EFFECT OF PHYSICAL FACTORS IN ECOLOGY OF INSECTS IN LOGS 25 
status of those problems dealing with the effect of the external factors 
upon insect activity, and it will therefore be necessary from time to 
time to refer to other works not previously mentioned. 
Most of the data included in this thesis were collected during 
the summer of 1920 at the field station of the University of Minne- 
sota Forest School. The prosecution of the work has been materially 
aided by the laboratory and other facilities provided by the School 
of Forestry and the helpful suggestions and criticisms by members 
of the University staff, particularly Dr. R. N. Chapman, under whose 
direction the work was carried on. 
The study of insects in their natural environment calls for a 
careful study of the environmental factors as they exist in nature. 
Under controlled experimental conditions it is possible to study very 
accurately the influence of a single factor upon insect activity by keep- 
ing uniform all but one of a series of factors. By varying this factor 
its influence upon the insects under experimentation may be deter- 
mined. Such a study under natural conditions, however, involves the 
accurate measurement of individual factors or groups of factors such 
as light, temperature, humidity, and evaporating power of the air, and 
the condition of the nutritive medium. 
Experimental Method 
In this investigation five species of logs were used. These were 
White pine, Pinus strobus, Linn.; Norway pine, Pinus! resinosa Aiton; 
Jack pine, Pinus banksiana Lambe; Black spruce, Picea mariana Brit- 
ton Stearns & Poggenberg; and balsam fir, Abies balsamea Miller. 
The logs were cut into four-foot lengths and placed in a north and south 
direction with the north end slightly raised. Representative logs of 
each species were placed under lath shades so constructed as to cut 
off three-fourths, one-half, and one-third of the direct solar radiation. 
One set of logs was left in full sunlight. The methods employed in 
supplementary experiments will be described as the results are pre- 
sented. The principal factors considered in this work were light, 
temperature, evaporation, and relative humidity. 
The measurement of light is always attended with considerable 
difficulty. Really accurate methods of light measurement are too 
complex and difficult for general field use, so it was necessary to 
adopt some simple means of measuring this factor even at the expense 
of exactness. The various methods used for measuring light in terms 
of chemical activity were considered as a possibility, but finally rejected. 
The instrument finally adopted was based upon the principle of the 
black body and measured the light in terms of heat units. It consisted 
