FLEAS 79 



Waterston remarked, " It is more than likely in a life so precarious as a 

 flea's speculative jumping plays a very large part." 



In many insects one sex, generally the male, hatches out before the 

 other, but in the case of fleas it is the female which emerges first. The 

 period it spends in the cocoon is shorter than that of the male. As it 

 also outlives the male by many months and can withstand spells of 

 adverse conditions to which the male promptly succumbs, there are times 

 when a breeding population of fleas consists almost entirely of females. 

 Most large collections in the past were made off' mammals theniselves 

 and the large excess of female fleas sometimes observed was attributed 

 not only to the greater agihty of males which more often escaped 

 capture, but to a deep seated divergence in the habits of the sexes— 

 the females supposed to cling to the body of the host and the males to 

 remain in the nest. Indeed there is some experimental evidence to 

 suggest that the females are more closely bound to the host's body. 

 Quite recently it has been shown that the proportion of male and 

 female fleas on rats in certain cities in the U.S.A. varies according to 

 the weather. On hot days males predominate and on cold days females. 

 In the case of bird fleas, however, some excess of females is also found 

 when specimens are taken direct from the nests. Waterston recorded 

 2,368 (or 56 per cent.) females to 1,672 (or 44 per cent.) males of 

 C.farreni from house-martins' nests and Rothschild found out of a total 

 of 1,218 fleas (five species were represented) from house-martins' and 

 swallows' nests that 732 (60 per cent.) were females. In many parasitic 

 insects there is a very marked tendency for the sex ratio to tip more and 

 more sharply in favour of the females. The reason for this is obscure. 

 Thus the males of certain Hce have so far never been found. Although 

 female fleas lay many unfertilised eggs, these do not develop. How- 

 ever, as one male flea has been known to fertihse thirteen females 

 their activity makes up to a certain extent for their numerical 

 inferiority. 



Because relatively few fleas are found on the bodies of the birds 

 themselves it is assumed that they spend the greater part of their time 

 in the nest or hiding in debris and only visit the host periodically when 

 they require a meal. There can be little doubt that the greatest danger 

 to a flea is the active and efficient defence put up by the host. It is no 

 mere coincidence that the hen stick-tight flea is only found round the 

 eyes and wattle of poultry— where the birds cannot preen diemselves 

 effectively. 



