THE MITES 



111 



will be influenced by age, condition, character of fleece, and the con- 

 ditions under which the sheep are kept. Animals debilitated from age, 



Fig. 68. — Psoroptes communis var. ovis, female; dorsal (left) and ventral 

 (right) surface. 



or other cause, offer little resistance, while lambs, due to the tenderness 

 of their skin and their dense fleece, are apt to be attacked more severely. 

 Sheep with a close wool, as the pure or grade merino, afford an ideal 

 habitation for the rapid multiplication of the parasites. The contagion 



Fig. 69. — Psoroptes communis var. ovis, male; dorsal (left) and 

 ventral (right) surface. 



of any form of acariases in sheep is facilitated by their habit of living 

 in flocks. The disease is, therefore, much more serious in winter when 

 the animals are huddled together, especially making rapid progress if 



