MUSCIDAE: THE MID-GUT 115 



mid-gut is attached to it at some distance from its periphery and on the 

 anterior side, there is an annular recess between the edges of the plug 

 and the anterior end of the cells belonging to the mid-gut. It is in this 

 recess that the peritrophic membrane arises. 



The structural arrangements at the communication of the oesophagus 

 with the mid-gut are evidently adapted to function as a valve to prevent 

 regurgitation of the contents of the gut. Pressure of fluid in the 

 anterior end of the mid-gut would result in dilatation of the annular 

 recess, and would, therefore, compress the sides of the opening of the 

 oesophagus. A sphincter muscle has been described in this region 

 in Musca. Its presence would not appear to be necessary for the 

 efficient working of the valve. 



The mid-gut is an elongated tube, extending from the proventriculus in 

 the thorax to the distal portion of the abdomen. It is many times the 

 length of the cavity in which it lies, and is thrown into 

 many coils and twisted upon itself in order to accom- 

 modate its shape to the space available. It is to the length of the mid-gut 

 that the elongation of the alimentary tract is due, for the fore-gut and 

 hind-gut are relatively no longer than those in the Orthorraphic flies. The 

 coils in Philaematomyia are closely similar to those in Stomoxys. The 

 separate turns are held loosely in position by the tracheae which supply 

 them with air. 



The mid-gut may be conveniently divided into three portions. The 

 first of these is a narrow tube immediately posterior to the proven- 

 triculus, which passes directly through the thorax without convolutions. 

 This expands after a short course to form the portion known as the 

 ventriculus, or cliyhfic ventricle, which is the widest part of the gut ; it 

 lies in the posterior part of the thorax and in the abdomen, and is twisted 

 into a double coil in its posterior part. The third part, corresponding to 

 rather less than half the total length of the mid-gut, is known as the 

 proximal intestine, a rather unfortunate term ; it should not be confused 

 with the hind-gut. It is about half the diameter of the ventriculus, and 

 is more loosely coiled. At its posterior end the Malpighian tubes enter 

 the gut. 



The wall of the mid-gut is lined throughout with a digestive epithelium 

 similar in its general appearance to that of Tabamts. The cells are 

 arranged in a single layer, and vary in height, the largest cells being 

 found in the ventriculus. Those lining the narrow anterior portion of 

 the gut are regularly cubical (Plate XXV, fig. 3). In the ventriculus, and 

 to a lesser extent in the proximal intestine, the wall is in many places 



