C. D. SOAR ON THE GENUS HYDRACHNA. 273 



blunted or pointed, skittle or cone-shaped, thorny or staff-shaped. 

 Piersig, in his key to this genus in Das Tierreich, very frequently 

 employs this character of skin formation to distinguish one 

 species from another ; but although of great assistance in this 

 respect it cannot always be relied on. There is great variation 

 in this characteristic, some specimens exhibiting the papillose 

 formation much more than others. It should be remembered, 

 also, that the angle at which the skin is folded when examined 

 under the microscope has much to do with the appearance of 

 these papillae. 



The most important character for the identification of species 

 is the dorsal chitinous plate, or plates, between and behind the 

 eyes, and upon this we may divide the genus into three groups. 

 (a) Those having only one large plate (e.g. Hydrachna scutata, 

 Pier.) which covers a considerable portion of the anterior dorsal 

 surface, surrounding the median eye and running backwards 

 some distance on the dorsal surface ; (b) those having two 

 distinct plates, one behind each eye as in Hydrachna globosa, de 

 Geer., and (c) those with a thin chitinous ridge running back 

 behind each eye, which riclge may be in one piece or split up into 

 two or more pieces. Although these plates are mentioned as the 

 most important character for the identification of species, it 

 unfortunately happens that in some cases they are very difficult 

 to see, the body skin and the plates and ridges being all of the 

 same colour, and the extra thickness of the plates not always 

 being enough to make them stand out in sufficient relief, so that 

 the most careful lighting will not always enable us to distinguish 

 the exact form which is so important for identification. But if 

 the mites when caught are placed alive in the preservative 

 solution suggested by Dr. Koenike,* the colour of the body skin 

 will gradually fade away after death, and leave the chitinous 

 plates and the other hard parts in their original colours. This 

 allows the dorsal plates and other parts to be seen to the best 

 advantage. 



* Ten parts pure glycerine ; ten parts distilled water ; three parts citric 

 acid. 



