The locust tegula: kinematic parameters and activity pattern during the wing stroke
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
* e-mail: hf4{at}st-andrews.ac.uk
Accepted 15 March 2002
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Summary |
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Key words: locust, sensorimotor system, tegula, insect, flight, wing stroke parameter, wing hinge element, Locusta migratoria
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Introduction |
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Contrasting with the rather detailed information available about tegula
sensorimotor pathways, little is known about the functional morphology of the
tegula organs or their mode of activation. The tegula is composed of two types
of mechanosensor, an external hair plate located on the cupola, which contains
approximately 40 sensory hairs, and a chordotonal organ inside the cupola,
which consists of approximately 30 scolopidial sensilla
(Kutsch et al., 1980). While
single (filiform) hairs are typical exteroreceptors responsive to the degree
and direction of hair bending (for reviews, see Thurm,
1982
,
1984
), hair plates (e.g.
Kent and Griffin, 1990
;
Mücke, 1991
;
Newland et al., 1995
) or hair
rows often subserve a proprioceptive function, for example in monitoring joint
position and movement (e.g. Wong and
Pearson, 1976
; Pflüger et
al., 1981
; Dean and Wendler,
1983
; Bässler,
1983
). This is achieved through the successive deflection of
adjacent hairs by skeletal elements such as neighbouring limb segments.
Chordotonal organs (for a review, see
Matheson, 1990
) are typical
proprioceptors responsive to the (relative) position and movement of skeletal
elements, including acceleration and vibration (e.g.
Zill, 1985
;
Hofmann and Koch, 1985
;
Kittmann and Schmitz, 1992
;
also in tympanal organs, Yack and Fullard,
1993
). In addition, chordotonal organs are employed to monitor the
position and movement of an appendage in the context of motor control (e.g.
Burns, 1974
;
Field and Pflüger, 1989
;
Matheson and Field, 1995
;
Büschges, 1994
).
The tegula would be equipped to encode almost every parameter of the locust
wing stroke important for aerodynamic force production, flight control and
steering (amplitude and angular velocity of the wing stroke, e.g. Lehmann and
Dickinson, 1998; timing of the stroke reversals, e.g.
Dickinson et al., 1999). The
tegula is also involved in phase-tuning muscle activity during the wingbeat
cycle, particularly regarding the wing elevators
(Wolf and Pearson, 1988
;
Pearson and Wolf, 1989
;
Wolf, 1993
,
Fischer and Ebert, 1999
).
Since the activation phase is one of the key features controlling the
mechanical output of synchronous oscillatory insect muscles (e.g.
Josephson, 1985
;
Stevenson and Josephson,
1990
), this would provide a direct functional context for
wingbeat-synchronous mechanosensory pathways, such as that of the tegula, in
flight pattern generation.
In the present study, we examined the functional morphology of the tegula organs, their timing and activity pattern during the wing stroke and possible stroke parameters encoded by the tegula. In a videographic analysis, the relationship between wing movement and tegula kinematics (including the kinematics of selected wing hinge elements) was examined, and electrophysiological and wing movement recordings were combined to analyse the relationship between tegula discharge and wing stroke parameters. The data suggest that the tegula does not just signal the downstroke movement, but rather monitors details of stroke timing and the angular velocity of the wing.
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Materials and methods |
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For high-speed video analysis, the tegula organs (encircled in Fig. 1A,B, which shows electron microscographs of the location of the tegula organs, the main structural components of the wing base and the pterothorax) were marked with circular dots of black ink (Texpen, USA, marked with open arrows in Fig. 1D) under a dissection microscope as a reference point for the examination of kinematic parameters. Ink dots were placed centrally on the organs, without covering the posterior hair fields of the tegulae or touching other structures of the wing base. To allow video recording of the forewing tegula, the posterior edge of the pronotum had to be clipped without damaging the ligament between the pro- and mesothorax.
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For electrophysiological experiments, a flight preparation was used
(Wolf and Pearson, 1987a) in
which the animal was glued to the holder in an inverted position. A flap of
the ventral cuticle was removed to provide access to either the mesothoracic
or the metathoracic ganglion and to the proximal segments of nerve 1 (N1;
nomenclature after Campbell,
1961
).
Aspects of functional morphology were studied either in freshly killed
animals or in isolated pterothoraces, macerated in concentrated KOH, to
investigate cuticular anatomy (see Pfau
and Koch, 1994). Anatomical descriptions are based on Albrecht
(1953
).
Data acquisition
A commercially available digital high-speed video system (HSVS; hardware:
Weinberger Systems, Switzerland; software; Speedcam, Fraunhofer Institute,
Erlangen, Germany) was used which allows synchronous recording by two separate
cameras (frame frequency adjustable between 1 and 1000 frames s-1).
