1956] 
Creighton — Ephebomyrmex 
59 
biculus exceeds that of pima both east to west and north 
to south. The vertical range of imberbiculus is almost 
twice as great as that of pima , for although imberbiculus 
does not descend to sea level, as pima does, it is capable 
of reaching levels above 6000 feet, while pima rarely 
reaches the 3000 foot level. The more restricted range 
of pima makes it easy to place that species in the Sonoran 
Desert biotic association as defined by Shreve (5) but no 
such single association is possible in the case of imber- 
biculus. Most of the middle of the range of imberbiculus 
lies in what Shreve calls the Chihuahuan Desert, a biotic 
association found on the Mexican plateau and adjacent 
portions of west Texas, New Mexico and Arizona. But 
it is scarcely possible to consider the stations in central 
Texas and Oklahoma in this category and those in western 
Arizona, Sonora and California are clearly in the Sonoran 
Desert association. It thus appears that the distribution 
of imberbiculus spans at least three different biotic areas. 
It is interesting to contrast the distribution of imber- 
biculus and pima with that of Novomessor cocker elli and 
albisetosus, since the latter two xerophiles occur in many 
of the stations where Ephebomyrmex is present. The 
writer has attempted to show (6) that the distribution 
of our two species of Novomessor is largely determined 
by their response to elevation. There can be no doubt 
that elevation is also a highly important factor in the 
case of imberbiculus and pima. The response of imber- 
biculus to elevation is very similar to that of N. cockerelli 
hence it is not surprising to find that the two species 
occur together over a very large area from western 
Texas to western Arizona and south along the Mexican 
plateau as far as Durango. With certain restrictions this 
range is true of N. albisetosus also. But pima behaves in 
an entirely different fashion. Its distribution is limited 
to Arizona and Sonora and in those states it occurs only 
in stations of low to moderate elevation. It is hard to 
escape the conclusion that this behavior is a result of the 
different elevational range possessed by pima. Since the 
upper limit of this range appears to be in the neighbor- 
hood of 3000 feet it follows that pima would, on this 
