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STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 421 



antero-ventral region of the abdomen. Its walls exhibit 

 muscular (unstriped) fibres; the flat epithelial cells have a 

 very thin cuticle. 



The proventriculus (Pv.) is circular and flattened dorso- 

 ventrally. Its structure will be understood by reference to 

 fig. 20. In the middle of the ventral side it opens into the 

 oesophagus, and on the dorsal side the outer wall is continued 

 as the wall of the ventriculus (Ven.). The interior is almost 

 filled up by a thick circular plug (Pv.p.), the cells of which 

 have a fibrillar structure, and it is pierced through the 

 centre by the oesophagus. The neck of the plug is sur- 

 rounded by a ring of elongate cells, external to which the 

 wall of the proventriculus begins, and, enclosing the plug at 

 the sides and above, it merges into the wall of the ventriculus. 

 I do not agree with Lowne in regarding the proventriculus 

 as "a gizzard and nothing more," but its structure suggests 

 a pumping function and also that of a valve. On the dorsal 

 side of the oesophagus, at its junction with the proventriculus, 

 a small ganglion, the proventricular ganglion (Pv.g.), lies, 

 communicating by a fine nerve with the cephalic ganglion. 



The ventriculus, or chyle stomach (figs. 17, 20, Ven.), 

 represents the anterior region of the mesenteron, the posterior 

 region of the latter being formed by the proximal intestine. 

 It is narrow in front, and widest in the posterior region of 

 the thorax, where it again narrows in passing through the 

 thoraco-abdominal foramen into the abdomen to become the 

 proximal intestine. Except in the anterior and posterior 

 regions, where columnar cells compose the digestive epi- 

 thelium, the walls of the ventriculus are thrown into a 

 number of transverse folds, which are again subdivided 

 longitudinally, the result being the formation of small crypts 

 or sacculi, which are lined by large cells. These sacculi 

 correspond to the digestive coeca of other insects. 



The proximal intestine (figs. 17, 21, p.int.) is the 

 longest region of the gut. It varies in length considerably. 

 In the normal-sized condition its course is as follows : 

 Beginning at the anterior end of the abdomen it runs dor- 



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