1972] 
Topoffj Lawson & Richards — Eciton 
361 
would cause the callows to reverse their direction of movement and 
return to the stimulus-saturated interior of the nest. By the end of 
the day’s raid, and immediately preceding the start of an emigration, 
a large portion of the adult workers are out of the nest. Then, as 
the full-scale emigration progresses, the remaining adult population, 
together with the brood and queen, also leaves the nest. Thus, at 
some time between the beginning of an emigration and its peak, the 
relative concentration of chemical and tactual stimulation outside of 
the bivouac may be sufficient to attract the callows out of the nest. 
This may occur because the callows are particularly attracted to the 
chemical secretions and tactual stimuli that arise from one subpopu- 
lation of the entire colony (such as the adult workers, the brood, 
the queen, or even the booty) . Or, the stimulative effect on the 
callows may be quantitative — that is the callows may leave the nest 
when the magnitude of stimulation outside the nest increases suffi- 
ciently, regardless from which particular segment of the colony the 
stimuli arise. 
We have recently made an observation in the field that is con- 
sistent with the hypothesis that high concentrations of social stimu- 
lation arising inside the nest serve to keep callow army ants inside 
the nest during the first few days of the nomadic phase. During a 
recent study of the nocturnal army ant species Neivamyrmex nigre- 
scens (Cresson) in southeastern Arizona, we located a statary col- 
ony bivouacked beneath a rock in the bank of a creek. The statary 
raids were unusually weak, with only several dozen workers partici- 
pating. On subsequent nights, the raids never increased in strength. 
One night we were surprised to find several dozen newly eclosed 
callow ants running sluggishly over the entire raiding route (which 
extended up to 50 m from the nest). No emigration occurred that 
night. The next night the callows were again observed along with 
the adults on the raiding trails. No emigration occurred the second 
night either. We then proceeded to dig up the nest, and we found 
that the entire colony consisted of between 100 and 200 mature 
adults and approximately 50 callows. There was no large mature 
adult or callow population, no brood, and no queen. As far as I 
know, this is the first report of callow army ants participating in 
raiding immediately after their eclosion from the pupal stage, and 
it is significant that in this one case there was no colony to speak 
of, and hence no large source of social stimulation. 
In the army ants, as in social species representing many levels of 
invertebrate and vertebrate evolutionary history, immature individ- 
uals are not immediately integrated into the society of their group, 
