Acoustic startle/escape reactions in tethered flying locusts: motor patterns and wing kinematics underlying intentional steering

We simultaneously recorded flight muscle activity and wing kinematics in tethered, flying locusts to determine the relationship between asymmetric depressor muscle activation and the kinematics of the stroke reversal at the onset of wing depression during attempted intentional steering manoeuvres. High-frequency, pulsed sounds produced bilateral asymmetries in forewing direct depressor muscles (M97, 98, 99) that were positively correlated with asymmetric forewing depression and asymmetries in stroke reversal timing. Bilateral asymmetries in hindwing depressor muscles (M127 and M128 but not M129) were positively correlated with asymmetric hindwing depression and asymmetries in the timing of the hindwing stroke reversal; M129 was negatively correlated with these shifts. Hindwing depressor asymmetries and wing kinematic changes were smaller and shifted in opposite direction than corresponding measurements of the forewings. These findings suggest that intentional steering manoeuvres employ bulk shifts in depressor muscle timing that affect the timing of the stroke reversals thereby establishing asymmetric wing depression. Finally, we found indications that locusts may actively control the timing of forewing rotation and speculate this may be a mechanism for generating steering torques. These effects would act in concert with forces generated by asymmetric wing depression and angle of attack to establish rapid changes in direction.Abbreviations ASR acoustic startle response - dB SPL decibel sound pressure level (re: 20 Pa RMS) - EMG electromyogram - FWA forewing asymmetry - HWA hindwing asymmetry     

Turning manoeuvres in free-flying locusts: two-channel radio-telemetric transmission of muscle activity

A device has been constructed allowing the simultaneous transmission of two separate electrical signals in unrestrained small animals. We employed this device to investigate the motor output in free-flying locusts. The activation pattern of several combinations of different muscles was recorded, including bilateral symmetric muscles and pairs of antagonists. Particular attention was paid to the recruitment of a specific set of flight muscles in both winged segments during rolling manoeuvres. The relationship of the muscle activation with wing movement was analysed in combination with a high-speed video-monitoring. The muscles are activated in advance of the relevant stroke directions, in opposition to previous studies of tethered flying locusts. During turning manoeuvres a statistically significant difference in timing of the bilateral symmetric muscles is not apparent; this contrasts with the distinct difference revealed for the bilateral wing movement. It is discussed that rolling might rely on the fine tuned interaction of several major flight muscles or on the precise activation of a specific wing hinge muscle. Correspondence with investigations of bird flight is discussed.     

Body temperatures in free-flying pigeons

We examined the relationship between body temperature (T_b) of free flying pigeons and ambient water vapor pressure and temperature. Core or near core T_b of pigeons were measured using thermistors inserted into the cloaca and connected to small transmitters mounted on the tail feathers of free flying tippler pigeons (Columba livia). Wet and dry bulb temperatures were measured using modified transmitters mounted onto free-flying pigeons. These allowed calculation of relative humidity and hence water vapor pressure at flight altitudes. Mean T_b during flight was 42.0 ± 1.3 °C (n = 16). Paired comparisons of a subset of this data indicated that average in-flight T_b increased significantly by 1.2 ± 0.7 °C (n = 7) over that of birds at rest (t = −4.22, P < 0.05, n = 7) within the first 15 min of takeoff. In addition, there was a small but significant increase in T_b with increasing ambient air (T_a) when individuals on replicate flights (n = 35) were considered. Inclusion of water vapor pressure into the regression model did not improve the correlation between body temperature and ambient conditions. Flight T_b also increased a small (0.5 °C) but significant amount (t = 2.827, P < 0.05, n = 8) from the beginning to the end of a flight. The small response of T_b to changing flight conditions presumably reflects the efficiency of convection as a heat loss mechanism during sustained regular flight. The increase in T_b on landing that occurred in some birds was a probable consequence of a sudden reduction in convective heat loss. Accepted: 2 February 1999     

