1962] 
Wilson — Erebomyrma 
7i 
her mate (or mates), is genetically the equivalent of two or several 
individuals, the female and sexual consorts in a normal population of 
non-social animals. Hence, what may be referred to as the “equivalent 
size” of the reproductive population, N d , in an idealized deme of 
social insects outside the nuptial season can be estimated as 
n a = -~~ (Q + m Q) 
where N t is the total adult population of the deme, including workers, 
m is the average number of males that fecundated the queens assuming 
approximately equal sperm contributions, Q is the average number of 
mother queens assuming approximately equal egg contributions, and 
N c is the average mature colony size. Thus in a cave deme containing 
10,000 adult individuals with an average mature colony size of 1000 
and single fertilizations of single queens, the reproductive population 
size equivalent would be only 20. We may note that a deme of 10,000 
adults, the non-social condition, would perhaps be large enough and 
contain enough genetic variability to be stable and self-maintaining. 
Demes of this size are quite common in nature. But a deme-equivalent 
of 20 adults, the social condition, must be below the critical threshold 
or dangerously close. 
In short, if species of social insects never become truly troglobitic 
it may be simply because they are unable to maintain sufficiently large 
cave demes. Due to limited habitable space, the Oropouche Cave could 
not have contained more than) 20' colonies of Erebomyrma urichi and 
probably held considerably fewer. Erebomyrma colonies are sometimes 
polygynous, as we know from the single Oropouche example. If ten 
queens per colony is accepted as the upper limit, and each were assumed 
to be singly fertilized, the reproductive-population equivalent of the 
Oropouche Erebomyrma is calculated not to exceed 400. Since the 
excavated colony was the only obviously populous colony found, the 
actual equivalent was probably far less. 
But let us suppose that social insects do occasionally become troglo- 
bitic; what characteristics might they be expected to have? In addi- 
tion to the well-known morphological changes common to most 
troglophilic animals, there are several features of social structure that 
might be affected. There would likely be one or more of three devices 
to increase N d ; namely, (1) decreased colony size, (2) increased poly- 
gyny, and (3) increase in m (polygamy). The Oropouche colony of 
Erebomyrma urichi was markedly polygynous, with the added result 
that the estimated N d of the colony was large (20). In fact, urichi, 
although not a troglobite, may be pre-adapted for marginal cave exis- 
