118 MEDICAL ENTOMOLOGY 



that is to say, concave longitudinally and convex transversely when 

 seen from above. Its lateral portions are wrapped round the oesophagus 



and the duct of the crop, which therefore appear to 

 Glossina ,,,,-, , , ,, 



be embedded in the substance of the organ. The crop 



is relatively large, and is always found to be full of fresh blood when the 

 fly has had a full meal. When the fly has not fed recently the crop con- 

 tains bubbles of gas. 



In all these flies it is the anterior portion of the mid-gut, or the 

 ventriculus, in which the fresh blood is found after feeding. The nar- 

 rower portion posterior to this, which, following the usual nomenclature, 

 has been termed the proximal intestine, frequently contains a darker 

 and more viscid material composed of partially digested blood. 



In Hippobosca and Melophagus the alimentary tract presents a strik- 

 ing peculiarity in the absence of a crop, a condition of affairs which is 



correlated to their method of feeding. Hippobosca 

 Hippobosca . . 



does not till itself with blood at one puncture, but nits 



about from place to place on the skin of the host, remaining with it for 

 long periods. Indeed, it is rare to find a fly of this genus apart from 

 its host, which it does not leave even for copulation. Melophagus, 

 which is wingless, cannot leave the host at all during the active 

 period of its life, but is associated with it after the manner of the fleas. 

 In neither case is there any necessity for the fly to take a large meal, and 

 consequently there is no necessity for a crop. 



The proventriculus in Hippobosca is represented by a small oval 

 swelling on the anterior portion of the gut. The mid-gut is very long 

 and is twisted into many coils, and has the same general character as that 

 of Musca. The anterior portion is much thicker than the rest, though 

 the gut as a whole is wider than that of any of the other flies referred 

 to. The rectum is large and capacious, and generally contains a milky 

 fluid. Its wall is thick and leathery, and of a dark brown colour, quite 

 unlike that of Musca. 



There are four Malpighian tubes, which arise independently from the 

 gut. They are simple and of a uniform diameter, and have not the bead- 

 like appearance of those in the Muscidae, and are distinctly narrower. 

 The hind-gut posterior to their origin is slightly swollen ; it contracts 

 again at the point where it joins the rectum. 



In the foregoing the term ' hind-gut ' has been used in the sense in 

 which it is ordinarily understood, as that part of the alimentary tract 

 posterior to the openings of the Malpighian tubes. The actual extent of 

 the posterior invagination, which is lined by a chitinous intima, is not as 



