118 RUPERT W. JACK. 



grass apparently occurs over wide areas. Conditions difier in various parts of the 

 fly-infested areas of Southern Rhodesia, but in the great majority mopani belts are 

 a feature of the country, and the fly certainly shows a strong preference for this type 

 of forest in the wet season, deserting its dry season haunts where the grass is usually 

 long and thick, sometimes very long. The sweet short grass of the mopani is very 

 attractive to many species of game in the wet season, though the reverse in the dry, 

 and the fly certainly has the best chance of meeting and perceiving game in this t}^e 

 of country during the rains. The mopani grass, however, dries up very quickly after 

 the cessation of rains, being shallow-rooted and of httle substance, so that it loses 

 its special attractiveness very shortly after the close of the wet season. Nevertheless, 

 this t3rpe of forest is much haunted by impala and wart-hog even at that time, and 

 many other species are commonly seen in it, if game is at all plentiful in the neigh- 

 bourhood. Whilst, therefore, it is probable that in Southern Rhodesia the long- 

 grass season is not in general a time of stress for the fly, the fact is in no way opposed 

 to Fiske's observations. In this connection it may be mentioned that the only sex 

 count made by the writer in the long-grass season was in April 1911, with fly taken 

 in mopani, and resulted in 53 males to 3 females, by far the most extreme disparity 

 yet recorded in the territory. The fly's habit of avoiding thick undergro^vth would 

 indeed suggest a difficulty in finding its hosts in such an environment. It is quite 

 possible that in certain parts of the fly areas in Rhodesia a time of stress for the fly 

 may occur in the long-grass season, as the mopani is not ubiquitous, being absent, so 

 far as the writer is aware, from most of the Jetjenini fly area and from certain parts 

 of the Sebungwe area. Prospectors and hunters in this territory have constantly 

 associated mopani and tsetse, and although obviously not essential to the tsetse, it is 

 quite possibly of value to the fly under certain conditions. According to Fiske's 

 observations it ought theoretically to be of value in the long-grass season, and possibly 

 it afiords a refuge from grass fires. 



The Question of Migration. 



The next point is the question of the migration of fly, corresponding to the move- 

 ments of game, or under stress of hunger. Now it is far from the writer's intention 

 to adopt a dogmatic attitude on this subject, but it would seem difficult to reconcile 

 any habit of this nature with a number of known facts. First, in regard to the ques- 

 tion of fly migrating with game, we have the phenomenon of restricted fly-areas and 

 their mode of extension. It has already been pointed out (Bull. Ent. Res. x, p. 88) 

 that the hmits of a fly area are not necessarily permanent. The permanent limits are 

 marked by the impingement of favourable on unfavourable country ; that is to say, 

 the country beyond the limit is for some reason or other unsuited to the tsetse. 

 Transitory limits are, however, formed by the high- water-mark, so to speak, of the 

 advancing flood when the fly is spreading, as has been the case in this territory since 

 the rinderpest. Transitory limits would also be apparent if the pest were receding. 



The advance of the pest is, however, comparatively slow, and in no way com- 

 parable with the powers of movement of the fly itself or of game. Much potential 

 fly area, known to have been infested in pre-rinderpest days, is still free from the 

 pest, although the latter is gradually occupying more and more of its old country. 



