186 



ARACHNIDA. 



to keep tlie spiracular orifices free from dirt, and thus 

 prevent any obstruction to the free ingress and 

 egress of the air. 



The above characters would in themselves be suffi- 

 cient to discriminate between the two classes; but 

 when we add that, in the Arachnidans, the eyes are 

 always simple, and the antennae of insects represented 

 by organs of a totally different description, we need 

 not enlarge further upon the distinctions between 

 them. 



The Arachnidans may be grouped in three prin- 

 cipal divisions, generally known by the names of 

 Mites, Scorpions, and Spiders. 



The Mites (Acari), breathe by means of air-tubes, 

 resembling those of insects, which are so arranged as 

 to convey air to every part of the system. 



'Fig. 145.— head of cheese-mite. 



These form a very numerous family, which is 

 extensively distributed. Some are parasitic in their 

 habits, infesting the bodies of insects ; many live in 

 cheese, and other provisions, where they multiply 

 prodigiously, and not a few inhabit leaves, or are 

 found under stones and beneath the bark of trees ; 

 while others, such as the pretty scarlet water-mite, 

 common in every pond, are aquatic. 



The Scorpions, foimiug the second division, are at 



