1970] 
Holldobler Wilson — Recruitment Trails 
389 
home by individual ants, some of which extruded their stings and 
touched them to the surface (Fig. 5). Simultaneously, another group 
of ants, mostly belonging to the smaller caste, ran with extruded 
stings back and forth between the nest and the prey, apparently re- 
inforcing the recruitment trail. A freshly killed cockroach usually 
caused a much stronger recruitment alarm than oat flakes. More- 
over, when all of the oat flakes were offered at one spot, the recruit- 
ment activity was more intense than if the same number of flakes 
were scattered over the whole arena. Fig. 6 gives the quantitative 
data from representative experiments in which ants were placed in 
the three feeding circumstances. 
Our observations on the recruitment behavior strongly suggest 
that chemical signals are involved. The next series of experiments 
tested this hypothesis: 
1 ) In one series the ants were counted as they passed over the 
bridge after a recruiting ant had returned on it from the arena to 
the nest. In a second series the old bridge was replaced with a new 
one immediately after the recruiting ant had passed over it, and 
again the number of ants running to the arena was recorded. The 
number of ants passing over the bridge in the second series proved 
to be far smaller than in the first series (Table 1). 
Number of ants that passed 
Number of ants that passed 
over the bridge after a 
over a new bridge, which 
recruiting ant returned to 
replaced the old one after 
the nest. 
a recruiting ant returned to 
the nest. 
M ± sd 
17.8 ± 5.3 
1.7 ± 1.9 
Range 
12 — 28 
0 — 5 
Table 1. During a 5 minute period all ants that passed over the bridge 
to the arena were counted (see Fig. 1). The data are derived from 8 
replications of each of the two experiments. 
2) If the arena was rotated after the recruiting ant had left the 
arena, thus removing the trail from the arena entrance, or if the 
surface was covered with a new layer of sand, the recruited ants no 
longer oriented accurately to the source of food. 
The results of these experiments left little doubt that P. badius 
utilizes special chemical recruitment and orientation signals. We next 
conducted a search for the anatomical source of the recruitment 
pheromone. 
The source of the recruitment pheromone 
In preliminary experiments we learned that P. badius workers 
