[ 55 ] 



British Ib^bracbniba^, cm* 



By Charles D. Soar. PI. III. 

 THE GENUS HYDRACHNA (0. F. Miiller, 1781). 



BODY, nearly circular in shape, soft and easily broken ; legs, 

 short, hairy, and adapted for swimming ; all the tarsi have 

 claws ; epimera forms two groups on each side ; mandibles 

 joined together, forming a beak, which under the microscope can 

 be seen to be lancet-shaped ; palpi, tolerably long, the third joint 

 being the longest. 



Miiller, in his work on Water-Mites, published in 1781, des- 

 cribes and figures forty-nine species of Hydrachna, using a specific 

 name only to distinguish one form from another. Since then the 

 family of Hydrachnidce has been divided into twenty distinct 

 genera, the name Hydrachna being retained for the genus we are 

 now considering, the characteristics of which are given above. 

 Duges was, I believe, the first to restrict this genus to those mites 

 possessing a beak, Koch, in his work, Deutschlands Crust., etc., 

 describes and figures five species, viz. : — H. geographica, H. 

 t/npressa, H. globosa, H. cruenta, and II. pufiicea. Neuman des- 

 cribes only two. 



I have not yet had an opportunity of observing the transition 

 of any of these from the egg to the imago ; but it has been 

 observed and described by other writers. The eggs are laid in the 

 spring on the stems and leaves of aquatic plants ; in a few days 

 these are hatched out. The larvae when hatched are hexapod, 

 with a large heart-shaped sucker in front, which might be taken for 

 the head ; but the eyes are situated behind on the anterior margin 

 of the body. In this stage they attach themselves to different 

 species of water insects, such as Nepa, Dytiscus marginalise etc. 



Several times in the pages of the earlier volumes of Science 

 Gossip may be found the query. What are the blood-red, pear- 

 shaped parasites firmly attached to the under part of the head of 

 the male Dytiscus} On page 255 oi Science Gossip, Vol. I., Lewis 

 G. Mills gives a figure of one found by him, which is no doubt the 

 larval form of a species of Hydrachna. After having spent a 

 certain time as a parasite on some other living object, they no 



