THE MORE COMMON AILMENTS OF SHEEP 387 



digestion than to deal with indigestion should it arise, 

 and it is vastly easier to keep the farmer's flock away 

 from infestation of scab mites than to deal with scab 

 should it come. 



Parasites that prey externally on sheep The prin- 

 cipal of these are three in number, viz., the sheep tick, 

 the scab mite and the sheep louse. Of these the scab 

 mite is altogether the most harmful. The sheep tick is 

 more or less in evidence wherever sheep are kept on this 

 continent, but not necessarily so. The sheep louse is not 

 greatly prevalent in America. 



The sheep tick (Melophagus avinnus) was introduced 

 from the Old World. It is a wingless fly. When full 

 grown it is less than one-fourth of an inch long. The 

 body is short, flattened and varying in color from white 

 to reddish. The color is influenced somewhat by the 

 amount of blood which it has taken from the sheep. The 

 ticks bear considerable resemblance to the spider in form, 

 but the limbs are very much shorter. The sheep tick 

 propagates by means of the eggs laid by the females. 

 These are relatively large, flat and ovoid in shape and 

 brown in color. Within each is an imperfectly developed 

 larva. The eggs, usually spoken of in common phrase 

 as "nits," are made to adhere to the wool by a sticky sub- 

 stance which covers them when they are first laid. But 

 one is deposited at a time, and it is thought the female 

 seldom lays more than 7 or 8 eggs in her lifetime, and the 

 number is probably less than the figures named, a fact 

 which has an important bearing on the possible eradica- 

 tion of the pest. They hatch in some instances in about 

 four weeks from the time when they are laid. 



They occur on sheep at all seasons, but are usually 

 most numerous and troublesome toward the approach of 

 spring. They cannot live long away from the body of the 

 sheep, the warmth of which seems to be essential to their 

 existence in conjunction with the food which they obtain 

 from it and the protection furnished by the wool. Their 



