Fig. 2. (A) Results from a control experiment with the channel removed. Walking trajectories of 49 ants (one run per ant). Left: box-plot representation of the lateral positions of the ants’ trajectories at a distance of 3.5 m north of the point of release, i.e. when the ants are half-way through the channel (compare with Fig. 1). The vertical blue line inside the box depicts the median of the ants’ positions. The left and right boundaries of the box indicate the quartiles, which demarcate the range including 25 % of the data to the left and right of the median; the horizontal bars indicate the 75 % range of the data. Red line, direct homeward course (R–N* in Fig. 1). Abscissa, lateral deviation from direct homeward course (in m). (B) Both sides of the channel are provided with vertically oriented black-and-white square-wave gratings (period 
=22.3° and height h=14.9° as seen from the centre of the channel). N=59. For conventions, see A.
