72 



(G. palpalis) are restricted to a few shady patches of bush along the 

 river banks, probably owing .... to their intolerance of the; 

 dryness of the atmosphere and the burnt-up condition of the country. 

 Of the flies brought to me, the proportion of males to females was as 

 three to one, but whether this was due to the females being actually 

 less numerous than the males, or to the fact that they were shyer 

 feeders or more cunning at avoiding capture, it is impossible to say, 

 The fact that in some of the temperature experiments the females 

 seemed to be more sensitive than the males to a raising of the tempera- 

 ture may have had something to do with it, as they may have been 

 less eager to fly out into the sunshine to feed, and may thus have 

 escaped capture. The converse, namely, that the female is apparently 

 less affected by cold, may account for the fact that on one occasion 

 only, on a singularly cool and sunless day, the proportion was reversed, 

 namely, three females being taken for every male. The flies were 

 also found to be susceptible to wind, and on the days when the 

 harmattan wind was blowing strongly few or no flies were caught." 



Fiske (54), who quotes Carpenter to a like effect, has shown as a 

 result of his work on islands in Lake Victoria, Uganda, that " When 

 any considerable number of flies of this species of Glossina are caught, 

 it is unusual to find the sexes evenly represented. Although they are 

 produced in equal or approximately equal numbers, they are caught 

 in unequal numbers." 



" This disparity between the sexes of Glossina palpalis," continues 

 Fiske, " is an extremely variable quantity, ranging in different localities 

 (in catches of 100 flies or more) from 1 -9 per cent, to 85-0 per cent, of 

 females, or from 15-0 per cent, to 98 1 per cent, of males. Females never 

 predominate to quite the extent of males, and most frequently the 

 males are in excess." 



Such variations in the sex ratio of G. palpalis are, according to Fiske 

 due to " purely local differences in conditions of life," and more espec- 

 ially to food supply and its nature. Man is not naturally a favoured 

 host of G. palpalis, consequently the destruction or expulsion of favoured 

 hosts such as crocodiles and Varanus (monitors) causes the fly, impelled 

 by hunger, to attack man to a greater extent than usual, the increase 

 of attack, and therefore of liability to capture, being most marked in 

 the case of the female. According to Fiske, " the most plausible 

 explanation of this phenomenon " is that " the male flies are normally 

 active and easily caught at all times during good weather, whether 

 they are hungry or not, but that the females are normally inactive 

 and not to be caught except when hungry and seeking food." This 

 hypothesis, supported by an observation made in the course of a 

 previous experiment, was directly tested " by banishing the host 

 animals from a small islet without catching off any of the flies, and 

 observing the effect on the sex ratio. 



: ' This experiment," proceeds Fiske, " was made on the small island 

 of Lugazi, in the following manner and with the following results. 



" 19th and 20th December 1913. A total of 197 flies were caught 

 on the islet, the sex was determined and they were then liberated (in 

 order that catching off of males should not affect the sex ratio) . The 

 ratio was, males : females : : 166 : 31 = 15' 5 per cent, females. 



" 22nd to 27th December 1913. All host animals known to be fed 

 upon by Glossina, consisting of several Varanus and crocodiles, were 

 systematically hunted from the islet. 



" 26th and 27th December 1913. A total of 208 flies were caught, 

 showing sex ratio, males : females : : 89 : 119 = 57-2 per cent, females. ' 



