296 MR. F. BALFOUR BROWNE ON AQUATIC COLEOPTERA. 
1904 and 5 are very different, while those of the Palpicornia 
are more or less similar. In the case of the Hydradephaga 
in 1904, the species per collection, that is the distribution of 
individuals, gradually increased throughout the year, attaining 
its maximum in September, while in 1905 the maximum was 
attained in J une, while there was a second but lower maximum 
in September. I have, of course, compared the second 
1904 curve — with July omitted — with the 1905 curve. In the 
1905 curve the difference between September and October is 
very marked, while in the 1904 curve there is no difference 
at all. In the case of the Palpicornia curve the difference 
between the two years is merely one of degree. The falling 
off in September and October, 1904, is small, whereas in 1905 
it is large. From May to August in both years, is the season 
“ par excellence ” for these Beetles, the maximum of species 
per collection and distribution of individuals being attained 
in each case in the latter month. 
The conditions, therefore, which injuriously affected the 
Hydradephaga in 1905 and so caused a difference from 1904 
in the process of distribution, did not so affect the Palpicornia, 
if we except the months of September and October. The 
Hydradephaga and Palpicornia curves agree in the decided 
falling off in species per collection in October, 1905. In 
September, 1905, it may be also said that they agree, since 
in the case of the Hydradephaga, what was a decided rise in 
the curve from August to September, 1904, is reduced to a 
very small rise, and in the case of the Palpicornia, what was 
a small falling off from August to September, 1904, is increased 
to a decided falling off. 
In these respects, therefore, I am inclined to attribute the 
differences between the 1904 and 5 curves to some conditions 
which similarly affected both the groups of Water-beetles; 
but beyond this the curves will not bear comparison. 
Before, however, considering what these conditions may 
have been, it is necessary to re-consider the life histories of 
the two groups in the light of my last season’s experience. 
I have already pointed out that when I suggested in my 
previous paper that the Hydradephaga were to some noticeable 
