AQUATIC COLEOPTERA OF THE OUTER HEBRIDES 107 



follows the struggle amongst the plants, and the peat-moss 

 pool community ultimately has complete possession. 



This movement of water-beetles is an interesting 

 phenomenon. It must be very intense, since several of the 

 ballast-pits I examined in Harris had been dug less than 

 a month, and yet all contained 2, 3, or even in one case, 

 6 species. In my garden in Cambridge, among a number of 

 tubs covered in with wire gauze, are two which have no covers 

 on them, and last spring I made some notes with regard to 

 the appearance of water-beetles in these tubs, which were 

 filled with clean water. I found that one or more beetles 

 appeared in them by the third day after filling. In April, 

 Hydroporiis lituratus seemed to be the most frequent visitor, 

 although several other species appeared, including one male 

 of Dytisais marginalis. After leaving one of the tubs 

 standing for a week in May, its contents were : — H. lituratus, 

 several; A. bipustulatus, 3; H. erythrocepJialus, i ; H. brevi- 

 palpis, several. There is no pond within at least half a mile 

 of my garden, yet a large number of individuals must have 

 been on the move in order that so many should discover 

 a tub full of water. 



Now if water-beetles are flying about as freely as these 

 records in Lewis and Cambridge seem to indicate, it would 

 seem quite reasonable to believe that many might be carried 

 by wind across a narrow stretch of sea such as lies between 

 the mainland of Scotland and the Outer Hebrides, and one 

 can almost imagine a rain of insects upon these outlying 

 islands which would easily account for the origin of their 

 water-beetle faunas. But, although I have made no special 

 observations on the point, I should think it very doubtful 

 if a water-beetle would attempt to fly in any but the calmest 

 weather, and I cannot imagine that extended seaward flights 

 would be normal occurrences. And yet, either water-beetles 

 do deliberately move to considerable distances or they are 

 accidentally transported, and of this we have evidence from 

 odd specimens of species captured now and again far beyond 

 their normal range. I have already referred to this point 

 in a previous paper, where I mentioned what I called 

 "ectopic" occurrences of brackish water species in inland 



