116 MEDICAL ENTOMOLOGY 



raised into small protuberances, between which there are shallow fossae. 

 In many sections one finds the cells undergoing changes connected with 

 the process of digestion, and discharging their contents into the lumen. 

 (Plate XXV, fig. 4.) 



External to the cells there are circular and longitudinal muscle fibres, 

 both of which are well developed. 



The hind-gut is similar to that of Tabanus, and is of about the same 

 length in proportion to the size of the fly. It is of uniform diameter, 



but has a slight constriction about its middle point, 

 Hind-gut . 



where the circular muscle fibres are strongly devel- 

 oped so as to form a sphincter. At the distal end it opens into a rectum 

 like that of the Orthorraphic flies, with four rectal papillae. 



The structure of the wall of the hind-gut is simple. There is a single 

 layer of regular cubical cells, with a chitinous intima which is best devel- 

 oped in the posterior portion, and external to this the usual two layers of 

 muscle, of which the circular fibres are the most strongly developed. 

 The chitinous intima in the lower part is developed into a series of 

 short spines, which project into the lumen of the gut, and are directed 

 towards the anus. (Plate XXIV, fig. 3.) 



The Malpighian tubes arise on each side by a common stem, and 

 pass forward and backward among the tissues. Like those of Tabanus 



, . ._. they are composed of a single layer of cells, but the 



Malpighian tubes .,..,,,, , : . 



individual cells are rounder, and their central portions 



project a little from the surface, thus giving to the tube a wavy 

 contour. At the distal end, for about one-quarter the length of the tube, 

 the lumen is dilated and filled with a densely white granular material, 

 which stretches the cells composing the wall and distorts the regular ar- 

 rangement found in the lower portions. The dilatation is most irregu- 

 lar, so much so that this part of the tubule appears to consist of a 

 series of elongated pouches connected together by narrow necks. The 

 wall is here very fragile and easily ruptured. The degree to which the 

 tube is distended and filled with this white material varies in different 

 specimens, and probably depends on the age of the fly. 



The peritrophic membrane (Plate XXIV, figs. 4 and 5), already referred 

 to, forms an inner tube which extends from the proventriculus to the 

 Peritrophic mem- anus, and completely separates the contents of the gut 

 brane from the epithelium. It hangs down from the pro- 



ventriculus, in fact, like a tubular curtain, and is in contact with the cells 

 at all points but nowhere attached to them except at its upper end. It is 

 composed of an extremely thin and translucent material which is very 



