THE SHEEP-TICK OR LOUSE-FLY--PHTHIRIASIS. 



Melopuagus oyinus, Linn. 



Plate IV. 



One of the best known of all the external parasites of sheep is the 

 sheep-tick, Melopliagus ovinus, Linn. This pest is a very common one in 

 the Eastern States, and although it seldom causes any serious damage 

 either to the sheep or to the wool, it is at all times an annoyance, and 

 occasionally causes decided losses to the flock-master. 



This tick, like a majority of the parasites of the domesticated animals, 

 was introduced into this country from Europe. The name '^sheep-tick," 

 though not a misnomer, as every one knows what a sheep-tick is, is 

 misleading. So much do they resemble the true ticks that they are 

 often classified together. The most superficial study will, however, 

 serve to show their differences. 



Description. — The sheep-tick is a wingless fly having but six legs, 

 whereas the true ticks are more closely related to spiders, and have 

 eight legs in their adult state. The adults are less than a quarter of 

 an inch long, and have a short, flattened, bristly, leathery body. The 

 head is slightly wider than the thorax, into which it is sunk. They 

 have very short antennae, which are sunk in sockets in the face; the 

 proboscis is tubular, and is protected externally by two flat, elongated 

 bristly pieces, the labrura ; its end is armed with teeth. The thorax 

 or limb-bearing portion of their bodies is nearly square when looked at 

 from above. It is composed of three pieces, the middle being the one 

 seen on the back. The legs are very stout, covered with bristles, and 

 each is provided with two strong, sharp claws. The last joint of each 

 foot bears a pinuiform or feather-like organ whose ofSce is as yet un- 

 determined, but is probably that of coiling around hair for better pre- 

 hension. There are no wings. On either side may be seen two small, 

 bristle-covered, round spots at points where the wings should be 

 attached. At the posterior outer corners of the thorax are two little 

 projections which remind one of balancers. The abdomen, usually 

 larger in females than in males, is flattened and bag-like, and is as 

 large or larger than the rest of the body, especially after the louse- 

 fly has eaten, when the red blood sucked from itfe host may be seen 

 through its skin. Its skin is tough, unsegmeuted, semi-translucent, 

 and permits the abdominal organs to show through. On each side 

 there are seven stigmata or breathing pores. The anus is situated on 



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