of the former has become well marked, so that its lateral edges curve inwards,
fitting inside the labial gutter, which has now become more V shaped. About the
middle of the labial gutter (Plate IV, Fig. 16) the edges of the groove are seen to
have approached still nearer to one another, so as to almost touch the hypo-
pharynx, which is now triangular in section, the rounded posterior angle fitting
into the bottom of the groove, the slightly concave base being opposite to the
groove in the labrum-epipharynx. At this level the inner and outer laminæ of
the labrum-epipharynx are firmly attached to one another by the median strand
of fibres. At the level of the " keel " of the labial gutter (Plate II, Fig. 7) the
labrum-epipharynx is considerably broader from side to side, and edges of the
groove are almost in contact with one another. The gutter is now much contracted
in its posterior half, forming a recess into which the hypopharynx, now also en
-larged and more distinctly concave on its anterior surface, is fitted. The anterior
half of each lateral wall of the gutter is bent outwards as well as forwards, so as to
form with the anterior surface of the hypopharynx, an even surface on which the
labrum-epipharynx, now broader than the labial gutter, rests.
Near the base of the labial gutter (Plate III, Fig. 8), where the keel is
becoming narrower the labrum-epipharynx appears as a closed. canal with a double wall, the
inner circular tube being continuous with the groove. The hypopharynx has
become an oval tube of thick chitin with a circular lumen, and lies between the
now contracted sides of the gutter. The theca is much reduced in its antero
- p o s t e r i o r d i a m e t e r , s o t h a t t h e u p p e r e n d o f t h e k e e l a p p r o a c h e s c l o s e t o t h e m e d i a n
limb of the Y shaped ridge on its anterior surface. At the extreme upper end of
the haustellum (Plate V, Fig. 19) the labial gutter and the hypopharynx are com
- p l e t e l y f u s e d t o f o r m a d e n s e m a s s o f c h i t i n , w h i c h e x t e n d s f r o m t h e t h e c a t o t h e
labrum-epipharynx, and to which the posterior surface of the food canal is fused.
The salivary duct runs through this expansion, towards its posterior border, as a
simple round tube.
It will be seen from the foregoing description that the food channel is formed
in the lower part by the labial gutter and the flattened tip of the labrum-epipharynx ;
in the middle portion, by the apposition of the edges of the groove in the
posterior surface of the labrum-epipharynx and the concave anterior surface
of the hypopharynx ; in the upper portion, by the closed canal in the labrum
- e p i p h a r y n x . T h e r e i s n o s y s t e m o f i n t e r l o c k i n g t e e t h s u c h a s t h a t f o u n d i nSto
- m o x y s ,
t h e c o n t i n u i t y o f t h e c h a n n e l d e p e n d i n g o n s i m p l e a p p o s i t i o n o f t h e p a r t s .
It will be noticed that the extreme tip of the hypopharynx projects into the lumen
of the canal ; the tip is extremely attenuated and ill-defined, and, as suggested by
Stephens and Newstead, it may function as a valve to Drevent the blood being
drawn up the salivary duct.