DIRECT INJURIES CAUSED BY INSECTS. 85 



never, in this country, have been able to meet with it ? Did it indeed 

 exist in our common scabies, it seems impossible that it could have escaped 

 the observation of the two last of these gentlemen ; Dr. Adams being so 

 well qualified to detect it from his observations in Madeira, and Mr. Baker 

 from' his expertness in microscopical researches. Dr. Bateman, in the 

 letter above quoted, says, " I have hunted it with a good magnifier in 

 many cases of itch, both in and near the pustules, and in the red streaks 

 or furrows, but always without success. In his work on Cutaneous Dis- 

 eases, he tells us, however, that he has seen it, in one instance, when it 

 had been taken from the diseased surface by another practitioner. And 

 though Dr. Willan in his book speaks of the Acarus as the concomitant 

 of this disease, yet his learned friend just mentioned observes, that he ad- 

 mitted that it was not to be found in ordinary cases, and indeed never 

 seemed to have made up his mind upon the subject. When I was at 

 Norwich, in 1812, Dr. Reeve very kindly accompanied me to the House 

 of Industry there, to examine a patient whose body was very full of the 

 pustules of this disorder ; but though we used a good magnifier, we could 

 discovernothing like an insect. I must observe, however, that our examin- 

 ation was made in December, in severe weather, when the cold mif^ht, 

 perhaps, render the animal torpid, and less easy to be discovered. 



From the above facts it seems fair to infer that this animal is not inva- 

 riably the cause of scabies, but that there are cases with which it has no 

 connection. Now, from this inference, would not another also follow, 

 that the disease produced by the insect is specifically distinct from that in 

 which it cannot be found ? Sauvages and Dr. Adams are both of this 

 opinion^, the former assigning to it the trivial name of vermicularis, and 

 the latter proving, by very satisfactory arguments, that it is different from 

 the other. If they were both animate diseases, but derived from two 

 distinct species of animals (for it seems not impossible that even our com- 

 mon itch may be caused by a mite 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 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 escaped both Linne 

 and De Geer, that the 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.^ 



But besides these Acarine 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 



1 This opinion Dr. Bateman thinks probably the true one. Catan. Bis. 197. 



' It may be mentioned here as a remarkable fact, that the Acarus Scahiei was discovered 

 by M. Latrielle upon a New Holland quadruped (Phascolomijs fusca Geoffr.) of the Marsu- 

 pian tribe. N. Did. d'Hist. JS'at. xx'i. 222. Much light has "recently been thrown on the 

 hisory of Acarus Scahiei by M. A. Dug^s, who regards it as forming the distinct genus 

 Sarcoptes {Ann. de Sci. Nat. 2d Serie, iii. 255.), and by MM. Bande, Rennucci, Sedillot. 

 and Blainville, the last of whom has given a critical history of this parasite in his report 

 in the Nouv. Ann. du Mus. iv. 213. See also Raspail's Mimoire Comparatif sur I' Hist. Nat. 

 de I'lnsecte de la Gale. s ^^a/i Ac. ubi supr. 101. 



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