Figure 3. Flagellar beat of capillary trapped specimens.

(a) A specimen of Euglena gracilis trapped at the tip of a capillary (bottom). The typical outline of its beating flagellum is that of a looping curve, which is consistent with the outline of a simple curve with two concentrated torsional peaks of alternate sign along the length of the curve, that is, a torsion dipole (top). (b) Close-up images of the same specimen of capillary-trapped (CT) E. gracilis as seen from different viewpoints, upon successive ∼90° turns of the capillary tube. The body orientation with respect to the objective is estimated from the anatomy of the cell, and in particular from the position of the eyespot (a visible light-sensing organelle present on the cell surface). Microscopy images are decorated with the tracked outlines of the flagellum in different phases (same color coding as in Figure 2). The outlines (2d projections) of the 3d reconstructed flagellar beat of freely swimming (FS) specimens are shown for comparison.