FLEAS 8l 



lairs is considered the rule. Although these mass movements have only 

 been noted rarely in Britain it seems likely that this type of migration 

 plays an important part in overcoming the hopeless situation of fleas 

 left in nests to which the hosts do not return. Scattered over a wider 

 area and able to fast for long periods, their chances of coming into 

 contact with a passing bird would be greatly increased. Fleas are 

 frequently noted in isolated situations apparently far removed from 

 either nest or host. For example there are records of two common 

 bird fleas, the hen flea and duck flea, collected from under stones, on 

 palings, under the bark of trees, among dried leaves, in hedge clippings, 

 in a pile of reeds, in moss, on rocks, in caves, in barns and even swept 

 from grass and flowers. 



Up till now no observations have been made regarding bird fleas' 

 preference — if any — for either sex of the host. Collectors rarely take 

 the trouble to record the necessary data. Linnaeus declared that women 

 harboured more fleas than men, but male squirrels on the other hand 

 seem more heavily parasitised than females. Where only the hen bird 

 incubates the eggs, the opportunity of becoming infested with fleas is 

 obviously greater in her case. 



In the butterfly collection varieties take a prominent place. 

 Individual fleas sometimes display variations which are just as remark- 

 able as, say, the black variety of the swallowtail, but so far they have 

 not attracted much attention. There are certain characters of fleas 

 which seem to vary more than others and in these — for example the 

 shape of the seventh sternite of the females — the arrangement of the 

 bristles is also variable. In fleas, as well as human beings, no two 

 individuals are exactly alike. The bristle formula on the abdominal 

 segments of a flea was worked out by Jordan who calculated that he 

 would require 14,482,000,000,000 specimens in order to be sure that he 

 had another one with the same arrangement on the sternites alone ! 

 Varieties, however, are well worth studying, for in them one often 

 catches a glimpse of the future evolutionary tendencies in the species or 

 even the family concerned. 



The Distribution of Bird Fleas 



Any attempt at discussing the distribution of bird fleas must to a 

 certain extent prove futile, because of the general lack of coUecting. 



