ORDER ACARINA 35 



where the eggs are laid and a new cycle begun. The disease 

 has some remarkable limitations in its distribution which 

 are probably associated with the distribution of the ticks or 

 the animals which serve as a reserve for the disease germs. 



Family Sarcoptidae. — This family includes parasitic mites, 

 affecting particularly birds and mammals. 



The Sheep Scab Mite (Psoroptes communis, var. ovis). — 

 The sheep scab mite produces a very serious condition 

 among sheep, evidenced by matting and tagging of wool 

 and the formation of thick, encrusting scabs. The eggs of 

 this mite are minute, glistening white specks, longer than 

 broad and nearly uniform in thickness. They may be 

 found under the scabs by careful inspection, and their 

 detection, even when mites are not seen, may be taken as 

 evidence of the disease. 



The larvse have nearly the same shape as the adults, but 

 are to be distinguished by the fact that only six legs are 

 apparent. 



The full-grown mites are nearly as broad as long, and are 

 characterized by their piercing mouth parts and the struc- 

 ture of the two posterior pairs of legs (see Fig. 15). In the 

 male the fourth is much reduced, and the third bears a 

 long thread-like appendage passing the sucker, while in 

 the female this leg carries two long, thread-like organs and 

 no sucker. 



The only treatment for this species worthy of recognition 

 is that of dipping, and this, if properly done, will secure the 

 extermination of the pest. A flock once freed will not 

 become again infested except by exposure to infected animals 

 or by the introduction of scabby individuals. 



So important is this parasite deemed that many of the 

 States have adopted stringent laws for the quarantine of 

 infected animals and for prescribing dips that must be used. 



The particular kind of dip is of less importance than the 

 thorough use of the one selected. The tobacco dips, sulphur 

 and lime dips, and also several of the patent dips prepared 

 by reputable firms can be recommended. The main objec- 

 tion to the latter, perhaps, is the fact that the user must 



