30 F^IGHTEENTH RePORT StATE EnTOMOLOGIST OF MINNESOTA 1920 



light conditions, varying from one-quarter to full sun, thus making 

 possible a comparison between the different species under varying con- 

 ditions. The shades were constructed of lath so spaced as to cut down 

 the direct solar radiation three c[uarters, one half, and one third, re- 

 spectively. 



All of these logs were lying north and south with the north end 

 slightly raised. Temperatures were taken three times a day on the 

 upper and lower sides of certain logs under each light condition. 



In addition, the progression of temperatures around logs of the 

 different species lying in different directions was observed, using six 

 thermometers to a log. The details of this experiment will be discussed 

 later. 



Extremes of Temper.\ture Observed Under the Bark of Logs 



The primary usefulness of the bark on a tree is apparently to 

 check evaporation from the growing tissues. In addition to this func- 

 tion it has generally been conceded that the bark, being a fairly good 

 non-conductor of heat, also serves the purpose of protecting the tissues 

 beneath from extremes of temperature. This is undoubtedly true in 

 certain cases, for example, with thick scaly bark, but the temperatures 

 observed under moderately thin bark in the course of the present work 

 force us to modify this conception decidedly. In fact, in most cases 

 the heat of the sun was absorbed so rapidly Ijy the bark that the sub- 

 cortical region showed temperatures so high as to be almost unbeliev- 

 able. For example, on July 3, which was a very bright day, the tem- 

 perature beneath the bark on the upper side of a white pine log was 

 60 degrees C. at 1 p. m., while air temperature was 40 degrees C. This 

 is of course far above the fatal temperature known for any insect. 

 The bark of this log was 5 mm. thick with a rough dark surface. 

 Several times during the summer the difference between the air and 

 the subcortical temperatures of white pine exceeded 20 degrees C, 

 while a difference of 15 to 20 degrees C. was a common 

 occurrence. So great a difference was not true of all species, as will 

 be shown later. 



Short exposures to a temperature of 48 degrees C". are fatal to most 

 insects. Experimental evidence indicates that a somewhat lower tem- 

 perature is fatal to certain bark beetles, while Chrysobothris is able to 

 endure temperatures slightly above 50 degrees C. for a short time. Thus 

 the temperature fatal to insects varies considerably with the species. 

 During June there was a considerable amount of cool cloudy weather. 



