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The visual centring response in desert ants, Cataglyphis fortis

Daniel Heusser and Rüdiger Wehner^*

Department of Zoology, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland

View larger version (21K): [in a new window]   Fig. 1. Experimental arrangement: the training (left) and test (right) situations. Ant foragers that have left the nest (N) and have walked straight to an artificial feeder (F) are captured at F and displaced to the test area. Upon release at R, they immediately head off in their home direction. During their homebound runs, they have to pass through a 4 m long and 1.5 m wide experimental channel. Having arrived at the fictive position of the nest (N*), they are returned to N. The visual patterns on the inner sides of the walls forming the channel and/or the heights of the walls can be varied independently on the left and right sides of the channel. At the entrance to the channel, a funnel formed by two narrow plates serves to guide the ants into the channel.

View larger version (19K): [in a new window]   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.

View larger version (11K): [in a new window]   Fig. 3. (A) Results from experiments in which the gratings presented on the left and the right walls are stationary, but the spatial period of the right-hand grating (=42.9°) is twice that of the left-hand grating. N=26. (B) The left-hand grating is moved at a speed of 0.1 m s–1 in the same direction as the ant’s direction of motion (left) or at a speed of 0.2 m s–1 in the direction opposite to the ant’s direction of motion (right). The right-hand grating has the same spatial period (=22.3°) but is stationary. Open arrow, direction of pattern motion. Red line, midline of channel. Green line, position at which the speeds of image motion experienced by the ant’s two eyes are balanced. N=30. For further conventions, see Fig. 2A.

View larger version (11K): [in a new window]   Fig. 4. (A) Results from experiments in which both walls are uniformly black and of equal height (h=0.2 m corresponding to 14.9° as seen from the midline of the channel). N=48. (B) Either the left-hand or the right-hand wall (left or right figure, respectively) is increased to double the linear height (h=0.4 m corresponding to 28.1° as seen from the midline of the channel). Orange line, position at which the heights of the left and the right walls appeared to subtend equal visual angles. N=22 (left figure) and N=21 (right figure). For further conventions, see Fig. 2A.

View larger version (12K): [in a new window]   Fig. 5. (A) Same as Fig. 2B (shown for comparison with B). (B) Results from experiments in which the left-hand and the right-hand walls carry stationary black-and-white square-wave gratings of equal spatial period (=14.9°), but one of the walls (the left-hand wall in the left figure and the right-hand wall in the right figure) is higher (h=0.19 m) than the other (h=0.11 m). Red line, midline of channel; orange line, position at which the heights of the left and the right walls appear to subtend equal visual angles (compare Fig. 4B). N=23 (left figure) and N=34 (right figure). For further conventions, see Fig. 2A.

View larger version (39K): [in a new window]   Fig. 6. (A) The natural alley formed by plants that had grown in the furrows used in the previous year to mount the walls of the channel. (B) The trajectories of ants passing through the natural alley. In the box-plot representation, the orange line marks the position at which the left and the right rows of plants appeared to subtend equal visual angles. Since the number (29 in the left row and 23 in the right row) and the linear heights (0.13±0.06 m in the left row and 0.18±0.07 m in the right row; means ± S.D.) of the plants varied between the left and the right sides of the alley, the orange line represents only an approximate mean value. Nevertheless, the ants’ trajectories are shifted towards it (see text). Red line, midline of alley. N=19.

View larger version (48K): [in a new window]   Fig. 7. (A) Walking paths of ants trained to walk from an artificial feeder to the nest, then captured at the feeder and displaced to the position marked by the open square. On their homeward runs towards the fictive position of the nest, they were presented with two black cylinders (0.40 m high, 0.25 m wide and 2.0 m apart from each other; see filled red circles). N=15 (left figure) and 16 (right figure). Recordings taken by B. Michel. (B) Walking paths of ants trained to return to a place (their nesting site) surrounded by three black cylinders (0.40 m high and 0.25 m wide, see filled red circles) positioned at the corners of an equilateral triangle and 2.0 m apart from the entrance of the nest. After training, the three-cylinder landmark array was established within a distant test area where 45 trajectories of 3 min search paths of 17 ants were recorded. While concentrating their search about the fictive position of the nest (the centre of the landmark array), the ants avoided the immediate neighbourhood of the landmarks. The three blue dots mark the positions at which the ants were released. Recordings taken by P. Antonsen.