Coming home: how visually navigating ants (Myrmecia spp.) pinpoint their nest.

Visually navigating Myrmecia foragers approach their nest from distances up to 25 m along well-directed paths, even from locations they have never been before ( Narendra et al., 2013). However, close to the nest, they often spend some time pinpointing the nest entrance, sometimes missing it by centimetres. Here, I investigated what guides homing ants in their attempt to pinpoint the nest entrance. As the ants approach the nest, their behaviour changes. At approximately 1 m from the nest, the ants slow down, their scanning amplitude becomes larger and their path direction changes more frequently. This change in scanning behaviour is not triggered by local olfactory, tactile or visual cues because ants tethered on a trackball 30-50 cm above ground also exhibit it at 0.6 m compared with 1.6 m distance from the nest. Moreover, the ants are able to pinpoint the nest when such local cues are removed by covering the ground around the nest or the nest entrance itself. Myrmecia ants thus rely on information from the global panorama when pinpointing the nest. During learning walks, these ants appear to systematically collect views directed toward and away from the nest ( Jayatilaka et al., 2018). Homing ants indeed change gaze and body axis direction appropriately with a delay when encountering views to the left or to the right of the nest. However, image analysis shows that close to the nest, opponent views with the same orientation become too similar, explaining the growing uncertainty reflected in the ants' increased scanning behaviour during homing.