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THE GENUS HYDRACHNA. 



By C. D. Soar, F>K.M.S. 



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{Read March 20th, 1908.) ^ 



Plate 21. 



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The name Hydrachna was first used by 0. F. Miiller in 1776, being 

 applied to the whole family of water-mites known at that date. 

 Later, in 1781, he figured and described forty-nine species, using 

 Hydrachna as the generic name in each case. With the exception, 

 however, of three mites (Hydrachna cruenta, H. geographica, and 

 H. impi'essa), the original species have all been removed and 

 placed in other genera. For example, the species which Miiller 

 named Hydrachna crassipes we now know as Atax crassipes, 

 and so on, the various species being grouped in several genera 

 according to the characteristics exhibited. 



The whole of the water-mites, or rather, we should say, fresh- 

 water mites — the marine forms being known as Halacaridae — 

 are placed under the family name Hydrachnidae. The word 

 Hydrachna is retained as a generic name only. Latreille, in 

 1796, was the first to use it in this restricted sense, regarding 

 H. crue7ita, Mull., as the type species of the genus. 



No better arrangement could have been made than the 

 adoption of the name Hydrachna for the genus with which it is 

 now associated, one of the included species being the largest mite 

 of the whole family, and one which was named by Miiller in his 

 work in 1776. 



Muller's book on water-mites, published in 1781, was the first 

 great work on this subject. Previous writers had mentioned 

 them, and had figured some of the species, but it was reserved for 

 Miiller to write a work dealing exclusively with the Hydrachnidae ; 

 and although only three of his species have been retained in the 



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