Muscle function in avian flight: achieving power and control

Flapping flight places strenuous requirements on the physiological performance of an animal. Bird flight muscles, particularly at smaller body sizes, generally contract at high frequencies and do substantial work in order to produce the aerodynamic power needed to support the animal's weight in the air and to overcome drag. This is in contrast to terrestrial locomotion, which offers mechanisms for minimizing energy losses associated with body movement combined with elastic energy savings to reduce the skeletal muscles' work requirements. Muscles also produce substantial power during swimming, but this is mainly to overcome body drag rather than to support the animal's weight. Here, I review the function and architecture of key flight muscles related to how these muscles contribute to producing the power required for flapping flight, how the muscles are recruited to control wing motion and how they are used in manoeuvring. An emergent property of the primary flight muscles, consistent with their need to produce considerable work by moving the wings through large excursions during each wing stroke, is that the pectoralis and supracoracoideus muscles shorten over a large fraction of their resting fibre length (33-42%). Both muscles are activated while being lengthened or undergoing nearly isometric force development, enhancing the work they perform during subsequent shortening. Two smaller muscles, the triceps and biceps, operate over a smaller range of contractile strains (12-23%), reflecting their role in controlling wing shape through elbow flexion and extension. Remarkably, pigeons adjust their wing stroke plane mainly via changes in whole-body pitch during take-off and landing, relative to level flight, allowing their wing muscles to operate with little change in activation timing, strain magnitude and pattern.     

Independently Controlled Wing Stroke Patterns in the Fruit Fly Drosophila melanogaster

Flies achieve supreme flight maneuverability through a small set of miniscule steering muscles attached to the wing base. The fast flight maneuvers arise from precisely timed activation of the steering muscles and the resulting subtle modulation of the wing stroke. In addition, slower modulation of wing kinematics arises from changes in the activity of indirect flight muscles in the thorax. We investigated if these modulations can be described as a superposition of a limited number of elementary deformations of the wing stroke that are under independent physiological control. Using a high-speed computer vision system, we recorded the wing motion of tethered flying fruit flies for up to 12 000 consecutive wing strokes at a sampling rate of 6250 Hz. We then decomposed the joint motion pattern of both wings into components that had the minimal mutual information (a measure of statistical dependence). In 100 flight segments measured from 10 individual flies, we identified 7 distinct types of frequently occurring least-dependent components, each defining a kinematic pattern (a specific deformation of the wing stroke and the sequence of its activation from cycle to cycle). Two of these stroke deformations can be associated with the control of yaw torque and total flight force, respectively. A third deformation involves a change in the downstroke-to-upstroke duration ratio, which is expected to alter the pitch torque. A fourth kinematic pattern consists in the alteration of stroke amplitude with a period of 2 wingbeat cycles, extending for dozens of cycles. Our analysis indicates that these four elementary kinematic patterns can be activated mutually independently, and occur both in isolation and in linear superposition. The results strengthen the available evidence for independent control of yaw torque, pitch torque, and total flight force. Our computational method facilitates systematic identification of novel patterns in large kinematic datasets.     

Genetic Variability of Flight Metabolism in DROSOPHILA MELANOGASTER. I. Characterization of Power Output during Tethered Flight

The mechanical power imparted to the wings during tethered flight of Drosophila melanogaster is estimated from wing-beat frequency, wing-stroke amplitude and various aspects of wing morphology by applying the steady-state aerodynamics model of insect flight developed by Weis-Fogh (1972, 1973). Wing-beat frequency, the major determinant of power output, is highly correlated with the rate of oxygen consumption. Estimates of power generated during flight should closely reflect rates of ATP production in the flight muscles, since flies do not acquire an oxygen debt or accumulate ATP during flight. In an experiment using 21 chromosome 2 substitution lines, lines were a significant source of variation for all flight parameters measured. Broadsense heritabilities ranged from 0.16 for wing-stroke amplitude to 0.44 for inertial power. The variation among lines is not explained by variation in total body size (i.e., live weight). Line differences in flight parameters are robust with respect to age, ambient temperature and duration of flight. These results indicate that characterization of the power output during tethered flight will provide a sensitive experimental system for detecting the physiological effects of variation in the structure or quantity of the enzymes involved in flight metabolism.     

