84 FLEAS, FLUKES AND CUCKOOS 



Therefore, all the suggestions which follow should be regarded as 

 tentative. Despite the relatively large numbers of specimens taken from 

 Ashton Wold and Tring Park, these two places are not particularly good 

 spots for fleas. In this case, as in so many others, the distribution 

 shown is that of the collectors rather than that of the fleas (see Map 3). 

 Bat fleas apparently show a strong predilection for cathedral cities but 

 this does not reflect their sectarian views; it merely demonstrates the 

 fact that their hosts are more numerous and thus easier to catch in 

 large belfries than in smafl ones. 



In studying the distribution of the fleas we are confronted with a 

 much more complicated problem than in the case of the feather lice. 

 Mallophaga pass their entire life-cycle on the bird and are so closely 

 linked to it that their own distribution closely parallels that of the host. 

 Whatever limits the range of the bird, whether it is chmate (past or 

 present), or food, or scarcity of nesting sites, or competition with other 

 birds, or geological history, these same factors limit the range of the 

 feather lice. They do not however exert a direct influence upon them. 

 The bird, not the external habitat, is their environment. In the 

 case of the adult fleas they are also closely hnked to the host, which 

 therefore must play an important role in determining their distribution. 

 However, the larval stages are free and are therefore influenced by all 

 the elements which aflect a free living organism. There are many 

 striking examples which illustrate this fact. The range of the hen 

 stick-tight flea, despite the wide distribution of the host, is restricted to 

 areas with a tropical or sub-tropical climate. The common rat flea 

 is rare or absent in the tropics and more or less confined to Europe. 

 Unlike its host it does not thrive in hot climates and has fafled to 

 spread aU over the inhabited portions of the globe with man and his 

 four-footed hangers-on. The rock-dove flea which is also a parasite of 

 the domestic pigeon, has not spread beyond Europe. The dovecots of 

 the United States harbour only hen fleas. In these cases the fleas fafl 

 short of the distribution of their host because the requirements of the 

 larvae limit them to certain ranges of temperature and humidity. 



It has already been pointed out that many fleas are not host specific 

 and are found on a wide variety of birds. Nevertheless there is often a 

 preference for certain birds with similar habits, and thus the fleas in 

 question are distributed according to the conditions, or microclimes, 

 found in diff"erent types of nest. Let us take for example three of the 

 commonest bird fleas in Britain, the hen flea (C. gallinae) the moorhen 



