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Insect flight

Insect flight is the powered aerial locomotion achieved by winged insects in the superorder Pterygota, representing a pivotal evolutionary adaptation that originated approximately 400 million years ago in the Early Devonian period and facilitated the diversification of over one million extant species across diverse ecosystems.^[1] This capability relies on specialized chitinous wings, asynchronous flight muscles, and intricate neural control systems that enable maneuvers ranging from hovering to rapid evasion, distinguishing insects as the most aerially dominant group of animals.^[1] Unlike vertebrate flight, insect wings operate through high-frequency flapping—often exceeding 100 Hz in smaller species—generating lift and thrust via unsteady aerodynamics rather than fixed-wing gliding.^[2] The evolution of insect flight traces back to the Early Devonian, with molecular clock estimates placing its emergence between 419 and 393 million years ago, predating the oldest known winged fossils from the Early Carboniferous around 325 million years ago.^[1] Key theories on wing origins include the paranotal hypothesis, positing that wings developed as extensions of thoracic terga (dorsal plates) for gliding before powered flight, supported by fossil evidence from primitive Palaeoptera like dragonflies; and the gill hypothesis, suggesting derivation from aquatic gill-like structures in crustacean ancestors, evidenced by genetic similarities in appendage development.^[3] The split into Palaeoptera (non-folding wings) and Neoptera (foldable wings with resilin for elasticity) estimated during the late Devonian to Carboniferous, around 350-400 million years ago, spurred explosive diversification, with flight enabling habitat expansion, predator escape, and resource exploitation, though secondary flightlessness has evolved repeatedly in stable environments.^[1]^[3]^[4] Insect wings consist of thin, veined membranes that provide structural integrity while allowing passive deformation for enhanced aerodynamics, with veins forming a hierarchical network that boosts fracture toughness and flexibility.^[1] Power is supplied by two primary muscle types: synchronous muscles in larger, basal insects like dragonflies, which contract once per neural impulse for precise control; and asynchronous (myogenic) muscles in advanced orders, which oscillate at frequencies up to 200 Hz with minimal neural input, achieving energetic efficiency through stretch-activation mechanisms.^[1] Wing kinematics involve bidirectional strokes—upstroke and downstroke—modulated by small steering muscles at the wing base, with body size influencing frequency: smaller insects compensate for low Reynolds numbers by increasing flap rates, while larger ones rely on broader wings.^[1] Passive elements, such as elastic resilin in hinges and bristles for wing coupling in moths and bees, further optimize stability and power storage during flight.^[5] Aerodynamically, insect flight exploits unsteady flow phenomena to generate forces at low speeds and small scales, where traditional fixed-wing lift principles falter due to viscous effects.^[2] A primary mechanism is the leading-edge vortex (LEV), a stable, low-pressure swirl that forms on the wing during translation and remains attached to augment lift by up to 50% at high angles of attack, as observed in fruit flies and hawkmoths.^[2] Additional contributions come from the clap-and-fling motion, where wings clap together at stroke reversal to fling apart, creating circulatory flow for enhanced circulation; rotational forces during pronation/supination; and wake capture from prior strokes.^[2] Wing flexibility passively induces camber and twist, improving efficiency by 10–20% and aiding disturbance recovery within 1–2 wingbeats, as seen in hoverflies and beetles.^[5] Flight control integrates sensory inputs from compound eyes, antennae, and halteres (gyroscopic organs in flies) via the brain's central complex, enabling real-time adjustments for navigation, obstacle avoidance, and stability amid gusts.^[1] Ecologically, insect flight underpins pollination, predation, and nutrient cycling, with its loss in groups like ants and termites reflecting trade-offs for ground-based lifestyles, yet its innovation remains central to insects comprising over half of all known biodiversity.^[1]^[3]

