96 DIRECT INJURIES CAUSED BY INSECTS. 



ing to it the trivial name of vermicularis ; and the lat- 

 ter proving, by very satisfactory arguments, that it is 

 different from the other. If they were both ainmate 

 diseases, but derived from two distinct species of ani- 

 mals, (for it seems not impossible that even our com- 

 mon itch may be caused by an A earns more minute than 

 the other, and so more difficult to find,) they would 

 properly be considered as distinct species; much more, 

 therefore, if one be animate and the other inanimate. 

 Nay this, I should think, would lead to a doubt whe- 

 ther even their genus were the same. I shall dis- 

 miss this part of my subject with the mention of a dis- 

 covery of Dr. Adams, which seems to have escaped both 

 Linne and De Geer — that the Acarus Scahici is en- 

 dowed with the faculty of leaping; (in this respect i"e- 

 sembling the insect found by Willan in Prurigo senilis 

 mentioned above,) for which purpose its four posterior 

 thighs are incrassated'^. 



But besides these Aearine diseases, there seems to be 

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

 class'') 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 parasites mistaken for lice. This, from the passage 

 above quoted, appears to have been Dr. Willan's opi- 

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



" Probably this Acarus in the modern system would form a distinct ge- 

 nus. Latreille places it in his Sarcoplcs with the Ac.pnsserinus, L. Latr. 

 Gen. i. 152. 2. " Amcen. Ac, ubi supr. 101. 



