PROPORTION OF SEXES 



81 



derived from a large number of bites gives a very erroneous 

 estimate. This fact has arisen from Fiske's investigations 

 into the reasons for the very varying proportions of the 

 sexes at different localities. 



It has been found that the females are much less inquisi- 

 tive than the males, and do not come to man in any great 

 numbers unless wishing to feed, whereas the male roams 

 about and investigates any new object without necessarily 

 being hungry. The percentage of females is therefore 

 an indication of the hunger of the flies. If, however, 

 a prolonged catch is made in one spot, the percentage of 

 females rises, because the more inquisitive males are 

 first caught ; the number of females per boy per hour, 

 however, remains fairly constant, since only the hungry 

 individuals are taken. 



An experiment such as the following illustrates this 

 point clearly. Flies were caught continuously at one 

 point on the small island of Tavu on January 13-16, 1914, 

 from early morning until the afternoon, and it is clear 

 that the number of males was soon affected, while the 

 catch of females remained constant. The percentage of 

 females, however, rose, owing to the males being caught 

 ofi ; not in this case because there were more females 

 eager to feed. 



The relation between the percentage of female flies in a 



