  Motivated by recent experimental work of Burd et al., we propose a model of
bi-directional ant-traffic on pre-existing ant-trails. It captures in a simple
way some of the generic collective features of movements of real ants on a
trail. Analyzing this model, we demonstrate that there are crucial qualitative
differences between vehicular- and ant-traffics. In particular, we predict some
unusual features of the flow rate that can be tested experimentally. As in the
uni-directional model a non-monotonic density-dependence of the average
velocity can be observed in certain parameter regimes. As a consequence of the
interaction between oppositely moving ants the flow rate can become
approximately constant over some density interval.
