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Escape manoeuvres in damsel-fly larvae: kinematics and dynamics

John Brackenbury

Department of Anatomy, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK

View larger version (27K): [in a new window]   Fig. 1. Kinematics of a simple flex manoeuvre in larva of Enallagma cyathigerum. (A) Successive 20 ms profiles seen from above as the larva reacts to a stimulus presented to the left anterior side of the body. (B) Angle of attack of the apical segment of the tail fin, represented by bars, during a simple flex manoeuvre. (C) Superimposed line representations of the longitudinal body axis corresponding to profiles in panel A. Open circles represent the head, crosses represent the centre of the body. In panels B and C, broken lines represent the motion of the head and tail ends of the body.

View larger version (28K): [in a new window]   Fig. 2. Kinematics of a twist manoeuvre in larva of Enallagma cyathigerum. (A) Successive 20 ms profiles seen from above as the larva reacts to a stimulus presented to the left anterior side of the body. Curved arrows represent the rolling motion of the body, straight arrows indicate the direction of bending of the apical segment of the tracheal plate, relative to the basal segment, along the flexion line. The dorsal plate of the tail fin is shaded for ease of reference. (B) Superimposed line representation of the longitudinal body axis corresponding to profiles in A. Open circles represent the head. Broken lines plot the motion of the head and tip of the tail.

View larger version (25K): [in a new window]   Fig. 3. Kinematic and hydrodynamic events during a twist manoeuvre in larva of Enallagma cyathigerum. (A) Successive 20 ms body profiles seen from above. (B) Paths followed by the head (open circles), fin tip (filled circles) and centre of the body (crosses) during the twist manoeuvre shown in A. Bars represent the apical segment of the tail fin. The curved broken outlines plot the motion of a ring vortex shed from the tail at the end of the manoeuvre. (C) Successive outlines of a ring vortex shed from the tail at the end of a twist manoeuvre. On the far right, the fin has just completed its extension (corresponding approximately to the 120 ms stage in A) and is shedding the vortex. The first indication of the vortex follows 20 ms later. Small, broken circles within the vortex outline represent the vortex core.

View larger version (19K): [in a new window]   Fig. 7. (A,B) Kinematic and hydrodynamic variables measured during the twist manoeuvre. Panel A shows the motion of the head (open circles) and tail tip (filled circles) during the manoeuvre. Panel B shows the initial orientation of the body, direction of exit of the head at the end of the extension phase and vortex propagation axis, all as straight arrows. V and Vjet, the head velocity and jet velocity of the vortex as measured at the ring plane, respectively; , angle between the vortex axis and a line projected dead-aft of the line of motion of the head, measured in the horizontal plane; , turning angle of the head and the anterior part of the body during the manoeuvre. (C) Scale drawing of the ring vortex. R, ring radius; Ra, external radius orthogonal to the ring plane; Rp, external radius parallel to the ring plane.

View larger version (33K): [in a new window]   Fig. 4. Kinematics of a twist manoeuvre in larva of Enallagma cyathigerum. (A) Consecutive 20 ms profiles seen from in front as the larva reacts to a stimulus presented to the left anterior part of the body. Curved arrows represent the rolling motion of the body onto its side. (B) Consecutive profiles seen from the right side of the larva as it reacts to a stimulus presented to its left side. (C) Superimposed line representations of the longitudinal body axis corresponding to the profiles shown in B. Open circles represent the head. Broken lines plot the motion of the head and tail tip.

View larger version (29K): [in a new window]   Fig. 5. Kinematics of the tail fin during a twist manoeuvre. (A) Successive series of images of the tail fin viewed from above. In the resting insect (0 ms stage), the tracheal plates are seen in edge-on view and there is little indication of any flexure of the apical segments relative to the basal segments. Successive stages show various degrees of movement of the tracheal plates in response to the forces experienced during elevation and twist (20-100 ms stages), flexion (120-140 ms stages) and extension (160-180 ms stages). Arrows indicate flexion of the apical segments relative to the basal segments of the tracheal plates. See text for further details. (B) Schematic representation of tracheal plate kinematics shown in A. The drawings represent the view seen in tail-fixed co-ordinates, i.e. the view seen from immediately above the tail assuming that the viewer travels with the tail.

View larger version (114K): [in a new window]   Fig. 6. Visualisation of hydrodynamic events during a twist manoeuvre in which the larva receives a light touch to the left side of the head and executes an escape to its right. In the first frame (0 ms), the insect has just begun to react to the stimulus from the syringe needle tip shown in the lower right of the panel; the end of the abdomen is beginning to elevate and twist to the insects right. At 100 ms, the twist and elevation stages have been completed, the insect has been rolled onto its right side and the abdomen is beginning to flex to its left. By 160 ms, extension of the abdomen is completed, driving the insect directly to the left of the frame. The vortex being released from the tail has not yet made contact with the tracer on the bottom. In the 200 ms-stage image, a ring vortex is propagating along its axis, marked by the arrow, and leaving its impression in the tracer.

View larger version (17K): [in a new window]   Fig. 8. Schematic representation of the mechanism of thrust vortex generation during a simple flex manoeuvre. The larva is viewed from above. The head and tail ends of the C-flexing body (stage 2) generate mutual starting vortices; the body vortex is then propagated down the body during the extension phase (stages 3-5) and finally released, along with the tail-generated vortex, as a ring vortex. The inset below shows the corresponding configuration of the body, at the end of the flexion phase, during the twist. The two configurations are functionally equivalent, although in the latter case the body has rolled onto its left side and the C-shaped cavity into which water will be drawn and thrust caudally has been formed by a right-side body flexure rather than a left-side flexure.

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