DIRECT INJURIES CAUSED BY INSECTS. 95 



and the other inanimate. Nay this, I should think, would: 

 lead to a doubt whether even their genus were the same. 

 I shall dismiss this part of my subject with the mention 

 of a discovery of Dr. Adams, which seems to have es- 

 caped both Linne and De Geer — that tiie Acarus Scabiei 

 is endowed with the faculty of leaping ; (in this respect 

 resembling the insect found by Willan in Prurigo senilis 

 mentioned above ;) for which purpose its four posterior 

 thighs are incrassated a . 



But besides these Acarine diseases, there seems to be 

 one (unless with Linne we regard the plague as of this 

 class b ) more fearful and fatal than them all. You will, 

 perhaps, conjecture I am speaking of that described by 

 Aristotle and Sir E. Wilmot as the Phthiriasis, and your 

 conjecture will be right. But some think, and those men 

 of merited celebrity, that Acari have nothing to do in 

 these and similar cases, for that maggots were the para- 

 sites mistaken for lice. This, from the passage above 

 quoted, appears to have been Dr. Willan's opinion, to 

 which, in the letter so often referred to, Dr. Bateman 

 subscribes ; adding as a reason for excluding Acari from 

 being concerned, that " they are too minute, and never 

 have been seen in such numbers as to be mistaken for 

 lice." But both Acari and Pediculi vary in size, some 

 of the former being larger than some of the latter. And 

 allowing them to be ever so minute, yet when they issue 

 in swarms, as mites from a cheese, they would be very 

 visible, were it only from their motion. Besides, as they 

 are furnished with legs, their motions resemble those of 



a Probably this Acarus in the modern system would form a distinct 

 genus. Latreille places it in his Sarcoptes with the Ac. passerinus, L. 

 Latr. -Gen. I 152. 2. * Amcen. Ac. ubi supr. 101. 



