Fig. 4. Tritonia diomedea detects and turns away downstream from upstream predators in the flow tank. T. diomedea tracks with a predator (solid lines) and controls (dotted lines) with similarly shaped latex tubing in the odour stimulus chambers (osc). With an upstream predator, those slugs that crawled into a plume zone downstream of the predator had a greater tendency to turn downstream. The inset shows the downstream turn magnitude (P=predator; C=control) measured by comparing mean crawling headings for the 2 min on either side of the estimated plume zone entry points (+). Slugs in four experimental trials (x) and two control trials (o) crawled upstream and contacted the second upstream grille without entering the plume zone. A further three animals (2 controls, 1 experimental) reached neither the plume zone nor the upstream grille. Tracks are projected onto a scaled and simplified flow tank diagram. Flow tank width, 1 m.