1960] 
Brown — A rmy Ants 
27 
Labidus praedator (Fr. Smith) found in the Laboratory Clearing on 
Barro Colorado, and results were again similar to those of the first 
trials with E. hamatum, I could not be sure in this case that I 
detected any odor at all from the crushed soldier heads of L. praedator , 
but the intensity of attack on these heads indicated that an odor 
must have been readily detectable by the workers participating. 
In summary, the detached heads of workers or soldiers of these 
army ants from three different genera were attacked by their nest- 
mates when placed in the foraging column, while the remainder of 
the body evoked little or no response. Objects smeared with the 
substance from the crushed heads of soldiers drew intense attack, 
while untreated control objects did not. It seems likely that a phero- 
mone originating in the head, and most likely specifically in the 
mandibular glands, occurs in the army ants as well as in Pogonomyr- 
mex, and that its function is similar. Apparently, the ants will attack 
any strange object presented close to the center of concentration of 
the alarm odor. 
The question arises as to how the individuals that are disturbed 
into producing the alarm odor are not themselves attacked in the 
normal course of events. Two of the most likely answers that sug- 
gest themselves to me are: (1) workers and soldiers, when normally 
disturbed, give off much lesser amounts of the alarm odor than is 
produced when a head is severed from the body or crushed, and the 
attack on severed heads or crushed heads represents a supernormal 
stimulus; (2) the postcephalic part of the ant’s body bears some 
quality, probably again pheromonal in nature, that neutralizes attack 
behavior that might otherwise be released in nestmates. The second 
answer is favored as an hypothesis in the absence of further experi- 
ments. If a given soldier were protected by such a neutralizer or 
“identification pheromone,” or by a “nest odor,” it would presum- 
ably be able to indicate by means of alarm odor release the locality 
of an intruding disturbance without itself being subject to attack by 
its nestmates. 
I should like to acknowledge helpful suggestions made by Prof. 
E. O. Wilson during the course of the work and the preparation of 
the manuscript. 
References 
Schneirla, T. C. 1956. The army ants. In Report of the Smithsonian 
Institution for 1955, pp. 379-406 (with bibliography). 
Wilson, E.O. 1959. (1958). A chemical releaser of alarm and digging 
behavior in the ant Pogonomyrtnex badius Latreille). Psyche. 65: 41-51. 
