and the two mason bees.

In the cells of these three species, I see the number of larvae put
out to nurse vary in so elastic a fashion that I must abandon all
idea of proportionate adjustment. The mother, without troubling
unduly whether there be an excess or a dearth of provisions for her
family, has filled the cells as her fancy prompted, or rather
according to the number of ripe ovules contained in her ovaries at
the time of the laying. If food be over-plentiful, the brood will
be all the better for it and will grow bigger and stronger; if food
be scarce, the famished youngsters will not die, but will remain
smaller. Indeed, with both the larva and the full grown insect, I
have often observed a difference in size which varies according to
the density of the population, the members of a small colony being
double the size of their overcrowded neighbors.

The grubs are white, tapering at both ends, sharply segmented and
covered all over their bodies with a coat of fine, soft hairs which
is invisible except under the lens. The head consists of a little
knob much smaller in diameter than the body. In this head, the
microscope reveals mandibles consisting of fine spikes of a tawny
red, which spread into a wide, colorless base. Deprived of any
indentation, incapable of chewing anything between their awl-shaped
ends, these two tools serve at best to fix the grub slightly at
some point of the fostering larva. Useless for carving, therefore,
the mouth is a pure osculatory sucker, which drains the provisions
by a process of exudation through the skin. We see here repeated
what the Anthrax and the Leucospis have already shown us: the
gradual exhaustion of a victim which the parasite consumes without
killing it.