Tegula movement was recorded from the dorsal side with one camera (Sigma
macrophoto lens, f=90 mm, Fig.
1D); the other camera was equipped with a zoom telephoto lens
(Cosimar, f=1.4-50 mm) and recorded the stroke movements of the fore-
and hindwings from a lateral view (not shown). The frames of both cameras were
system-internally synchronised during recording. Between the two cameras,
frames corresponding in time were identifiable by the displayed frame numbers.
Recordings were stored on-line on computer disc. For analysis, the digitally
recorded episodes of both cameras were transferred onto VHS videotape. In each
individual, the lengths of the fore- and the hindwings (base to tip) were
measured, and the dimensions of wing hinge components and tegula organs were
determined with an ocular micrometer after the experiments.
Flight motor activity was monitored by bipolar electromyographic (EMG)
electrodes (30 µm stainless-steel pins) from the first basalar depressor
(forewing, M97; hindwing, M127) and a tergosternal elevator (M83/84 and M113,
respectively; nomenclature according to
Snodgrass, 1929)
(Fig. 2Ai,Bi). To record tegula
activity, bipolar hook electrodes were placed either on nerve N1 or on nerve
branch N1C (nomenclature after Campbell,
1961
), which contains the afferent axons from the wing base. The
recording site was isolated with silicone grease. In this experimental
arrangement, wing position during flight was recorded by an optical position
detector (von Helversen and Elsner,
1977
) (Fig. 2Ai,Bi)
in parallel with the extracellular nerve recordings. For each animal, the
detector was calibrated by positioning the wing passively at given stroke
angles after the experiment. In addition, after the completion of
electrophysiological recordings, the recorded tegula organ was severed as a
control (Fig. 2Aii,Bii). The
data were stored on compact disc (CD recorder, Pioneer PDR 04) and transferred
onto a computer hard disc using an analog/digital converter (Biologic
DRA-800).
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Data evaluation
The VHS recordings were monitored on a 27 inch Sony Trinitron colour video
monitor. To measure wing stroke parameters, recordings by the lateral camera
were screened frame by frame, and the instantaneous stroke position and stroke
deviation (see Fig. 1C for
explanation) were transferred to overhead transparencies for further analysis
(see also Baker and Cooter,
1979). The distance between the upper and lower reversal points of
the wing was measured, and the total (peak-to-peak) stroke amplitude (
,
see also Sane and Dickinson,
2001
) was calculated from these data as a cosine function of wing
length (base to tip; see Fischer and
Kutsch, 1999
).
The pterothorax (i.e. the fused meso- and metathorax with fused sterna and
pleura, which is further stabilised by several sternal and pleural apostemata;
e.g. Albrecht, 1953) was
studied after maceration by applying mechanical stress, which revealed
rigidity along the longitudinal axis. Experiments in which the thorax was
filmed during flight showed that, by using a ventral attachment of the locust
by both meso- and metathoracic sterna (e.g.
Zarnack and Wortmann, 1989
),
flight activity did not result in any longitudinal (i.e. foreward and
backward) movements or lateral displacements of the pterothorax relative to
the tether. The tether or the margin of the video frames was therefore used as
a reference for measuring movements of the tegula and other skeletal elements
of the wing hinge. In contrast, suspension of the animals by the pronotum
(e.g. Dugard, 1967
;
Baker, 1979
) resulted in strong
oscillatory displacement of the body relative to the tether during flight and,
thus, prevented accurate focusing on the tegulae and other wing
structures.
The degree of tegula rotation in the wingbeat cycle was estimated from the transparencies, according to Fig. 1Diii, with the orientation of the ink dot at the upper reversal point of the wing serving as a reference. Initially, this procedure was tested using a Styrofoam sphere marked with a circular ink dot and rotated through known angles while being filmed from the same view as the tegula during experiments. The degree of tegula inclination (i.e. the inclination of its longitudinal axis) was estimated by measuring the relative changes in the visible area of the ink dot using the area of the dot at the upper reversal point of the wing as a reference. For individual calibration, the wing was positioned at given angles in quiescent locusts before flight experiments.
The evaluated parameters of wing stroke and tegula activity are explained
in the legend to Fig. 2. The
mean amplitude of the tegula discharge was calculated as the integral of the
rectified tegula burst divided by burst duration (e.g.
Chau et al., 1998). The data
were analysed using the Spike 2 data software package (Cambridge Electronics,
UK) and the Data View signal-analysis program (W. J. Heitler, University of St
Andrews, UK).
