16 REPORT 108^ U. S. DEPARTMENT OF AGRICULTURE. 



Kramer in 1877 originated an ingenious scheme, which has been extended by 

 Canestrini, whereby 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 classification is as follows: 



Astigmata: Vermiformia and Sarcoptina. 



Hydracarina: Water mites. 



Prostigmata: The Trombidioidea and Eupodoidea of this paper. 



Cryptostigmata: The oribatid mites. 



Metastigmata : The ticks. 



Mesostigmata : The Gamasoidea of this paper. 



Trouessart has modified this scheme somewhat. He di\ddes the mites into two 

 orders, Acarina and Vermiformia; the latter group again into Octopoda (Demodicidne) 

 and Tetrapoda (Eriophyidse). The true Acarina he divides into three suborders, 

 the Prostigmata (including the Hydracarina of Canestrini), the Metastigmata (includ- 

 ing the Mesostigmata and Cryptostigmata of Canestrini), and the Astigmata (including 

 only the Sarcoptina). In the writer's opinion the Oribatidse and Tyroglyphidae are 

 more closely related than indicated in either of these classifications. 



The classification of Renter (1909) does not differ greatly from that here adopted; 

 the Oribatidse and Sarcoptoidea are united into one group, the Sarcoptiformes. The 

 Trombidioidea includes the Tarsonemidse, while the Gamasoidea and Ixodoidea are 

 united into the Gamasiformes. 



Lahille proposed to use three suborders in the Acari, Stylocerata for the Prostig- 

 mata (except Bdellidae), Arpagostoma for the ticks, and Chelicerata for all other 

 groups. The fact that Lahille devoted his attention almost wholly to the ticks 

 explains the classification. 



In 1906, Oudemans arranged the mites in four subclasses, viz : Xemiostigmata for 

 the Eriophyidse, Lipostigmata for the Demodicidae, Octostigmata for the Oribatoidea, 

 and Distigmata for all the others. The Distigmata he divided into fi^-e orders for 

 known forms, and another for supposed forms to connect two of the other groups. 



Thor has arranged the Acarina in four subclasses, Vermiformia, Sarcoptina, 

 Metastigmata and Prostigmata. The water mites are placed in the Prostigmata, and 

 the Gamasidse, Ixodidae, and Oribatidse in the Metastigmata. 



Oudemans has proposed divisions of the Prostigmata with unwieldy names. The 

 Eleutherengona for most of the group except Trombidiidse and Rhyncholophidae, and 

 the water mites, which he calls Parasitengona ; the Halacaridse form the Pleuromeren- 

 gona. The Parasitengona he di\ddes into Apobolostigmata for the Rhyncholophidir, 

 and Eugonostigmata for the remainder; this latter is again divided into Phanerostig- 

 mata for the Trombidiidse, and some parts of the Hydrachnidiie, and Calyptostigmata 

 for the other forms. 



Recently Ewing has proposed new names for various groups. The Prostigmata are 

 divided into Hydracarina (water mites), Adactylognatha ( Eupodoidea \ and Dactylo- 

 gnatha for remaining families. The Parasitoidea and Oribatoidea are united under 

 Peri tremata;. the Heterotracheata include the Heterostigmata (Tarsonemida?), 

 Ginglylosoma (Hoploderma), and Scleroderma (Oribatida?); the sarcoptid and 

 tyroglyphid mites are classed in Atracheata, and the Eriophyidic in the Tetrapoda. 



In the arrangement used below, the main divisions are about the same as in several 

 recent classifications, 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 provided for them would heavily 

 encumber the synopses. The characters used in defining genera and higher groui)8 

 vary greatly according to the group. In some cases genera are based on very minute 

 structures, which in other families are of no value. In several groups the habitat is 



