PARASITIC INSECTS AND MITES 243 



face of the skin where they are sheltered by scabs and scurf; and 

 the Symbioies, which also live on the surface of the skin, but 

 prefer the regions of the hind feet and legs. 



Acari multiply rapidly and live their entire life on the body 

 of the host. A new generation is produced in about fifteen days. 

 Gerlach has estimated the natural increase in three months at 

 1,000,000 females and 500,000 males. Scab and mange are 

 exceedingly contagious diseases. 



Fig. 6S. — riheep scab. 



Common sheep scab is caused by that specie of mites known 

 as the Psoroptes communis var. oris (Fig. 67). Any part of the 

 body may become affected. The bites of the mites greatly 

 irritate the skin, and the animal scratches, bites and rubs the part 

 in its effort to relieve the intense itching. The skin becomes in- 

 flamed and scabby, the wool is pulled and rubbed out, and the 

 fleece becomes ragged (Fig, 68). By pulling wool out of the 

 newly infested area, or collecting skin scrapings and placing 

 this material on black paper in a sunny, warm place, the mites 



