262 GLOSSINA PALPALIS [CH. 



on their persons or on the water pail. On the other hand 

 Bagshawe has shewn that the fly may travel at least a mile 

 along the banks of water, and the observations on seasonal 

 distribution suggest that G. palpalis is capable of travelling for 

 very considerable distances. 



During the rainy season the flies ascend to the upper limits 

 of the rivers and thus may appear in valleys that are dry for 

 the greater part of the year. In this manner the range of 

 G. palpalis may be considerably extended and there is a danger 

 that during an exceptionally rainy season the fly may spread 

 from the Congo to the Zambesi, as the upper tributaries of 

 these two rivers are not widely separated. 



As the rivers dry up after the cessation of the rains, the flies 

 follow the receding waters and thus forsake those valleys that 

 have been temporarily filled with streams. The flies, however, 

 may persist in the neighbourhood of any more or less permanent 

 water, such as small pools left in the river-bed. It is possible 

 therefore to distinguish such temporary haunts, only occupied 

 by the fly during the rainy season, from the permanent haunts 

 in which the fly is present the whole year round. Needless to 

 add, the latter constitute the sources from which the flies 

 migrate to the temporary haunts and are worthy of more 

 careful attention. 



In addition the flies are subject to seasonal variations even 

 in their permanent haunts, the numbers greatly diminishing 

 during the dry summer and increasing during the rainy season. 

 The reason for this decrease in numbers during the dry season 

 has not been satisfactorily explained, but it may depend to 

 some extent on the breeding habits of the insect {vide infra). 



The habits of G. palpalis are fairly well known and its 

 presence can readily be detected when the flies are abundant. 

 The peculiar buzzing noise made by the insects whilst flying 

 and also the quick darting manner in which they flit from place 

 to place when not actually feeding, enable experienced ob- 

 servers to identify the flies without actually catching specimens. 

 The best way of detecting the presence of flies is to coast along 

 slowly in a canoe, when if the weather is favourable, the insects, 

 if present, soon appear in the boat. The attraction of moving 



