Escape manoeuvres present the locomotory system of a swimming animal with a severe test because their successful execution demands a change in direction, which may sometimes be up to 180°, combined with rapid acceleration, all within the duration of one or two strokes of the body or the fins or both. Drucker and Lauder (2001a) used the term 'manoeuvrability' to cover a variety of unsteady locomotor behaviours involving controlled dynamic instability of the body. Such behaviours include turning,

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... braking, negotiating obstacles, controlling the vertical position of the body in the water and stopping and starting. In many fish, an escape manoeuvre is essentially a start coupled with a turn, and the best documented of these is the rapid C-start, in which the body is rapidly flexed, usually away from the threatening stimulus, then rapidly extended to accelerate the body in the new direction (reviewed by Domenici and Blake, 1991 , 1993 Blake et al., 1995) . The two-stage response involves a reorientational phase (the flex) and a translational phase (the extension) usually followed by a variable stage consisting of continuous or coasting swimming. Recent flow studies by Wolfgang et al. (1999) suggest that the C-start is a continuum in the generation of a thrust vortex, the first stage of which is the drawing of a jet into the C-shaped cavity formed by the flexing body. The kinematics and dynamics of the rapid flex are determined by the movements of the body and the caudal fin, although sometimes the pectoral fins may also be used to pivot the body in the vertical plane. A more specialised role for the pectoral fins in turning has been described by Drucker and Lauder (2001a) in the sunfish Lepomis macrochirus. This involves co-ordination between the near-side, or strong-side, fin, which produces a jet to rotate the body onto a new heading, and the far-side, or weak-side, fin, which produces a jet to drive the body away from the stimulus. As in the case of the C-start, the sequential division of the response into re-positional and translational phases is evident. In a previous study on the hydrodynamics of steady swimming in damsel-fly larvae (Brackenbury, 2002) , the author noted a rapid escape response similar in kinematics and dynamics to the rapid C-start of fish. Subsequently, a related, although more complex, rapid escape manoeuvre has also been identified that involves specialised movements within the three-lobed caudal fin. The present study was undertaken to analyse the dynamics of the escape responses of damsel-fly larvae against a background of the knowledge of steady swimming gained from the earlier investigation. Materials and methods The experiments were carried out on final-stage larvae of Enallagma cyathigerum L., which were collected from permanent pools in the Fenland area to the northwest of Cambridge, UK. The larvae were maintained indoors at 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.