STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 525 
similar to that portion of the pharynx of the fly which lies in 
the fulcrum. The whole length of the floor of the pharynx is 
traversed by aseries of eight grooves separated by bifurcating 
ribs which are T-shaped in section (fig. 18, ¢.7.), and are called 
the “T ribs” by Holmgren (1904); they form a series of 
eight tubular grooves. Holmgren believes that they may 
have been derived from a condition similar to that found in 
the pharynx of the larva of Phalacrocera, where the floor 
of the pharynx is traversed by a number of deep but closed 
longitudinal fissures.. These pharyngeal grooves probably 
have a straining function, but they may also be of use in 
allowing a certain amount of the salivary secretion to flow 
backwards towards the csophagus. The musculature and 
action of the pharynx has been described. On the dorsal 
side of the pharyngeal mass and attached laterally to the 
layer of cells covering the lateral sclerites there is a loose 
membrane (m.), whose function, I believe, is to accommodate 
the blood contained in the pharyngeal sinus (p.s.) when the 
roof of the pharynx is raised. Posteriorly the floor of the 
pharynx curves dorsally and opens into the cesophagus. 
The oesophagus (fig. 29, w.) is a muscular tube beginning 
at the posterior end of the pharyngeal mass. It describes a 
dorsal curve when the larva is contracted, and then runs in 
a straight line through the cesophageal foramen between 
the cerebral lobes of the ganglonic mass and dorsal to the 
eanglion to the posterior region of the sixth larval segment, 
where it terminates and opens into the proventriculus. It is 
of a uniform width throughout and is lined by a layer of flat 
epithelial cells (fig. 25, w. ep.) whose internal faces are lined 
by a chitinous sheath (ch. ¢.), which is thrown into a number 
of folds. There is nothing of the nature of a ventral diver- 
ticulum forming a crop such as Lowne describes in the larva 
of the blow-fly. 
The proventriculus (fig. 29, pv.) varies slightly in shape 
according to the state of contraction of the alimentary tract ; 
in the normal condition it is cylindrically ovoid and its axis 
is parallel with that of the body. As will be seen from the 
