274 CD. SOAR ON THE GENUS HYDRACHNA. 



Dr. Wolcott, the American writer on the Hydrachnidae, suggests 

 that, as the genus is getting so numerous in species, it would be de- 

 sirable to split it up into subgenera, using the papillae and eye-plates 

 for that purpose. This is a good suggestion, and no doubt will be 

 adopted when a monograph on the whole genus is produced ; but 

 as we are here dealing with British species only, it will not 

 be necessary to make the suggested subgenera now. 



One of the most striking characteristics of this genus is the way 

 in which the rostrum or capitulum is carried forward nearly level 

 with the tips of the palpi. This appears, on examination, to be a 

 sheath in which the mandibles are enclosed and hidden when not 

 in use. The rostrum appears to be split on the upper edge, for 

 after death it is often found that the mandibles have sprung out 

 of the sheath and project forward along the dorsal portion of the 

 rostrum. The mandibles are long and lancet-shaped, and are 

 serrated on one edge. The stock of the mandibles extends 

 some distance down into the body of the mite. 



The epimeral plates are in four groups, the smallest in front 

 and the largest behind. These plates vary in shape in different 

 species, some being much rounded and others being very angular. 

 Between the two posterior pairs of epimeral plates is placed 

 the genital area. The shape of the genital plates is the only 

 great difference in the external structure of the males and 

 females. These plates also show different forms in different 

 species, some having a deep bay in the anterior margin and 

 others a slight indentation. The acetabula are also different, 

 in some cases being all of one size, whilst in others two or 

 more >are much larger. In Hydrachna dissimilis, Halbert, the 

 female has four larger acetabula in a row on the genital field. 



We have, at present, twenty-one species of Hydrachna in 

 Britain, fourteen of which have already been recorded, the re- 

 maining seven embracing three species new to the British fauna 

 and four species new to science. Many probably remain to be 

 discovered, as but few districts in the British Isles have been 

 at all systematically worked. 



