ITCH MITES. 



293 



The specific characters are chiefly drawn from the bristles and 

 hooks on the tarsi, the number of short stumpy thorns or spines 

 on the back, and the proportions of the different chitonous bands 

 and other parts. This species, for example, has six thorns on the 

 back of the thorax, and fourteen on the back of the abdomen. 

 These have some resemblance to a short Roman sword. There 

 are also a great number of smaller papillae or raised projections, the 

 form and disposition of which are used as subsidiary characters. 



When examined with the naked eye, the mite looks white and 

 shining, and was aptly described by Bonomo, one of its first 

 observers, as like a little bladder of water ; when seen running, 

 however, upon the surface of a plate of gla^s, it may be perceived 

 that its anterior margin presents a dusky tint of colour, and the 

 examination of this part of the creature with the microscope 

 brings into view a head not unlike that of a tortoise, and a pair 

 of large and strong legs on each side of the head. These organs 

 are more or less encased in a moderately thick layer of chytine, 

 and have consequently the reddish-brown tint of the cases of certain 

 insects, or of the bright part of a thm layer of tortoise-shell. 



Two of the posterior legs of the male of Sarcoptes 

 scabiei. Copied from Farstenberg. 



Two of the posterior legs of the female of 

 Sarcoptes scabiei. Ditto. 



