INSECT ENEMIES OF LIVE STOCK 187 



posterior processes, each furnished with stiff bristles. 

 Again, the last pair of legs in the male are very short and 

 without sucker feet, whereas in the female it is the third 

 pair of legs that are without sucker feet, and in their place 

 are two very long curved bristles. Wherever the host is 

 thickly covered with wool, on back, shoulders, sides, and 

 rump, there the mites congregate most thickly. With 

 their barbed mouth organs they pierce the skin of their 

 host, and, by working these barbs up and down they cause 

 intense irritation and a flow of blood and matter; this 

 hardens to form a scab, beneath which the mites hide, 

 hence their popular name, scab mites. The damage does 

 not end here, for infected sheep have soiled and matted 

 wool which finally falls off in large patches. The disease 

 is exceedingly contagious, both directly, from sheep to 

 sheep, and indirectly, from fallen wool, rubbing posts, 

 etc. ; in fact, healthy sheep have become infected in places 

 where no sheep have been kept for as long as twenty- 

 four months, though how the mites retain their vitality 

 during this period is not known. Birds probably aid in 

 spreading the disease; they constantly settle on the 

 backs of scabby sheep, and, passing thence to healthy 

 animals, they may carry on their feet a tag of wool or 

 even a scab. 



The female mite lays about a score of eggs, either near 

 the roots of the host's wool or actually on the skin, and 

 after doing so she dies. From these eggs six-legged larvae 

 arise, and after moulting they become mature. The dura- 

 tion of the egg and larval stages varies considerably under 

 varying conditions, but, as a rule, the eggs hatch in about 

 six days, and, on the third day of larval life, a moult may 

 occur, when the larva obtains its fourth pair of legs and 

 becomes adult ; pairing then takes place, followed by two 

 moults and oviposition. So that the average period for a 

 complete life-cycle is about fifteen days, and, of ten females 

 and five males, " in three months' time the sixth generation 



