ACARINA OR MITES. 



129 



Sheep-scab. 



The Scab or Soab of sheep is of three types, produced Ijy the 

 throe genera of Sarcoptidm. The commonest form of scab is 

 that produced by Psoroptes communis v. ovis. It is sometimes 

 called Demodectic Scabies. The acarus (fig. 56) can just be 

 seen with the naked eye, and may readily be found under 

 one of the scabs in all stages. The female Psoropt of the 

 sheep has a large round 

 body with four pairs of 

 short legs, the third and 

 fourth pairs being of nearly 

 equal size ; but the third is 

 devoid of the curious sucker 

 on the feet. The male has 

 the fourth pair of legs very 

 short, the third pair being 

 long, and armed with the 

 sucker. The ova laid by 

 the female are comparatively 

 large ovoid bodies, which 

 are deposited at the edges 

 of the scab. I have kept 

 the ova for two months, at 

 a temperature of 30° Fahr., 

 without their incubating ; 

 but when the temperature 

 was raised to 67° Fahr. they soon hatched out. In the summer 

 five to eight days is, however, the period of the egg-stage. The 

 larva is much narrower than the adult, and has only six legs, 

 the fourth pair not being developed until later in life. In 

 fourteen days the larva casts its skin for the first time, and then 

 ten days later another moult takes place. By the thirty-eighth 

 to the fortieth day of its existence a third moult brings it 



I 



Fio. 50. — Fkmale. Sheep-scab Mite. 

 (Greatly enlarged.) 