Statistical analyses
Statistical analyses were computer-aided (KaleidaGraph, MS Excel, StatView)
and followed the criteria described by Sachs
(1978). Correlation and linear
regression analyses were tested for significance levels of P<0.05,
with r indicating the linear correlation coefficient. Partial
correlation coefficients (r*) were determined according to
Sachs (1978
). The statistical
significance of non-linear regressions of data is given by the coefficient of
determination, r2. Mean phase values are given as
± mean angular deviation, with r describing the mean vector.
Circular two-sample comparison was performed using the WatsonWilliams
test (Batschelet, 1981
). Unless
stated otherwise, data are given as mean ± S.D.
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Results |
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Kinematic parameters of thoracic structures and wing hinge sclerites
surrounding the tegula
The cupola of the tegula organ is integrated into a common ligament
attached to the scutum, basalar sclerite, pleura and leading edge of the wing.
This is illustrated for the forewing in
Fig. 3A. The schematic forewing
diagram in Fig. 3B shows that,
during the downstroke, the scuta of the wing segments moved dorsally but were
also displaced posteriorly along the body axis. During this posteriorly
directed movement of the scutum, the wing was promoted, i.e. shifted in the
anterior direction (anterior stroke deviation, see
Fig. 1C). The changes in stroke
position and stroke deviation of the wing were strictly phase-coupled during
the wingbeat cycle (Fig. 3C; at
the upper reversal point, the wing tip has reached its posterior extreme
position; the anterior extreme position is reached when the wing passes
through the lower reversal point). This appears to be due to the tight
mechanical coupling between most elements of the wing hinge
(Pfau, 1982). The
pterothoracic scuta are connected by an elastic ligament. Thus, during the
downstroke, the two scuta moved posteriorly at slightly different times;
during the upstroke, their anterior-directed movements were almost synchronous
(and in phase with the hindwing upstroke, shaded area in
Fig. 3D). Furthermore, both
scuta also underwent a vertical displacement during the wingbeat cycle,
roughly in anti-phase to the wing movement, because of their location on the
inner side of the wing hinge. The scuta were displaced dorsally during the
downstroke and returned ventrally during the upstroke. We were, however,
unable to quantify this vertical movement because both scuta showed
considerable dorso-ventral deformation superimposed on their vertical
movements, which appeared to be caused mainly by the contraction of the dorsal
longitudinal muscles.
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In the video recordings taken from the dorsal side, the mesothoracic first
basalar sclerite performed rotational, rather than horizontal, movements
during the wingbeat cycle (determined via its horn-shaped anterior
process, 1ba in Fig. 1; shown
schematically in Fig. 3B using
the orientation of the process at the upper reversal point of the wing as a
reference). This is probably (i) because the first basalar depressor muscle
attaches to the posterior part of the sclerite
(Albrecht, 1953) and (ii)
because the sclerite itself is attached to the scutum by the medial and
anterior edges of the common ligament. The posteriorly and upward-directed
components of the scutum movement, together with the contraction of the first
basalar muscle during the downstroke, are thus transformed into an `inward
rotation' of the first basalar sclerite. This change in orientation was
slightly phase-shifted with respect to the stroke position of the forewing
[Fig. 3C, advanced by a mean
phase (
) of 0.18±0.03, r=0.967, N=45, data pooled
from five animals] but occurred almost in synchrony with the horizontal
movement component of the scutum (
=0.02±0.02, r=0.945,
N=45). The first basalar sclerite of the hindwing could not be
investigated because it is located below the plane of the hindwing and was not
visible in the video recordings.
Kinematic parameters of the tegula organ in the wingbeat cycle
The tegula organ followed a complex three-dimensional trajectory during the
wingbeat cycle in both the fore- and hindwings. During the downstroke, the
longitudinal axis of the oval-shaped tegula was inclined horizontally. This is
illustrated schematically for the forewing in
Fig. 4A (the longitudinal axis
of the organ is indicated by a dashed line). At the same time, the organ was
rotated around its longitudinal axis (anti-clockwise on the animal's
right-hand side, i.e. anterior margin upwards; indicated by the elliptical red
arrow in Fig. 4A). During the
upstroke, this movement was reversed; the longitudinal axis of the organ moved
vertically, with a synchronous downward rotation of the anterior tegula
margin.
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The temporal pattern of tegula inclination and rotation, with respect to stroke position and deviation movements, is shown for four consecutive wingbeat cycles in Fig. 4B (forewing parameters in Fig. 4Bi and hindwing parameters in Fig. 4Bii; both panels represent typical experimental animals, data confirmed in all animals studied). Movements of the wing and tegula organ exhibited a stable phase relationship, with the tegula reaching maximum rotation (and minimum inclination) near the lower reversal point of the wing beat (right in Fig. 4A) and vice versa near the upper reversal point (left in Fig. 4A).