Anatomy of Insect Wings and Flight Muscles

Wing Structure and Venation

Insect wings are composed of a thin, flexible membrane supported by a framework of tubular veins. The membrane primarily consists of an epicuticle layer, with varying presence of exocuticle and mesocuticle in species such as those in Odonata, while chitin is notably absent from the membrane in locusts but present in vein cuticles across many taxa.^[6] Veins form multilayered cuticles (up to six layers) containing chitin and resilin, serving as hollow tubes that transport hemolymph for nutrient delivery, oxygenation, and waste removal to the living wing tissues.^[6] These veins also house tracheae for oxygen supply and nerves for sensory functions, with hemolymph flow patterns including looped circulation in crane flies or oscillatory flow in butterflies, driven by thoracic pulsatile organs and vein elasticity.^[6] Wing attachment occurs via hinge regions, often reinforced by flexion lines that act as one-way hinges promoting controlled bending and twisting during deployment.^[7] The leading edge typically features denser venation for enhanced rigidity, while the trailing edge is thinner and more flexible, allowing dynamic camber changes of up to 12% in species like hoverflies.^[7] Wing venation follows a primitive pattern of six symmetrical pairs of veins—costa (C), subcosta (Sc), radius (R), media (M), cubitus (Cu), and anal (A)—each with a convex anterior branch and concave posterior branch arising from blood sinuses.^[8] In archaic Paleoptera (e.g., Odonata), venation is retained in a straight hinge series with wings held extended, whereas Neoptera exhibit a derived V-shaped hinge enabling wing folding over the body.^[8] This venation network functions as a lightweight mechanical skeleton, with tubular struts optimizing stiffness-to-weight ratios by resisting cracks and forming corrugation vertices that enhance structural integrity without excessive mass.^[9] Spanwise flexural stiffness scales with the cube of wing span due to leading-edge veins, achieving 1–2 orders of magnitude greater rigidity than chordwise bending across diverse species.^[10] Venation patterns vary markedly across insect orders to suit ecological roles. In Coleoptera (beetles), forewings are thickened into rigid elytra up to 1 mm thick for protection, while hindwings remain membranous and folded with resilin aids at hinges.^[7] Odonata (dragonflies and damselflies) possess flexible, reticulate-veined wings with extensive secondary cross-veins forming net-like patterns over areas of 20–725 mm².^[9] In Diptera (flies), hindwings are modified into clubbed halteres—stiff basal structures serving as mechanosensory organs rather than flight surfaces.^[7] Surface features further adapt wings for airflow interaction. Scales on Lepidoptera wings create rough textures that generate micro-turbulence, reducing drag and enhancing lift through vortex formation, as seen in the peacock butterfly (Inachis io).^[11] Hairs on Odonata wings produce micro-vortices to improve lift and stability by altering local boundary layer flow.^[11] Corrugations, prominent in Odonata leading edges, increase spanwise stiffness while modifying airflow by trapping vortices in profile valleys, thereby boosting aerodynamic force production without compromising flexibility.^[7]

