NOTES ON THE BIONOMICS OF CLOSSINA MORSITANS. 121 



and there are certainly many reasons why such a habit is, to say the least of it, 

 unlikely. The question is intimately connected with that of fly migrating with 

 game ; for if the fly followed herds of game within the infested area, there appears 

 to be no obstacle to its following the herds beyond it, provided that the country 

 entered is potential fly country. This, as an attempt has been made to show, is 

 at least not its general habit. The further difficulties in the way of accepting 

 this idea, mostly already mentioned, may be briefly summed up as follows : — 



(1) female flies are apparently not known to follow to any great distance at any time ; 



(2) gravid females seek seclusion, and are not in the least likely to attempt to follow 

 a moving herd ; (3) hungry flies tend to feed fully, and full-fed flies abandon their 

 hosts, which are unlikely to remain in the vicinity until the flies recover ; (4) the 

 fly is diurnal in habit, whilst game moves largely at night. It would seem therefore 

 that the only individuals capable of following game even for a few hours are those 

 which do not desire to feed. The majority of observations indicate that only the 

 males exhibit a tendency to follow, although the possibility of non-gravid females 

 doing so is not altogether excluded. In any case it is well known that even the 

 males do not follow human beings for more than an hour or so, but gradually fall 

 away, and there seems little reason to think that they would exhibit a different 

 habit in respect to game. Even supposing they followed till nightfall, they must 

 surely lose touch with the herd after dark. 



It appears probable that the tsetse-fly neither ranges the forest in search of its 

 prey, nor follows it when encountered for any great length of time, but that it waits 

 for the animal to come within the range of its perceptive powers. The fact that 

 fly is constantly met with at the same spots* strongly suggests ambush rather than 

 pursuit, and the writer has certainly no experience of having encountered fly in 

 circumstances that suggested anything else. 



The Maximum Distance at which the Fly can detect its Hosts. 



Direct experiments are needed and planned to determine this point as far as possible. 

 It appears necessary to distinguish between the maximum distance at which the fly 

 readily perceives its prey, so that a man or animal would not pass by without being 

 observed, and the distance at which it might locate say a herd of game grazing for 

 several hours in one spot. 



Some of the early explorers and hunters who encountered tsetse-fly were most 

 emphatic that a very short distance might separate complete safety from certain 

 death for their cattle, a narrow stream being sometimes mentioned as the dividing 

 interval. Like other statements emanating from untrained observers this assertion, 

 though undoubtedly greatly exaggerated, appears to have some foundation in fact. 

 It would be possible to mention numerous personal observations of having been very 



*The writer is aware that observations have been published of fly having been 

 encountered in quantity at certain spots on one occasion and only in very small numbers 

 at the next visit. During the wet season this is quite likely, as the fly is not confined 

 by lack of shade, and when carried even for a short distance is hardly likely to return to 

 exactly the same spot. Even in the dry season the passage of a herd of game miglit 

 make a very material difference for a few hours, the huno;ry flies feeding and seeking 

 seclusion, and the non -hungry males following the herd. Nevertheless, allowing for the 

 difference in distribution in the wet and dry seasons, the statement is substantially 

 correct. 



