1928] Effect of Extreme Temperature on Criocerus 231 
The average annual minimum isotherm of —10° bisects 
Iowa, crosses northern Illinois, cuts thru Michigan and includes 
the region of Ontario influenced by the lakes, cuts thru New York 
state and the center of New England. Allowing for seasonal de- 
viations of warm winters, etc., the beetle has hovered along this 
invisible barrier during its westward spread. 