Water influx and efflux in free-flying pigeons

We used tritium-labeled water to measure total body water, water influx (which approximated oxidative water production) and water efflux in free-flying tippler pigeons (Columba livia) during flights that lasted on average 4.2 h. At experimental air temperatures ranging from 18 to 27 °C, mean water efflux by evaporation and excretion [6.3 ± 1.3 (SD) ml · h⁻¹, n = 14] exceeded water influx from oxidative water and inspired air (1.4 ± 0.7 ml · h⁻¹, n = 14), and the birds dehydrated at 4.9 ± 0.9 ml · h⁻¹. This was not significantly different from gravimetrically measured mass loss of 6.2 ± 2.1 g · h⁻¹ (t = 1.902, n = 14, P>0.05). This flight-induced dehydration resulted in an increase in plasma osmolality of 4.3 ± 3.0 mosmol · kg⁻¹ · h⁻¹ during flights of 3–4 h. At 27 °C, the increase in plasma osmolality above pre-flight levels (ΔP _osm = 7.6±4.29 mosmol · kg⁻¹ · h⁻¹, n = 6) was significantly higher than that at 18 °C (ΔP _osm = 0.83±2.23 mosmol · kg⁻¹ · h⁻¹, (t = 3.43, n = 6, P < 0.05). Post-flight haematocrit values were on average 1.1% lower than pre-flight levels, suggesting plasma expansion. Water efflux values during free flight were within 9% of those in the one published field study (Gessaman et al. 1991), and within the range of values for net water loss determined from mass balance during wind tunnel experiments (Biesel and Nachtigall 1987). Our net water loss rates were substantially higher than those estimated by a simulation model (Carmi et al. 1992) suggesting some re-evaluation of the model assumptions is required. Accepted: 8 April 1997     

Auditory interneurons in locusts produce directional head and abdomen movements

Locusts (Locusta migratoria) were stimulated with pulses of pure tones of frequencies between 5 kHz and 25 kHz. Interneurons responding to these stimuli (auditory interneurons) were recorded intracellularly and identified by dye injection. Their output functions were investigated by injection of depolarizing current during simultaneous registration of components of flight steering behavior of the animals, i.e. movements of the head and the abdomen and flight activity. Three different types of effects were found, corresponding to 3 functional classes of interneurons:

Gliding behaviour elicited by lateral looming stimuli in flying locusts

We challenged tethered, flying locusts with visual stimuli looming from the side towards one eye in a way that mimics the approach of a predatory bird. Locusts respond to the lateral approach of a looming object with steering movements and a stereotyped, rapid behaviour in which the wingbeat pattern ceases and the wings are swept into a gliding posture. This gliding behaviour may cause the locust to dive. The gliding posture is maintained for 200 ms or more after which flight is resumed with an increased wingbeat frequency or else the wings are folded. A glide begins with a strong burst of activity in the mesothoracic second tergosternal motor neuron (no. 84) on both sides of the locust. Recordings of descending contralateral movement detector (DCMD) activity in a flying locust show that it responds to small (80-mm diameter) looming stimuli during tethered flight, with a prolonged burst of spikes that tracks stimulus approach and reaches peak instantaneous frequencies as, or after, stimulus motion ceases. There is a close match between the visual stimuli that elicit a gliding behaviour and those that are effective at exciting the DCMD neuron. Wing elevation into the gliding posture occurs during a maintained burst of high frequency DCMD spikes.     

Variability in odor-modulated flight by moths

Based on previous studies of odor-modulated flight where track parameter data was lumped and averaged, the speed and orientation of the moths' movement along their flight tracks have been said to be controlled to maintain certain “preferred” values. The results from our fine-scaled analysis of this behavior show that none of the track parameters typically measured are held constant. The moths' speed along the flight track is modulated substantially and predictably: fastest along the straight legs and slowest around the turns. In addition, about half of the individuals studied progressively reduced the peak speed along the straight legs as they approached the pheromone source. While most of the track legs between the turns were directed upwind, their orientations were widely distributed, indicating no preferred direction. Small fluctuations of orientation along some straight legs suggest corrective maneuvers to stabilize flight direction about an internal set point. The visual inputs hypothesized to control steering and speed, transverse and longitudinal image flow, changed continuously during upwind flight in pheromone, but no regular relationship between them was observed. We found that the orientation of the longitudinal body axis and the direction of thrust (course angle) were only rarely coincident during upwind flight to the odor source, suggesting that moths receive sensory input which differs quantitatively from that calculated by conventional methods. Our results strongly suggest that the long-accepted hypothetical mechanisms of control for this behavior do not operate in the manner in which they have been proposed. Accepted: 11 July 1997     