The mean values of inclination and rotation were lower in the forewing than in the hindwing organs (P<0.05, data not shown). For both tegulae, total inclination and rotation movements during a wingbeat cycle (determined as peak-to-peak values, Fig. 4B) were significantly correlated with wing stroke amplitude in all animals investigated (0.66<r<0.73, P<0.05, N=10, data not shown). Furthermore, the angular velocity of tegula inclination and tegula rotation during a wingbeat cycle was significantly correlated with cycle period (0.68<r<0.83, P<0.05, N=10, not shown) and with the angular velocity of the wing itself (0.71<r<0.89, P<0.05, N=10, data not shown). These findings indicate that tegula movement reliably reflects wing movement, albeit slightly differently in the fore- and hindwings.
In addition to the rotational movements described above, the tegula organs of both pairs of wings shifted in the horizontal plane during the wingbeat cycle (Fig. 4C). During the downstroke, the tegulae were displaced posteriorly in synchrony with the posteriorly directed movement of the adjacent segmental scutum (Fig. 3) and the anterior stroke deviation. During the upstroke, these movements were reversed. The tegulae also showed a proximo-distal component of movement during the wingbeat cycle since the distance between the right and left thoracic pleurae decreased during the downstroke and increased during the upstroke (ordinate in Fig. 4C), this effect being more pronounced in the metathorax (open circles in Fig. 4C).
Pattern of tegula activity with respect to wing stroke parameters:
burst duration
The pattern of tegula activity was investigated with respect to specific
wing stroke parameters in 20 animals (10 for the forewing organs, 10 for the
hindwing organs). For both pairs of wings, the relationships between stroke
parameters and tegula burst duration are given in
Fig. 5.
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In contrast, in the majority of animals, tegula burst properties were not correlated with stroke amplitude in either pair of wings (P>0.05; results shown for four individuals in Fig. 5Bi,ii). In the remaining animals, burst duration was either slightly negatively (one hindwing) or slightly positively (three forewings, one hindwing) correlated with stroke amplitude (see Table 1).
The duration of tegula bursts was significantly related to the angular
velocity during the downstroke (, rad s-1) in the majority
of individuals (P<0.05; hindwing: 8/10 animals; forewing: 6/10
animals; data from four individuals each are shown in
Fig. 5Ci,ii). Tegula burst
duration decreased by 0.28±0.13 ms rad-1 s-1
(N=6), on average, in the forewing, and by 0.22±0.11 ms
rad-1 s-1 (N=8) in the hindwing organ. The
remaining (two and four, respectively) locusts exhibited no significant
relationship between burst duration and angular velocity (P>0.05,
Table 1).
Pattern of tegula activity: latency and phase of discharge onset
The tegula organs are activated with some delay after the beginning of the
downstroke movement. In the hindwing, this latency was, on average,
15.5±2.6 ms (N=10); it was 11.6±4.1 ms (N=10)
in the forewing. These two values are significantly different
(P<0.05, N=10). In both sets of wings, the latency
between the start of the downstroke and the onset of tegula activity was
related to the stroke parameters examined above. The results are shown in
Fig. 6. Latency was related to
downstroke interval in all 20 animals examined (P<0.05;
Fig. 6A,Aii shows data from
four individuals; see Table 1).
In the hindwing, the latency increased by an average of 0.67±1.5 ms per
millisecond increase in downstroke interval (N=10). Comparable values
were observed in the forewing organ (0.72±1.9 ms ms-1, mean
values not significantly different, P>0.05, N=10). In
contrast, latency was not significantly related to stroke amplitude for the
forewing in seven out of 10 individuals and in the hindwing in eight out of 10
individuals (P>0.05; Fig.
6Bi,Bii illustrates data from four individuals; see
Table 1). In the remaining
animals, the relationship between latency and stroke amplitude was not
consistent: two locusts showed a positive relationship, the remaining three a
negative relationship (P<0.05).
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In all 20 animals investigated, the latency of tegula discharge was dependent on the angular velocity of the wing during the downstroke in a non-linear manner. For both the fore- and hindwing organs, the typical characteristics of latency, as dependent on angular velocity, are shown in Fig. 6Ci,ii (data from four animals; r2 significantly different from zero in all animals investigated, P<0.01, Table 1). In seven of the 10 animals, the latency of hindwing tegula activation reached a minimum at approximately 66.5±9.2 rad s-1 (N=7). The corresponding minimum value was 60.7±5.9 rad s-1 (N=6) in the forewing organ, with six of the 10 animals reaching such a minimum (minimum values were calculated from the equations used to fit the data points). In the remaining seven animals, the graph did not reach a consistent minimum value within the angular velocities recorded.