Direct and Indirect Flight Muscles

Insect flight muscles are classified into direct and indirect types based on their anatomical attachments and functional roles in wing movement. Direct flight muscles attach directly to the bases of the wings, typically via sclerites or ligaments, enabling precise control over individual wings.^[12] These muscles are prevalent in more primitive insect orders, such as Odonata (dragonflies and damselflies) and certain small insects like Blattodea (cockroaches), where they allow for independent manipulation of each wing for maneuvers like hovering or rapid directional changes.^[13] In these insects, key direct muscles include tergosternal (dorsoventral) and pleurosternal muscles that connect to wing base elements, such as the axillary sclerites, to drive downstrokes and upstrokes.^[14] Direct muscles operate synchronously, with each contraction triggered by a single action potential from motor neurons, limiting wingbeat frequencies to typically less than 100 Hz.^[15] In contrast, indirect flight muscles do not attach to the wings but instead connect to the exoskeleton of the thorax, deforming its structure to indirectly transmit motion to the wings through notal and pleural sclerites.^[16] These muscles are dominant in more advanced insect orders, such as Diptera (flies), Hymenoptera (bees), and Coleoptera (beetles), facilitating high-power output for sustained flight.^[15] The primary indirect muscles consist of dorsal-ventral (tergosternal) muscles, which extend from the dorsal notum to the ventral sternum and elevate the wings during contraction, and dorso-longitudinal muscles, which run along the length of the thorax between anterior and posterior apodemes and depress the wings by arching the notum.^[13] This antagonistic pairing causes oscillatory deformation of the thorax, with up to 85% of the energy for wing downstrokes stored as elastic potential in the sclerites.^[13] A key distinction between direct and indirect muscles lies in their contraction mechanisms: synchronous versus asynchronous. Synchronous contraction, characteristic of direct muscles in basal insects like Odonata, involves one muscle twitch per neural action potential, with calcium release and reuptake tightly coupled to each cycle for controlled but lower-frequency motion.^[12] Asynchronous contraction, typical of indirect muscles in higher insects, allows multiple wingbeat cycles per action potential through stretch activation, where mechanical stretch of the muscle during the opposing phase triggers delayed force enhancement without repeated neural input.^[15] In asynchronous systems, motor neuron impulses occur at low frequencies (e.g., 3–5.5 Hz in fruit flies), yet wingbeat frequencies reach 185–195 Hz due to resonant thoracic oscillations and sustained calcium levels in the sarcoplasm.^[15] This mechanism enables wingbeat rates exceeding 100 Hz, up to 1,000 Hz in small insects like midges, far surpassing synchronous limits.^[15] Asynchronous indirect muscles feature specialized fibrillar fiber types, where myofibrils are loosely packed with abundant mitochondria (e.g., 43% in bee flight muscles) and a highly ordered, near-crystalline arrangement of sarcomeres to support rapid oscillations.^[15] These fibrillar muscles contain elastic C-filaments linking myosin filaments to Z-lines, enhancing stretch sensitivity and minimizing energy loss from calcium cycling, which contrasts with the more compact, cross-striated fibers in synchronous muscles.^[15] This structural adaptation allows for high-frequency performance while maintaining efficiency, with myofibrillar volume comprising about 53% of the muscle in asynchronous types.^[15]

Aerodynamic Principles

Direct Flight Mechanics

Direct flight mechanics in insects involve the direct attachment of flight muscles to the wing bases, allowing for synchronous contraction that pulls the wings through their strokes. This mechanism is characteristic of more primitive insect orders, where muscles contract once per neural impulse, enabling precise control over wing motion. In dragonflies (Odonata), for instance, these direct muscles facilitate independent movement of the fore and hind wings, which are not coupled as in more derived insects.^[17] The kinematics of direct flight typically produce complex trajectories, such as figure-eight paths traced by the wing tips during flapping. In dragonflies, wing motion involves a downstroke and upstroke in a near-vertical plane, with the figure-eight pattern arising from the rotation and feathering of the wings at stroke reversal, optimizing lift generation. This path allows for effective hovering and rapid directional changes, as the wings can adjust amplitude, phase, and angle of attack independently on each of the four wings.^[18]^[19] Force generation in direct flight relies on synchronous muscle activation, where each wingbeat corresponds to a single neural signal to the muscles, matching the wingbeat frequency. In Odonata, this frequency typically ranges from 20 to 40 Hz, varying with body size and temperature, lower than in asynchronous systems due to the direct neural pacing. Neural control occurs via motor neurons that innervate the muscles for each stroke, providing fine-tuned adjustments for maneuvers.^[20]^[21]^[15] This synchronous direct control offers advantages in precise maneuvering and hovering, as seen in the agile flight of dragonflies, which can execute sharp turns and stationary positions with independent wing actions. However, it limits efficiency and maximum frequency compared to indirect mechanisms, as the slower contraction rates constrain power output for sustained high-speed flight. In Ephemeroptera (mayflies), direct flight similarly enables short, erratic flights with uncoupled fore and hind wings, though at frequencies around 25 Hz, suited to their brief adult lifespans. The wing structure, with muscles attaching directly to the base via sclerites, supports this mechanics by allowing pivotal motion without thoracic deformation.^[22]^[23]^[20]^[24]