Activity of the forewing stretch receptor in immature and mature adult locusts

Maturation of the flight system of Locusta migratoria occurs during the first two weeks following imaginal ecdysis. One aspect of maturation is an increase in the wingbeat frequency from about 13 Hz to about 23 Hz. We investigated physiological and anatomical mechanisms that may contribute to this process. The difference between the frequencies of the central flight rhythms of immature and mature deafferented preparations was not as great as that between the wingbeat frequencies of immature and mature intact animals. Results from static and dynamic wing elevation showed that the intensity of the forewing stretch receptor response to a given stimulus increased during maturation. The diameter of the main stretch receptor axon was larger and the conduction velocity of signals conveyed along the forewing stretch receptor and the dorsal longitudinal motoneuron was faster in mature than in immature animals. We conclude that during maturation of the flight system the forewing stretch receptor responds to wing elevation with a higher frequency signal that reaches the central circuitry faster. These findings are discussed in the context of a model that describes the influence of stretch receptor input on wingbeat frequency along with other potential mechanisms involved in flight maturation.Abbreviations fDLMn forewing dorsal longitudinal motoneuron - fSR forewing stretch receptor - SR stretch receptor     

Flight duration and flight muscle ultrastructure of unfed hawk moths

Flight muscle breakdown has been reported for many orders of insects, but the basis of this breakdown in insects with lifelong dependence on flight is less clear. Lepidopterans show such muscle changes across their lifespans, yet how this change affects the ability of these insects to complete their life cycles is not well documented. We investigated the changes in muscle function and ultrastructure of unfed aging adult hawk moths (Manduca sexta). Flight duration was examined in young, middle-aged, and advanced-aged unfed moths. After measurement of flight duration, the main flight muscle (dorsolongitudinal muscle) was collected and histologically prepared for transmission electron microscopy to compare several measurements of muscle ultrastructure among moths of different ages. Muscle function assays revealed significant positive correlations between muscle ultrastructure and flight distance that were greatest in middle-aged moths and least in young moths. In addition, changes in flight muscle ultrastructure were detected across treatment groups. The number of mitochondria in muscle cells peaked in middle-aged moths. Many wild M. sexta do not feed as adults; thus, understanding the changes in flight capacity and muscle ultrastructure in unfed moths provides a more complete understanding of the ecophysiology and resource allocation strategies of this species.     

Escapes with and without preparation: The neuroethology of visual startle in locusts

Locusts respond to the images of approaching (looming) objects with responses that include gliding while in flight and jumping while standing. For both of these responses there is good evidence that the DCMD neuron (descending contralateral movement detector), which carries spike trains from the brain to the thoracic ganglia, is involved. Sudden glides during flight, which cause a rapid loss of height, are last-chance manoeuvres without prior preparation. Jumps from standing require preparation over several tens of milliseconds because of the need to store muscle-derived energy in a catapult-like mechanism. Locusts’ DCMD neurons respond selectively to looming stimuli, and make connections with some motor neurons and interneurons known to be involved in flying and jumping. For glides, a burst of high-frequency DCMD spikes is a key trigger. For jumping, a similar burst can influence timing, but neither the DCMD nor any other single interneuron has been shown to be essential for triggering any stage in preparation or take-off. Responses by the DCMD to looming stimuli can alter in different behavioural contexts: in a flying locust, arousal ensures a high level of both DCMD responsiveness and glide occurrence; and there are significant differences in DCMD activity between locusts in the gregarious and the solitarious phase.     

