Figure 1. Schematic of the two-surface test illustrating the geometric details for deriving the path prediction model.

The ants crossed two kinds of surfaces to gather food. The dotted yellow line indicates the trail expected according to Fermat’s principle. With decreasing walking speed ratio (v ₁/v ₂) point D (associated with the predicted trail angle ρ) will move to the left. If v ₁< v ₂, ρ will be larger than α (the crossing angle of the shortest line between food and nest).