The Function of Dipteran Flight Muscle

The flight muscles of flies are separated into two physiologically, anatomically, and functionally distinct classes: power muscles and control muscles. The large indirect power muscles sustain the high level of mechanical energy required to flap the wings up and down during flight. The contractions in the asynchronous power muscles are initiated by stretch, and their slow presynaptic motor drive serves only to maintain a tonic level of cytosolic calcium. Although providing the mechanical energy for flight, the power muscles are not directly attached to the wings. Instead, their mechanical energy is transmitted to the base of the wings through the complex linkage system of the wing hinge. In contrast, the small control muscles insert directly onto the skeletal elements at the base of the wing. Through their mechanical effects on the hinge, the control muscles act collectively as a transmission system that determines how the mechanical energy produced by the power muscles is transformed into wing motion. The control muscles are activated by motor spikes in the conventional one-for-one fashion. Thus, although the control muscles can generate little mechanical power, they provide the means by which the nervous system can rapidly alter wing kinematics during sophisticated aerial maneuvers.     

Genetic Variability of Flight Metabolism in DROSOPHILA MELANOGASTER . II. Relationship between Power Output and Enzyme Activity Levels

The major goal of the studies reported here was to determine the extent to which genetic variation in the activities of the enzymes participating in flight metabolism contributes to variation in the mechanical power output of the flight muscles in Drosophila melanogaster. Isogenic chromosome substitution lines were used to partition the variance of both types of quantitative trait into genetic and environmental components. The mechanical power output was estimated from the wingbeat frequency, wing amplitude and wing morphology of tethered flies by applying the aerodynamic models of Weis-Fogh and Ellington. There were three major results. (1) Chromosomes sampled from natural populations provide a large and repeatable genetic component to the variation in the activities of most of the 15 flight metabolism enzymes investigated and to the variation in the mechanical power output of the flight muscles. (2) The mechanical power output is a sensitive indicator of the rate of flight metabolism (i.e., rate of oxygen consumption during tethered flight). (3) In spite of (1) and (2), no convincing cases of individual enzyme effects on power output were detected, although the number and sign of the significant enzyme-power correlations suggests that such effects are not totally lacking.     

Muscle Function in vivo: A Comparison of Muscles used for Elastic Energy Savings versus Muscles Used to Generate Mechanical Power1

SYNOPSIS. The function of muscles used to generate force economicallyand facilitate elastic energy savings in their tendons is comparedwith muscles that function to produce mechanical power. Theunderlying architectural design of the muscle and its tendon(if present) dictate much of their functional capacity and rolein animal locomotion. Using methods that allow direct recordingsof muscle force and fiber length change, the functional designof muscle-tendon systems can now be investigated in vivo. Thesestudies reveal that, in the case of wallaby hindleg muscles,the fibers can maintain sufficient stiffness during tendon stretchand recoil to ensure useful elastic energy recovery and savingsof metabolic energy. In the case of the pectoralis muscle ofpigeons, although isometric or active lengthening of the muscle'sfibers may occur late in the upstroke of the wing beat cycleto enhance force development, the fibers shorten extensivelyduring the downstroke (up to 35% of their resting length) toproduce mechanical power for aerodynamic lift and thrust. Oscillatorylength change, with force enhancement during active lengtheningmay be a general feature of muscles that power aerial and aquaticlocomotion. Similarly, force enhancement by active lengtheningis likely to be important to the design and function of musclesthat primarily generate force to minimize energy expenditure/unitforce generated, as well as for elastic energy savings withina long tendon. Architectural features of muscle-tendon unitsfor effective elastic energy savings, however, are likely toconstrain locomotor performance when mechanical work is required,as when an animal accelerates, either limiting performance orrequiring the recruitment of functional agonists with greatermechanical power generating capability (i.e., longer fibers)     

Comparative characteristics of the wing apparatus and flight of Syrphid flies (Diptera: Syrphidae)

Comparative analysis of the wing apparatus and flight in nine species of flower flies (Syrphidae) has been performed. Data on the flight velocity, aerodynamic force, wing-beat frequency, stroke amplitude and stroke plane angle, wing area, body mass and volume, as well as correlations between these parameters at the intraspecific and family levels, have been obtained. Based on the obtained data, the subfamilies Syrphinae and Eristalinae have been compared.     