In 12 of the 20 animals, the phase of the onset of tegula activity in the
wingbeat cycle (insets Fig. 6)
was not significantly correlated with the cycle period (P>0.05).
The results were inconsistent among the remaining animals (positively
correlated in five and negatively correlated in three individuals, see
Table 1). Similarly, there was
no clear relationship between the phase of tegula discharge and stroke
amplitude in the majority of animals (15/20,
Table 1; P>0.05,
insets Fig. 6B). The findings
that latency was inversely related to angular velocity and that this
relationship, shown in Fig. 6C,
was hyperbolic, suggest that the tegula is activated at a nearly constant
phase irrespective of the wing's angular velocity. Indeed, in 10 of 20
animals, phase was not significantly correlated with angular velocity
(P>0.05, Table 1). In the majority of the remaining animals, the phase of tegula activation
varied little over a wide range of angular velocities (insets in
Fig. 6C). Consistent with these
observations, the mean coefficient of determination r2
between phase and angular velocity
(, N=20) was
much lower than that between latency and angular velocity
(
, N=20,
Table 1).
Pattern of tegula activity: mean burst amplitude
In 12 animals (six forewings, six hindwings), the `mean amplitude' of the
rectified and integrated tegula burst was calculated and related to
instantaneous wing stroke parameters. In all 12 animals investigated, mean
burst amplitude was correlated with the angular velocity of the wing
(P<0.05, r ranging from 0.41 to 0.77 in the forewings and
from 0.40 to 0.68 in the hindwings). For each wing, data from four animals are
shown in Fig. 7Ai,ii. In eight
of the 12 animals, mean burst amplitude was not significantly related to the
stroke amplitude of the wing (P>0.05,
Fig. 7Bi,ii), while in the
remaining four animals, such a correlation was observed
(0.45<r<0.56). To examine whether this dependency of tegula
burst amplitude on angular velocity was based on a common influence related to
the correlation between stroke amplitude and angular velocity reported above,
the partial correlation coefficients r* were calculated
(to remove the interfering variable). In three of the four animals,
r* was significantly different from zero
(P<0.05, r* ranging from 0.41 to 0.49),
indicating a stronger influence of wing angular velocity on mean burst
amplitude than on stroke amplitude.
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Effects of tegula ablation on wing stroke amplitude
In five out of nine animals examined, the stroke amplitude () of the
hindwings did not change significantly after removal of the hindwing tegulae
(control
c=121.5±21.2°, deafferented
d=121.6±22.6°; N=5, P>0.01).
In these animals, however, tegula removal significantly delayed the start of
wing elevation with respect to the preceding downstroke (determined as the
phase of elevation onset in the wingbeat cycle defined by the start of the
downstroke,
;
c=0.531±0.033, r=0.924;
d=0.587±0.026, r=0.963; P<0.01).
This indicates that tegula removal in the hindwings prolongs the downstroke
interval (cf. Büschges and Pearson,
1991
; Wolf, 1993
;
Fischer and Ebert, 1999
). In
the remaining animals, the effects of hindwing tegula removal on stroke
amplitude were inconsistent: in three of the nine animals,
d
decreased (on average by 13%, P<0.01), and
d
increased by 9% in one animal (P<0.01).
remained unchanged
in three of these animals (P>0.05) and decreased in one individual
(P<0.01). The ablation of the forewing organs had a small and
inconsistent effects on forewing stroke amplitude:
increased, on
average, by 4% in three out of seven animals
(
c=108±9.4°), remained unchanged in two and
decreased in two. In six out of seven animals,
did not change after
ablation of the forewing organs (
c=0.462±0.022,
r=0.924;
d=0.464±0.021, r=0.963,
P>0.05; see, for example,
Büschges and Pearson,
1991
).
Tegula excitation in relation to wing stroke parameters
In both the fore- and hindwings, a failure of tegula discharge was usually
observed when the downstroke movement was terminated prematurely (examples are
shown for the hindwing in Fig.
8Ai,ii). In both pairs of wings, stroke amplitude () and
angular velocity (
) were determined for such wingbeat cycles and in a
number of cycles where premature termination was suspected. Histograms of
these data are given in Fig.
8B. A failure of the hindwing tegula was observed if the amplitude
of the wing beat remained within 50° of the upper stroke reversal
(Fig. 8Bi, indicated by the
grey shaded area). This is less than 40% of the mean wingbeat amplitude
(
hw=119.9±21.8°, N=8). Similarly,
excitation of the forewing tegula failed at stroke amplitudes below 44°,
or 40% of the mean stroke amplitude of the forewing
(
fw=110.9±18.6°, N=8). Occasional failures
were also observed at higher stroke amplitudes for reasons as yet
undetermined. In both sets of wings, tegula failure was apparently unrelated
to a minimum angular velocity of the downstroke movement
(Fig. 8Bii).
