460 WHEELER. 



ture in California. The ants of the Transition and Boreal Zones re- 

 quire a considerable amount of humidity and warmth during their 

 breeding season. These conditions are not realized simultaneously in 

 California, where the rainy season comes during the winter and the 

 summer is rainless except in the high Sierras. The more perfect 

 adaptation of the species of the Sonoran zones to a smaller amount of 

 moisture and to winter temperatures not sufficiently low to inhibit 

 completely the activities of the worker ants, probably accounts for 

 the greater number of species and colonies at lower altitudes in 

 Southern California, where the conditions are much like those of 

 Arizona. Even moderately low temperatures, when coupled with 

 considerable humidity, a condition which prevails in California dur- 

 ing the winter months, is very imfavorable to ants, and when such 

 conditions are most accentuated, the ant-fauna is reduced to a mere 

 remnant, although the vegetation, if the temperature is not too low, 

 may be luxuriant. This is the case in New Zealand w^here I some- 

 times searched m vain for an ant-colony in forests whose luxuriance 

 rivalled those of the tropics. But we have a striking example of the 

 depressing effects of cold and moisture on ant-life much nearer home. 

 The cool Selkirk Mts. of British Columbia have an abundant supply 

 of moisture and an unusually rich flora, but their ant-fauna is reduced 

 to a few boreal species. The adjacent Canadian Rockies, however, 

 though in the same latitude, are less humid and have a poorer flora, 

 but their ant-fauna is decidedly richer in species and colonies. 



In mountain regions slope exposure in its relation to insolation is 

 a very important factor in the local distribution of ants, but it is 

 impossible at present to give more than a general statement in regard 

 to this matter. Northern slopes in the northern hemisphere are usu- 

 ally, for very obvious reasons, almost or quite destitute of ants. In 

 regard to the other slopes my observations in the Alps of Switzerland 

 and the mountains of the United States, British America, Mexico and 

 Central America confirm those of Forel in the Alps and the mountains 

 of North Carolina. He finds that ants prefer the eastern and southern 

 slopes as these are the situations in which they have the longest day 

 for their activities during the breeding season, since they are early 

 awakened by a sufficiently high temperature of the soil and air from 

 the lethargy induced by the chill night hours, and even though the 

 slope may be in shade during the afternoon the warmth is sufficient 

 to sustain their activities till sun-set. On western slopes, however, 

 the morning hours are too cool and are therefore practically lost to 

 the ants, whereas the afternoon hours are too warm. 



