LIFE-HISTORY OF SCAB PARASITES 



647 



which grow wool. Head scab is produced by another distinct 

 species of mite, Sarcoptes scabiei, de .Geer, var. ovis, which 

 lives on any part of the skin growing short hair, but most 

 frequently on the head. Burrowing more deeply into the 

 skin, it resembles the Sarcopt which attacks the common 

 goat, and it is not so easily reached by external dressings 

 or dips as the Psoropt. It is with Psoroptes communis, the 

 cause in this country of great pecuniary loss and much labour 



PSOROPTES OVIS, 

 SCAB MITE OF SHEEP 



FIG. 53. SCAB-MITES MALE AND FEMALE (MAGNIFIED). 



(Drawn from nature by Mrs D. Hutcheon.) 



which might be rendered unnecessary, that we have now 

 to deal. 



The life-history of the parasite is simple ; but an accurate 

 knowledge of it is nevertheless all-important to those con- 

 cerned, as upon this knowledge rests the chance of ordering 

 aright the course of the treatment to be adopted. In the 

 active forms it is easily destroyed by the poisons which 

 effective dipping materials contain ; but the eggs are, like 

 the germs of so many low-life forms, practically indestructible 

 by the ordinary means at the disposal of the farmer. It is 

 therefore necessary to destroy all the moving acari by a 

 preliminary dipping, and then, having waited until the eggs 



