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 I refer the Acarus niarginatus and the Ac testudi- 

 narius, of Hermann, Jun., Mem. Apter., p. 76, pi. vi, fig. 6, and p. 

 80, pi. ix, fig. 1. 



Trogulus, Lat. 



Anterior extremity of the body projecting 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 simjjly 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, leaA'^es, 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, Ijin. Trans., Xll, xxii, 16; from Brazil. 



f Siro rubens, Lat., Gener. Crust, et Insect., I, vi, 2 ; — Acarus crassipes, Herm., 

 Mem. Apter., Ill, 6, and IX, U. N. 



X Trogulus neptefonnis, Lat. Gener. Crust, and Insect., I, vi, 1 ; Phalangium 

 tricarinatum, L. — South of France, Spain. 