In Vivo Time-Resolved Microtomography Reveals the Mechanics of the Blowfly Flight Motor

Dipteran flies are amongst the smallest and most agile of flying animals. Their wings are driven indirectly by large power muscles, which cause cyclical deformations of the thorax that are amplified through the intricate wing hinge. Asymmetric flight manoeuvres are controlled by 13 pairs of steering muscles acting directly on the wing articulations. Collectively the steering muscles account for <3% of total flight muscle mass, raising the question of how they can modulate the vastly greater output of the power muscles during manoeuvres. Here we present the results of a synchrotron-based study performing micrometre-resolution, time-resolved microtomography on the 145 Hz wingbeat of blowflies. These data represent the first four-dimensional visualizations of an organism''s internal movements on sub-millisecond and micrometre scales. This technique allows us to visualize and measure the three-dimensional movements of five of the largest steering muscles, and to place these in the context of the deforming thoracic mechanism that the muscles actuate. Our visualizations show that the steering muscles operate through a diverse range of nonlinear mechanisms, revealing several unexpected features that could not have been identified using any other technique. The tendons of some steering muscles buckle on every wingbeat to accommodate high amplitude movements of the wing hinge. Other steering muscles absorb kinetic energy from an oscillating control linkage, which rotates at low wingbeat amplitude but translates at high wingbeat amplitude. Kinetic energy is distributed differently in these two modes of oscillation, which may play a role in asymmetric power management during flight control. Structural flexibility is known to be important to the aerodynamic efficiency of insect wings, and to the function of their indirect power muscles. We show that it is integral also to the operation of the steering muscles, and so to the functional flexibility of the insect flight motor.     

Aerial locomotion in flies and robots: kinematic control and aerodynamics of oscillating wings

Flight in flies results from a feedback cascade in which the animal converts mechanical power produced by the flight musculature into aerodynamic forces. A major goal of flight research is to understand the functional significance of the various components in this cascade ranging from the generation of the neural code, the control of muscle mechanical power output, wing kinematics and unsteady aerodynamic mechanisms. Here, I attempted to draw a broad outline on fluid dynamic mechanisms found in flapping insect wings such as leading edge vorticity, rotational circulation and wake capture momentum transfer, as well as on the constraints of flight force control by the neuromuscular system of the fruit fly Drosophila. This system-level perspective on muscle control and aerodynamic mechanisms is thought to be a fundamental bridge in any attempt to link the function and performance of the various flight components with their particular role for wing motion and aerodynamic control in the behaving animal. Eventually, this research might facilitate the development of man-made biomimetic autonomous micro air vehicles using flapping wing motion for propulsion that are currently under construction by engineers.     

The constraints of body size on aerodynamics and energetics in flying fruit flies: an integrative view

Reynolds number and thus body size may potentially limit aerodynamic force production in flying insects due to relative changes of viscous forces on the beating wings. By comparing four different species of fruit flies similar in shape but with different body mass, we have investigated how small insects cope with changes in fluid mechanical constraints on power requirements for flight and the efficiency with which chemical energy is turned into aerodynamic flight forces. The animals were flown in a flight arena in which stroke kinematics, aerodynamic force production, and carbon dioxide release were measured within the entire working range of the flight motor. The data suggest that during hovering performance mean lift coefficient for flight is higher in smaller animals than in their larger relatives. This result runs counter to predictions based on conventional aerodynamic theory and suggests subtle differences in stroke kinematics between the animals. Estimates in profile power requirements based on high drag coefficient suggest that among all tested species of fruit flies elastic energy storage might not be required to minimize energetic expenditures during flight. Moreover, muscle efficiency significantly increases with increasing body size whereas aerodynamic efficiency tends to decrease with increasing size or Reynolds number. As a consequence of these two opposite trends, total flight efficiency tends to increase only slightly within the 6-fold range of body sizes. Surprisingly, total flight efficiency in fruit flies is broadly independent of different profile power estimates and typically yields mean values between 2–4%.     

