no fissure, no
opening that can facilitate access. A miner's drill penetrates the rock
only by pulverizing it. This method is not admissible here; the extreme
delicacy of the implement is opposed to it. The frail stem requires, so
it seems to me, a ready-made way, a crevice through which it can slip;
but this crevice I have never been able to discover. What about a
dissolving fluid which would soften the mortar under the point of the
ovipositor? No, for I see not a trace of humidity around the point where
the thread is at work. I fall back upon a fissure, a lack of continuity
somewhere, although my examination fails to discover any on the
Mason-bee's nest. I was better served in another case. Leucopsis
dorsigera, FAB., settles her eggs on the larva of the Diadem Anthidium,
who sometimes makes her nest in reed-stumps. I have repeatedly seen her
insert her auger through a slight rupture in the side of the reed.
As the wall was different, wood in the latter case and mortar in the
former, perhaps it will be best to look upon the matter as a mystery.

My sedulous attendance, during the best part of July, in front of the
tiles hanging from the walls of the arch, allowed me to reckon the
inoculations. Each time that the insect, on finishing the operation,
removed its probe, I marked in pencil the exact point at which the
instrument was withdrawn; and I wrote down th