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wound is made. I have referred in a previous paper to the rapidity of action of
which insect muscle is capable, and the action of the muscles of the bulb in
undoing the invagination at the end of the labella, and so causing the teeth to
press upwards, appears to me to be another instance of it. The muscles which
are inserted into the furca are not only much larger in this fly than they are in
Musca, but they are proportionately very much larger than those of the anterior
set, and are undoubtedly the dominant motive force in the proboscis.

   Retraction of the furca is not the only result of the action of these muscles.
It has already been pointed out that the labrum-epipharynx and the hypo-
pharynx do not reach to the distal end of the proboscis in the resting position.
In fact, they are so much shorter than the combined labium and labella that
their distal ends only appear in sections about the level of the furca, and distal
to them there is the whole length of the discal sclerite. Now the discrepancy
in length is partly compensated for by the retraction of the labellar walls, by
means of which the distal end of the discal sclerite becomes the terminal point
of the proboscis, but even this is not sufficient, as is clearly shown by actual
measurements of cleared proboscides, to bring their distal ends into such a position
that they can perform their functions, and further shortening is required before
they can reach the wound. This is accomplished by the shortening of the whole
of the mentum. In the early part of this paper it was stated that the whole of
the chitinous wall of the mentum is traversed by a closely set series of ridges
and furrows. Now these do not present the same appearance in all prepara-
tions, and a careful comparison of those in proboscides which are in the position
of rest and those in the position of action shows that the more the lateral arms
of the furca are displaced backwards the more marked the ridges and furrows
become, and it follows that in each individual specimen the more marked the
furrows are the shorter the mentum will be, for the crinkling of the wall must
reduce its length. I have been unable to obtain a cleared preparation in which
complete evagination has occurred, but I have no doubt that if one could obtain
one it would show that the evagination of the labella and the crinkling of the
mentum taken together had reduced the total length of the labium just suffi-
ciently to bring the labium-epipharynx and the hypopharynx to the distal end
of the proboscis. The muscles of the bulb, the retractors of the furca, bring
this about by pressing the furca on the ventral sclerites, so that the movement
of the furca is an actual displacement as well as a rotation on its fixed point.

   The labrum-epipharynx and the hypopharynx are of course quite incapable
of being moved on the labium, owing to the close attachments at their proximal
ends. The labial gutter cannot move on the mentum, for, although there is no
firm chitinous union, they are closely held together by means of the membrane
which unites their lateral borders throughout their length, and at the proximal