54 



ovoid eggs. After two or three days these eggs hatch, and in fifteen days 

 the progeny become adult. Each adult female is estimated to lay about 

 fifteen eggs, two-thirds of which produce females. When hatched the 

 young invade new territory and lead the life of their parents. The ex- 

 tension of the invading hosts is due to their migration and rapid propa- 

 gation. The disease usually spreads as a constantly growing patch. 

 The infected sheep sometimes scatter the scabs by scratching ; those, 

 in turn, become new centers of infection. The multiplication of the 

 invading pests ceases only at the death of the host, or when they 

 are killed by the use of proper remedies. To illustrate the rapid 

 increase of the tSarco])tes, Gerlach, a scientist, computed that in three 

 months a single female would produce 1,500,000 progeny. He esti- 

 mated that each adult female laid fifteen eggs, of which ten were 

 females, and that the eggs became adults in fifteen days. The result is 

 shown in tabular form as follows : 



This table presents a very moderate estimate of the rate of propaga- 

 tion. A microscopic examination of minute particles of scab shows 

 them to be teeming with young and old parasites, and would seem to 

 confirm the estimate given. As but few of the parasites may be trans- 

 ferred to a healthy animal, it is evident that up to the second month 

 but little advance in the disease will be noticed, but after that time the 

 tenfold increase every two weeks produces an enormous number of the 

 parasites and causes the disease to advance with wonderful rapidity. 



THE HEAD SCAB. 

 Sarcoptes scabiei, de Geer, var. ovis. 



Description. — The insects which cause this variety of scab are almost 

 invisible to the unaided eye, and are among the smallest of the scab- 

 making insects. They are know n as Sarcoptes scabiei, de Geer, var. 

 ovis. They may be recognized by their rounded or somewhat oval 

 bodies, their small heads, which are furnished with a biting apparatus, 

 and by the adult having four pairs of legs. The young have but three 

 pairs of legs. Other anatomical characters, which are different in differ- 

 ent species, are present, but for the flockmaster these are unessen- 

 tial, as the species can be separated by biological characters which are 

 plainer and easily understood. 



Disease. — Head scab begins on the upper lip, and about the nostrils; 

 more rarely it may show itself for the first time about the eyelids and 



