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.