ON THE COLOURS OF WATER-MITES. 1231 



54. On the Colours o£ Water-Mites. By C. S. Elton, 

 New College, Oxford *. 



[Received August 31, 1922 : Read November 7, 1922.] 



Contents. 



Page 



1. Introduction 1231 



2. The Mites of Railway Ditch, Oxford 1232 



3. The Evidence for Warning Coloration 1233 



4. The Evidence for Miillerian Mimicrj^ 1235 



5. Discussion 1238 



6. Summary 1239 



1. Introduction. 



There are in the British Isles over 250 species of Hydrachniclse 

 or Water-mites, belonging to about 40 genera [7]. The life- 

 history is interesting, as the larva is usually parasitic upon some 

 other fresh-water animal (e. g., Hydrachna globosa or Bytiscus, 

 Repa, etc.), although the adult is free-living. By such means 

 dispersal is often effected. One result of this rather elaborate 

 and risky life-history is that comparatively few adults are 

 usually found. We should expect, a priori, to find some special 

 method of defence among these mites, since they are not able to 

 increase rapidly in numbers, in order to counteract the depre- 

 dations of enemies. 



The colours of Water-mites are veiy varied. As this paper 

 deals mainly with the scarlet species, the genera may be roughly 

 divided into four groups in order to give some idea of the colours 

 which occur : 



1. All the species of the genus bright red, e. g., Eylctis, Diplo- 



dontus, Hydryphantes, etc. 



2. Most of the species of the genus bright red, but a few non- 



red, e. g., Hydrarachna. 



3. A few of the species of the genus bright red, but most of 



them non-red, e. g., Linmesia, Arrhenurus, Fiona. 



4. All the species of the genus non-red, e. g., Ata.v, Hygrobates. 



This is a large group and the colours are extremely varied. 



(These groups are not intended to express the natural relations 

 of the genera.) 



The ground-colour of the mites is due to pigment present in 

 the skin. Markings of various sorts are produced by the internal 

 organs showing through the skin. 



It is the object of this paper to prove that warning coloration 

 and probably Miillerian mimicry exist among some at least of 

 the Hydrachnidse. 



I am much indebted to Prof. E. B. Poulton for help and 

 encouragement during the preparation of this paper, and to 

 Mr. C. D. Soar for help in identifying the mites, and for other 

 valuable information mentioned later on. 



* Communicated by Professor E. B. Poulton, F.R.S., F,Z,S 



