AQUATIC COLEOPTERA OF THE OUTER HEBRIDES 19 



spot. Hence, when one collector works a large area, his 

 results can only produce an approximately complete list, the 

 actual approach to completeness depending upon experience 

 and method of work. 



In all my collecting I keep records of all the species 

 found in every collection made, a collection consisting in a 

 careful examination of a particular pool or part of the shore- 

 line of a loch or of a river, and I do my best to record every 

 species of water-beetle in the place under examination. Such 

 a collection may be completed in as little as ten minutes, or 

 it may take as much as half or even three-quarters of an 

 hour, according as species are scarce or abundant, and I 

 usually continue working the place until no new species occurs 

 in three successive hauls of the net, so that my results give 

 a fairly complete list of the community of species in the 

 spot examined. These results can be utilised in various 

 ways ; for instance, one can work out the average number 

 of species per collection, and thus obtain an idea of the rich- 

 ness or poverty of the fauna. In the island of Lewis the 

 average number of species per collection was 6-i, it being 

 slightly higher in the north (6-4) than in the south (5-8). 

 One can also compare the richness of different collections 

 from different types of ground. In Lewis the richest collec- 

 tion contained 16 species, and was made in a ballast-hole 

 which was covered with glyceria, and had therefore not 

 reached the acid-water condition of the Sphagnum pool 

 which, at its best, seldom produced even as many as 10 

 species. By keeping records of collections made in all parts 

 of the country, one can compare the results from different 

 localities, although for various reasons such a comparison 

 cannot be a very accurate one. In a paper on the water- 

 beetles of East Norfolk,^ I shouted that the results obtained 

 month by month in two successive years differed remarkably, 

 and I endeavoured to trace the differences to variations in 

 climatic conditions. Now climate varies from locality to 

 locality, so that even if several localities were worked at the 



1 " A Study of the Aquatic Coleoptera and their surroundings hi the 

 Norfolk Broads District," part ii,, Trans. Norfolk and Norwich Nat. 

 Soc, viii., Part II., 1906. 



