Fig. 2. Tritonia diomedea detect and locate upstream prey odour sources in the flow tank. T. diomedea tracks with (solid lines) and without (dotted lines) their prey in an upstream odour stimulus chamber (osc). With upstream prey, eight of nine slugs crawled upstream to make direct contact with the upstream grille, and seven of nine found the osc (x marks point of first contact with upstream grille). In controls, only one slug crawled directly upstream, three eventually reached the upstream grille crawling after contact with the sidewalls, and none found the empty osc (o marks point of first contact with upstream grille). Tracks are projected onto a scaled and simplified flow tank diagram. Flow tank width, 1 m.