many a time I find the Dioxys and the
Mason-bee in almost equal proportions. The parasite has wiped out
half the colony. To complete the disaster, it is not unusual for the
grub-eaters, the Leucopsis and her rival, the pygmy Chalcis, to have
decimated the other half. I say nothing of Anthrax sinuata, whom I
sometimes see coming from the nests of the Chalicodoma of the Sheds; her
larva preys on the Three-horned Osmia, the Mason-bee's visitor.

All solitary though she be on her boulder, which would seem the proper
thing to keep away exploiters, the scourge of dense populations, the
Chalicodoma of the Pebbles is no less sorely tried. My notes abound in
cases such as the following: of the nine cells in one dome, three are
occupied by the Anthrax, two by the Leucopsis, two by the Stelis, one
by the Chalcis and the ninth by the Mason. It is as though the four
miscreants had joined forces for the massacre: the whole of the Bee's
family has disappeared, all but one young mother saved from the disaster
by her position in the centre of the citadel. I have sometimes stuffed
my pockets with nests removed from their pebbles without finding a
single one that has not been violated by one or other of the malefactors
and oftener still by several of them at a time. It is almost an event
for me to find a nest intact. After these funereal records, I am haunted
by a gloomy thought: the weal of one means the woe of another.

CHAPTER 11. THE LEUCOPSES.

(This chapter should be read in conjunction with the essays entitled
"The Anthrax" and "Larval Dimorphism", forming chapters 2 and 4 of "The
Life of the Fly."--Translator's Note.)

Let us visit the nests of Chalicodoma muraria in July, detaching them
from their pebbles with a sideward blow, as I explained when telling the
story of the Anthrax. The Mason-bee's cocoons with two inhabitants, one
devouring, the other in process of being devoured, are numerous enough
to allow me to gather some dozens in the course of a morning, before the
sun becomes unbearably hot. We will give a smart tap to the flints so as
to loosen the clay domes, wrap these up in newspapers, fill our box
and go home as fast as we can, for the air will soon be as fiery as the
devil's kitchen.

Inspection, which is easier in the shade indoors, soon tells us that,
though the devoured is always the wretched Mason-bee, the devourer
belongs to two different species. In the one case, the cylindrical form,
the creamy-white c