3*20 ARACHN1DES. 



They have two filiform palpi, composed of five joints, the last of which is 

 terminated by a small nail; two distinct eyes; two jaws formed by the pro- 

 longation of the radical joint of the palpi, and frequently four more, which are 

 also a mere dilatation of the hip of the two first pairs of legs. The body is 

 oval or rounded, and covered, the trunk at least, with a firmer skin; there is 

 also an appearance of annuli or plica; on the abdomen. The legs, of which 

 there are always eight, are long, and distinctly divided, like those of Insects. 

 At the origin of the two posterior legs, at least in several Phalangium are 

 two stigmata, one on each side, but hidden by their hips. 



Most of them live on the ground, at the foot of trees, and on plants, and are 

 very active; others conceal themselves under stones and in moss. 



PHALANGIUM, Linnaeus, Fabricius. 



The chelicera projecting, much shorter than the body; eyes placed on a 

 common tubercle. Their legs are very long and slender, and when detached 

 from the body show signs of irritability for a few moments. 



There are some other genera, viz., Siro, Macrocheles and Trogulus. 



In the second tribe of the Holetra, that of the ACAKIDES, we sometimes 

 find chelicerae, but they are simply formed of a single forceps, either didactyle 

 or monodactyle, and are hidden in a sternal lip; sometimes there is a sucker 

 formed of united lancets; or finally the mouth consists of a simple cavity 

 without any apparent appendages. This tribe is composed of the genus 



ACARUS, Linnceus. 



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 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 experi- 

 ments 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 chelicera' . 



TROMBIDIUM, Fabricius. 

 The chelicera? monodactyle, or terminated by a moveable hook ; salient palpi, 



