NO. 1382. THE ACARINA OR MITES— BANKS. 9 



There are, however, various cases of protective reseinbhmce, especially 

 among the inunatiire forms. No examples of mimicr}', I think, are 

 known. 



A g-reat many mites are more or less injurious to the property of 

 man. Three, at least, can be ranked as pests of great importance, 

 namely, the cattle-tick, the sheep-scab, and the pear-leaf blister-mite. 

 The classification of mites has in recent years been developed to a 

 considerable degree. The group is usually held to be an order, includ- 

 ing about thirty natural groups. An excellent historical review of 

 the classitication of Acarians has been presented l)y Trouessart." The 

 value of these natural groups of mites has l^een variously estimated 

 by different authors as tribes, subfamilies, and families. Kramer in 

 1877 originated an ingenious scheme, which has been extended by 

 Canestrini, wherel)y the mites are arranged according to the position 

 of the opening of the tracheal system. By this method there are six 

 main divisions of the Acarina. 



Canestrini's classitication is as follows: 



AsTiG:\rATA — Vermiformia and Sarcoptina. 



Hydkacarina — Water mites. 



Prostigmata — The Trombidoidea and Eupodoidea of this paper. 



CkyptostKtMATA — The Oribatid mites. 



Metasticjmata — The ticks. 



Mesostigmata — The Gamasoidea of this paper. 



Trouessart has modified this scheme somewhat. He divides the 

 mites into two orders, Acarina and Vermiformia; the latter group 

 again into Octopoda (Demodicidie) and Tetrapoda (Eriophj'idtv). The 

 true Acarina he divides into three suborders, the Prostigmata (includ- 

 ing the Hydracarina of Canestrini), the Metastigmata (including the 

 ^Nlesostigmata and Cryi^tostigmata of Canestrini), and the Astigmata 

 (including only the Sarcoptina). In the writer's opinion the Oribatid?e 

 and Tyroglyphidtv^ are more closely related than indicated in either of 

 these classilications. 



In the arrangement used below, the main divisions are about the 

 same, although based largely on other characters. These characters 

 will be found defined under the various families; and although there 

 are exceptions to the tables they are not prominent, and to have pro- 

 vided for them would heavily incumber the svnopses. The characters 

 used in defining genera and higher groups vary greatly according to 

 the group. In some cases geilera are based on very minute structures, 

 which in other families are of no value. In several groups the habi- 

 tat is the l)est clew to the systematic position. About three himdred 

 and lift}'- species are known from this country, where there are doubt- 

 less a thousand or more, so that the student must not be surprised to 



"Revue des Sciences naturelles «le I'Ouest, 1.S91, pp. 289-308; 1892, pp. 21-56. 



