ACARINA OE MITES. 113 



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 ; hut 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 nearly to maturity. 

 When about two months old the acarus commences to lay. 

 Some authorities say they mature much sooner. The larvae 

 become eight-legged before the nyinph stage is assumed. They 

 remain attached to the leg-bristles of the ovigerous female for 

 some little time after birth. The adult mites live for three or 

 four weeks. Psoroptes may be found amongst the wool and 

 under the scabs, but never tunnel in the trua ski7i. One can 

 watch a young mite burrow into the outer skin or cutis : in 

 about ten days a small nodule will arise over the point of entry ; 

 from this vesicle a yellowish exudate appears, which hardens 

 over the skin, forming the scab, beneath which the mites breed, 

 being especially prevalent at the edges of the psora. The scabs 

 usually commence to be noticeable about the sixteenth day after 

 entry. This psoroptio mite, the true scab acarus, is found 

 chiefly affecting the woolly regions of the sheep ; it is seldom 

 found on the head or legs, or on the abdomen beneath or 

 breast region. The disease seems to die out spontaneously, 

 especially after dipping and shearing. The acari often become 

 detached on the wool the sheep rub off, when trying to relieve 

 the pruritus set up by them. Other sheep rubbmg against the 

 wattles, pens, hedges, &c., where this diseased fieece is, wiU 

 soon contract the malady, and so the disease is spread from one 

 flock to another. 



The Sarcopt known as S. scabiei v. ovis produces the head- 

 scab. This mite attacks those regions devoid of wool, or at 



