EFFECT OF PHYSICAL FACTORS IN ECOLOGY OF INSECTS IN LOGS 23 
development of methods for forest insect control resolves itself into a 
study of the ecological relationship of insects in the forest envirnoment. 
The first single factor of the environment to attract the attention 
of entomologists was temperature. Several more or less successful 
attempts were made to correlate variations in the length of the develop- 
mental period and the geographic distribution of animals on the basis 
of this factor alone. Judeich and Nitsche (1895) report a rather 
unsuccessful attempt of Forester Uhlig-Therand to correlate the length 
of the life cycle of Ips typographus Linn. with temperature by compar- 
ing with the length of the insect’s life cycle, the total midday tempera- 
ture and the total mean daily temperature, as computed from three 
readings a day. Merriam (1808) established life zones on the basis 
of the sum of positive temperatures (above 43 degrees Fahrenheit for 
the entire season of growth and reproduction) which, he believed, 
limited the northward distribution of terrestrial animals. The southern 
limits of distribution he believed to be governed by the mean tem- 
perature during the hottest part of the year. Sanderson (1908) points 
out that low winter temperatures limit the northward distribution of 
many insects, since many species do not occur in every locality where 
the sum of positive temperatures is sufficient for their development. 
He also points out that the effective temperature may vary with differ- 
ent insects. Later, Sanderson (1910), in a paper treating of the effect 
of temperature on the growth of insects, recognizes the importance 
of humidity in influencing the rate of development. 
More recently Sanderson and Peairs (1913) published results 
showing that the variation in velocity of development at different con- 
stant temperatures within normal limits was, other things being equal, 
increased directly as the temperature, the curve for the increase being 
a true mathematical hyperbole. “The factor, or index of development, 
for any point on this curve is the reciprocal for the point. Thus the 
reciprocal, . . . takes by definition the form of a straight line; 
the inclination of this line to the axes of the curve being governed by 
the rapidity of development of the insect and stage.” In this’ way it 
is possible to compute the complete development curve by establishing 
any two points on the curve and plotting the reciprocals. 
The first extensive investigation of temperature and humidity as 
applied to forest insects was that of Hennings (1907-10), who pub- 
lished a series of articles on the biology of certain bark beetles. His 
results were based upon experiments under controlled conditions of 
air temperature and humidity. His work has materially contributed 
