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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 a series of eight grooves separated by bifurcating 

 ribs which are T-shaped in section (fig. 18, t. r.), 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 oesophagus. 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 oesophagus. 



The oesophagus (fig. 29, OB.) 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 ganglionic mass and dorsal to the 

 ganglion 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, OB. ep.) whose internal faces are lined 

 by a chitinous sheath (ch. i.), 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 



