232 FLEAS, FLUKES AND CUCKOOS 



can prove fatal to birds. Blindness sometimes follows attachment in the 

 region of the eye, and the flesh of the bird becomes "pulpy" and semi- 

 liquid in the immediate vicinity of the tick. Many fatal diseases of 

 mammals are carried by ticks and both the pigeon tick and fowl tick 

 {A. persicus) are vectors of fowl relapsing fever. The causative agent is 

 a spirochaete {Borrelia anserinum), which undergoes development in the 

 body cells of the tick. Many birds are susceptible to this fatal disease 

 which is passed on in the eggs of the tick. The larvae are thus already 

 infected at birth. 



Some birds, besides the "delousers" mentioned in Chapter 3 

 destroy large numbers of ticks. Jackdaws have been seen feeding 

 avidly on the guillemot tick and in many places chickens are run 

 in with cattle and sheep to keep the pastures free of them. They are 

 also eaten by ants and are parasitised by small Hymenoptera which lay 

 their eggs in the bodies of young nymphs which are then consumed 

 alive by the developing insects. 



Tongue-Worms (Pentastomida) 



The tongue-worms in the adult stage inhabit the nasal fossae and 

 respiratory tract of vertebrates — chiefly reptiles. They feed on blood 

 and slimy exudates. The only species known from birds is found in the 

 air sacs (see Fig. 4 (i) p. 196). The larval form, which is in- 

 gested in the o^gg stage, invades the viscera of a much wider range of 

 vertebrates which act as intermediate host, but there are only a few 

 stray records from birds. 



These curious parasites appear like worms — cylindrical, blind and 

 pale. The body, which is generally a few centimetres in length, is often 

 divided into rings but these do not correspond to any internal segmenta- 

 tion. Two pairs of retractile hooks which superficially resemble cats' 

 claws, and are thought by some to be vestigial appendages, are placed 

 on either side of the mouth. Respiration is cutaneous. The sexes are 

 separate. The females are larger than the males and may have over a 

 million eggs developing simultaneously in their overies. 



As we have said, the cheif hosts of tongue- worms are reptiles, which 

 include some of the largest forms, such as the boa-constrictors, pythons 

 and crocodiles. Related species from certain genera parasitising 

 crocodiles and various large snakes, are found in widely separated 



