Fig. 1. (A) Paths traced out by the tip of the caudal fin during fast swimming in five damsel-fly larvae. Solid bars on the paths represent the orientation of the caudal fin. (B) Parameters used in the hydromechanical model (see text for details). 
, angle of incidence of the tail fin to the instantaneous line of travel of the fin tip; 
, angle of inclination of the tail fin to the mean swimming line; 
, perpendicular velocity of the tail tip; U, instantaneous forward velocity; V, absolute velocity of the fin tip; W, lateral velocity of the tail tip. (C) Flow parameters used in the calculation of thrust from the vortices produced during fast swimming (see text for details). R, ring radius; Ra, external radius of the vortex measured along the axis; Rp, external radius of the vortex measured along the ring plane; 
, momentum angle of the jet measured in the frontal plane relative to dead aft of the swimming line.
