1979] Chadab — Warning Cues for Social Wasps 121 
Another indication that the wasps discriminate army-ant odor 
with some specificity is the use by the wasps of primarily one alarm 
behavior in response to army ants. P. exigua possesses a repertoire 
of alarm responses: for example, disturbances which simulate a 
vertebrate predator such as my gently shaking the nest or my 
approach to the nest caused the wasps to retreat to the space 
between the upper surface of the envelope and the leaf; a sudden rap 
on the nest leaf caused the wasps to rush out onto the envelope 
flipping open their wings; while arthropods flying or walking next to 
the nest evoked buzzing and fanning by a single wasp and in some 
instances group fanning. Apparently, the wasps distinguish among 
types of disturbances and respond appropriately. This specific 
recognition of predator is similar to the “enemy specification” of the 
ant Pheidole dentata (Wilson, 1975). 
Detection of army ants by sight and odor may be adaptive to the 
wasps for the following reasons. Once army ants discover a nest the 
wasps are unable to fend off the attack. The wasp brood cannot be 
rescued, but the adult population can flee and recolonize (Naumann 
1975, Chadab 1979). Speed in evacuating the nest is crucial since the 
ants attack suddenly and in great numbers (Chadab and Retten- 
meyer 1975). Using the sight or smell of army ants as a warning cue 
aids the wasps in preparation for a rapid evacuation: alarm is spread 
through the colony, the wasps run out of the tiny entrance in single 
file and spread out on the outside of the envelope. Flight occurs only 
after contact with the army ants, but the threshold for flight is 
reached with only one to several army ants. The wasps are able to fly 
instantly in almost a single wave. In most raids, nonetheless, several 
adults are seized by the ants because they are trapped inside the nest, 
become embattled with the first invading ants, or are sluggish ten- 
eral adults.. Without an advance warning many more wasps would 
probably be trapped inside, and the threshold for flight might 
depend on considerably more direct contact with the ants. The 
result would be higher mortality of adult wasps. 
The adaptive ability of P. exigua to detect Eciton odor probably 
resulted from selective pressure exerted by army ants. Since army 
ants prey upon numerous species and prey colonies are usually 
cropped rather than destroyed, the effect of army ants on any one 
species might be considered weak or negligible. However, response 
of P. exigua to army ant predation is evidence that army ants have 
had a tangible effect on at least that prey species. 
