EFFECT OF PHYSICAL FACTORS IN ECOLOGY OF INSECTS IN LOGS 35 
character of the log. They occur rarely on the sides of logs and almost 
never in heavy shade. 
The insects of Group II are more generally distributed. In full 
sun they occur most commonly on the side of the logs, whereas in three 
quarters shade they find optimum conditions on the top surfaces. They 
are not usually abundant on the lower surfaces. 
The insects of Group III are confined for the most part to the 
lower side of the logs where moist conditions maintain. Few of them 
are injurious. 
The results so far obtained seem to indicate that logs in heavy 
shade are less subject to insect injury than logs in more exposed 
situations. This appears to be due primarily to the reduced rate of 
development under cool, shady conditions. 
Effect of Relative Humidity and the Evaporating Power of the Air 
When this problem was undertaken it was thought probable that 
relative humidity and the evaporating power of the air would be found 
to have a marked effect upon the activity of insects in logs. For this _ 
reason careful observations of these factors were made using the 
instruments already described, 1.e., the Livingston porous cup atmom- 
eter and the cog psychrometer. The daily records of these factors 
were plotted in graphic form on profile paper together with the daily 
records of light, air temperatures, and subcortical temperature of typical 
logs. The graphs, however, failed to prove any definite effect of 
either the evaporating power of the air or relative humidity. Altho 
from this experiment no conclusions can be drawn as to the effect of 
relative humidity and evaporating power of the air, it still appears 
possible that both these factors have a decided influence upon the 
activity of xylophagous insects. 
Conclusions 
These experiments have shown that the activities of insects work- 
ing in logs are greatly influenced by the environmental conditions to 
which they are exposed. These conditions are in many cases controlled 
or modified by the action of external physical factors such as light, 
heat, and moisture. The effect of these factors upon the conditions 
within the log to which the insects are exposed varies according to 
the characteristics of the species and the characteristics of the indi- 
vidual log within the species. i 
The food value of logs is determined primarily by the chemical 
composition, which is in turn dependent upon the species of tree and 
the part of the log under consideration. The cambium and phloem 