Indirect Flight Mechanics

Indirect flight mechanics in insects involve specialized muscles that do not attach directly to the wings but instead deform the thoracic exoskeleton, causing the wings to pivot on hinge-like articulations at their bases. This system, prevalent in advanced flying insects, relies on antagonistic pairs of indirect flight muscles: the dorsal longitudinal muscles (DLM), which run along the length of the thorax, and the dorsoventral muscles (DVM), which span vertically. When the DLM contract, they compress the thorax longitudinally, bowing the dorsal notum downward; conversely, DVM contraction elevates the notum, expanding the thorax vertically. These deformations are amplified through the rigid yet elastic thoracic structure, transmitting forces to the wing hinges without direct muscular control, enabling rapid oscillations.^[15]^[25] A hallmark of indirect flight is the asynchronous operation of these muscles, where wingbeat frequency decouples from neural firing rates due to stretch activation. In this mechanism, sustained low-frequency neural impulses maintain tonic calcium levels in the muscle, but actual contractions are triggered by mechanical stretch from the antagonistic muscle pair, leading to delayed force enhancement and self-sustained oscillations. This allows one neural impulse to drive multiple contraction cycles—often 3 to 100 or more—facilitating wingbeat frequencies far exceeding neural limits, such as 185–195 Hz in fruit flies (Drosophila melanogaster) and up to 1,000 Hz in midges. In contrast, primitive direct flight muscles operate synchronously, with contractions tied 1:1 to neural impulses at lower frequencies.^[15]^[26]^[25] The thorax's architecture is crucial for force transmission in indirect flight, featuring the notum as the dorsal plate that bows under muscle contraction and the pleura as lateral walls that flex to accommodate deformation. Elastic elements, including the rubber-like protein resilin integrated into myofibrillar structures and cuticular regions, provide resilience and energy return, preventing fatigue during high-frequency cycles; for instance, resilin pads at pleural hinges enhance deformability. In Diptera, this setup produces a characteristic "click" during rapid thorax buckling, observable in tethered flight preparations.^[15]^[27] This indirect asynchronous system powers flight in most Endopterygota, including Hymenoptera (e.g., bees with wingbeats around 200–250 Hz) and Lepidoptera (e.g., moths and butterflies achieving 10–50 Hz but with efficient thorax leverage for sustained hovering). These insects, comprising over 90% of flying species, evolved this mechanism to support agile maneuvers and high power output, distinct from the synchronous direct muscles in more basal orders like Odonata.^[15]^[25]

Leading Edge Vortex

The leading edge vortex (LEV) is a prominent unsteady aerodynamic feature in insect flight, characterized by a stable, low-pressure vortex that forms and remains attached to the dorsal surface of the wing during the translational phase of the flapping stroke, particularly at high angles of attack exceeding 45 degrees.^[28] This vortex generates substantial lift by creating a region of low pressure above the wing, enabling insects to achieve forces far beyond those predicted by steady-state aerodynamics. The LEV was first visualized experimentally in the hawkmoth Manduca sexta using smoke-wire flow visualization on a dynamically scaled mechanical flapper, revealing an intense vortex on the downstroke sufficient to account for observed high-lift production.^[28] Formation of the LEV occurs primarily during the downstroke and upstroke translations, where the wing's rapid motion separates flow at the leading edge, initiating dynamic stall—a process where the vortex detaches from the boundary layer but stabilizes due to the wing's three-dimensional geometry and spanwise flow.^[2] Unlike two-dimensional airfoils, where such vortices shed periodically and cause stall, the insect LEV grows to a constant size without bursting, sustained by a spanwise velocity component comparable to the flapping speed that transports vorticity outward toward the wingtip, mimicking the conical vortex on delta wings.^[28] This stability is further reinforced by the spanwise pressure gradient and the absence of additional vorticity generation at the trailing edge, adhering to a modified Kutta condition that prevents premature shedding.^[2] The LEV contributes substantially to the total lift during translation in model experiments simulating fruit fly (Drosophila melanogaster) kinematics, with peak circulatory forces enhanced by up to 70% over quasi-steady estimates.^[29] In hovering flight, this mechanism allows insects to maintain equilibrium by balancing weight against mean lift, as demonstrated in robotic wings mimicking Drosophila strokes where LEV attachment persisted across Reynolds numbers typical of small insects (100–10,000).^[29] The vortex's persistence is conserved across diverse insect taxa, from hawkmoths to hoverflies, despite variations in wing planform, underscoring its generality in flapping aerodynamics at low Reynolds numbers.^[2] Experimental validation through particle image velocimetry (PIV) on tethered insects and computational models confirms that LEV strength scales with wing velocity and angle of attack, but its attachment is modulated by wing flexibility, which can delay vortex bursting and enhance endurance.^[2] For instance, in butterflies with flexible wings, the LEV remains stable longer than on rigid models, contributing to efficient gliding transitions. Overall, the LEV exemplifies how unsteady effects dominate insect flight, integrating with rotational and wake-capture mechanisms to enable agile locomotion.^[29]

