DIRECT INJURIES CAUSED BY INSECTS. 95 



capecl both Linne and De Geer that the Acarus Scdbiei 

 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 mites 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. Batemaii 

 subscribes ; adding as a reason for excluding mites from 

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

 have been seen in such numbers as to be mistaken for 

 lice." But both 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 lice infinitely 

 more than do the contortions of maggots. So that a 

 mite would be deemed a louse much sooner by an un- 

 entomological observer than would a maggot. Whether 

 mites have ever been seen in such numbers as to be mis- 



a It may be mentioned here as a remarkable fact, that the Acarus 

 Scabiei was discovered by M. Latreille upon a New Holland qua- 

 druped (Phascolomys fusca, Geoflfr.) of the Marsupian tribe. N. Diet. 

 d'Hist. Nat, xxi. 222. b Amcen. Ac, ubi supr. 101. 



