FLEAS 85 



flea {D. gallinulae) and the duck flea (C. garei) (Maps 2, 3, 4). The hen 

 flea is apparently much more tolerant of the dry conditions found in 

 hen-houses and also in dry, loosely built nests placed in elevated situa- 

 tions. It is found more frequently than any other flea in the nests of the 

 sparrow, starling, sparrow-hawk, swallow and so forth. D, gallinulae^ 

 however, prefers nests situated in relatively low positions such as those 

 of the robin and warblers. It also seems partial to the closely built nests 

 of finches, and in those of blackbirds and song-thrushes the relative 

 frequency of these two fleas is about the same. C. garei on the other hand 

 is essentially a ground flea, and can survive in wet swampy situations 

 which prove fatal to the other two mentioned above. Thus it is the only 

 one of these three fleas met with in the nests of ducks and geese, and 

 certain waders and sea birds. No doubt this type of distribution reflects 

 the larval adaptations of the fleas in question, and results in different 

 horizontal zones of distribution within the same locality. Birds them- 

 selves show marked habitat preferences and we do not find rooks 

 nesting on the ground or partridges in the tree tops. 



There are certain cases where the distribution of the flea probably 

 closely parallels that of the host, but collecting has been so inadequate 

 that it is impossible to make any definite statement to that effect. The 

 common house-martin flea (C. hirundinis) is found in Europe and the 

 Himalayas and North Africa, and it seems likely that it accompanies 

 the bird throughout its range. 



The house-martin is divided up into several geographical races or 

 subspecies. That is to say in certain areas where it is found the birds 

 show marked variations common to the population of house-martins in 

 that particular district. Thus an expert would be able to tell whether 

 certain house-martins had bred in Algeria or the Himalayas or North 

 Europe, by noting small differences in the colour, size, weight and so 

 forth. 



There are two house-martin fleas, C. hirundinis and C. farreni, which 

 are known to extend their range beyond Europe. C. hirundinis, as we 

 have already explained, is found on all three subspecies of the martin, 

 but itself remains unchanged. At any rate there are no visible mor- 

 phological changes connected with its geographical distribution. We 

 have no method of estimating physiological differences which may be 

 present. C. farreni, however, has split into two subspecies — one in 

 Europe and one in North Africa. Only females are known from the 

 latter region but these show constant differences in the arrangement of 