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Discussion |
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The present study focuses on an insect mechanosensory organ, the tegula,
the knob-shaped cupola of which is integrated into a common ligament attached
to the scutum, basalar sclerite, thoracic pleura and leading edge of the wing
(Fig. 3). This wing-associated
sense organ plays an important role in the generation and modulation of the
flight motor pattern. However, in contrast to other wing-related sense organs,
which usually consist of one morphological type of mechanoreceptor, the tegula
houses two morphologically distinct sensory systems. Each consists of a
relatively large number of primary mechanosensory axons, approximately 40 from
mechanosensory hairs located on the posterior cupola and approximately 30
scolopidial sensilla from a chordotonal organ attached to the inner surface of
the posterior cupola. These sensory cells each project into the central
nervous system in a single afferent axon
(Kutsch et al., 1980), which
makes (excitatory) monosynaptic connections with motoneurons driving the wing
elevator muscles (Pearson and Wolf,
1988
) and also supplies all known interneurons of the flight
oscillator in parallel (see Pearson and Wolf,
1988
,
1989
).
At present, however, little is known about what wingbeat parameters might be encoded by the tegula organs or how the organ might be activated during flight. To address these questions, the present study employed electrophysiology and high-speed video recordings to monitor the collective activity patterns of tegula afferents, the kinematic movements of the wing and of the tegula organs themselves as well as of the cuticular structures of the wing hinge attached to the tegula organs.
Excitation of the tegula organs during flight
It has been hypothesised that the tegula is excited during flight by the
organ touching a membranous fold during the downstroke, probably resulting in
the bending of the mechanosensory hairs on the posterior cupola
(Kutsch et al., 1980). The
high-speed video recordings confirmed that, during the downstroke, the
posterior region of the tegula on which the hair plate is located touches a
membrane fold located just ventral to the subcosta. This contact is
intensified by the anterior deviation of the wing
(Fig. 1C) during the downstroke
and by the synchronous, posteriorly directed shift of the scutum
(Fig. 3). Since the hair plate
region was covered partly by the membrane fold itself and partly by the
ligament (and, thus, was not visible in the video recordings) when the wing
approached its lower reversal position, we were unable to quantify accurately
the total time of contact between the hair plate region and the membrane fold
from the high-speed recordings. We conclude from our recordings, however, that
at least part of the hair plate region touches the membrane fold during
approximately half of the cycle period, including one-third of the upstroke
interval. The nerve recordings (Figs
2,
6) show that tegula activity
starts with a brief delay after the upper stroke reversal, thus roughly
matching the time in the cycle when part of the hair plate makes first contact
with the membrane fold. Nevertheless, stimulation of the hair plate may not
play a key role in tegula excitation because (i) a tegula discharge during the
wingbeat cycle cannot be prevented by covering the hair plate with wax
(Neumann, 1985
) and (ii)
tegula bursts reliably terminated at the lower reversal point of the wing
(Fig. 2), i.e. at a time when
the hair plate was still in contact with the membrane fold.
The mechanisms that, apparently, limit the tegula discharge to the downstroke interval are not clear at present. The obvious coincidence between the posterior stroke deviation, which starts when the wing passes its lower stroke reversal (Fig. 4B), and tegula burst termination might indicate that the tegula is also sensitive to wing motion in a plane perpendicular to the stroke plane and, thus, that the anterior stroke deviation during the downstroke (see Fig. 4B) might limit tegula activity to the downstroke interval. However, this hypothesis now needs to be addressed by further experimentation.
For both wings, a failure in tegula excitation was almost exclusively
observed during stroke cycles in which the wing did not reach its normal lower
reversal point (Fig. 8B).
However, tegula failure was not restricted to cycles in which the angular
velocity was particularly low (Fig.
8B). In addition, moving the wing passively at angular velocities
far below those observed during active flight, but at comparable amplitudes,
activated the tegula organs (see Fig.
2 in Fischer and Ebert,
1999). This indicates that excitation of the tegula relies on the
wing passing a `critical' position during the downstroke rather than the wing
reaching a certain angular velocity (Fig.
8Bii). Together with the observations of Neumann
(1985
), this suggests that the
response of position-sensitive afferents in the chordotonal organ of the
tegula might play a role in the activation of the organ during the wing
stroke.
