ANIMAL PARASITES AND PARASITIC DISEASES 331 



There they attach to the lining membrane by two small hooks 

 and feed on the slimy secretion of the nasal mucous membrane. 

 When ready to pass out for the next stage in their life cycle, they 

 release their hold, crawl out of the air chambers, and drop to the 

 ground. Here they bury themselves and remain in a dormant 

 state for forty to fifty days or longer, depending on temperature 

 and moisture conditions. Country slaughter houses are un- 

 doubtedly the source from which these flies are often spread. 

 If all sheep's heads were properly destroyed, many cases of " grub 

 in the head" would be prevented. 



The sheep like the other animals has an instinctive fear of 

 bot-flies and will hold its nose close to the ground and seek the 

 center of the flock whenever any flies are around. One or two 

 flies are said to alarm a whole flock, sometimes causing a stampede. 

 No effective method of prevention has been found. The grubs 

 may be removed from the heads of sheep by a surgical operation 

 known as trephining. 



The sheep tick (Melophagus ovinus) is a common parasite, 

 but should be called the sheep louse, as it belongs to the insects 

 instead of to the arachnids. It differs from most insects in 

 that it does not possess wings. The adult is reddish or brownish 

 in color, and about 34 inch long. It has three pairs of jointed 

 legs instead of the four pairs characteristic of true ticks. This 

 is a permanent parasite, its whole life being spent on the body 

 of one host. It lays 8 to 10 pupse in the fleece. In about four 

 weeks these hatch and release a small tick which soon becomes 

 capable of reproduction. The pupae can be readily found by 

 anyone who will separate the wool along the neck, shoulders, 

 and belly. All sheep are attacked, but the long wooled breeds 

 more than the fine wooled. At shearing season the parasite is 

 most in evidence, lying close to the skin. When brushed or 

 picked off, a red irritated spot is seen where the tick has bitten 

 the sheep and injected a little poisonous substance. Consider- 

 able itching is produced and the sheep bite, rub, and scratch. 

 The ticks live on the blood of the host and the yolk of the wool. 

 At the shearing season they leave the sheep in large numbers and 

 go to the lambs on which they find better protection. Nearly 

 all sheep harbor some ticks. When they are present in large 

 numbers they cause unthriftiness and poor condition. Treat- 



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