106 MEDICAL ENTOMOLOGY 



when the organ is distended with blood. It lies in the second and 

 third segments of the abdomen, and is embedded in a dense coating of 



fat body. Next to the ovaries, it is the part of the 

 Mid-gut ..... . 



body with the richest tracheal supply. 



The wall consists of a single layer of columnar epithelial cells, set 

 on a basement membrane, and two layers of muscular fibres, an inner 

 circular layer and an outer longitudinal one. 



The layer of columnar cells is collected into numerous villi, which 

 project into the lumen on every side, giving it a stellate appearance. 

 In the intervals between the villi the cells are regularly columnar, 

 but those which compose the villus are altered in shape so as to 

 adapt themselves to its contour ; the attached ends are narrowed and 

 compressed together, while the part of the cell towards the lumen is 

 correspondingly increased in size, its free border projecting into the 

 lumen, and giving to the border of the villus a crenulated appearance. 

 The basement membrane of the cells is tucked inwards at the base 

 of the villus, and at this point a small trachea enters the wall and at 

 once breaks up into minute branches. It is to the presence of these 

 villi that the mid-gut owes its mamillated appearance. 



The cells of the mid-gut present very different appearances at different 

 stages of the process of digestion, and comparatively little is known 

 about the changes which occur in them, in this or in any other blood- 

 sucking fly, during the digestion of blood. In the resting condition, 

 in which the fly is preparing for its meal of blood, the part of the 

 cell anterior to the nucleus is filled up with vacuoles, of various sizes 

 in different preparations ; they may be so minute as to impart only 

 a granular appearance to the cell, or they may be so large as to break 

 down the cell substance. The border of the cell is striated, and in 

 many preparations appears to be ciliated, though it is doubtful if there 

 are really free cilia present. The nucleus is large and oval, and is 

 situated in the posterior part of the cell in those cells which lie 

 between the villi, but near the lumen in the rest. At the base of many 

 of the cells a second small and deeply staining nucleus can be dis- 

 tinguished ; this represents an immature cell which will eventually grow 

 to replace a degenerated one. 



The muscular coat of the mid-gut is of considerable thickness. The 

 inner fibres are arranged a little obliquely, and interlace with one 

 another. The outer longitudinal fibres are straighter, and are 

 continuous with those of the hind-gut, the whole of the alimen- 

 tary tract posterior to the proventriculus being enveloped in the same 