Disrupting the myosin converter-relay interface impairs Drosophila indirect flight muscle performance

Structural interactions between the myosin converter and relay domains have been proposed to be critical for the myosin power stroke and muscle power generation. We tested this hypothesis by mutating converter residue 759, which interacts with relay residues I508, N509, and D511, to glutamate (R759E) and determined the effect on Drosophila indirect flight muscle mechanical performance. Work loop analysis of mutant R759E indirect flight muscle fibers revealed a 58% and 31% reduction in maximum power generation (P_WL) and the frequency at which maximum power (f_WL) is generated, respectively, compared to control fibers at 15°C. Small amplitude sinusoidal analysis revealed a 30%, 36%, and 32% reduction in mutant elastic modulus, viscous modulus, and mechanical rate constant 2πb, respectively. From these results, we infer that the mutation reduces rates of transitions through work-producing cross-bridge states and/or force generation during strongly bound states. The reductions in muscle power output, stiffness, and kinetics were physiologically relevant, as mutant wing beat frequency and flight index decreased about 10% and 45% compared to control flies at both 15°C and 25°C. Thus, interactions between the relay loop and converter domain are critical for lever-arm and catalytic domain coordination, high muscle power generation, and optimal Drosophila flight performance.     

On the mechanism of speed and altitude control in Drosophila melanogaster

ABSTRACT The total power output of tethered flying Drosophila melanogaster in still air depends on translational velocity components of image flow on the eye, whereas the orientation of the average flight force in the midsagittal plane of the fly is widely independent of visual input (Götz, 1968). The fly does not seem to control the vertical and the horizontal force component independently. Freely flying flies nevertheless generate different ratios between lift and thrust, simply by changing the inclination of their body. By the combined adjustment of the body angle and the total power output a fly appears to be able to stabilize height and speed (David, 1985). Here a possible mechanism is proposed by which the appropriate torque about the transverse body axis could be generated. Translational pattern motion influences the posture of the abdomen and the plane of wing oscillation. Thus the position of the centre of gravity relative to the flight force vector is changed. When abdomen and stroke plane deviate from an equilibrium state, a lever is generated by which the force vector will rotate the fly about its transverse axis.     

Aerodynamic corrections for the flight of birds and bats in wind tunnels

Few wind tunnel studies of animal flight have controlled or corrected for distortions to behaviour, physiology or flight aerodynamics representing the difference between flight in the tunnel and flight in free air. Aerodynamic correction factors are derived based on lifting-line theory and the method of images for an animal flying freely within closed- and open-section wind tunnels; the method is very similar to that used to model flight in ground effect, and as in ground effect the corrections to induced drag may be substantial. These correction factors are used to estimate bound wing circulation, drag and mechanical power for comparison with free flight, and to derive testable predictions of optimum flight strategies for an animal in a tunnel. In an open-section tunnel, mechanical power is increased compared to free flight, and the animal should fly at the tunnel centre. In a closed tunnel mechanical power is usually reduced, and substantial savings are available, particularly at low speeds, if the animal flies close to the tunnel roof. Anecdotal observations confirm that birds and bats adopt this strategy. The mechanical power-speed curve in a closed tunnel is flatter than the curve for free flight, and this may explain the flat metabolic power-speed curves for birds and bats obtained in some measurements.     

Haltere-mediated equilibrium reflexes of the fruit fly, Drosophila melanogaster.

