66 
Psyche 
[September 
North American Desert, is pertinent to the discussions which follow. 
The physical features, vegetation and flora of this desert have been 
well reviewed by Shreve (1951). The Sonoran Desert lies in a 
region of plains, hills and mountains, and extends in elevation from 
approximately 3500 feet in eastern Arizona and northern Sonora to 
sea level on the coasts of Sonora and Baja California, and to below 
sea level in the vicinity of the Salton Sea in southern California. The 
Colorado River is the only permanently flowing stream in the entire 
region. A fairly uniform, continental type of climate prevails, in 
which the temperature varies from some of the highest records for 
North America to occasionally heavy frosts in the northern and 
eastern sections. Annual rainfall generally increases in amount from 
west to east, with less than 5 inches falling in the lower Colorado 
Valley and much of Baja California to slightly more than 15 inches 
in eastern Arizona and Sonora. The seasonal distribution of rainfall 
also varies importantly, with the extreme western areas receiving 
virtually all of their moisture during the winter from December to 
March. Going eastward the pattern shifts so that the eastern borders 
of the desert receive their major rainfall during a well-defined 
summer season from July to September. 
In Arizona where more precise information is available, Ptero- 
termes has been found from approximately 4000 feet on the lower 
slopes of a few mountain ranges, across the bajadas and onto the 
adjacent valley floors. It is apparently restricted to the more open 
foothills and bajadas rather than to the recesses of deep canyons. 
Here it is found in the larger woody plants of the desert scrub or 
spinose desert plant communities (Figs. 2 and 3). It is hardly 
necessary to point out that nearly all the collections from Baja Cali- 
fornia have been made in the lower, less rugged areas which are 
more readily accessible, either by sea or by the few passable roads 
over the peninsula. 
Emerson (1955) has mentioned that temperature and moisture 
are the major physical factors limiting termite dispersal, largely by 
their effects in determining vegetation types. Pterotermes is a primi- 
tive, monotypic genus, apparently endemic to the hot, dry Sonoran 
Desert. Although almost pure speculation, it may be of interest to 
suggest that Pterotermes is so restricted not so much by high tempera- 
ture and low moisture as by the higher rainfall of the surrounding 
regions. Approximately 12 vigorous and healthy colonies have been 
personally examined in extremely desiccated wood. A single large 
colony was completely removed from a dead palo verde tree in 
February, 1966, after a series of unusually heavy rains. This 
