186 INSECTS AND MAN 



them are suspected of transmitting a bovine trypano- 

 somiasis, known as galziekte or gall sickness, the patho- 

 genic agent of which is Trypanosoma theileri. Three 

 closely related flies, Lynchia maura, Lynchia capensis, 

 and Ornithomyia avicularia, are parasitic on the pigeon, 

 and, of these, maura, in Algeria, where it is common, trans- 

 mits a pathogenic blood parasite, known as Hcemoproteus 

 columbce. 



Biting lice, Trichodectes ovis, are common parasites of 

 sheep. Like the keds, they are truly parasitic, and spend 

 their whole existence on their host. Their feet are remark- 

 ably well adapted for their mode of living, being armed 

 with powerful claws, which enable their owners to obtain 

 a firm grasp of the wool in which they hide. Their food 

 consists of skin secretions and possibly wool, for though 

 possessed of biting mouths, they lack all provision for 

 sucking blood, thereby differing from the true blood-suck- 

 ing lice. 



The most serious parasitic disease of sheep, known as 

 scab, is caused, not by an insect, but a mite ; it is also the 

 oldest known of all diseases attacking this animal. The 

 disease-causing mite is known as Psoroptes communis, and 

 varieties of the same species cause scab in horses, cattle, 

 goats, and rabbits. So similar are these varieties that it 

 is a matter of exceeding difficulty to identify them ; but, as 

 the scab mite of one host will not cause scab on a dissimilar 

 host, there is no doubt that the varieties are distinct, and 

 they are known as Psoroptes communis, var. ovis, var. equi 

 (fig. 52), var. bov is, var. caprce or var. cuniculi, according to 

 whether they attack sheep, horse, ox, goat, or rabbit. 



Though one of the largest of these mites, the female 

 sheep scab mite is only about one-fortieth of an inch long 

 by one-sixtieth of an inch broad, whilst the male is a little 

 smaller; they are just visible to the naked eye. Apart 

 from a difference in size, the sexes may be distinguished 

 by the fact that in the male only there are a pair of 



