34 Eighteenth Report State F.ntomologist of Minnesota — 1920 



the density of the wood. l'\jr example, heat is conducted around a 

 l)alsani loii^ somewhat more rapichv than around a spruce log. The 

 slower this conduction, the sharper the line separating the areas of 

 high and low temperature. This point is graphically illustrated in 

 Plate II. This division line is often so sharp that there may he a dif- 

 ference of more than a degree, C, for each centimeter of circum- 

 ference along this line. It is perfectly possible for a good-sized Cer- 

 aml)\-cid or l!u])restid larva, Iving transverselv beneath the bark, to 

 have one part of its body in an area where favorable conditions of 

 temperature exist while another part may be exposed to extreme con- 

 ditions. Bark beetles such as Pifxoktciiics sparsiis Lee, which con- 

 struct transverse egg galleries, are able to avoid extreme temperatures 

 to some extent by moving from one part of the gallery to another. 

 The larvae are, of course, unable to change their positions quickly in 

 this way, and so must endure existing temperatures or perish. Thus 

 it is that we may find completed egg galleries in an area where larval 

 development is impossible. 



Effect of Solar Radiation 



The efifect of the intensity of solar radiation upon the sulx'or- 

 tical temperatures is very marked throughout the entire season. In 

 general, the stronger the sunlight the higher the subcortical tempera- 

 ture, altho this is somewhat modified by other factors. 



TABLE II* 

 Solar Altitudes for 48 Degrees North Latitude 



'Vva\\\ Kiiiil);ill. IIitImtI. II,. Moiillily A\ Ci t licr Iti'vicw, Vol. 47. No. 11. p 



