View-based navigation in insects: how wood ants (Formica rufa L.) look at and are guided by extended landmarks

Bees, wasps and ants learn landmarks as views from particular vantage points, storing the retinal positions of landmark edges. By moving so as to minimise the difference between their stored and current view, they can return to the vantage point from which a view was taken. We have examined what wood ants learn about a laterally placed, extended landmark, a wall, while walking parallel to it to reach a feeder and how they use this stored information to guide their path. Manipulation of the height of the wall and the ant's starting distance from it reveals that ants maintain a desired distance from the wall by keeping the image of the top of the wall at a particular retinal elevation. Ants can thus employ image matching both for returning to a place and for following a fixed route. Unlike many flying insects, an ant's direction of motion while walking is always along its longitudinal body axis and, perhaps for this reason, it favours its frontal retina for viewing discrete landmarks. We find that ants also use their frontal retina for viewing a laterally placed wall. On a coarse scale, the ant's path along the wall is straight, but on a finer scale it is roughly sinusoidal, allowing the ant to scan the surrounding landscape with its frontal retina. The ant's side-to-side scanning means that the wall is viewed with its frontal retina for phases of the scanning cycle throughout its trajectory. Details of the scanning pattern depend on the scene. Ants scan further to the side that is empty of the wall than to the side containing the wall, and they scan further into the wall side when the wall is of a lower apparent height. We conclude that frontal retina is employed for image storage and for path control.