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 Sarcoiifes also lives on the goat. 



Common Scab. 

 (PsorojJtes 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 esfcjs 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 

 Oerlach. 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 tlie 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 tlie 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 avooI 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 



