982 
Journal of Agricultural Research 
Vol. XXIII, Not 12 
During the summer of 1919 the highest daily evaporation of 78.6 cc. 
occurred on September 21. This was probably due to continuous wind 
movement rather than to unusually high temperature or low humidity. 
The highest evaporation recorded during 1920 occurred on August n, 
being 50.9 cc. As would be expected, there is a marked correlation 
between the amount of evaporation and precipitation and temperature, 
the highest evaporation occurring during periods of high temperature 
and little precipitation. Wind is also an important factor, and the 
character and duration as well as the actual amount of precipitation has 
a great influence upon evaporation. A long period of light rainfall 
retards evaporation more than a short period of heavy rainfall, although 
the actual amount of precipitation may be greater in the latter case. 
A study of the data here presented shows that there is a general 
correlation between the rate of evaporation and the rate of development 
of Aphis pomi. On the whole, a high rate of evaporation was accom¬ 
panied by a rapid development of the aphids, and a low rate of evapora¬ 
tion by a comparatively slow development of the insects. While this 
correlation seems to be true in a general way, there is considerable varia¬ 
tion from the rule. The variation which may occur in the average daily 
rate of evaporation during any given length of developmental period is 
illustrated graphically in figure 7. 
Big. 7.—Variation in Average daily rates of evaporation occurring during 14, 11, and 9 day developmental 
periods. White bars represent average daily evaporation, individual records. Black bars represent 
the means of all records of their respective periods. 
These results show that under the conditions of this investigation, 
evaporation, as registered by the standard evaporimeter used, is not a 
satisfactory measure of aphis metabolism. This condition apparently 
results from the fact that the combination of factors, humidity, tempera¬ 
ture, wind, etc., which influence evaporation, affect evaporation from 
the standard porous cup in a manner which is not closely comparable 
to their effect upon the metabolism of Aphis pomi . 
As pointed out by Livingston, 6 the rates of evaporation from different 
types of evaporimeters under any given complex of atmospheric condi¬ 
tions are not comparable. It is, therefore, not surprising that evapora¬ 
tion from an instrument as used in these experiments would not give an 
accurate index of the effects of the atmospheric conditions upon aphis 
metabolism. It is possible that an evaporimeter more closely simulating 
the conditions of the aphid body might give a closer correlation between 
atmospheric evaporation and insect metabolism. 
• Livingston, Burton Edward, op. cit. 