Oxygen partial pressure effects on metabolic rate and behavior of tethered flying locusts

Resting insects are extremely tolerant of hypoxia. However, oxygen requirements increase dramatically during flight. Does the critical atmospheric P (O)(2) (P(c)) increase strongly during flight, or does increased tracheal conductance allow even flying insects to possess large safety margins for oxygen delivery? We tested the effect of P(O)(2) on resting and flying CO(2) emission, as well as on flight behavior and vertical force production in flying locusts, Schistocerca americana. The P(c) for CO(2) emission of resting animals was less than 1 kPa, similar to prior studies. The P(c) for flight bout duration was between 10 and 21 kPa, the P(c) for vertical force production was between 3 and 5 kPa, and the P(c) for CO(2) emission was between 10 and 21 kPa. Our study suggests that the P(c) for steady-state oxygen consumption is between 10 and 21 kPa (much higher than for resting animals), and that tracheal oxygen stores allowed brief flights in 5 and 10 kPa P(O)(2) atmospheres to occur. Thus, P(c) values strongly increased during flight, consistent with the hypothesis that the excess oxygen delivery capacity observed in resting insects is substantially reduced during flight.     

Plasticity of olfactory-guided behaviour and its neurobiological basis: lessons from moths and locusts

The sense of smell plays an important role in guiding the behaviour of many animals including insects. The attractiveness of a volatile is not only dependent on the nature of the chemical, but might change with the physiological status (e.g., age/hormone or mating status) or environmental conditions (e.g., photoperiod or temperature) of the individual. Here we summarize our studies focused on the plasticity of olfactory-guided behaviour and its neurobiological basis linked with the physiological status in Lepidoptera and migratory locusts. In moths and locusts, age and juvenile hormone changed the behavioural responses to pheromones. In moths, mating had an effect on pheromone responses in males and plant odour responses in females. In all cases of behavioural plasticity studied, we found changes in the sensitivity of olfactory interneurons in the antennal lobe, whereas the peripheral system does not seem to show any plasticity in that context. Changes in the central nervous system were slow under the influence of juvenile hormone (days) or fast after mating (minutes). The olfactory system seems thus to adapt to the physiological or environmental situation of an animal to avoid a waste of energy. We discuss possible mechanisms underlying the observed plasticity.     

Neuronal co-processing of course deviations and head movements in locusts

Summary In Locusta migratoria, the major pathway from descending deviation detectors (DDNs; preceding paper, Hensler 1992) to wing motoneurons involves a population of thoracic interneurons (TINs). Nine TINs are characterized which receive input from cervical proprioreceptors. Responses to the combination of exteroreceptive input (signalling course deviation) and proprioreceptive input (monitoring movement and position of the head) are described and compared to those of DDNs to the same stimuli.Abbreviations AP action potential - DDN descending deviation detector neuron - TCG tritocerebral giant neuron - TIN thoracic interneuron     

Neuronal co-processing of course deviation and head movements in locusts

Summary Descending deviation detector neurons (DDNs) of Locusta migratoria are characterized physiologically by their responses to light on/off stimuli, simulated course deviation (rotation of an artificial horizon), passive rotation of the head, frontal wind, and flight activity. The investigation emphasises on the co-processing of exteroceptive input signalling course deviation (mainly movement of the retinal image, but also wind), and proprioceptive input signalling head movement and position. Stimuli were presented in combinations as expected during natural behavior. Eight DDNs are described for the first time, and 3 previously described DDNs are characterized further. Responses to horizon rotation and imposed head movements are assigned to one of 4 response types: (1) the horizon-only type codes retinal slip and/or the position of the horizon in the visual field but ignores cervical proprioception; (2) the head-only type ignores visually simulated course deviation but codes for movement or position of the head; (3) in the compensating type, head rolling causes visual input and cervical proprioceptive input of opposite signs, so that head movements themselves are ignored, whereas course deviations are recognized; (4) in the amplifying type, head rolling causes visual input and cervical proprioceptive input of the same sign, i.e. one input amplifies the other. This classification does not take the various responses to wind into account. In several DDNs, responses to phasic and tonic stimuli of the same modality, and/or responses to deviations about different axes could be assigned to different response types. Activity in DDNs has been shown previously to result in steering responses of wings, legs, abdomen and/or the head. It is proposed that different kinds of flight steering (e.g. corrective course control, intentional steering, orientation towards or away from a target) may be controlled by selective enhancement or suppression of responses or motor effects of DDN-subpopulations.Abbreviations AP action potential - DDN descending deviation detector neuron - DNI, DNC, DNM descending deviation detector neurons receiving major input from the ipsilateral, contralateral, and median ocellus respectively - PDDSMD protocerebral, descending direction-selective motion-detecting neuron - PI(2)5 descending deviation detector neuron with the cell body in the pars intercerebralis medialis - TCG tritocerebral commissure giant neuron     