The pterothoracic scuta are attached to each other by a flexible ligament. The two scuta move posteriorly at different phases during the downstroke, but they shift anteriorly almost in synchrony during the upstroke (Fig. 3). The hindwing tegula is integrated into this ligament, which inserts in a mesothoracic fold located posteriorly between the scutum and the dorsal border of the epimeron. One might expect, therefore, that hindwing tegula activity would be affected by the kinematics of the forewing. However, we found no indication of a mechanical influence of the mesothoracic scutum on hindwing tegula activity and vice versa. Apparently, the two tegula organs are functionally quite separate.
Relationship between wing stroke parameters and tegula activity
The mechanical elements of the locust thorax and wing hinge are tightly
coupled and move in strict phase relationships during flight (e.g.
Fig. 3). The movement of these
elements, to which the tegula organ is attached by a common ligament, results
in the rotational and tilting movements of this organ, which occur at stable
phase values with respect to wing movement
(Fig. 4). Both the latency of
tegula activity with respect to the onset of the wing downstroke and the burst
duration decrease when the downstroke interval is reduced with increasing
wingbeat frequency (Figs 5,
6). This implies that the
tegula organs are activated at an almost constant phase during the wingbeat
cycle (Fig. 6;
Table 1). These observations
are in accord with the phase-locked kinematics of the organ's movements.
The present results thus seemingly disagree with previous findings
suggesting a relatively constant latency of tegula discharge, at least at
lower wingbeat frequencies (Wolf and
Pearson, 1988). However, latency was determined with respect to
the activity of single wing depressor muscles (first basalar or subalar) in
previous studies, and the relationship between the activity of a particular
muscle and wing movement can be variable
(Wilson and Weis-Fogh, 1962
;
Pfau, 1978
,
1982
; Möhl,
1985
,
1988
). Evaluation of the
present data set with regard to first basalar muscle activity indeed
demonstrated considerable variability between the individuals tested,
including, almost equally, no, positive or negative relationships between
latency and cycle period (data not shown). A variable discharge pattern was
observed in particular in the first basalar muscle, which is involved in a
number of tasks (e.g. flight steering,
Zarnack and Möhl, 1977
;
Baker, 1979
; climbing flight,
Fischer, 1998
;
Fischer and Kutsch, 1999
;
adjustment of the angular setting of the wing, Pfau,
1978
,
1982
). The quantitative data
presented by Wolf and Pearson
(1988
) were from just one
animal, and the slope of the relationship between latency and cycle period,
although small (approximately 0.1; H. Wolf and K. G. Pearson, unpublished
data), is well within the range of slopes observed in the present study (data
not shown, Table 1). The
effects of the tegula on the flight motor pattern reported previously
(Wolf and Pearson, 1988
;
Fischer and Ebert, 1999
) thus
appear to result to a large extent from the constant delay required to elicit
elevator activity in response to a tegula discharge, particularly at lower
wingbeat frequencies (Wolf,
1993
), rather than from a constant latency of tegula activation
(see also the effects of tegula ablation reported above).
Key parameters of tegula discharge, such as latency and phase
(Fig. 6), duration
(Fig. 5) and the mean amplitude
of the tegula burst (Fig. 7),
were related to the stroke amplitude, downstroke interval and angular velocity
of the wing, which are important for aerodynamic force production during
flight (e.g. Ellington, 1984;
Lehmann and Dickinson, 1997, 1998;
Thüring, 1986
).
Apart from the fact that a minimum amplitude seems to be required for tegula activation (Fig. 8), stroke amplitude does not appear to play an important role in determining the pattern of tegula discharge during flight since the duration, the latency and the amplitude of the tegula bursts were not significantly related to the amplitude of the wing stroke (Figs 5B, 6B, 7B). Furthermore, removal of the tegula organs had no consistent, if any, effect on stroke amplitude in either pair of wings.
In contrast, the latency and duration of the tegula discharge were
significantly related to the downstroke interval and, thus, to the cycle
period, since these two parameters are correlated during flight
(Table 1; see also
Wolf, 1993). The dependency of
the duration and latency of tegula discharge on the downstroke interval is
evident when considering the fact that the tegula is excited by (e.g.
Wolf and Pearson, 1988
)
(Fig. 8Ai) and is active during
the downstroke, and it suggests that the tegula encodes parameters of the
downstroke such as timing (Wolf,
1993
) and velocity (see also Figs
5C,
6C).
In both sets of wings, latency (Fig.
6C), burst duration (Fig.
5C) and burst amplitude (Fig.
