100 WILLIAM F. FISKE — THE BIONOMICS OF GLOSSINA ; 



It is therefore definitely known that in this locality there were marked flies 

 in precisely the above proportions ; the proportions of the sexes among marked 

 flies recaptured ought, then, to furnish an answer to the question. They were as 

 follows : — 



The number of marked flies recaptured between 2nd August and 7th February, 

 inclusive, totalled 998. 



487 c?, 511 Q = c?48'8#, Q 51*22 % . 

 These figures are strikingly like those of the equation first given, but it must 

 not be forgotton that a correction is necessary, because actually a higher per- 

 centage of the marked males was recaptured than of the marked females. Thus : — 

 487^ = 16'5% of those liberated after marking. 

 511 Q =15'7% „ „ 



16.5 

 1fi , ' . ? = 51*1, which figure represents the actual percentage of males 



caught on a basis of equal numbers of both sexes liberated. 



The difference between this figure and the 50 per cent, which ought to have 

 been caught, if it were to be the true index of the proportion of males known to 

 exist, is too small to be of conclusive value. 



There is, however, to be taken into consideration a possible greater longevity 

 of the females, which would act to increase the number of that sex recaptured 

 over so long an interval. To ascertain the importance of this factor, the totals 

 of flies recaptured during the two weeks immediately following the cessation of 

 the marking of each individual lot were determined, with the following results. 

 The total flies recaptured during these two weeks was 599 (considerably more 

 than half the total number recaptured) made up of : — 

 C?298, Q301 ; ^49.7, Q 50*3 

 Corrected, the figures are — 



(5298 = 10*1 of those liberated after marking. 

 Q301 = 9-2 „ „ 



* q.o == e( l lla l s 52.3, representing the actual percentage of males caught 



on a basis of equal numbers of both sexes liberated. 



Again the difference (1*1 #) between this figure and that representing the 

 actual percentage of males in the total recaptured during the entire period, is too 

 small to be indicative of any considerable error arising in this instance through 

 the greater longevity of the females. 



There is still another possible source of error, in the chance that more pro- 

 nounced migratory habits of the females would lead a larger relative proportion 

 to leave the locality. This, if it were at all considerable in this instance, would 

 make it appear that, sexes being evenly divided, more females than males would 

 be caught. There seems no way to demonstrate whether this did or did not take 

 place. If it did take place, it was almost immediately after the flies were marked, 

 otherwise there would probably have been a greater discrepancy between the 

 percentages for the two week period as compared with those for the total period ; 

 unless it were that migration of the females was almost precisely coincident with 

 a greater rate of mortality amongst the males. For the present, however, we 



