14
closed positions. When the labelia are not in use, the oral surfaces are contracted
and in contact with one another, the lower limit of the organ being formed by the
scaly border of the external surface. The labella are rotated backwards, the
heart-shaped sclerite in the posterior surface coming to lie in contact with the
posterior surface of the theca. The labellar rods rotate backwards on the fixed
point provided by the attachment of their tubercles to the end of the labial gutter,
carrying the teeth and the serrated blades with them. The rods continue to lie
in the long axis of the labella as they are turned backwards, so that, when the
labella are closed, the upper ends of the rods project forwards and slightly down
- wards
.
The method by which the wound is made is best studied by the dissection of'
freshly killed flies, the actions of the muscles being tested by pulling on them with a
needle. The anterior or mesial sets are found to erect the labellar rods and to
bring them into line with the labial gutter. The posterior muscles pull the furca
upwards, and so exercise a tension on the labellar walls to which the furca is so
intimately attached, pulling the pseudo-tracheal membrane away from the teeth,
a n d b y t r a c t i o n o n t h e m e m b r a n e t o w h i c h t h e y a r e a t t a c h e d r o t a t i n g t h e m i n t o a
horizontal plane. When this occurs their cutting edges are turned upwards.
What appears to occur when the fly commences to feed is this. The labellar rods
are erected and the tip of the proboscis pressed against the skin of the host with the
teeth i n the cl osed posi ti on; the l abel l ar rods bei ng fi xed i n thi s posi ti on by thei r
muscles; the f urea and the walls of the labella are pulled upwards so that the pseudo-
tracheal membrane is drawn away from, the teeth, which by a continuation of the
traction are rotated through a right angle into a horizontal plane, their cutting
edges being pressed against the skin of the host. This is repeated with very great.
rapidity, and imparts to the head of the fly a vibratory motion which can often be
observed at the commencement of feeding. At each successive contraction the
teeth are thrust a little further into the skin, until eventually blood is reached. The
action is rather a scarification than `` biting." Whether the blood once it begins
to flow is sucked up through the medium of the pseudo-tracheal membrane or
whether it goes directly into the mouth it is difficult to say, but I am inclined to
think that the membrane is drawn entirely out of the way, and that its function as
a filter is performed by the remarkable serrated blades. These structures, like the
teeth, are attached to the membrane, and must be drawn upwards in the same
way, but in advance of them, and will therefore sweep away the cellular debris
produced by the teeth, each time they are drawn through the wound, and will
prevent the ingress of large particles into the mouth. When the teeth are drawn
up the axial apophysis becomes the lowest part of the proboscis, and projects
beyond the teeth; it probably materially assists in the cutting operations by fixing
the distal ends of the labellar rods.