Flies display a sophisticated suite of aerial behaviours that require rapid sensory-motor processing. Like all insects, flight control in flies is mediated in part by motion-sensitive visual interneurons that project to steering motor circuitry within the thorax. Flies, however, possess a unique flight control equilibrium sense that is encoded by mechanoreceptors at the base of the halteres, small dumb-bell-shaped organs derived through evolutionary transformation of the hind wings. To study the input of the haltere system onto the flight control system, I constructed a mechanically oscillating flight arena consisting of a cylindrical array of light-emitting diodes that generated the moving image of a 30 degrees vertical stripe. The arena provided closed-loop visual feedback to elicit fixation behaviour, an orientation response in which flies maintain the position of the stripe in the front portion of their visual field by actively adjusting their wing kinematics. While flies orientate towards the stripe, the entire arena was swung back and forth while an optoelectronic device recorded the compensatory changes in wing stroke amplitude and frequency. In order to reduce the background changes in stroke kinematics resulting from the animal's closed-loop visual fixation behaviour, the responses to eight identical mechanical rotations were averaged in each trial. The results indicate that flies possess a robust equilibrium reflex in which angular rotations of the body elicit compensatory changes in both the amplitude and stroke frequency of the wings. The results of uni- and bilateral ablation experiments demonstrate that the halteres are required for these stability reflexes. The results also confirm that halteres encode angular velocity of the body by detecting the Coriolis forces that result from the linear motion of the haltere within the rotating frame of reference of the fly's thorax. By rotating the flight arena at different orientations, it was possible to construct a complete directional tuning map of the haltere-mediated reflexes. The directional tuning of the reflex is quite linear such that the kinematic responses vary as simple trigonometric functions of stimulus orientation. The reflexes function primarily to stabilize pitch and yaw within the horizontal plane.     

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.     

Aerodynamics and Energetics of Intermittent Flight in Birds

Hypotheses explaining the use of intermittent bounding and undulatingflight modes in birds are considered. Existing theoretical modelsof intermittent flight have assumed that the animal flies ata constant speed throughout. They predict that mean mechanicalpower in undulating (flap-gliding) flight is reduced comparedto steady flight over a broad range of speeds, but is reducedin bounding flight only at very high flight speeds. Lift generatedby the bird's body or tail has a small effect on power, butis insufficient to explain observations of bounding at intermediateflight speeds. Measurements on starlings Sturnus vulgaris inundulating flight in a wind tunnel show that flight speed variesby around ±1 m/sec during a flap-glide cycle. Dynamicenergy is used to quantify flight performance, and reveals thatthe geometry of the flight path depends upon wingbeat kinematics,and that neither flapping nor gliding phases are at constantspeed and angle to the horizontal. The bird gains both kineticand potential energy during the flapping phases. A new theoreticalmodel indicates that such speed variation can give significantsavings in mechanical power in both bounding and undulatingflight. Alternative hypotheses for intermittent flight includea gearing mechanism, based on duty factor, mediating musclepower or force output against aerodynamic requirements. Thiscould explain the use of bounding flight in hovering and climbingin small passerines. Both bounding and undulating confer otheradaptive benefits; undulating may be primitive in birds, butbounding may have evolved in response to flight performanceoptimization, or to factors such as unpredictability in responseto predation.     

