Fig. 2. Illustration of the video analysis technique used for tracking antennule movement. (A) Video images of a flicking right antennule show the motion of the lateral flagellum (lf) during the closing phase; the medial flagellum (mf) remains stationary. The location of the antennule edge was determined by superimposing a line tangent (tl) to the motion of the arc swept by the lateral flagellum. To clearly show the lateral flagellum's motion, this illustration presents images and tangent lines at the start (tl₀) and end (tl₁) of a 22 ms time period (images were actually sampled every 2 ms). Scale bar, 4.9 mm. (B) The pixel intensity profile from the tangent line (tl) was used to locate the lateral flagellum's edge and to calculate position and speed during a flick. For illustration purposes, we show a small number of pixels with an intensity profile including only white (background) or dark stippling (flagellum) rather than the hundreds of pixels and continuous shades of gray actually used in the analyses. Times tl₀ and tl₁ correspond to the tangent lines denoted as tl₀ and tl₁ in panel A. Arrows indicate the moving edge of the lateral flagellum.