7A) depend on the angular velocity of the wing. The relationship
between latency and angular velocity is non-linear. Towards higher angular
velocities, latency approaches or reaches a minimum value and often stays near
this minimum if angular velocity increases further. The minimum is reached
between approximately 45 and 75 rad s-1. The corresponding wingbeat
frequencies are between approximately 15 and 19 Hz, i.e. they mark the lower
limit of frequencies observed during free flight. The latency of the tegula
discharge thus appears to be kept within a narrow range during normal flight,
indicating that a feedback loop is functioning. Furthermore, the tegula organs
are sensitive to a very wide range of angular velocities
(Fig. 8), including very low
values (e.g. 2-5 rad s-1) that are not observed during flight (data
shown in and extracted from Fig.
2 in Fischer and Ebert,
1999). In principle, angular velocity could be adjusted by
controlling stroke amplitude or cycle period or both. However, in 85 % of the
animals examined, angular velocity was (negatively) related to the cycle
period (Table 1), while in only
60 % of the locusts was it (positively) related to stroke amplitude (data not
shown). It appears, therefore, that angular velocity is primarily related to
wingbeat frequency.
The above conclusions are based on the changes in the collective activity
of the tegula afferents with respect to variation in particular wingbeat
parameters. It has previously been suggested that the tegula might work as a
functional unit since the two sensory systems in the tegula are in close
vicinity to each other (Kutsch et al.,
1980). However, since the tegula consists of a large number of
afferents (in contrast to other wing-associated sensory organs, e.g. the
single-cell stretch receptor), there is the distinct possibility of different
response properties and range fractionation (for reviews, see
Field and Matheson, 1998
;
Newland et al., 1995
;
Neumann, 1985
) among the 70-80
sensory cells of the tegula. This might allow population-coding of wingbeat
parameters, although at present this possibility must remain speculative
because of the absence of data concerning tegula receptor physiology or the
the specificity of the central connections of different receptor cell
types.
In the present study, a meaningful distinction of different spike amplitudes, or discharge properties of different axon populations, was not possible because of the rather homogeneous distribution of both spike amplitudes and discharge characteristics in the compact tegula bursts (data not shown, but see Fig. 2). Even in the few cases where large, small and sometimes intermediate spike amplitudes could be differentiated in the tegula discharge, these groups of sensory axons had very similar discharge patterns (data not shown).
The locust pterothorax is a compact mechanical structure composed of a
large number of coupled cuticular elements that move in strict phase
relationships during flight (e.g. Fig.
3). The mechanical composition and properties of the pterothorax
(including flight musculature) both determine, to a major extent, the complex
three-dimensional wing movements (Pfau,
1978,
1982
) and limit the power
output of the flight oscillator through their mechanical damping properties
(Roeder, 1951
;
Soltavalta, 1952
). A more
global assessment of flight performance should consider at least two points.
First, the activation phase of the flight muscles, which controls their
mechanical power output (e.g. Stevenson
and Josephson, 1990
), and in turn is translated into basic flight
parameters, such as wingbeat frequency, stroke amplitude, the angular velocity
of the wing and finally into aerodynamic force production, should be examined.
Second, an appropriate tuning of these flight parameters may be important not
only for flight performance per se but also in the context of a
possible resonance stabilisation of the flight oscillator
(Greenwald, 1960
; Scharstein,
1998a
,b
).
Insects need to activate their flight muscles at appropriate times and
phases to generate a functional flight pattern under a variety of conditions,
e.g. during changes in stroke frequency. This provides a direct functional
context for wingbeat-synchronous (i.e. phase-locked,
Fig. 6) sensorimotor pathways,
such as that involving the tegula, which are important in flight motor control
(e.g. Wendler, 1974;
Möhl, 1985
;
Wolf and Pearson, 1988
;
Wolf, 1993
;
Fischer and Ebert, 1999
).
Thus, the respective sense organs may serve to monitor, and control, the
appropriate tuning of critical flight variables, e.g. the angular velocity of
the wing, which plays an important role in aerodynamic performance (e.g.
Ellington et al., 1996
;
Lehmann and Dickinson, 1998; Dickinson et
al., 1999
). If there is not just a correlation between particular
parameters of wing movement and tegula discharge, but if this correlation is
non-linear and a preferred parameter combination is maintained during normal
flight (e.g. the phase of tegula activation during the cycle or the
relationship between latency and angular velocity,
Fig. 6), this may indicate that
a feedback loop is involved. Tegula afferent pathways might thus be part of a
feedback loop controlling not only the basic flight motor pattern
(Wolf and Pearson, 1988
) but
also the angular velocity of the downstroke, with the variations in tegula
activity serving as error signals for these control loops. However,
experiments demonstrating conclusively such feedback loops can be obtained
only by examining the control circuit in cybernetic experiments (e.g.
Wendler, 1974
).
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References |
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