How Insect Flight Steering Muscles Work

Insights into how exactly a fly powers and controls flight have been hindered by the need to unpick the dynamic complexity of the muscles involved. The wingbeats of insects are driven by two antagonistic groups of power muscles and the force is funneled to the wing via a very complex hinge mechanism. The hinge consists of several hardened and articulated cuticle elements called sclerites. This articulation is controlled by a great number of small steering muscles, whose function has been studied by means of kinematics and muscle activity. The details and partly novel function of some of these steering muscles and their tendons have now been revealed in research published in this issue of PLOS Biology. The new study from Graham Taylor and colleagues applies time-resolved X-ray microtomography to obtain a three-dimensional view of the blowfly wingbeat. Asymmetric power output is achieved by differential wingbeat amplitude on the left and right wing, which is mediated by muscular control of the hinge elements to mechanically block the wing stroke and by absorption of work by steering muscles on one of the sides. This new approach permits visualization of the motion of the thorax, wing muscles, and the hinge mechanism. This very promising line of work will help to reveal the complete picture of the flight motor of a fly. It also holds great potential for novel bio-inspired designs of fly-like micro air vehicles.The ability for powered flight has evolved four times in the animal kingdom and, thanks to their ability to fly, insects have diversified and moved into new regions and habitats with enormous success [1]. Powered flight requires an integrated system consisting of wings to generate aerodynamic force, muscles to move the wings, and a control system to modulate power output from the muscles. Insects are bewilderingly diverse with respect to flight morphology and behaviors, which in turn provides a real challenge to researchers wishing to understand how insects fly. In particular, the impressive flight maneuvers in flies, such as blowflies and fruit flies, have inspired scientists for many years [2]. The ability of a fly to accelerate, make tight turns, rolls, and loops that allow the creature to land upside down on a ceiling is unparalleled in any other organisms, as well as any manmade aircraft. Everybody knows how difficult it is to swat a fly with bare hands—the fly''s capacity for rapid take-off and accurate movement away from a perceived approaching threat is exquisite [3].The flight muscles of many insects, including flies, bees, and mosquitoes, are divided into a few large power muscles that simply contract cyclically to generate sheer power output and a greater number of smaller steering muscles that control the force transmission from the power muscles to the wing [4]–[6]. The power muscles of a fly consist of two sets of antagonistic muscles attached to the inside of the thorax (exoskeleton) (Figure 1). In many insects, including flies, these muscles are asynchronous, which means their contractions are uncoupled to the firing rate of the associated motor neuron [6],[7], i.e., the muscles continue to contract as long as the nerve tickles them. Another characteristic feature of the power muscles is that they are stretch-activated and contract as a response to being lengthened. Both sets of power muscles deform the thorax when contracted such that when the dorso-ventral muscles contract, the thorax is squeezed together dorso-ventrally while expanding longitudinally, and vice versa when the dorsal-longitudinal muscles contract as a response to prior lengthening. The result is an alternate contraction and lengthening of these perpendicular muscle groups and a resonance of the entire thorax that drives the wingbeat. Typical wingbeat frequencies are in the range from 100 Hz and even up to 1,000 Hz in the smallest species [5],[8].Open in a separate windowFigure 1The thorax with and dorsal longitudinal (upper left) and dorso-ventral (upper right) power flight muscles of a fly.The cartoon (bottom) shows a transverse section through the thorax with dorso-ventral muscles (DVM) and dorsal longitudinal muscles (DLM) indicated. The two upper illustrations are redrawn from [6].The forces from the flight muscles are transmitted to the wing through an intricate hinge mechanism (Figure 2). The hardened plates of cuticle between the thorax and wing (sclerites) are mobile and their positions relative to the thoracic outgrowths and wing determine the extent of the wing motion, i.e., the angular amplitude of the wingbeat [6].Open in a separate windowFigure 2Cartoon illustration of a transverse section of the thorax of a fly in rear view, showing some elements of the complex wing hinge of a fly, consisting of ridges and protrusions on the thorax and a number of hardened plates of cuticle (sclerites) between the body (thorax) and the wing root.The basalare sclerite (not shown) is positioned anterior of the first axillary sclerite (Ax1). The indicated structures are dorso-ventral power muscle (DVM), pleural wing process (PWP), post-medial notal process (PMNP), parascutal shelf (PSS), axial wing sclerites (Ax1, Ax2, Ax3), and radial stop (RS). Redrawn and modified from [18].Flight maneuvers arise owing to asymmetric force generation between the left and right wing. Aerodynamic force is proportional to the angle of attack (the angle between the wing surface and the airflow) and the speed squared relative to the air [9],[10]. Except from the turning points of each half-stroke, when the wings rotate about their span wise axes, the angle of attack is usually quite constant during the translational phases of the wingbeat [10], while asymmetric forces are mainly created by changing the wingbeat amplitude in flies [11]–[14]. With wingbeat frequency kept constant, changed amplitude changes the speed and hence force generated.The control of the elements forming the hinge mechanism of the wing is achieved by the steering muscles, which are tiny in terms of mass (<3% of the power muscle mass), but mean everything when it comes to making flight maneuvers. In contrast to the power muscles the steering muscles are synchronous, i.e., there is a 1∶1 correspondence between neural