PAEASITIC DISEASES OF THE SKIN. 



235 



cause of foot scab. In studying the life-history of these para- 

 sites we find they are virtually alike, in that they attack the 

 skin of the sheep, and live on the fluid arising from the irritation 

 from their bites. The scabs form over the irritated surfaces; 

 and beneath these crusts the insects deposit their eggs. In the 

 course of from two to three days the eggs hatch, the newly born 

 parasite becoming adult in fifteen days. Each female parasite, 



©re up of emaciated and Scabby Sbccp. 



according to Gerlach, will lay fifteen eggs, ten of which will 

 bring forth females, the balance males. The young parasites 

 as soon as they hatch spread and invade new territory, which 

 gives the affected part the appearance of spreading by a constant 

 advance of its circumference. The eggs being embedded in 

 these crusts or scabs, are deposited on posts, feeding troughs, or 

 bedding, etc., and become fresh centers of infection, it only 



