1969] 
Topoff — Army Ant 
379 
oratory experiments by Plsek et al. (1969), showed that individ- 
uals of a related species, H. texanus (Le Conte) can follow chemical 
trails deposited by workers of N. nigrescens. In the field,, army ant 
trails are reinforced by thousands of ants running to and from their 
bivouac during the course of each night’s raiding. This results in 
the deposition of a very strong and relatively non-volatile chemical 
trail. If a beetle accidentally crossed a trail used by army ants during 
the previous night, it could run along the trail until it encountered a 
colony of 2 V. nigrescens. 
After locating a colony of N. nigrescens , beetles may forage in the 
ant columns by responding to combinations of visual, chemical, and 
tactual stimuli. Both species have large protruding compound eyes, 
which could enable them to orient visually at night. Our field ob- 
servations indicate that tactual orientation is important in the beetles’ 
foraging pattern, and is accomplished by their continuous responses 
to physical encounters as they “plow” through ants running to and 
from their bivouac. 
Our field and laboratory observations of contacts between beetles 
and army ants, has also explicated the role of tactual stimulation in 
army ant group raiding and emigrations. When individuals of both 
species of beetles ran through raiding columns, foraging and booty- 
laden ants returning to their nest were repeatedly pushed aside, or 
knocked off their feet. Surprisingly, these ants were only momen- 
tarily interrupted by the activities of the beetles ; they quickly resumed 
their positions on the trail, without appreciably disrupting other ants 
in the column. 
In order to test the effect of mechanical stimulation on workers of 
N. nigrescens in the field, tactual interactions between beetles and 
ants were simulated by running the tip of a flexible wire probe through 
a column of raiding ants. Ants displaced by the moving probe were 
only momentarily aroused, and the column remained intact. These 
reactions to mechanical stimulation were then contrasted with the 
ants’ behavior when subjected to stimuli not normally present in their 
nocturnal environment. Tests showed that even low intensity stimu- 
lation by light (from a headlamp) or by air currents (created by 
blowing) causes the running ants to disperse from the column. 
The lack of significant arousal of N. nigrescens workers by beetles 
foraging in their columns, may be explained, in part, by consider- 
ing the stimulus-environment characteristic of army ant colonies. 
Throughout their development and adult life, army ant workers live 
in an environment in which they are continuously stimulated by 
