12 
Psijche 
[March 
ing the dominant oak involved. In Arizona this is Q. 
emoryi, in Chihuahua it is Q. santaclarensis. Both these 
oak associations appear to reach their maximum develop- 
ment in areas where the average minimum annual temper- 
ature is not less than 15 °F. or more than 20 °F. In such 
areas light winter snows are not uncommon and minimum 
January temperatures as low as -6°F. have been recorded 
at several weather stations (4). The ayerage annual rain- 
fall in such areas is from 15 to 20 inches. Of this total 
more than half falls during the period from the first of 
July to the middle of September. Spring rains are excep- 
tionally light, seldom comprising more than 10% of the 
total annual rainfall. Over most of the year there is a 
difference of at least 40°F. between the daily minimum and 
maximum temperatures. The humidity is low and the 
evaporation rate very high, since the area has an unusually 
large percentage of cloudless days. 
Fig. 1. Map sliowing the known distribution of Pseudomyrmex apache 
in tlie southwestern United States and northern Mexico. 
It seems clear from the above data that apache can tol- 
erate lower temperatures than most of the other species of 
Pseudomyrmex which occur in the southern United States. 
