94 FLEAS, FLUKES AND CUCKOOS 



not known. Such species as the sparrow flea and the hen flea have been 

 taken off cats and many other predatory animals such as rats, 

 stoats, weasels, and foxes. Conversely certain mammal fleas are not 

 infrequently found as stragglers on birds of prey, especially owls (Plate 

 VI), or on small birds which to a certain extent share their habitat. 

 Thus the rabbit flea straggles on to burrow- and ground-nesting birds, 

 and has been taken off ducks, puffins, shearwaters and partridges. The 

 American and British squirrel fleas are found in a variety of arboreal 

 nests, and occasionally on the bodies of birds themselves. Bat fleas have 

 been found on the swift, the human flea on wild duck, and the hen flea 

 and house-sparrow flea on man himself. 



It is not difficult to imagine new species arising in this way, providing 

 some accident supplies the necessary isolation. On Skomer Isle one can 

 conceive our rabbit flea establishing itself permanently on the shear- 

 waters, and thus giving rise to a situation which may puzzle and confuse 

 future systematists. 



Even among such closely related species as our eleven Ceratophyllus 

 we can trace certain evolutionary trends and try to construct a phylo- 

 genetic "tree" by studying details of morphology. For example by 

 grouping them according to the degree of thickening and hardening 

 (sclerotisation) of certain internal organs of the female, and the shape 

 of the receptaculum seminis (Plates XIII and XIV) we find they fall 

 into two main groups. The first contains the three British species 

 C. garei, C. borealis and C. columbae from the British fauna and one 

 species from Turkestan and another from North America. This group 

 shows the least sclerotisation and has a kidney shaped body to the 

 receptaculum. It is descended from the Monopsyllus sciurorum group of 

 squirrel fleas (Plate XIIIc). The second group, which falls into two 

 distinct sub-groups, shows sclerotisation and a progressively more 

 vermiform body to the receptaculum. These fleas represent a second 

 evolution from the same genus of rodent fleas, this time from the Mono- 

 psyllus anisus group (Plate XI Vg), which are found on rats and squirrels 

 in the Pacific area of the Palaearctic region. Our bird fleas may have 

 originated from that area via North America. The first of these sub- 

 groups contains C. hirundinis and C. rusticus from our fauna. The 

 second sub-group, with the most pronounced sclerotisation, includes 

 all our other Ceratophyllid bird fleas and many foreign species, 

 and incidentally all those which have become adapted to relatively 

 dry nests. 



