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The locust tegula: kinematic parameters and activity pattern during the wing stroke

Hanno Fischer^1,*, Harald Wolf² and Ansgar Büschges³

¹ School of Biology, Bute Medical Buildings, University of St Andrews, St Andrews, Fife KY16 9TS, Scotland
² Neurobiologie, Universität Ulm, D-89069 Ulm, Germany
³ Zoologisches Institut, Universität Köln, Weyertal 119, D-50923 Köln, Germany

View larger version (117K): [in a new window]   Fig. 1. Location of the tegula. Electron microscographs showing dorsal views of the mesothoracic (A) and metathoracic (B) wing hinge of an adult female Locusta migratoria. The locations of the tegula organs are circled; orientation and wing hinge area are indicated by the locust outline (left; lower image margin, body midline). (A) The location of the forewing tegula (tg) in the downstroke position, with the organ located in a fold of the subcostal (sc) membrane; (B) the hindwing organ in the upstroke position. fw, forewing; hw, hindwing; lg, ligament; lba, anterior process of the first basalar sclerite; mssc, mesothoracic scutum (anterior border); mtsc, metathoracic scutum; pn, pronotum. (C) Schematic drawing of the wing tip (black dots) trajectories, reconstructed from the superimposed video frames recorded during three wingbeat cycles by the lateral camera. The upper (URP) and lower (LRP) reversal points of the forewing (fw) and the hindwing (hw) are indicated by open circles. sd, stroke deviation with respect to the stroke plane (indicated by the line connecting the URP and LRP). (D) Dorsal view of the metathoracic wing hinge; pictures are single frames from a high-speed video recording (500 frames s-1). The tegula organ (circled) is shown in the upstroke (Di) and downstroke (Dii) position (approximate area shown in Di is indicated by the dashed frame in B). The tegulae were marked with black ink dots (open arrows). (Diii) Schematic drawing of the tegula in dorsal view, with the ink dot indicated in black, (a) when the wing was folded in the resting position, (b) at the upper reversal point (URP) of the wing, (c) in the horizontal wing position (mid-downstroke) and (d) at the lower reversal point (LRP) of the wing. The arrow indicates the longitudinal axis of the organ, pointing towards the wing tip. The instantaneous angle of tegula rotation was measured as the orientation of the small bisector (line marked by a dot) of the ink dot ellipsoid in the plane view, with the orientation of the ellipsoid at the upper reversal point of the wing used as a reference. (e) Superposition of b and d showing the total angle of rotation during a wingbeat cycle ().

View larger version (30K): [in a new window]   Fig. 2. Discharge pattern of the forewing (A) and hindwing (B) tegula in relation to wing position during tethered flight. Wing position (top traces in i) was monitored by an optical position detector. Tegula activity (bottom traces) was recorded extracellularly from nerve branches N1 (B) or N1C (A) (details in Pearson and Wolf, 1988), which supply the tegula organs. Black arrowheads indicate cross-talk between motoneurons innervating the dorsal longitudinal depressor muscles. Electromyographic (EMG) recordings were made from wing depressor (M97, M127) and elevator (M83, M113) muscles. Shaded areas indicate tegula burst duration and latency (time between onset of wing downstroke and start of tegula discharge). Cycle period was determined as the time between consecutive downstroke movements. The downstroke interval was measured between the beginning and end of the downstroke movement. Stroke amplitude was determined from the distance between the upper and lower reversal points of the wing. The angular velocity of the wing (, rad s-1) was calculated from the change in wing position during the 10 ms period following tegula activation (indicated by the open boxes in Ai and Bi). The phase of onset of tegula activity within a cycle was calculated as latency divided by cycle period. (Aii,Bii). Nerve recordings after ablation of the tegula organs recorded in Ai and Bi (traces selected and aligned according to EMG activity, which is not shown).

View larger version (48K): [in a new window]   Fig. 3 . Morphology and movement of the wing hinge area. (A) Tegula position in the folded (left) and unfolded (right) forewing, frontal view. mssc, mesothoracic scutum; mtsc, metathoracic scutum; tg, tegula; lg, ligament; 1ba, first basalar sclerite; eps, episternum; sc, subcosta; scmf, subcostal membrane fold (also for B—D). The tegula is integrated into a common ligament (lg, blue shading) attached to the basalar sclerite, scutum, subcosta and leading edge of the wing. During the downstroke, the organ slides into a subcostal membrane fold, indicated by a dashed circle (see Fig. 1Diii). (B) Schematic drawing of the forewing hinge in dorsal view (the diagram on the left indicates the area shown). The posterior displacement of the scutum (black arrow) and synchronous rostral stroke deviation (white arrow) of the wing near the lower stroke reversal are indicated by reference lines; the change in the orientation of the first basalar sclerite is indicated by a green arrow. See text for details. fw, forewing. (C) Displacement of mesothoracic wing hinge components during the wingbeat cycle; three consecutive cycles are shown. The upstroke phase of the forewing is indicated by the shaded area. AEP, anterior extreme position of movement; PEP, posterior extreme position of movement; URP, upper reversal point; LRP, lower reversal point of the wing (see also Fig. 1C). (D) Displacements of the meso- and metathoracic scuta in relation to fore- (fw) and hindwing (hw) movements; three consecutive wingbeat cycles are shown. The upstroke of the hindwing is indicated by the shaded areas.

