1906.] Wheeler, The Ants of the Grand Canon. 333 



beds is very meager and, so far as I have observed, represented only 

 by the following, of which the second is the most common and char- 

 acteristic : 



Pheidole maricopa, Dorymyrmex pyramicus, 



Pogonomyrmex rugosus, Iridomyrmex analis. 



4. The warm desert zone, extending from the Angel Plateau to 

 the river, comprises some half dozen species, of which the California 

 harvester {Pogonomyrmex californiciis) is the most abundant and 

 characteristic : 



Ponera opaciceps, Dorymyrmex pyramicus, 



Solenopsis aurea, • Prenolepis guatemalensis, 



Pogonom,yrmex calijornicits, Myrmecocystiis melliger. 



A comparison of these lists shows that only a single species, 

 Dorymyrmex pyramicus, ranges from the top to the bottom of the 

 canon. Of the other species only a few extend over more than one 

 of the zones. Thus the Kohonino and Indian Garden zones have in 

 common only the widely distributed Lasius americanus and Mono- 

 morium m-inim^um-. Iridom^yrmex analis is common to the Indian 

 Garden and Angel Plateau and Ponera opaciceps to the former and 

 the desert zone of the canon bottom. Excluding the humid zones 

 and patches represented by the Indian Garden, we may say that each 

 of the canon zones is characterized by a species of Pogonomyrmex, 

 the uppermost and coldest by P. occidentalis , the lowermost and warm- 

 est by P. californiciis, and the intermediate zone by a form of the 

 widely distributed P. harhatus. Each of these species occupies on 

 the canon walls a position which, with respect to altitude, moisture 

 and temperature, corresponds very closely with its distribution over 

 the vast stretches of country to the north and south of the canon. 



Whence came the species found in the different zones of the canon? 

 There can be no doubt that the colonies were founded by isolated 

 queens that had drifted into the canon while on their marriage flight. 

 Although these insects are certainly carried long distances by the wind 

 and must fall into the canon in great numbers during the late spring 

 and early summer months, it is equally certain that they can establish 

 their colonies only under the most favorable conditions of soil, tem- 

 perature and moisture. In this respect they resemble the wind-borne 

 seeds of plants. Queens falling on inhospitable ground must either 

 perish or run about till they find the proper sites for the establish- 

 ment of their colonies. This is indicated very clearly in the sharp 

 zonal demarcation of the various species on the walls of the canon. 



