14 
Psyche 
[March 
United States also occur along the coastal plain of eastern 
Mexico, this region has served as a pathway for northern 
migration of some members of this genus. It is possible 
that apache might have come north by this route and 
reached southern Arizona by turning west up the Rio 
Grande Valley. But if this has been the case then the 
climatic conditions along the eastern coastal plain in Mexi- 
co must have been different from what they are now or 
apache must have acquired its low tolerance for annual 
rainfall after it turned west from the coastal plain. In 
either case it is difficult to see why apache should have 
stopped its migration along the coastal plain at the Rio 
Grande River. For, on either count, areas north of the Rio 
Grande along the Gulf Coast should have been available to 
it. It can be argued that subsequent climatic change elim- 
inated apache from the Gulf Coast except for the narrow 
strip of arid territory in the Rio Grande Valley area. The 
writer finds it difficult to believe that there would not be 
some traces of apache left in other parts of the Gulf Coast 
region under such circumstances. A much more acceptable 
explanation of the present range of apache can be made 
if it is assumed that the insect migrated north along the 
Mexican Plateau. The traces of this northern progress are 
present along the western edge of the Plateau, for the de- 
creasing incidence of apache south through Chihuahua 
may be regarded in this light. There is additional evidence 
from the responses of apache that it has had a long and 
extensive acquaintance with conditions on the Plateau. It 
has been shown elsewhere (1) that apache customarily 
nests in sizeable limbs or the trunk of the tree, and that 
it rarely, if ever, nests in hollow twigs as do many species 
of Pseudomyrmex. It has also been shown (5) that larger 
limbs, particularly those stubs which point upward, ac- 
cumulate much moisture after a rain fall. It seems clear 
that apache has lived under arid conditions long enough 
to have restricted its nesting habits to the parts of the tree 
which provide the maximum conservation of moisture. 
Coupled with this is the large tolerance of apache for vari- 
ous sorts of trees as nest sites. To date this ant has been 
taken from six species of evergreen oak, two species of 
deciduous oak and mesquite. It is interesting to note that 
