Fig. 2. (A) Side view of video images of the downstroke flick (lasting 100 ms) by the lateral flagellum of a P. argus antennule, showing its position at the start (1) and end (2) of the flick as it passes through an odor plume. The lateral flagellum of the antennule appears light because it was flicking in a plane illuminated by a sheet of laser light. The medial flagellum of the antennule, which did not move during the flick, appears black because it was not in the plane illuminated by the laser. The lobster was facing upstream in a flume (flow from left to right) and the antennule was flicking in a turbulent plume of odor mixed with fluorescent dye (fluorescein), which appears as pale filaments swirling in the water around the antennule. (B) Diagram of a cross-section of the lateral flagellum, showing the orientation (angle between dotted lines) of the aesthetascs with respect to the direction of the rapid antennule downwards flick (left diagram) and slower upwards return stroke (right diagram). The direction of water motion relative to the antennule, which changes as a result of the antennular motion is indicated by the solid arrows.