TRACHEARLE. 339 



the mouth consists of a simple cavity without any apparent appen- 

 dages. This tribe is composed of the genus 



ACARUS, Lin. 



Most of these animals are very small or nearly microscopical. They are 

 observed everywhere. Some of them are errant, and of these some are 

 found under stones, leaves, bark of trees, in the earth, in water, dried meat, 

 old cheese, and putrescent animal matters. Others are parasitical, living 1 

 on the skin or in the flesh of various animals, which they often, by their 

 excessive multiplication, reduce to a state of great debility.- The origin 

 of certain diseases, such as the itch, is attributed to particular species. The 

 experiments of Dr Galet prove that if the Acari of the human psora be 

 placed on the body of a perfectly healthy person, they will inoculate him 

 with the virus of that disorder. Various species of Acari are also found 

 on Insects, and some of the Coleoptera that feed on cadaverous substances 

 are frequently covered with them. They have even been observed in the 

 brain and eye of Man. 



The Acari, or Mites, as they are vulgarly termed, are oviparous, and ex- 

 cessively prolific. Several of them at first have but six legs, the remaining 

 two being developed shortly after. Their tarsi terminate in various ways, 

 according to their habits. 



Some ACARIDES, Lat. or the Acari proper, have eight legs, solely des- 

 tined for walking, and chelicerse. 



TROMBIDITJM, Fab. 



The chelicerae monodactyle, or terminated by a movable hook; salient 

 palpi, pointed at the end, with a movable appendage or species of finger 

 under their extremity, two eyes, each placed on a little immovable pedicle. 

 The body is divided into two parts, the first of which, or the anterior, is very 

 small, and bears the two first pair of legs, together with the eyes and mouth. 



Tromb. holosericeum, Fab. Very common in gardens in the spring; blood- 

 red; abdomen nearly square, posteriorly narrowed, with an emargination; 

 the back loaded with papillae, hairy at base, and globular at the extremity. 



There are various other subgenera, such as ERITHR.EUS, GAMASUS, CHET- 

 LETUS, ORIBATA, UROPODA, &c. 



Here we find our Common Ticks. The most remarkable is that of the 

 sub genus 



IXODES, Lat. Fab. 



The palpi forming a sheath to the sucker, and with it constituting a pro- 

 jecting and short rostrum, truncated and slightly dilated at the extremity. 



The Ixodes are found in thick woods abounding in brushes, briars, &c.; 

 they hook themselves to low plants by the hind legs, keeping the others 

 extended, and fasten on Dogs, Oxen, Horses and other Quadrupeds, and 

 even on the Tortoise, burying their suckers so completely in their flesh, 



