56 BRITISH HYDRACHNID^. 



doubt turn into the perfect mite. A short time ago a Quekett man 

 was surprised to find, on examining a Dytiscus marginalis which 

 he had preserved in spirit some time before, a perfect mite (genus 

 Bydrachna), partly emerged from the body of the beetle. As I 

 have not seen it, I cannot say what species it was. I am inclined 

 to think that, when this subject has been more studied, the same 

 species of mite will always be found on the same species of 

 water insect. As a typical species of Bydrachna, I propose to 

 describe and illustrate Hydrachna geographica (Miiller). 



Bibliography. 

 1 78 1. — Hydrachna geographica. Miiller, Hydrachna, etc., p. 59, 



Tab. viii., Figs. 3, 4, 5. 

 1793. — Trombidmtn geographicwn . J. C. Fabricius, Ent. Sysf., 



Tom. 11., p. 405, N. 32. 

 1835-41. — Hydrachna geographica. C. L. Koch, Deutschlands 



Crust., etc.. Haft. 14, Fig. 13. 

 i^']^.— Hydrachna geographica. Neuman, Sveriges Hydrachnider, 



p. Ill, Tab. xiv.. Fig. 2. 



This species is, I believe, the largest of the water mites, speci- 

 mens having been found measuring as much as one-third of an 

 inch in length. 



The female from which this drawing was made is about one- 

 fifth of an inch long. Body nearly circular, black and red in 

 colour. On the dorsal side are eight spots, or pores, arranged in 

 two rows, as shown in the drawing. Legs red, with yellow joints. 

 In some drawings which I have seen of this mite, the legs are 

 shown as black with red joints ; but those which I have examined 

 have not been so. The first pair of legs are without swimming- 

 hairs. It is a very beautiful mite alive, and is a powerful swimmer. 



The male is pear-shaped, and a little smaller than the female. 

 I have not at present been able to detect any difference in the 

 sexes. The male of H. geographica has not the peculiar process 

 on the fourth pair of legs nor the modified third pair of feet met 

 with in males of other species of Hydrachna, as described in the 

 previous papers on this subject. Both these mites were taken by 

 me at Snaresbrook on March 23rd, 1895, on one of the excur- 

 sions of the Quekett Microscopical Club. I believe this is the 

 first time the male has been figured. 



