15
At the upper end of the haustellum the labial gutter becomes much reduced
in thickness, and becomes attached to the hypoharynx at the point where the
latter is itself becoming narrower. Finally the two fuse together, and a few
sections further up only the thickened salivary duct remains, and this, as
previously described, becomes adherent to the food canal. This is precisely
what happens in the case of Musca, except that in the latter fly the hypo-
pharynx does not become adherent to the food canal. The two laminæ of the
hypopharynx cannot be distinguished from one another, but this method of
termination makes it probable that here, as in the Nematocerous flies, the
salivary duct lies between one lamina which is a true outgrowth from the
s t o m o d æ u m a n d a n o t h e r w h i c h i s d e r i v e d f r o m t h e a n t e r i o r s u r f a c e o f t h e
labium, that is, from the labial gutter. The chief difference between Lyperosia
and Muscai n t h i s r e s p e c t i s t h a t i n t h e f o r m e r t h e s e p a r a t i o n o f t h e l a b i a l
element from the labium has gone much further than in Musca. Separation
of the hypopharynx from the labium appears to occur with the adaptation to
blood-sucking habit, as one finds a similar state of affairs in the mosquito,
where the hypopharynx, though not capable of taking part in the making of
the wound, is completely separated from the labium in the female, and remains
united with it in the male.
The distal end of the labial gutter (Figs. 4 and 5, Plate I, and Fig. 10,
Plate II) is a somewhat complex structure, having become moulded in an intri
- c a t e m a n n e r i n o r d e r t o t a k e p a r t i n t h e a r t i c u l a t i o n b e t w e e n t h e l a b i u m a n d .
the labella, and to assist in supporting them during the act of feeding. In the
first place, the distal third of the gutter is much thicker than the remaining
portion, and is curved a little downwards, that is, towards the furca. It forms
a prominent chitinous bar bounding the upper side of the membraneous trian
- g l e i n t h e r e g i o n o f t h e f u r c a . T o w a r d s i t s t e r m i n a t i o n i t b e c o m e s s e p a r a t e d
into two distinct portions, with a narrow interval between them. The lower
portion, coming from the bottom of the groove, retains the shape of a shallow
gutter when seen in cross section, while the upper portion appears as a stout
rod, flattened from side to side so that its vertical diameter is about twice its
width. In other words, the gutter becomes deeper and at the same time its
bottom portion becomes separated from its lateral walls. At the extreme end
the lateral roads are slightly swollen, and terminate in blunt points. The
lower portion is much more highly specialised. The most ventral portion is
continued onwards to the level of the termination of the lateral rods, with a
slight upward curve, but the lateral portions cease abruptly at the level of the
swollen portion of the upper rods, so that when seen in profile the end appears
as if it were sharply cut away from above, leaving a wide interval between the
upper and lower portions of the gutter. Further details of the articulation