16
between this pointed end and the discal sclerite will be reserved until the
chitinous framework of the labella has been discussed.
The muscles of the labium are very well developed, as they have to
actuate the biting apparatus. The most important one is the retractor of the
f u r c a . T h i s l a r g e a n d p o w e r f u l m u s c l e o c c u p i e s t h e w h o l e o f t h e c a v i t y o f t h e
bulb, arising from the internal surface of the mentum and from the " keel " of
the labial gutter, which separates the muscles of the two sides. As it passes
forward a tendon is developed in its middle, and as the diameter of the cavity
d e c r e a s e s t h e mu s c l e f i b r e s g i v e p l a c e a s t o a l a r g e r o u n d b u n d l e o f f i b r o u s t i s s u e ,
very conspicuous in all sections of the narrower part of the proboscis. This
tendon is inserted mainly into the inner and posterior surfaces of the tip of
each lateral arm of the furca, but it also sends out numerous branching fibres.
to the internal surface of the external plate of the labellum in thee neighbour
- h o o d o f t h e f u r c a . I n t e r m i n g l e d w i t h t h e m o s t i n t e r n a l f i b r e s o f t h i s m u s c l e
in the middle and distal portions of the labium there are other fibres having an
oblique direction, and passing from the internal surface of the mentum inwards
and forwards ; this band of muscle corresponds to that described by Stephens
and Newstead, but in Lypevosia it does not appear to be inserted into the
labial gutter. It has no tendon at any part of its extent, so that it is difficult
to trace it to its insertion, and in any case it is a very small band. In addition
to this there is in the distal part of the labium, in the region of the thick part
of the " keel, " a definite band of fibres passing from the posterior surface
of the mentum in the region of the ventral sclerites and also proximal to them,
directly across the cavity of the labium, to the base of the labial gutter, so
that the bands of fibres are cut in their long axis in transverse sections of the
proboscis.
It will be well to note here the relations of these muscles to those of
M u s c a
, i n o r d e r t o b e a b l e t o u n d e r s t a n d t h e i r f u n c t i o n i n c o n n e c t i o n w i t h
the mechanism of feeding when we come to discuss the complex structures
on which they act. The large retractor muscles in the bulb are quite evidently
similar in their origin and insertion to the retractor muscles of the furca in
M u s c a
, t h o s e t e r m e d b y K r a e p e l i nr
e t r a k l o r
d e r u n t e r e n C h i t i n g a b e l;
t h e y
are, however, enormously enlarged. The second set of fibres appears to
correspond to the retractors of the discal sclerites ( oberen Chi ti ngabel o f
Kraepelin), while the third is evidently the same as the transverse set in
M s c a
. T h e t h r e e s e t s o f m u s c l e s a r e t h e r e f o r e r e t a i n e d , b u t o n e o f t h e m
has increased to a very great degree, while the other two sets are much less
conspicuous than in Musca, or even in Ph i l œma t o my i a. I n a d d i t i o n t h e r e a r e
a few fibres in the middle and posterior portions of the labium, which pass
from the lateral extremities of the gutter across the front of the cavity to.