174 DR. SCHWETZ. 



During this journey I recorded for the first time a curious fact. The hearing is, 

 as is well known, sometimes a very good means of detecting certain kinds of insects, 

 and particularly flies, without their being seen. A tsetse, a Hymenopteron, an 

 ordinary fly, etc., can be recognised by the sound of their flight. Moreover, one 

 can distinguish by this means different species of Hymenoptera (wasps, bees), different 

 gadflies (Haematopota. Tabanus), different ordinary flies (house-fly, Sarcophaga), 

 and different mosquitos. With regard to Glossina, it is easy to distinguish the flight 

 of a large tsetse (brevipalpis, fusca) from that of a small one (morsitans, palpalis). 

 Even with certain practice one is able to distinguish the flight of & fusca from that of 

 a brevipalpis. The sound produced by brevipalpis in flying over the ground or over 

 grass by night is so typical that one is scarcely ever deceived. I will not say never, 

 because I have just been deceived. Starting about 12.30 p.m. in search of Glossina 

 in* the district of km. 123, I soon heard, the typical sound of the flight of numerous 

 brevipalpis. But on looking carefully on the ground I found quite different flies. 

 I observed the same phenomenon the next day in the district of km. 105. This 

 other fly flew in exactly the same manner and produced the same sound as brevipalpis. 

 The confusion occurred particularly because at km. 105 this fly was associated with 

 the real brevipalpis. The fly in question is probably a Choeromyia, or at least closely 

 allied to the genus Auchmeromyia. 



Considering this study as a preliminary and incomplete note, I will abstain for the 

 moment from drawing any conclusions, and will limit myself to two considerations : 

 one theoretical and the other practical. 



It is a fact that G. morsitans exists along the railroad between the Lualaba 

 and the Luizi in great numbers, and between Tanganyika and the Niemba in rather 

 less numbers, but not at all (or hardly at all) between the Luizi and the Niemba. 

 As, in my opinion, it is the difference in the species of vegetation that governs the 

 tsetse question, it is in the vegetation of the three sections that I have sought the 

 explanation of the presence or absence of morsitans. I have not mentioned the 

 much-discussed big game, which constitutes, in the opinion of many, the real reason 

 for the presence of morsitans, because I do not believe it ; and I do not believe it 

 because, as I have already stated at length elsewhere, game exists everywhere in 

 Central Africa, equally where morsitans occurs and where there are neither morsitans 

 nor other tsetse. 



However, in science, we have no preconceived ideas. Therefore, auditur et altera 

 pars. And this is the explanation of the distribution of G. morsitans along the railway 

 of Kabalo-Albertville given to me by a colleague who knows the region well. 



" Many morsitans exist between the Lualaba and the Luizi, because game is very 

 abundant there ; there are no morsitans between the Luizi and the Niemba, because 

 in this section game is rare." 



This explanation has the great advantage of being short and clear, but it is too 

 theoretical and categorical and not sufficiently accurate. That game is more abundant 

 in the first section than in the second, I quite admit, but game exists also throughout 

 the second section. Moreover, if game is rarer here, it is because the trees grow here 

 more thickly. 



