Escape manoeuvres in damsel-fly larvae: kinematics and dynamics.

The kinematics and hydrodynamics of rapid escape manoeuvres executed by final-stage larvae of Enallagma cyathigerum were investigated using videography combined with a simple wake-visualisation technique. Two kinds of escape manoeuvres were identified: first, a 'rapid flex', comparable with the rapid C-start of fish, and, second, a 'rapid twist' that involves a helical contraction of the body inducing motion in the yaw, pitch and roll planes. In both cases, the initial flexion phase is concerned with re-orientating the body, the extensional phase with acceleration of the body in the new direction. The behaviour of the caudal fin during twist indicates considerable independence of movement and aspect control within the three constituent lobes. Dye deposited beneath the resting larvae showed a thrust jet shed into the wake at the end of the extension phase. The estimated momentum of the ring vortex containing the jet was similar to that imparted to the body at the start of the translational phase. Similarities between the swimming dynamics of damsel-fly larvae and fish are discussed, as well as the wider implications of these findings to other aquatic invertebrates whose normal, steady swimming appears to be based on unsteady manoeuvres.