SHEEP SCAB. 
5 
mal. The spreading of the disease is greatly helped by the 
rubbing and pulling of the wool, which often removes num- 
bers of both mites and eggs. 
These mites are never winged and their power of locomo- 
tion is not great, so that I do not think it likely that one of 
these parasites would be able to travel more than a very few 
rods in its lifetime. 
HOW LONG YARDS MAY RETAIN THE INFECTION. 
It is important to know how long these mites or their eggs 
may remain alive in the yards or corrals after the sheep have 
been removed. My experiments have all been conducted since 
last November, so they are not as complete as could be de- 
sired. I feel very safe in concluding from them, however, that 
it would be impossible to carry the disease over in the cor- 
rals from one year to another, or from fall to spring or spring 
to fall, and it seems highly improbable that the eggs or mites 
can be kept alive more than a few weeks under ordinary con- 
ditions. In my experiments, a temperature of 0, or 4 or 5 de- 
grees below, have killed both eggs and mites in every case. 
Eggs kept at a temperature near that of the body will hatch 
in from four to eight days, and mites kept at the same tem- 
perature will seldom live more than five days without food. If 
kept in a temperature below that at which the eggs will hatch 
or the mites be active, both will retain vitality for a much 
longer time, but just how long I have not yet fully determined. 
For farther information on these points, see tabulated state- 
ments and notes. 
DESCRIPTIONS OP THE MITES AND THEIR EGGS. 
Figure 1 will show the structure of these mites to the 
average reader better than a technical description. In all stages 
they are nearly white in color; the females are a little larger 
than the males, and are about one-fortieth of an inch in length, 
or almost exactly the size of the dot of this letter (i) when fully 
grown. The mature insects have four pairs of legs, like the 
spiders, but the last pair is small, and in the young they are 
entirely wanting. A very noticeable peculiarity in these mites 
is the long gossamer threads attached to the third pair of feet, 
and which trail behind them as they travel along. In a newly- 
hatched mite I have seen these threads fully two and one-half 
times the length of the body, and so slender that it required 
a rather high power of the microscope to see them at their 
distal ends. The males can be distinguished from the females 
