TABANUS: THE HIND-GUT 107 



continuous sheet. At the lower end of the mid-gut, immediately above 

 the attachment of the Malpighian tubes, there is a small dilatation, 

 specially well provided with circular fibres, which act as a sphincter 

 muscle. 



The hind-gut of Tabanus is naturally separated into two parts, 

 an upper which is narrow and convoluted, and a lower which is 



wider and straight. The former, which may be termed 



, . . Hind-gut 



the ileum, passes downwards and to the right, then 



transversely upwards and to the left ; the wider part, or the colon, com- 

 mences at the last turn, and from this point the gut passes directly 

 backwards to the posterior end of the abdomen. The wall of the hind- 

 gut (Plate XXII, fig. 5) consists of a single layer of regular cubical 

 cells on a basement membrane, an inner layer of circular or oblique 

 muscle fibres, and an outer layer of longitudinal ones ; neither of these 

 are very well developed. The layer of chitin which is present on the 

 internal aspect of the cells is much thicker in the posterior part of 

 the canal than in the ileum, and can only be distinguished with difficulty 

 in the part near the mid-gut. 



The colon is separated from the rectum by a short constriction. 

 The rectum is typically pear-shaped, and opens between the last pair 

 of dorsal and ventral plates by a narrow neck. Its wall has the same 

 structure as that of the rest of the hind-gut, but the circular muscle 

 fibres are much more strongly developed, and the internal chitinous 

 layer is more conspicuous. In the wall of the rectum there are six rectal 

 papillae or glands ; these are stout, curved, wedge-shaped bodies, each 

 a little shorter in its long diameter than the rectum itself. They are 

 inserted into the wall with their broad ends directed outwards, and 

 project a little from the outer surface through the muscular coat ; their 

 pointed ends are directed towards the anus. Each consists of a mass 

 of large cells arranged in a radiating manner around a central lumen 

 which opens into the body cavity, and into which there passes a small 

 tracheal twig. The papillae occupy a considerable proportion of the 

 total area of the lumen of this part of the gut. Their function is un- 

 known, but they are believed to be homologous with similar structures, 

 of common occurrence in aquatic larvae, which function as respiratory 

 organs by absorbing air from water. In many such larvae the papillae 

 can be protruded from the rectum at will. 



In the hind-gut, from the ileum downwards, it is common to find, 

 in this and other blood-sucking flies, a number of coarse granules of 

 a reddish colour. These are the residue from the last meal of blood. 



