AQUATIC COLEOPTERA OF THE OUTER HEBRIDES 109 



Ilybius SKbceneus, Er. — A single specimen from the Isle of Man. 

 Other records are for east and south-east of England (^ide 

 "The Aquatic Coleoptera of the Isle of Man, etc.," The 

 Naturalist, March 191 1, p. 133)- 



Gyri/ii/s iiri/iator, 111. — A single specimen from Co. Antrim, taken 

 by Mr R. Welch and brought to me. Otherwise the species 

 has only occurred in Ireland in South Kerry and West Cork. 



Hydr'oits {Bydrocharis) caraboides, L. — A single specimen in South 

 Lanes, taken by the late Dr G. W. Chester on the roof of a 

 greenhouse at night. Normal range of the species apparently 

 limited to a few south-eastern counties from Essex to Surrey. 

 I found a single $ specimen this year in Hunts. 



Helophflrus nanus, Sturm. — A single specimen in Co. Meath, the 

 only one so far found in Ireland. The normal British dis- 

 tribution is entirely S.E. of a line drawn from N. Lincoln to 

 East Gloucester. 



There are innumerable other records of the same .sort, and 

 in many cases, on account of their improbability, these are 

 put down as due to errors in identification ; but in my 

 opinion there is ample evidence to support the view that — 

 whether by accidental transportation or by deliberate migra- 

 tion — the water-beetle fauna of these western islands might 

 have reached them across an intervening sea, especially 

 remembering the enormous time during which such a 

 method of colonisation may have been going on. 



It may be that the absence from Lewis of some species 

 quite common on the mainland is to be explained on the 

 ground that they have so far failed to arrive. For instance, 

 CcElavibus iii((3qualis, F., is common throughout the Brittanic 

 area; it was found on Skye, but not on Eigg. It is not 

 recorded from Mull — this may be due to the fact that 

 comparatively little work has been done there — but it 

 occurred on Coll. Hydroporus rivalis, Gyll., occurred on 

 Eigg, Skye, and Mull, but has apparently not reached Coll. 



Hydroporus lepidus, Ol., did not occur on Eigg, though 

 present on Skye and Coll, while H. tiinbrosus, Gyll., was 

 absent from Eigg, but occurred on Coll and Tiree. 



On the other hand, it does not necessarily follow that 

 the mere arrival of a species is all that is necessary to enable 



