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Psyche 
[Vol. 88 
this is the panic-alarm defense employed by these species against the 
army ants. 
That defense behavior in ants can be both enemy specific and 
complex was established with the discovery of the alarm-recruit- 
ment defense of Pheidole dentata against the fire ant Solenopsis 
geminata (Wilson 1975 and 1976). Although more evidence is 
necessary, the defense behavior of P. hyatti and P. desertorum 
appears to be both enemy specific and complex. The defense, which 
begins when a Pheidole forager contacts an army ant and runs back 
into the nest raising an alarm, occurs in two phases. In the first, or 
“milling”, phase, workers carrying brood well out of the nest but 
remain in close contact near the nest’s entrance. In the second, or 
absconding, phase, the workers flee from the nest. P. desertorum's 
flight is protean in nature (Humphries and Driver 1970) with 
workers scattering in all directions. In P. hyatti the exodus is more 
organized with the workers fleeing in columns which appear to 
follow recently-laid chemical trails. 
After evacuating from their nest the fleeing workers tend to 
concentrate at temporary shelters such as that provided by leaf 
litter, fallen branches, rotting logs and tufts of grass. Some workers 
eventually find some or all of the former nests and begin to recruit 
other workers to them. After the raid is over workers will also start 
to return to the evacuated nest. In this manner the colony becomes 
fragmented with various proportions of the colony in some or all of 
the available nests. The colony then begins the process of reorgan- 
izing with segments in one nest emigrating to join segments in 
another nest until the colony becomes reunited in one nest. Hence it 
appears that the surplus nests resulting from the frequent emigra- 
tions of these species serve a dual purpose after an evacuation: they 
provide shelter and centers for reorganization. 
After a nest evacuation, finding a place of suitable moisture 
before the lethal surface temperatures and low surface humidity of 
the approaching day is undoubtedly of vital importance for these 
nocturnal species. This problem becomes particularly severe in the 
desert-grassland where the lack of ground cover makes nests 
excavated in the ground the only suitable shelters. Having alternate 
nests becomes a necessity when an army ant colony bivouacs in the 
evacuated nest. The hypothesis we are proposing then is that the 
surplus nests which result from the emigrations of these species 
increases the effectiveness of the panic-alarm defense by reducing 
