1960] 
Creighton — Pheidole militicida 
9 
The ants selected as intruders were Pogonomyrmex maricopa Wh. 
and Pogonomyrmex calif ornicus Buck. This choice was made because 
both species occur in close proximity to militicida colonies in the 
field and the harvesting activities of all three species lead to frequent 
encounters outside the nests. The results secured from the experi- 
mental nest described above were highly interesting. On the removal 
of the blocking cotton plug both groups would begin to explore the 
communicating passage. They did so in an entirely different fashion. 
The Pogonomyrmex workers moved slowly into the passage but 
rapidly backed out of it when they became aware of the advancing 
militicida workers. In most cases the militicida minors first entered 
the passage. Some of them would usually be seized and killed by 
the Pogonomyrmex workers but others returned to the nest and 
alerted the majors. When these entered the passage they showed 
precisely the reactions that they had exhibited around their nest 
entrances. They advanced very cautiously, with the jaws wide open, 
and made frequent short lunges in the direction of the Pogonomyrmex 
Fig. 1 — Major of Pheidole militicida in the defensive posture. 
workers. As the militicida majors wedged themselves tightly into 
the passage, three or four ranks deep, the passage was completely 
blocked and the front face of this block was a highly dangerous area 
for the Pogonomyrmex workers for it consisted of the closely ap- 
proximated heads and wide open jaws of the militicida majors. As 
to what happened next depended on the Pogonomyrmex workers, 
who would charge up to the barrier and slash at the militicida majors 
with their mandibles. These attacks were usually futile, for the only 
exposed parts of the militicida major which could be damaged were 
