1 8 Second Report on Economic Zoology. 
by its position; secondly, by being very much smaller than the 
common scab-mite ; it is almost invisible to the naked eye. Crusts 
are produced like the common species, which run together. The 
parasites sink into the skin, grow and produce their young. The 
young move about and then penetrate the skin, as did the parents. 
The scabs later become thick, grey, and hard ; the sheep rub the 
scabby places and the scabs get torn off, and open cracks and bleeding 
surfaces are caused. The mites occur in the moist layers under the 
scabs and so get detached. 
This Sarcojptes also lives on the goat. 
Common Scab. 
( Psoroptes communis, var. ovis.) 
The life-history of all these three acari is roughly the same ; the 
Common Scab Mite lays from fifteen to twenty or more eggs on the 
skin and under the scabs ; now and then they appear to be fastened 
to the wool close to the skin. The eggs hatch in from two to five 
days, as far as those were concerned which I have kept under obser- 
vation, but the usual time is from three to four days, according to 
Gerlach. The length depends on the temperature and moisture. The 
larvae are six-legged, but they soon moult, for in four days they have 
assumed the eight-legged stage. At the end of from seven to eight 
or nine days the mites are ready to reproduce. After copulation the 
male dies, but the female casts her skin, this second stage lasting 
four days, sometimes five (according to Stiles) ; a third moult follows 
immediately, and eggs are laid and the adult dies. Stiles says a 
fourth moult may take place, but apparently without any further 
production of eggs. 
Speaking roughly, a generation may be completed in fifteen days, 
allowing for variations. 
The mites live under and near the edges of the scabs, and are 
removed with the wool and crusts when the sheep rub themselves 
against wattles, hedges, posts, gates, etc., or the scabs may fall off on 
to the ground. 
The mites possess great vitality under favourable conditions, but 
I have not been able to keep them alive more than three weeks when 
detached from scab and wool under normal conditions. I think it is 
an exception for them at an ordinary temperature to live more than 
a month — more often they die in a few days. But there are cases 
recorded where they have lived up to from six to eight weeks away 
from the sheep. There are cases where sheep have become infected 
