304 MR. F. BALFOUR BROWNE ON AQUATIC COLEOPTERA. 
what I have said with regard to the Beetle fauna of temporarily 
dry ponds I am inclined to think that the third hypothesis 
is the most probable ; but the evidence in favour of it is so 
far quite insufficient to establish it as a fact. 
I will now re-consider the curves showing the number of 
species per collection in the light of the preceding remarks. 
I have shown that there seems to be evidence in favour 
of believing that the Water-beetles, both Hydradephaga 
and Palpicornia, only go through one cycle in the year 
and that the hatching time of the imagos is during the 
summer and early autumn months. We should, therefore, 
expect the distribution curve to rise steadily throughout the 
season, first in the spring because of the re-appearance of 
individuals which have hibernated, and later because of the 
emergence of new imagos. In the case of the Palpicornia 
the curve appears to be according to our expectation, and 
in the case of the Hydradephaga I have accounted for the 
July drop by assuming that these beetles tend to leave the 
water in the hot season. 
Before we can explain the curves further we have to consider 
whether the Hydradephaga curve for 1904 or that for 1905 is 
to be taken as the normal one and how we are to account for 
the abnormality of the other curve. I have said that on 
certain premises we should expect the curve to rise steadily 
throughout the season, and I have endeavoured to account 
for the July depression, so the 1904 curve would appear 
to be the normal one. How then can we account for the 
abnormality of the 1905 curve ? We have seen that it is 
probable that the months of September and October are to be 
accounted for by some conditions which affected similarly 
and injuriously both the Hydradephaga and the Palpicornia, 
so that what we have at present to account for is the low 
August distribution. 
I have already referred to the parched condition of the 
marshes in August, 1905, and to the fact that the rainfall 
was actually greater than in the same month 1904. For the 
purpose of comparing the rainfall during the two seasons 
I have mapped out a chart from the figures given for Yarmouth 
