MR. F. BALFOUR BROWNE ON AQUATIC COLEOPTERA. 299 
although few are, I think, interesting. Whenever I came 
across a crowded centre, that is, a place where any species 
was abundant, 1 carefully examined a number of the indi- 
viduals for the purpose of seeing whether they were mature, 
or soft and recently emerged. In March. April and May 
every such centre examined was composed of mature 
individuals, and it is worthy of notice that more such centres 
were observed in March than in April, and very few were 
discovered in May. I do not know how long after hatching 
from the pupa an imago remains sufficiently soft to be recog- 
nised as immature, but I should imagine that it would not be 
more than a few weeks at the most, possibly indeed only 
a few days, so that it is quite possible that if a “ hatching ” 
occurred in a dyke or pond shortly after my month’s visit 
had been paid, I would find a centre of mature individuals 
on my next return there. It seems, however, significant that 
whereas at least many of these centres found in the autumn 
of 1904 remained crowded in the early spring of 1905, the 
crowded condition did not persist, but usually cleared in 
April or May. This state of things was not invariable, as 
one or two crowded centres observed in September, 1904, 
were not crowded in October, e.g., one of Hydrcena testacea . 
Curt., referred to in my previous paper, where the species 
had entirely disappeared from a dyke in October, after being 
abundant in September, and where it did not occur again 
until August, 1905. On the other hand, although as a rule 
what were crowded centres in the autumn of 1904, cleared 
in the spring of 1905, there were one or two cases where the 
clearing up process was very slow, occupying most of the 
season. Although I have very little evidence at present, it 
appears to me that when the clearing up process is very slow 
the species ultimately disappears completely from the dyke 
or pond, a phenomenon which, if true, is very interesting. 
One or two interesting cases occurred in 1905 where I 
actually found the specimens soft and watched the progress 
of the centre. On June 3rd I visited a certain ditch in Potter 
Heigham which was then merely damp, there being no visible 
water, although on May 10th, the occasion of my previous 
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