48 NATURAL HISTORY OF GLOSSINA PALPALIS 



the curve for male flies shows great variation, the female 

 curve little, but on Kimmi the female curve is the 

 greater variant and the male curve more constant. It 

 may be that the supply of food on Tavu is so good that 

 the females are well fed and do not come in any numbers 

 to be caught by the fly boys, whereas on Kimmi the food 

 supply is more erratic : it must be said, however, that 

 there is no direct evidence for this. 



A very interesting relation was made out on Tavu 

 between the number of flies caught at the weekly visits 

 and the number of crocodiles seen when the island was 

 circumnavigated in the canoe before a landing was made 

 at each visit. There was a large flat topped rock on the 

 east coast forming a little plateau that was always in use 

 by crocodiles as a basking place ; the number on it 

 varied from one to four, and others were seen in the water. 

 On one visit as many as fifteen large crocodiles were 

 seen on or about this small island, and on that day the 

 number of flies was at the rate of ninety-three per boy 

 per hour ! The lowest figure was fifty-six per boy- 

 hour when only two crocodiles were seen. The accom- 

 panying chart shows well the concurrent variation in 

 numbers. It must be confessed that the explanation of 

 this is not at all clear. One would have rather expected 

 that when there was an abundant supply of food on 

 the island the flies would have been less hungry, so that 

 fewer would have been attracted by men, and the catch 

 would have been less instead of greater the more crocodiles 

 there were within reach. On the other hand, had this 

 relation been noticed at one spot on a continuous length 

 of mainland shore it could quite well be explained by 

 the wanderings of flies in search of food which, finding 

 abundance at one spot, remained there. 



But, as has been pointed out, there is no ground for 

 believing that flies cross to Tavu over the open water 

 separating it from the larger Kome and Bulago. 



