24

pharynx, the stream of blood being aided by the negative pressure in the food
canal, which will also tend to draw the two organs together. The entrance to
the food channel is not a fixed and rigid one, but varies in size according to the
distance between the labrum-epipharynx and the hypopharynx, this being
regulated by the muscle at the base of the labrum. The adjustment is a very
fine one, for the more powerfully the pharyngeal muscles contract the more
will the two parts of the tube be drawn together, and more the valve-like end
of the hypopharynx will tend to close the aperture.

   A word is necessary with regard to certain minute spines found in the in-
ernal surface of the groove of the labrum-epipharynx. I have only found
these in Tabanidœ and in Simulium, but it is likely that they also occur in the
other members of the group. They are short, stout, and gently curved struc-
tures, arising from expanded and globular bases which lie in the substance of
the chitin of the epipharynx, and are seen as clear spaces when the structure is
examined in optical section. The base of each of these spines is perforated
where it projects into the space between the labrum and the epipharynx, and
with careful focussing a fine canal can be seen in the lower part of the free
portion of the hair. They closely resemble the spines described in a correspond-
ing situation in Stomoxys by Hansen and by Stephens and Newstead, and
one is inclined to regard them, from their situation, as organs of taste, though
of course there is no direct evidence as to their function. Possibly it may be
through them that communication between the prestomal orifice and the muscle
of the labrum is established for the purpose of regulating the flow of blood.
Their occurrence in such a widely separated flies as Simulium and Stomoxys is
suggestive.

   The mechanism of the labium appears to have escaped the notice of
workers on this subject. It is generally stated that when the mosquito feeds
the labium becomes bent backwards like a bow, and the labella separated to
allow the piercing parts to pass between them, but no mention is made of
the forces which bring about this change in position, and one is left to
assume that the labium is simply pushed back because it cannot enter the
skin. The process can be readily watched in the mosquito, and is faithfully
depicted in Nuttall and Shipley's drawing. In all the other flies without
exception the same thing must take place, since the labium and labella are
always long enough to conceal the mouth parts in the condition of rest.
There is no reason to suppose that there is any such haphazard bending as
that indicated above, for the labium has its intrinsic muscles, some arising
within the head capsule and some within the cavity of the organ, which are
inserted into its integument at the distal end and into the labella. These
are familiar from the many diagrams of the cross-section of the proboscis