114 FLEAS, FLUKES AND CUCKOOS 



C. Styx is a large species about 4 mm. in length and is the hairiest of 

 the British bird fleas. It appears to be a very faithful companion of the 

 sand-martin, and we know of no colony where its absence has been 

 established with certainty. It is one of the most suitable fleas upon 

 which to make studies of population density, sex ratio, hibernation, 

 migration, breeding cycle, but so far the opportunity has been neglected. 

 On the continent this species provides one of the extremely rare cases 

 among fleas of polymorphism. That is to say there are two distinct 

 morphological forms of the female, each of which shows a characteristic 

 seventh sternite. This confused Rothschild who thought the second 

 type of female was a distinct species and gave it another name. The 

 error was subsequently spotted and the true nature of the "new" 

 species revealed. Perhaps this second type of female will be turned up 

 in Britain if it is searched for. Sometimes, however, as in the case of the 

 well-known polymorphic female butterfly, var. valesina of the silver- 

 washed fritillary {Argynnis paphia), one form is restricted to certain 

 localities. 



C. Styx swarms in the burrows of the sand-martin and no other flea 

 seems able to compete with it. Only once has another species been 

 found sharing a nest in Britain, and that needless to say, was the ubiqui- 

 tous C. gallinae. On the other hand C. styx seems fairly closely adapted 

 to life in sand quarries and is not found as a straggler except on the 

 dipper {Cinclus cinclus). It has been taken four times from this bird, in 

 considerable numbers, and from widely separated areas. On the whole, 

 however, it can be considered one of our strictly host specific bird fleas. 



It has already been noted (p. 80) that C. styx over-winters in 

 the nest. It can hibernate either as a pupa or adult. Large numbers 

 have been observed in burrows immediately before the return of the 

 hosts. 



It has been claimed that certain mammal fleas develop finer and 

 longer bristles on their legs if they parasitise rodents living in holes in 

 sandy soil. C. styx certainly has finer and more numerous bristles than 

 other British bird fleas, and this may be a direct result of the type of 

 soil in which the birds make their nests. The same can be said of the 

 closely related species off' the sand-martin in North America. A closer 

 study of fleas will certainly reveal the effect of other external influences, 

 besides the rather obvious ones of the temperature and humidity in the 

 nest. 



