290 OBSERVATIONS ON INSECTS. 



assist its march by them, which is entirely effected 

 by the other three pair. I suspect, however, that 

 there are several distinct species of these Acari, 

 tertiis femoribus incrassatis, which have been hitherto 

 confounded :* some that I have taken from the green 

 woodpecker are of a much more oblong form than the 

 more common sort figured by De Geer, with the 

 third pair of legs of more moderate size, and not so 

 enormously thick. 



Acarus domesticus.-^ The mites which infest 

 museums, and make such havoc in collections of 

 dried specimens, appear to me to be of two or more 

 kinds. One, however, seems identical with the 

 cheese-mite, as described and figured by De Geer ; 

 at least, I can discover no difference between them 

 on the closest comparison. These little creatures 

 are of active habits, and run with considerable speed. 

 They appear to shift their skin very often, as appears 

 by the numberless empty sloughs that may be found 

 in their haunts. When first hatched, and for some 

 time after, like many others of this tribe, they have 

 but six legs, the two others not sprouting till they 

 are near full-grown. The deficient pair is always 

 the third. 



Cheyletus eruditus.% This little mite, which seems 

 scarcely to have been noticed by English authors, is 

 remarkable for its very large and conspicuous sickle- 



* They will probably be found eventually to constitute a 

 peculiar genus by themselves, subordinate to Sarcoptes of Latreille. 

 f De Geer, torn. vii. p. 88. pi. v. if. 1-4. 

 J Lum. Hist. Nat. des An. sans vert. (2nd edit.) torn. v. p. 75. 



