Fig. 1.

Assay plate and acidic pH avoidance behavior. a Assay plate used in this study. Because of the pH-sensitive BPB dye added to the assay plate, the less acidic region became blue and the acidic region (approximately pH 4.0) turned yellow (see “Methods” for detail). Schematic drawings of b long reversal avoidance, c short reversal avoidance, d gradual curve avoidance, and e deep curve avoidance behavior. The angle of encounter into the acidic region (theta), the distance of worm position relative to the acidic yellow color border (d), and directional bias after each behavioral maneuver are indicated. Because deep curve avoidance was rarely observed, directional bias was not indicated. f Positive and negative directions of distance relative to the color border used in the following figures. Scale in f is not identical to that in (b–e)