122 ZOOLOGY. 



condition of other genera. Duges divides the order into seven families, the 

 succession of which is here followed in reversed order. The genera are 

 numerous. 



Fam. 1. Oribatidoi. These have a remarkably hard exterior, the back 

 being formed like a carapace, whence Hermann's generic name Notaspis. 

 This carapace sometimes has a transverse division, and Duges mentions a 

 species in which the feet can be hidden by its lateral borders. The palpi 

 have five articulations, of which the second is the thickest and longer than 

 the rest conjointly. The mandibles are cheliform or didactyle, and the eyes 

 are usually absent. 



Fam. 2. BdellidcG. Body oblong and tumid posteriorly, contracted behind 

 the eyes ; rostrum in the form of a head, and elongated ; palpi resembling 

 antennie, mandibles with a claw, or cheliform; feet cursorial. 



Fam. 3. Acaridce. In this family the feet are carunculated, the mandibles 

 cheliform, and the palpi indistinct, on account of their adherence along the 

 inside margin of the labium. These animals increase rapidly, and various 

 species infest provisions and living animals, including man. Others roam 

 at large upon the ground, or on plants and other objects. Acarus siro {pi. 

 77, fig. 70) is the cheese-mite, which is mentioned by Aristotle. In Sar- 

 coptes., the body is soft and tumid, the four posterior feet are rudimentary 

 and provided with long bristles, the collar below and the base of the feet 

 are armed with hooks, and the anterior feet terminate in vesicles. This 

 genus is parasitic under the skin of various animals, including man, causing 

 the disease named itch, and although this fact has been often doubted, it 

 seems now to be established beyond dispute. The species which infests 

 man has been known to the Arabs, and is mentioned by an Ai-abic author, 

 Abenzoar, of the twelfth centurj'. It has been known from time immemorial 

 in Southern Europe, where it has been considered to be the cause of the 

 itch, under names which are diminutives of the terms used for lice, with 

 which they were naturally confounded in the absence of microscopic 

 examination. Linnasus confounded this species with Acarus siro, consider- 

 ing the two to be varieties of a single species, a view which has been 

 contended for in later times. It seems probable that Acarus siro is 

 occasionally found about the itch pustules, but the Sarcoptes must be 

 searched for at the bottom of the tortuous galleries which it forms under the 

 skin, extending from one to six lines. The common Sarcoptes scabiei {pi. 

 77, fig. 68) is marked with curved lines above, the middle has small eleva- 

 tions, and there is a small bristle on each side, and two large ones 

 posteriorly. 



The itch, caused probably by different species of Sarcoptes, is sj)read 

 over a great portion of the earth, and is endemic in some localities, and 

 although it is readily destroyed, there are those who consider the irritation 

 in the light of a luxurv, and refuse to have it removed. 



Various species of Sarcoptes infest man, monkeys, horses, and drome- 

 daries. That of the last-named animal {S. dromedarii) is larger and better 

 armed than the ordinary species, and in several instances it has been com- 

 municated to man. 

 326 



