268 INSECTS AFFECTING DOMESTIC ANIMALS. 
insects themselves can be found among the hairs at but little distance from the bites. 
As time passes and the insects multiply in numbers these elevations become more 
and more numerous and closer and closer together, until they finally unite over a con- 
siderable extent. From the summit of each elevation or papule a watery, serous fluid . 
exudes and accumulates, which transforms them into vesicles and pustules, and 
which in drying cover them over with a thin crust. In a few days the whole surface 
is covered with a yellowish, greasy, scaly layer, under which the parasites are hidden. ~ 
As the disease proceeds this layer gradually increases in thickness by an increase of ~ 
the serous exudate, and in circumference by the extension of inflammation produced 
by the ever-muitiplying parasites which live beneath it, forming scaly crusts. These 
crusts in being torn out, mainly by the rubbing with which the sheep endeavors to 
allay its intense itching, carry with them the tags of the wool, the loss of which is 
an early symptom of the disease. Ata later period the crusts are replaced by another 
set of thicker, firmer, adherent scabs, which are still further enlarged by the out- 
ward migration of the parasites. As they abandon the center of the scabs these are 
again replaced by a peeling off of the external layers of the skin, which gradually 
heals, while the disease slowly progresses at the outside. The complete cure is very 
slow, and the skin remains thick and folded for along time. In sheared sheep the 
skin becomes covered by a thick, dry crust, like parchment, while beneath it remains 
much swollen. 
The fleece of scabby sheep presents a characteristic rough look. In places the 
wool is stuck together in masses; in others it falls, while in others, which are 
apparently sound, it can be easily plucked off. The rubbing and scratching 
indulged in by the sheep not only tend to tear away the wool but increase the irri- 
tation of the skin, which may be intensely inflamed and finally end in superficial 
death of the part. Unlike Sarcoptes, the Psoroptes seeks the longest, thickest wool. 
It begins its attack along the back and extends to the neck, flanks, and rump. The 
Psoroptes are seldom found in the region of the chest and abdomen. They are col- 
lected in masses on circumscribed surfaces. The scabs they produce constantly 
increase at their edges, and their number depends on the number of places invaded. 
Owing to the closeness in which the sheep congregate and to their violent scratch- 
ing, the parasites become very generally scattered, and finally, the scabs may run 
together. 
While few of the parasites are present in the older diseased parts, at the edges of 
the scabs they can be found in swarms. They look like little white points with a 
brownish extremity. If picked up by the point of the knife or a sharp stick and 
placed on the hand they will be seen to move. The six-legged young, the eight- 
legged adults, the sexes, couples joined together, and the eggs of this interesting 
insect can easily be identified by the aid of a low-power magnifying glass. 
DESCRIPTION AND LIFE HISTORY. 
The eggs of this mite are minute, glistening white specks, longer than 
broad, and nearly uniform in thickness. They may be found under 
the scabs as before mentioned, and their detection, even when mites 
are not seen, may be taken as evidence of the disease. 
The larvee have nearly the same shape as the adults, but are to be 
distinguished by the fact that only six legs are apparent. 
The full-grown mites are nearly as broad as long, and are charac- 
terized by their piercing mouth parts and the structure of the two 
posterior pairs of legs (see fig. 159). In the male, the fourth is much 
reduced, and the third bears a long thread-like appendage passing 
the sucker, while in the female this leg carries two long, thread-like 
organs and no sucker. 
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