Activation phase ensures kinematic efficacy in flight-steering muscles of Drosophila melanogaster

During tethered flight in Drosophila melanogaster, spike activity of the second basalar flight-control muscle (M.b2) is correlated with an increase in both the ipsilateral wing beat amplitude and the ipsilateral flight force. The frequency of muscle spikes within a burst is about 100 Hz, or 1 spike for every two wing beat cycles. When M.b2 is active, its spikes tend to occur within a comparatively narrow phase band of the wing beat cycle. To understand the functional role of this phase-lock of firing in the control of flight forces, we stimulated M.b2 in selected phases of the wing beat cycle and recorded the effect on the ipsilateral wing beat amplitude. Varying the phase timing of the stimulus had a significant effect on the wing beat amplitude. A maximum increase of wing beat amplitude was obtained by stimulating M.b2 at the beginning of the upstroke or about 1 ms prior to the narrow phase band in which the muscle spikes typically occur during flight. Assuming a delay of 1 ms between the stimulation of the motor nerve and muscle activation, these results indicate that M.b2 is activated at an instant of the stroke cycle that produces the greatest effect on wing beat amplitude.     

Visual control of steering in locust flight: the effects of head movement on responses to roll stimuli

The contribution of head movement to the control of roll responses in flying locusts (Locusta migratoria) has been examined (i) on a flight balance, recording the angles through which the locust turns when following an artificial horizon; (ii) by recording activity in a pair of flight muscles in restrained conditions; and (iii) by observations on free flying locusts. Responses were compared when the head was free to turn about the thorax, as normal, and when the head was waxed to the thorax, blocking any relative motion between the two (head-fixed). These experiments suggest that the major signal generating corrective roll manoeuvres is the visual error between the angle of the head and the horizon, rather than a signal that includes a measure of the head-thorax angle.

Collision avoidance during group evasive manoeuvres: a comparison of real versus simulated swarms with manipulated vision and surface wave detectors

Coordinated group motion has been studied extensively both in real systems (flocks, swarms and schools) and in simulations (self-propelled particle (SPP) models using attraction and repulsion rules). Rarely are attraction and repulsion rules manipulated, and the resulting emergent behaviours of real and simulation systems are compared. We compare swarms of sensory-deprived whirligig beetles with matching simulation models. Whirligigs live at the water''s surface and coordinate their grouping using their eyes and antennae. We filmed groups of beetles in which antennae or eyes had been unilaterally obstructed and measured individual and group behaviours. We then developed and compared eight SPP simulation models. Eye-less beetles formed larger diameter resting groups than antenna-less or control groups. Antenna-less groups collided more often with each other during evasive group movements than did eye-less or control groups. Simulations of antenna-less individuals produced no difference from a control (or a slight decrease) in group diameter. Simulations of eye-less individuals produced an increase in group diameter. Our study is important in (i) differentiating between group attraction and repulsion rules, (ii) directly comparing emergent properties of real and simulated groups, and (iii) exploring a new sensory modality (surface wave detection) to coordinate group movement.     

Flight physiology of migrating Urania fulgens (Uraniidae) moths: kinematics and aerodynamics of natural free flight

Summary Air speeds and wing kinematics were determined for the Neotropical moth Urania fulgens in natural migratory flight over Lake Gatun, Republic of Panama. Morphological parameters are presented for the same insects filmed in free flight. A quasi-steady aerodynamic analysis was used to show that unsteady mechanisms of lift generation are probably not necessary to produce the forces necessary for fast forward flight. Mechanical power requirements of forward flight were estimated from the biomechanical and morphological data. Over an airspeed range of 1.5 to 4.5 m/s, the mechanical power required to fly is predicted to increase dramatically with forward speed. A comparison of estimated metabolic rates with endogenous lipid reserves suggests that U. fulgens feeds extensively on flower nectar during migration from Central into South America.