View larger version (48K): [in a new window]   Fig. 4 . Movements of the tegula during the wingbeat cycle. (A) The forewing hinge in frontal view; schematic drawings of upper (left) and lower (right) stroke reversals. Abbreviations as in Fig. 3. The dashed line marks the longitudinal axis of the tegula organ; rotational movement around this axis is indicated by an elliptical red arrow. Black arrows indicate displacement of the scutum and the change in orientation of the basalar sclerite. (B) For four wingbeat cycles of the forewing (Bi) and the hindwing (Bii), the time courses of tegula rotation and inclination are shown in relation to stroke position and stroke deviation of the wing. (C) Displacement of the tegula organ in the horizontal plane during the wing stroke (data from four consecutive wingbeat cycles superimposed). The black arrow marks the direction of movement during the downstroke; the white arrow indicates upstroke. URP, upper reversal point; LRP, lower reversal point of the particular wing; AEP, anterior extreme position of movement; PEP, posterior extreme position of movement.

View larger version (40K): [in a new window]   Fig. 5. Relationships between tegula burst duration and wing stroke parameters (see Fig. 2) for the fore- (i) and hindwing (ii) tegulae (data from four individuals shown, each given in a different colour). In both pairs of tegula organs, the burst duration depends on the duration of the downstroke interval (A) (P<0.05, regressions indicated by solid lines); however, burst duration is not significantly related to stroke amplitude (B) (P>0.05, regressions indicated by dashed lines). (C) Tegula burst duration is significantly related to the angular velocity during the downstroke (P<0.05). For each individual, data points represent means ± S.D. (indicated as caps of error bars only) calculated from 5-21 observations during 3-5 flight episodes. Relationships for all animals investigated are given in Table 1.

View larger version (47K): [in a new window]   Fig. 6. Latency (main panels A—C) and phase (insets in A—C) of the onset of tegula activity in the wingbeat cycle and their relationship to wing stroke parameters (see Fig. 2) in the fore- (i) and hindwing (ii) (data from four individuals shown, each in a different colour). (A) In both pairs of sense organs, latency depends on the downstroke interval (P<0.05; solid regression lines). However, the phase of tegula activation was not related to cycle period (P<0.05; broken regression lines). (B) Neither the latency nor the phase of the tegula discharge depends on stroke amplitude (P>0.05). (C) The relationship between the latency of tegula activity and wing angular velocity is non-linear (r2 significantly different from zero, P<0.01). Within the range of angular velocities observed, latency approaches or reaches a minimum value at higher angular velocities. In contrast, the phase of tegula discharge is almost independent of angular velocity. For details, see text. Relationships for all animals investigated are given in Table 1.

View larger version (30K): [in a new window]   Fig. 7. Relationships between the mean amplitude of the tegula burst (calculated from rectified and integrated recordings) and the angular velocity (A) and stroke amplitude (B) of the fore- (i) and hindwings (ii) (data from four individuals shown, each in a different colour). In both pairs of sense organs, mean burst amplitude depends on the instantaneous angular velocity of the wing (A; solid regression lines), whereas burst amplitude is not significantly related to instantaneous stroke amplitude (B; broken regression lines). Data points represent samples pooled from 2-4 flight episodes. Data shown in A and B are from the same animals. For details, see text.

View larger version (25K): [in a new window]   Fig. 8. Downstroke amplitude () and angular velocity () of the wing as determinants of tegula excitation. (Ai,ii) Two sample recordings from the hindwing organ illustrate failures of the tegula during single wingbeat cycles with incomplete downstroke movements. For each cycle shown, the values of and are given. Wing position (top trace) and tegula activity recorded extracellularly from nerve branch N1 (lower trace) are shown. The shaded areas indicate tegula burst duration. (Bi) Excitation of the tegula organs in relation to stroke amplitude. Data for each wing were pooled from eight animals. For the hindwing (hw), no excitation of the tegula was observed during strokes of less than 50° in amplitude; in the forewing (fw), the tegula failed at amplitudes of less than 44° (illustrated by the shaded area). (Bii) Excitation of the tegula organs in relation to the angular velocity of the downstroke (same data set as above). In both pairs of wings, the tegula organs were active at any given angular velocity within the range of values investigated.

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