FLEAS 97 



Despite the fact that this trend is not present in the family Cerato- 

 phyllidae as a whole, the kittiwake flea (Mioctenopsylla arctica) also 

 shows a weakening and loss of tarsal bristles — at least in the hind 

 tarsus. In this case the mysterious influence which produces these 

 parallel changes in bird fleas seems to affect certain species of the whole 

 order irrespective of family trends. 



It has already been seen (p. 42) that among parasites there is 

 often a tendency towards loss of eyes, and this is particularly marked in 

 animals which live in the host's nest. In the case of fleas, however, the 

 matter is by no means so simple. It is true that many nest dwelhng 

 species are blind but the bird fleas, which are nest dwellers par 

 excellence, all have well developed eyes. There appears to be some 

 connection between the life of the host and the degree of development 

 of the flea's eyes. Thus broadly speaking nocturnal animals — for 

 example bats — are parasitised by bhnd fleas and diurnal animals by 

 fleas with eyes. It is remarkable nevertheless that all bird fleas, even 

 the sand-martin and shearwater fleas which live in nests in burrows, 

 should have retained their eyesight. The question is a comphcated one 

 and further research into the problem would be of great interest. 



The changes referred to above are morphological ones and therefore 

 relatively easy to detect. There must be many other changes hnked with 

 a parasitic life on birds, physiological ones for example, about which 

 httle or nothing is known. One such adaptation has already been 

 mentioned, namely the development of a well defined breeding season 

 in C. gallinae to coincide with that of the host. It is most probable 

 that this feature is common to all the bird fleas of the temperate 

 zones. 



From time to time isolated observations are made on bird fleas 

 which may bear some relation to their particular mode of life. As we 

 have noted, one of the most vital problems which annually confronts 

 the majority of bird fleas is the question of survival when the nest is 

 deserted at the end of the breeding season. In this situation widespread 

 scattering is an advantage. It is, therefore, not surprising that C. gallinae 

 and C. garei are so frequently collected far removed from either host or 

 nest (p. 81). This ability to hibernate or rest, fasting, under the bark 

 of trees, among leaves, in cracks on fences or in piles of rubbish, there to 

 wait for a passing host, seems especially well developed in bird fleas. 

 As we have already suggested, the need for wide dissemination may 

 also account for the mass migration of certain species. One of the 



