68 MR. F. BALFOUR BROWNE ON AQUATIC COLEOPTERA 
dispersal are the only alternatives. Without doubt large 
numbers die off, but others no doubt take their chance in 
new regions. 
So, on the small scale, the individuals of one qf the normal 
crowds devour all the food in their immediate neighbourhood 
and then spread over the land in search of more. The places 
where these normal crowds occur are then the centres of 
distribution of the species concerned. 
I have observed more than fifty such centres during the 
past season, all, with the exception of two or three, having 
been recorded in the spring or autumn. In some of these 
cases in which I counted the number of individuals of each 
species taken in the collection, the dominant species composed 
from 50 % to 60 % of the individuals, in one case even 69 %. 
Such cases of abundance were sometimes common to several 
collecting places at a station — as in the case of Ccelambus 
impressopunctatus on the Hickling marshes — but were more 
commonly confined to one collecting place. To give a few 
instances of such abundance — in April, Bidessus unistriatus. 
an uncommon species, was exceedingly common on some 
flooded marshes in the Hickling neighbourhood, although in 
no other locality all through the season could I find more than 
two or three individuals in a collection. In the same month 
Hydroporus neglectus, also somewhat rare, turned up in such 
numbers in a ditch in Potter Heigham parish that I counted 
seventy in the collection. In May this species was common, 
but not abundant in the same ditch, and in the latter part 
of the year only one or two individuals could be found there 
each month. Again, Philhydrus maritimus was very plentiful 
at a collecting place in the Horsey parish in May, after which 
it became almost scarce there. Similarly Hydrcena teslacea 
appeared in abundance in a dyke at Sutton in September, 
where only one or two individuals had previously occurred, 
and where it was not to be found in October. 
Most of the centres of distribution of the Hydradephaga 
were observed in March and April, and again in September 
and October, while those of the Palpicornia, of which I noticed 
only a few, were at the very end of April and in May, and in 
