MR. I'. BALFOUR BROWNE ON AQUATIC COLEOPTERA. 305 
in the Weekly Weather Reports issued by the Meteorological 
Office. The six months March to August, 1905, were actually 
wetter than the same period in 1904, the mean being 2.23 
inches last season as against 1.5 1 in 1904. 
In the face of this the dry condition of the marshes in 
August last as compared with August, 1904, was perplexing, 
and it was not until I had mapped out a temperature chart 
and a chart showing the relative humidity of the atmosphere 
that the explanation occurred to me. The amount of water 
vapour that the atmosphere is capable of holding in suspen- 
sion varies with the temperature, warm air holding more than 
cold air. At any given temperature a certain amount of 
aqueous vapour can be held in suspension and no more. 
The air does not, however, always hold in suspension the 
maximum amount, and the quantity actually held is measured 
by the hygrometer and recorded as the percentage of the 
possible amount and is called the relative humidity. If, 
therefore, we take the mean temperature for each month 
and also take the mean percentage of possible aqueous vapour 
held in suspension we get the mean relative humidity for 
each month. Now when the relative humidity is lower, the 
air is drier and consequently evaporation of water from the 
earth’s surface will be greater. By referring to Chart 4 we 
see that for March, April, May and July the relative humidity 
was lower in 1905 than in 1904, while in June it was the same. 
Thus the dryness of the atmosphere during these five months 
was greater in 1905 than in 1904, the evaporation from the 
marshes was therefore greater, and the parched condition 
last year as compared with 1904 can be accounted for. 
I believe that this excessive dryness accounts for what 
may be described as the failure of the autumn hatch of 
Hydradephaga imagos, which so far as I can see. is the only 
possible explanation of the low August distribution. The 
larvae of both the Hydradephaga and Palpicornia, just previous 
to becoming pupae, leave the water and burrow into the damp 
soil at the edge. Lyonnet found in the case of Hydrophilns 
that unless the earth is sufficiently moist the transformation 
cannot take place, and it seems not unreasonable to assume 
