320 
ARACHNIDES. 
legs are very large, soldered, and form a plate under the body. These 
legs are separated from the others and placed behind*. In Phalan- 
gium properly so called, the palpi are filiform, spineless, and termi- 
nated by a joint much longer than the preceding one, with a little 
terminal hook. All the legs are approximated, with similar coxae 
contiguous at their origin. Such are all the species indigenous to 
Europe. 
SiRO, Lat. 
Projecting chelicerae nearly as long as the body ; eyes separated and 
placed on different insulated tubercles f . 
Macrocheles, Lat. 
Extremely salient and very long chelicerae ; but the eyes null or ses- 
sile; the two anterior legs very long and antenniform; the top of the 
body forming a plate or scale without distinct annuli. 
To this genus 1 refer the Acarus marginatus and the Ac. testudi- 
narius, of Hermann, Jun., Mem. Apter., p. 7^5 pl- vi, fig. 6, and p. 
80, pi. ix, fig. 1. 
Trogulu.<3, Lat. 
Anterior extremity of the body jjrojecting like a clypeus, and re- 
ceiving the chelicerae and other parts of the mouth into an inferior 
cavity. 
The body is flat and covered with a very firm skin J. 
In the second tribe of the Holetra, that of the Acarides, we some- 
times find chelicerae, but they are simply formed of a single forceps, 
either didactyle or monodactyle, and are hidden in a sternal lip; some- 
times 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, 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, the 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 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 
* Gonoleptes horridus, Lin. Trans., XII, xxii, 16; from Brazil. 
•b Siro rubens, Lat., Gener. Crust, et Insect., I, vi, 2 ; — Acarus crassipesj Herm., 
Mem. Apter., Ill, 6, and IX, Q. N. 
t Trogulus nepaformis, Lat. Gener. Crust, and Insect., I, vi, 1 ; Phulangium 
tricarinutum, L. — South of France, Spain. 
