214 ARACHNIDES. 



Siro, Lat. 



Projecting chelicerse nearly as long as the body; eyes separated and 

 placed on different insulated tubercles(l). 



Macrocheles, Lat. 



Extremely salient and very long chelicerse; but the eyes null or sess- 

 ile; 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 marginatus 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 chelicerse and other parts of the mouth into an inferior 

 cavity. 



The body is flat and covered with a very firm skin(2). 



In the second tribe of the Holetra, that of the Acarides, 

 we sometimes find chelicerse, 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, 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 ani- 

 mal matters. Others are parasitical, living on the skin or in the 

 flesh of various animals, which they often, by their excessive multi- 

 plication, reduce to a state of great debility. The origin of certain 



(1) Sirorubens, Lat., Gener. Crust, et Insect., I, vi, 2; Jicarus crassipes, Herm., 

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



(2) Trogulus nepseformis, Lat. Gener. Crust, and Insect., I, vi, 1,- Phalangium 

 tricarinatum,!.. South of France, Spain. 