spikes and muscle contraction. No less than some 22 pairs of steering muscles are involved in the force transmission; a few of these indirectly modulate the output by affecting the resonating properties of the thorax, while others are directly attached to the sclerite elements of the hinge mechanism [6],[15]. Three small muscles (b1–b3) are attached to the basalare plate that is directly involved in wing articulation (Figure 3). The actual wing sclerites (Figure 2) are also controlled by specific steering muscles, also with the function of moving the sclerites in relation to required wing motion. The main control function of the hinge mechanism appears to be of the downward movement of the wing, i.e., the angle at the turning point at end of downstroke. For a detailed review about the steering muscles and their function see Dickinson and Tu [6].Open in a separate windowFigure 3The position of the three steering flight muscles b1–b3 inserted to the nail-shaped basalare sclerite.Contraction by the b1 and b2 muscles move the basalare forward and their antagonist b3 moves it backwards when contracted. Redrawn from [6].To date, the function of the steering muscles has been revealed mainly by electrophysiological studies on tethered subjects. Tethering means that the animal is glued to the end of a thin rod, often with force sensors attached to it, and then stimulated to “fly.” In many insects this can be achieved by simply blowing at them or placing them in a wind tunnel. On the tether the insect can either be presented with a visual stimulus or be rotated, which flies can sense via their halteres (hind wings modified to sensory gyroscopic sensory organs) [16]. By inserting electrode wires into the steering muscles, the neural impulses are measured at the same time as the wingbeat kinematics is recorded [13],[17]. What we know about the function of the steering muscles comes from the meticulous studies of correlations between muscle activity and the associated wing movement, including how the hinge mechanism works [6],[18]. Needless to say, such experiments are extremely difficult to achieve in small insects like blowflies and fruit flies that flap their wings at high frequencies. Recent studies of the wing and hinge kinematics provide some support for the hypothesis that the hinge may have a gear function that affects stroke amplitude, as well [18]. However, there are still many open questions regarding the exact function of the steering muscles and how they help in generating laterally asymmetric forces during a fly''s flight maneuver [6].In an article published in this issue of PLOS Biology, Walker and colleagues take a new approach for studying how steering muscles regulate the power output from power muscles [19], using time-resolved x-ray microtomography [20]. By rotating tethered blowflies (Calliphora vicina) in the X-ray beam, a 3D-movie was captured that shows how the steering muscles move. This by itself is a grand achievement at a wingbeat frequency of 145 Hz. As the flies could sense being rotated the steering muscles acted accordingly to achieve an asymmetric power output as a response to a perceived turn. The movies that accompany the article show how several of the key steering muscles and their sclerites operate in concert during the course of a wingbeat, and the visual results are supported by advanced statistical analyses of muscle strain rates and their phase offset. For example, the b1 and b3 muscles (Figure 3) work antagonistically, as was known before, but on the low-amplitude wing the oscillations are delayed by about a quarter of a wingbeat. The strain amplitudes of b1 and b3 were different between the two wings, which were found to be due to dorso-ventral movement of the basalare sclerite on the high-amplitude side and rotation on the low-amplitude side. This shows even higher complexity of the wing hinge than was previously envisaged.The measurements of strain rate in the muscle confirmed the results of a previous study, which showed that asymmetric power output is partially achieved by negative work [21], i.e., absorption of work, by the b1 muscle on the low-amplitude wing. As with other muscles, the steering muscles insert on the skeletal parts and sclerites by tendons. The tendon of the muscle (I1) associated with the first axillary sclerite was observed to buckle when the wing was elevated above the wing hinge, indicative of compressive force acting on it near the top of the wing stroke. This buckling of the tendon forces a reinterpretation of the function of this muscle: it is involved in reducing stroke amplitude at the bottom of the downstroke rather than exerting stress near the opposite end of the stroke. Tendon buckling was seen in some other muscles as well, and although this is its first observation, it may be a more general mechanism involved in control of insect wingbeat kinematics.What are the wider implications of this new study? First, it demonstrates the utility of a new approach to examine the in vivo operation of several insect flight muscles. This alone signals a methodological breakthrough that promises more. So far the flies were tethered and studied during one behavioral treatment (rotation about the yaw axis). Real flight maneuvers, however, also involve angular rotation about pitch and roll axes, acceleration, and braking. Thus, it remains to be seen how the steering muscles operate to control more subtle changes in wing kinematics during the turning saccades and advanced flight maneuvers that take place during free flight. The method involved exposure to lethal X-ray doses, which of course limits how long the experiments can be. Second, tethering is the prevailing paradigm for studying insect flight, but because it interrupts the sensory feedback loop [22], it would be useful for future studies to compare tethered and free flight in some commonly studied species. Furthermore, a more complete understanding of the flight muscle-hinge mechanism may help bio-inspired design of wing articulation systems for fly-like micro air vehicles. Until then, we can enjoy the stunning videos of the oscillating thorax and flight muscle system of the blowfly [19]. See the video from the related research article here (http://youtu.be/P6lBkK3J9wg) or [19].     

