1969 ] 
Hangartner — Solenopsis 
65 
myrmecine ant Solenopsis saevissima , Wilson (1962b) demonstrated 
that carbon dioxide acts as a weak attractant that finally leads to 
settling. Because the same behavior could be observed during these 
studies on Solenopsis geminata, I consider the digging activity re- 
leased by C 0 2 as a by-product of attraction enabling the ants to get 
closer to the source of the stimulus. This interpretation is supported 
by the observation that if there is no hindrance between the workers 
and the source of C 0 2 , the ants are merely attracted to the place 
where the concentration is most convenient; no digging behavior can 
be observed in such a case. 
The results presented in this paper offer a possible explanation to 
all the observations of digging behavior toward trapped ants as cited 
in the introduction. This of course does not mean that the same 
mechanism works in all ant species. The finding that single individ- 
uals of Pogonomyrmex badius or even parts of a worker release dig- 
ging behavior (Spangler, 1968) indicates that this species is extremely 
sensitive to C 0 2 , or it could also be that other chemical stimuli are 
involved. The whole surface of the ant’s body as well as larvae 
and pupae could be contaminated by 4-methyl-3-heptanone, the com- 
pound identified as releaser for alarm and digging behavior (McGurk 
et al., 1966). 
Because C 0 2 -concentrations up to 1-2% were demonstrated in 
the interior of ant nests (Poitier and Duval, 1929; Raffy, 1929), 
this simple molecule could accomplish the following functions in a 
Solenopsis geminata colony: 
1. It acts as a pheromone to control settling inside the nest (Wil- 
son, 1962b). 
2. It diffuses through the nest entrance and serves as an orienta- 
tion aid, at least in the near vicinity of the nest. [Wilson (1962b) 
concluded that fire ant workers are able to move up C 0 2 gradients.] 
3. It acts as a “rescue”-pheromone in case groups of ants are 
trapped following a major cave-in. 
4. The C 0 2 produced by large piles of brood attracts the workers 
necessary to take care of the larvae and pupae. 
Although there may exist other or stronger stimuli which control 
these behavior patterns, C 0 2 at least plays an important supportive 
role. 
Summary 
Workers of Solenopsis geminata are attracted to low concentrations 
of C 0 2 and try to dig through all obstacles in order to get close 
enough to the source of this chemical stimulus. 
