MR. F. BALFOUR BROWNE ON AQUATIC COLEOPTERA. 297 
extent double-brooded, I drew my conclusions from two 
sources, one the shape of the curve of the species per collection, 
and the other, the crowded centres which I had only observed 
in the spring and autumn, and which I considered to be hatch- 
ing centres and thus centres of distribution of the particular 
species. That some of these crowded centres are indeed 
hatching centres for the imagos, I have since been able to 
prove, as I shall show directly, and it is, I still think, probable 
that they all are ; but I do not now think that the discovery 
of such a centre is evidence that a hatch of imagos of the 
particular species has taken place immediately previous to 
the discovery. For instance, in September and October, 
1904, there were two crowded centres of Hydroporus granu- 
lans, L., in two dykes in Catfield Fen, and in these same 
two places the same species was abundant in March, 1905. 
whereas in April its numbers were not abnormal. Hydroporus 
incognitus, Sharp, swarmed in a certain ditch in Potter 
Heigham in the Autumn of 1904. and was again abundant 
in the same ditch all through the spring of 1905, gradually 
getting scarcer after April and having entirely disappeared 
by October. I have noticed a few other similar cases and 
they certainly suggest that it is not safe to rely upon the date 
of discovery of a crowded centre as indicating the time of 
the hatch of imagos, unless we have some evidence that the 
particular collecting place had not been crowded previously. 
Now ii there are two seasons in the year at which the imagos 
of various species appear, we should find some evidence of this 
(1) in the egg-laying period, (2) in the records of well-grown 
larvae, and (3) in the observations of the times when soft or 
recently emerged imagos are to be found. 
I have not been able to collect much evidence upon any 
of these points, but such as I have collected does not seem to 
bear out the suggestion of there being two cycles in the year. 
Almost the only evidence 1 have obtained as to the breeding 
period of the Water-beetle was got by watching Helochares 
lividus, Forst., and punctatus, Sharp. These species carry 
their eggs about with them attached in a mass to the hind 
femora. The eggs are apparently thus carried until the larvae 
