BRITISH HYDRACHNID^. 383 



margin ; in fact, both in dorsal and ventral surface, the figures of 

 Miiller's quite agree with our own. I mention this because 

 Koenike in 1881 describes and figures a mite of this genus in the 

 Zeitschrift f. Wissenoch Zoologie, XXXV., B.A., p. 600, PI. XXX., 

 Figs. I — 6, which he calls Midea elliptica (Miill.), the name given 

 by Miiller to the other mite, Figs, i and 2 on PI. VII. of his work. 

 On looking carefully at Koenike's figures, I cannot help thinking 

 it is the same mite as our own. It is nearly circular, the depressed 

 line is near the margin, and in all other respects is like the male, 

 as we know it, of M. orbiculata. Now, the figure of M. elliptica^ 

 as its name implies, is a longer-shaped mite, according to Miiller, 

 and differently marked. The depressed line is much nearer the 

 centre of the body, as is the case in Arrenurus. 



Average length of body, about o"64 mm. ; width, about 0*58 

 mm. ; the first leg, 0*28 ; the fourth leg, about o"68 mm. ; palpus, 

 about 0*16 mm. Body nearly circular, with a depressed line in 

 dorsal surface. On the surface of the inner portion of the dorsal 

 covering are twelve dermal glands, arranged in four sets of three 

 (see Fig. 5). Legs similar to other water mites, well supplied with 

 swimming-hairs. Palpi, five joints, the fourth being the longest. 



It should always be remembered that the colouring of these 

 creatures is due to two causes : — first, the colouring pigment in 

 the skin ; secondly, the contents of the body. In no other mite 

 that I have observed at present is this more apparent than in 

 M. orbiculata. The outside margin is yellow and green. The 

 inside portion of the dorsal plate is blue, white, purple, and red. 

 This varied colouring is due to the two causes just mentioned. 

 The skin is blue, with a large white patch on the anterior portion ; 

 under this in the body is a large patch of bright red (due, no 

 doubt, to the contents of the stomach). This, being seen through 

 the blue, undoubtedly give it a purple colour, where seen through 

 the white part the bright red is observed. These colours are all 

 very brilliant and must be seen to be appreciated, it being impos- 

 sible to give any idea of them in a black and white drawing. The 

 ventral surface is yellow, with green patches. Legs and palpi a 

 pale straw colour. Eyes a bright red. The little spots on the 

 back (see PI. XXL, Fig. 5) are yellow, which makes them, in the 

 living mite, very conspicuous. 



