THE ACAEINA OR MITES. 15 



widely separated regions, and it appears that water birds are partly responsible for 

 their distribution. 



In our country the mites of New York, Virginia, and Illinois are fairly well col- 

 lected, but from what we know of the western species there is no reason to suppose 

 they are more closely related to the European fauna than are those of the Eastern 

 States. 



Of the exotic mites the Ixodidre and Analgesidce are mostly described : of the other 

 families very few; but Berlese has lately published many new species, largely of the 

 Parasitidae. 



In the vicinity of any one locality in the temperate regions one may expect to find 

 from 300 to 500 species of mites. Triigardh, who has studied the vertical distribution 

 of mites on a mountain in Sweden, finds that the mites are fewer in number and in 

 species as one ascends, and that in the low birch zone there are many mites in moss 

 and among dead leaves, while in the lichen zone the great proportion are under stones. 

 In the birch zone the predaceous species are in the ascendency, but in the willow and 

 lichen zones the herbivorous species are most abundant. In going up the mountain 

 the Oribatidse decrease the least in species and in specimens, the Trombidiidse next, 

 and the Parasitidoe decrease very rapidly — less than one-fifth as many species in the 

 lichen zone as in the birch zone. 



Most mites have but few enemies outside of their predatory relatives; some hymen- 

 op terous parasites have been bred from ticks, and chrysopid, cecidomyiid, and coni- 

 opterygid larvae sometimes attack red spiders. There are various cases of protective 

 resemblance, especially among the im mature forms. No examples of mimicry, I 

 think, are known, but there are cases of structural convergence, due to habitat. 



A great many mites are more or less injurious to the property of man. Five, at least, 

 can be ranked as pests of great importance, namely, the cattle tick, the "moubata" 

 bug, the sheep scab, the red spider, and the pear-leaf blister mite. 



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

 degree. They are usually considered to be an order, including about 30 natural 

 groups. An excellent historical review of the classification of acarians has been pre- 

 sented by Trouessart.^ The value of these natural groups of mites has been variously 

 estimated by different authors as tribes, subfamilies, and families. Linnaeus placed 

 all the mites known to him in the genus Acarus. In 1796 Latreille, in his "Precis," 

 established the group "Acephales," which in 1806 he charged to "Aceres" for the 

 mites. He divided it into four families, as follows: 



Acaridae {Trombidium, Erythraeus, Carpais, Oribata, Acarus, Tyroglyphus). 

 Riciniae (Sarcoptes, Cheyletus, Smaris, Bdella, Argas, Ixodes, Uropoda). 

 Hydrachnellidae {Eylais, Hydrachna, Limnochares) . 

 Microphthira (Caris, Leptus, Astoma, all six-legged mites). 



In 1816 von Heyden published a synopsis of his proposed classification, which in its 

 completed form was never issued. A number of his genera are without species and 

 80 are unknown. He arranged the mites in four groups according to the number of 

 legs and presence or absence of eyes. 



In 1834 Dugfes gave the first real classification of the acarians; he had seven families 

 similar to the groups used to-day. They were Trombidiei, Hydrachnei, Gamasei, 

 Ixodei, Acarei (Sarcoptes), Bdellei, and Oribatei. These families wera based on the 

 shape of the palpi, a character which, in general, is very useful, but which presents 

 exceptions in almost every group. Thus the genus Argas was by Dug^s placed in the 

 Gamasei. He used the genus Acams for what is now Tyroglyphus. 



Koch, in general, followed Duges, but made many new genera, often heterogeneous, 

 and poorly characterized. 



1 Revue des Sciences naturelles de I'Ouest, 1891, p. 289-308 ; 1892, p. 21-56. 



88854°— 15 2 



