THE SHEEP-TICK.--This is not a true tick. It resembles a fly more than it does a tick, and its right name is _Melophagus ovinus_ (Fig. 66). Louse-fly is a better name for this parasite than tick, as its entire life is spent on the body of a sheep. The general color of the body is brown. The legs are stout, covered with hair and armed with hooks at their extremities. The mouth parts consist of a tubular, toothed proboscis with which the parasite punctures the skin and sucks the blood. Within a few hours after birth, the larvae develop into pupae, which are hard, dark brown in color and firmly glued to the wool. The young louse-fly emerges from the pupa in from three to four weeks. The sheep-tick is a very common external parasite. The adult parasites and the pupae are large and easily found. When badly infested with ticks, a sheep will rub, dig and scratch the skin and fleece. This results in pieces of wool becoming pulled out and the fleece appears ragged. After clipping the ticks migrate from the ewes to the lambs, which may become unthrifty and weak. _The treatment_ consists in dipping the flock in a one or two per cent water solution of a coal-tar dip. Dips containing arsenic are most effective in ridding sheep of ticks. [Illustration: FIG. 67.--Sheep scab mite, ventral view. (From Farmers' Bulletin No. 159, United States Department of Agriculture.)] SCABIES.--This parasitic disease is one of the oldest and most prevalent diseases of the skin. It is commonly known as scab or mange. The animals most commonly affected are sheep, horses and cattle. _The disease is caused_ by _small mites_ or _acari_ that are naturally divided into the _Sarcoptes_, which burrow under the epidermis, forming galleries; the _Psoroptes_, which live on the surface of the skin where they are sheltered by scabs and scurf; and the _Symbiotes_, which also live on the surface of the skin, but prefer the regions of the hind feet and legs. Acari multiply rapidly and live their entire life on the body of the host. A new generation is produced in about fifteen days. Gerlach has estimated the natural increase in three months at 1,000,000 females and 500,000 males. Scab and mange are exceedingly contagious diseases. [Illustration: FIG. 68.--Sheep scab.]