280 GLOSSINA MORSITANS [CH. 



and premature at the present time, considering our incomplete 

 knowledge of the subject. No direct evidence has yet been 

 published, supporting the view that morsitans takes in any 

 food other than blood, but the flies have been observed 

 apparently drinking dirty water at the edges of puddles. If 

 placed in a bottle with ripe water-melon the flies may be seen 

 to thrust their proboscides into it, but there is no evidence of 

 any food being ingested. 



The effect of climate on the numbers of flies is very marked. 

 Within a " fly-area " the tsetse will not necessarily be found 

 there at all times of the year. It is often possible to go through 

 such a region without seeing a single fly. For example, 

 in Nyasaland, between the settlement of Zomba and the 

 Mlange Mountains, a distance of about 40 miles, lies an extensive 

 plain. During the months of May or June flies are practically 

 absent, whereas in October, it is necessary to pass through 

 about 25 miles of " fly." On the advent of the first rains in 

 November, they begin to disappear, but may still be found 

 until the arrival of the cold weather in April and May. In the 

 Luangwa Valley, North Eastern Rhodesia, Lloyd observed that 

 the fly was very common during the early part of the dry 

 season, but the numbers diminished until the commencement 

 of the rains, when the numbers at once increased. At Nzoa, 

 in the higher ground of the Congo Zambesi watershed (altitude 

 4000 ft.), the fly only became numerous after the rains had 

 ceased. It is evident, therefore, that within any particular 

 fly-area, the number of tsetse may vary according to the season 

 of the year. 



Although many observers state that G. morsitans is more 

 or less independent of moisture, Dr J. O. Shircore, who has 

 devoted a great deal of attention to this question, has clearly 

 shewn that in Nyasaland, and probably in other regions, the 

 fly is greatly affected in its distribution by the presence or 

 absence of moisture. At the height of the dry season the tsetse 

 is only found in certain restricted areas to which the term 

 " Primary Fly Centres " has been applied. These primary 

 centres are the only regions where, in the dry season, water is 

 actually above the earth's surface, or at no great distance below it. 



