August, '08] JOURNAL OF ECONOMIC ENTOMOLOGY 253 



annual-minimum and maximum annual-minimum isotherms been 

 drawn from records from all the voluntary observers, the dip in 

 southern New York would have extended farther south and many 

 isolated spots in the Alleghanies would stand out with lower tem- 

 peratures than here indicated. 



Upon comparing these isotherms with the boundaries of the life 

 zones (see map 6), charted by Dr. C. Hart Merriam, many similarities 

 become apparent, but also a number of important differences, and 

 upon comparing the distribution of several well known injurious in- 

 sects with the average annual-minimum isotherms, they were found 

 to define the northern limits in some instances rather better than 

 the life zones of Doctor ]\Ierriam. 



The basis for the establishment of these zones has been stated by 

 Doctor Merriam as follows ■} 



''Investigations conducted by the Biological Survey have shown that 

 the northward distrihution of terrestrial animals and plants is gov- 

 erned hy the sum of the positive (or 'effective temperatures,' i. e., 

 over 43° F. — B. D. S.) temperatures for the entire season of growth 

 and reproduction, and that the southtvard distribution is governed hy 

 the mean temperature of a brief period during the hottest part of 

 the year." 



Isotherms plotted by Doctor IMerriam on this basis were found 

 "to conform in the most gratifying manner with the northern boun- 

 daries of the several life zones." In Bulletin 10 (1. e.) the "govern- 

 ing temperatures" of the zones are given and the maps previously 

 published were slightly modified in agreement with this hypothesis, 

 which map does not seem to have been revised in any subsequent pub- 

 lication. 



The distribution of many common insects, some of which will be 

 noted below, shows that there are numerous exceptions to the first 

 part of this law, and leads us to question its validity as regards north- 

 ward distribution. Is the sum of the positive temperatures for the 

 season of growth and reproduction, the only or most important fac- 

 tor governing distribution northward? At least three fundamental 

 objections to this law being of first importance will be illustrated by 

 the examples below. 



First. Many insects which have two or three generations at 35° 

 to 40° N. Lat. might readily reproduce in southern New Hampshire 

 (about 43° N. Lat.) were their existence merely dependent upon a 

 sufficient summation of temperature over 43°F. (6°C.) which do not 



iBuIletin 10, Division of Biological Survey, U. S. Dept. Agr. (1898), p. 54. 



