NOTES ON THE BIONOMICS OF GLOSSINA MORSITANS. 119 



Is it possible for these conditions to obtain if the fly had a habit of migrating with 

 herds of game, apart, of com'se, from the males' habit of temporarily accompanying 

 moving animals and human beings ? Herds of certain species of game are more 

 or less constantly on the move in and out of the fly area, and if the fly accompanied 

 them in their wanderings the whole of the potential fly area would certainly be very 

 quickly occupied. 



It may be contended, however, that the fly only follows " trekking " game under 

 stress of hunger. This appears to imply a rather marvellous instinct on the part 

 of the fly. Any fly suffering from hunger that encounters a herd of game is hkely 

 to be a fuU-fed fly in a space of time measured by minutes, and is then not in a 

 condition to follow anything for some time. Must we, therefore, suppose that the 

 fly restrains its appetite in order to accompany the herd into another part of the 

 country ? Again, game travels as a rule by night, and though there are numerous 

 records of fly " biting " on warm nights, it is essentially of diurnal habit. Finally, 

 we have apparently no records to show that the female flies follow animals or human 

 beings for any appreciable distance at all, whether they are hungry or not. The 

 Avriter's observations are all to the contrary, and pubUshed accounts apparently 

 give no record of other than males exhibiting an obvious following habit. Much 

 argument has in fact been based upon the supposition that the female flies only seek 

 animals and human beings for the purpose of feeding, whilst the males follow for 

 other reasons. 



These considerations have led the writer to be extremely sceptical concerning 

 the fly's alleged habit of migrating with game. Regular movements of game in a 

 given direction might, it is admitted, help the spread of the fly in that direction. 

 The allusion is to the daily movement of game from its feeding grounds to water, 

 as, for instance, game which feeds in the neighbourhood of a dry river and moves 

 across to a flowing one to drink (e.g., the Mzola and Kana Rivers in this territory). 

 In the same way, regular traffic along a road would tend to spread the fly, if condi- 

 tions admit of spread, but this is quite apart from the question of fly migrating 

 wdth migrating game. 



The second aspect of the question is that of fly migrating without help from 

 moving game, when pressed by hunger. This would be a serious state of affairs ; 

 as they might be attracted by the settler's herds of cattle, take up their residence 

 in the vicinity of his " kraal " and deposit their larvae under his orange trees. 

 Such a development might well have happened in the writer's experience if hungry 

 fly had a habit of migrating even three or four miles to a food supply. The important 

 point is that it did not happen. The idea of this form of migration has, as a matter 

 of fact, always puzzled the writer. How is it supposed to take place ? Do the 

 tsetse send out scouts hke bees to locate a better piece of country and then rise in a 

 swarm to follow these guides to the new locahty ? Does some instinct inform 

 them that better conditions prevail elsewhere, so that they rise with a common 

 impulse and migrate thither ? Do they, under stress of hunger, migrate en masse 

 at random guided by chance, or perhaps the direction of the wind ? Do they migrate 

 by a series of short journeys, gradually tending day after day in one direction ? 

 Unless the movement takes place at random, this presupposes a knowledge of where 



