78 MR. F. BALFOUR BROWNE ON AQUATIC COLEOPTERA 
bution of Water Beetles in this pit is very marked. Omitting 
those species of which only a few individuals occurred during 
the season, two species are peculiar to the western ponds 
and five to the eastern, while five are markedly more common 
in the western than in the eastern, and twelve in the eastern 
than in the western. Now in an area of ground, perhaps 
200 yards long by 50 yards wide, the discovery of such a 
localisation can scarcely be considered imaginary when it is 
the outcome of regular monthly observations from April until 
November and the result of sorting out 74 collections, 32 from 
the western ponds and 42 from the eastern. Including all 
species, 55 occurred in the eastern ponds and only 38 in the 
western. 
Now the pit is fully exposed to the east winds, and the 
banks are scarcely sufficient to form a protection to any of 
the ponds. There is however one fact which I learnt in 
conversation with the foreman of the brick -works which may 
have some bearing upon the localisation of the Beetles. The 
sand in the western part of the pit is unsuitable for using in 
the brick-making owing to its coarseness and all the sand 
used in the works is taken from the eastern end. This 
difference in the nature of the sand at the two ends of the pit 
may of course have nothing to do with the distribution of 
the beetles, but it seems reasonable to believe that it may 
affect them either through their food or in some other way. 
It has seemed to me more than once that the orientation of 
a dyke may affect the distribution of the species, for when 
a dyke turns off at right angles to its original course I have 
several times noticed that a species which in one branch was 
common was either absent or scarce in the other branch. 
Possibly in considering the direction in which a dyke runs we 
are coming back to a question of temperature ; but I have so 
far made no attempt to verify this. I have, however, 
occasionally compared the temperatures of some of the dykes 
in which I have been collecting, and have found differences 
even to 5 0 Fahrenheit. I hope, however, to pay more 
attention to this part of the subject during the present season. 
Now there are possibly groups of species governed as to 
their_ distribution by definite sets of determining factors. 
