( cxlvii ) 



ctenidael A comparison with other groups of ectoparasites 

 will enable us to answer the question. Among the Siphon- 

 aptera the very intensely parasitic Echidnophaga and Derma- 

 tophilus penetrans have not lost the eye ; they fasten them- 

 selves on the naked or comparatively bare parts of the skin of 

 the host, in birds, for instance, around the eyes and ears, and 

 on the wattles of fowls, in mammals on the ears and feet. 

 The ? of Dermatophilus penetrans burrows into the bare skin, 



Fig. 8. Head of Xenopsylla eridos Roths. (1904). 

 9. ff ,, ,, niloticus Roths. (1908). 



" 10. " ., „ niloticus Roths. (1908). 



11 " , ,, brasiliensis Baker (1905) 



\\ 12." ,',' „ „ tortus Roths. (1908). 



" 13. „ „ Booseveldtiella georychi Fox (1913). 



but even that habit is not accompanied by the loss of the 

 eyes. The nearest allied species, D. caecata, however, has a 

 reduced eye : and this species swells up in such a curious 

 way in the $, that the abdomen almost completely envelopes 

 the head and thorax. In both species the $ <J do not burrow 

 into the skin. Again, among the Anoplura, the human lice 

 are at least as intensely parasitic as the Polyctenids, but they 

 have nevertheless preserved the eyes, while other species of 

 lice, living in the fur of burrowing mammals, have the eyes 

 much reduced or absent. All this appears to show that the 



