1954] 
Creighton — Pseudomyrmex apache 
13 
For the range of Ps. elongata is confined to areas where the 
average annual minimum temperature is 30°F. or more 
(southern Florida). That of Ps. gracilis mexicanum is 
limited to areas where the above temperature is 25°F. or 
more (southern Texas). Ps. brunnea is restricted to areas 
where the average annual minimum temperature does not 
go below 20°F., hence, while it occurs as far north as the 
Carolinas and thence south through the Gulf States into 
Mexico, it does not occur in southern Arizona. Only Ps. 
pallida, whose tolerance for low temperatures is equal to 
that of apache, does so, and pallida occurs through the Gulf 
States and north to the Carolinas. 
The question immediately arises as to why, since apache 
can tolerate low temperatures as well as pallida, does not 
apache occupy the same area as the latter species. The 
restricting factor in this case appears to be rainfall rather 
than temperature. Although apache can live in areas where 
the average annual rainfall is as low as ten inches, it has 
never been taken in an area where the annual average rain- 
fall is more than twenty-four inches. Beginning in west 
Texas (10 inches) and running east to Alabama (65 
inches) there is a gradient of rainfall which increases to 
the east. It is interesting to note that the line which 
marks the area where the average annual rainfall passes 
25 inches lies only a few miles north and east of the known 
eastern limit of the range of apache. It is further inter- 
esting to note that there is a comparable gradient of rain- 
fall which increases southward from the Rio Grande Val- 
ley down the coastal plain of Mexico. This gradient begins 
with 25 inches in the Brownsville area, rises to 45 inches 
at Tampico and reaches 64 inches at Vera Cruz. As has 
been noted elsewhere, apache appears to be absent in this 
area. This might be expected if the ant is unable to toler- 
ate an average annual rainfall in excess of 24 inches. 
If, as seems to be the case, it is rainfall rather than 
temperature which plays the major part in determining 
the range of apache, some interesting speculations can be 
advanced as to how apache reached its present geographical 
position. It may be taken as axiomatic that apache came to 
southern Arizona from tropical sources. Since three of 
the species of Pseudomyrmex which occur in the southern 