Plastic and Genetic Variation in Wing Loading as a Function of Temperature Within and Among Parallel Clines in Drosophila subobscura

Drosophila subobscura is a European (EU) species that was introducedinto South America (SA) approximately 25 years ago. Previousstudies have found rapid clinal evolution in wing size and inchromosome inversion frequency in the SA colonists, and theseclines parallel those found among the ancestral EU populations.Here we examine thermoplastic changes in wing length in fliesreared at 15, 20, and 25°C from 10 populations on each continent.Wings are plastically largest in flies reared at 15°C (thecoldest temperature) and genetically largest from populationsthat experience cooler temperatures on both continents. We hypothesizethat flies living in cold temperatures benefit from reducedwing loading: ectotherms with cold muscles generate less powerper wing beat, and hence larger wings and/or a smaller masswould facilitate fight. We develop a simple null model, basedon isometric growth, to test our hypothesis. We find that bothEU and SA flies exhibit adaptive plasticity in wing loading:flies reared at 15°C generally have lower wing loadingsthan do flies reared at 20°C or 25°C. Clinal patterns,however, are strikingly different. The ancestral EU populationsshow adaptive clinal variation at rearing a temperature of 15°C:flies from cool climates have lower wing loadings. In the colonizingpopulations from SA, however, we cannot reject the null model:wing loading increases with decreasing clinal temperatures.Our data suggest that selective factors other than flight havefavored the rapid evolution of large overall size at low environmentaltemperatures. However, selection for increased flight abilityin such environments may secondarily favor reduced body mass.     

Ambient temperature affects free-flight performance in the fruit fly Drosophila melanogaster

To gain insight into how temperature affects locomotor performance in insects, the limits of flight performance have been estimated in freely flying fruit flies Drosophila melanogaster by determining the maximum load that a fly could carry following take-off. At a low ambient temperature of 15 °C, muscle mechanical power output matches the minimum power requirements for hovering flight. Aerodynamic force production rises with increasing temperature and eventually saturates at a flight force that is roughly equal to 2.1 times the body mass. Within the two-fold range of different body sizes, maximum flight force production during free flight does not decrease with decreasing body size as suggested by standard aerodynamic theories. Estimations of flight muscle mechanical power output yields a peak performance of 110 W kg⁻¹ muscle tissue for short-burst flight that was measured at an ambient temperature of 30 °C. With respect to the uncertainties in estimating muscle mechanical power during free flight, the estimated values are similar to those that were published for flight under tethered flight conditions. Accepted: 5 January 1999     

Wing flexibility enhances load-lifting capacity in bumblebees

The effect of wing flexibility on aerodynamic force production has emerged as a central question in insect flight research. However, physical and computational models have yielded conflicting results regarding whether wing deformations enhance or diminish flight forces. By experimentally stiffening the wings of live bumblebees, we demonstrate that wing flexibility affects aerodynamic force production in a natural behavioural context. Bumblebee wings were artificially stiffened in vivo by applying a micro-splint to a single flexible vein joint, and the bees were subjected to load-lifting tests. Bees with stiffened wings showed an 8.6 per cent reduction in maximum vertical aerodynamic force production, which cannot be accounted for by changes in gross wing kinematics, as stroke amplitude and flapping frequency were unchanged. Our results reveal that flexible wing design and the resulting passive deformations enhance vertical force production and load-lifting capacity in bumblebees, locomotory traits with important ecological implications.