ax. It is a tiny worm about a millimeter long and
almost as slender as a hair. It is very difficult to see because of its
transparency. When tucked away in a fold of the skin of its fostering
larva, an excessively fine skin, it remains undiscoverable to the lens.
But the feeble creature is very active: it tramps over the sides of the
rich morsel, walks all round it. It covers the ground pretty quickly,
buckling and unbuckling by turns, very much after the manner of the
looper caterpillar. Its two extremities are its chief points of support.
When at a standstill, it moves its front half in every direction, as
though to explore the space around it; when walking, it swells out,
magnifies its segments and then looks like a bit of knotted string.

The microscope shows us thirteen rings, including the head. This head is
small, slightly horny, as is proved by its amber color, and bristles in
front with a small number of short, stiff hairs. On each of the three
segments of the thorax there are two long hairs, fixed to the lower
surface; and there are two similar and still longer hairs at the end of
the terminal ring. These four pairs of bristles, three in front and one
behind, are the locomotory organs, to which we must add the hairy edge
of the head and also the anal button, a sustaining base which might
very well work with the aid of a certain stickiness, as happens with
the primary larva of the Sitaris [a Parasitic Beetle noted for the
multiplicity of transformations undergone by the grub]. We see, through
the transparent skin, two long air tubes running parallel to each other
from the first thoracic segment to the last abdominal segment but one.
They ought to end in two pairs of breathing holes which I have not
succeeded in distinguishing quite plainly. Those two big respiratory
vessels are characteristic of the grubs of flies. Their mouths
correspond exactly with the points at which the two sets of stigmata
open in the Anthrax larva in its second form.

For a fortnight, the feeble grub remains in the condition which I have
described, without growing and very probably also without nourishment.
Assiduous though my visits be, I never perceive it taking any
refreshment. Besides, what would it eat? In the cocoon invaded there is
nothing but the larva of the mason bee; and the worm cannot make use
of this before acquiring the sucker that comes with the second form.
Nevertheless, this life of abstinence is not a life of idleness.