Clap and Fling

The clap and fling mechanism enables small insects to generate impulsive circulatory lift during the reversal between upstroke and downstroke phases of the wingbeat cycle. First identified through high-speed cinematography in the parasitic wasp Encarsia formosa, the process begins with the wings clapping together at the dorsal end of the upstroke, expelling air from between them and aligning their leading edges. As the downstroke initiates, the wings fling apart while remaining in close proximity near the trailing edges, forming a narrow slot that creates a low-pressure region and promotes rapid circulation around each wing, enhancing lift through delayed stall of the leading edge vortex.^[28] In insects with flexible wings, the fling phase employs a peeling mode, where separation propagates from the trailing edge toward the leading edge, rather than an abrupt rigid separation. This peeling delays the shedding of trailing edge vortices, preventing their interaction and cancellation with the beneficial leading edge vortices, thereby sustaining higher lift over a longer duration and reducing drag by up to 50% compared to rigid-wing flings. Computational simulations at low Reynolds numbers (Re ≈ 10) demonstrate that this mode maintains vortical asymmetry, crucial for efficient force production in tiny insects like thrips.^[30] High-speed imaging of free-flying Drosophila melanogaster reveals the clap and fling occurring over approximately 15% of the wingbeat cycle, providing a significant enhancement to lift at stroke reversal through amplified peak force.^[31]^[32] The fluid dynamics of the fling can be modeled as the separation of two closely spaced airfoils, yielding enhanced circulation relative to isolated wing translation and doubling lift in theoretical models.

Flight Dynamics and Performance

Hovering

Hovering represents a specialized form of insect flight characterized by zero net forward velocity, enabling stationary positioning in the air for tasks such as foraging or mating. In many insects, such as bees, this is achieved through symmetric wing kinematics involving near-horizontal strokes in the stroke plane, with the wings undergoing pronation—rotation such that the leading edge moves downward—at the beginning of the downstroke and supination—leading edge upward—at the start of the upstroke. These rotational adjustments allow the wings to maintain a positive angle of attack throughout both the downstroke and upstroke, generating approximately equal lift during each half-stroke to balance body weight without translational thrust. For instance, honeybees employ relatively low stroke amplitudes of about 90° combined with high wingbeat frequencies around 230 Hz to sustain this balanced lift production.^[33]^[34] Stability during hovering requires precise active control to counteract perturbations like body torques from asymmetric forces or wind gusts. Insects achieve this by dynamically modulating stroke amplitude, which primarily influences vertical force, and feathering angles (angle of attack), which adjust both lift and torque. These rapid adjustments, often on the scale of wingbeat cycles, enable corrections for pitch, roll, and yaw instabilities; for example, increasing stroke amplitude on one side can generate counter-torques to stabilize orientation. Such control mechanisms ensure long-duration hovering, with the leading-edge vortex playing a key role in augmenting lift efficiency across strokes.^[35]^[36] The metabolic demands of hovering are exceptionally high due to the continuous power output required for static lift, often exceeding resting rates by 50–100 times in many species. In hummingbird hawkmoths (Macroglossum stellatarum), hovering elevates metabolic rates up to 170 times the resting level, reflecting the intense aerobic demands fueled primarily by carbohydrate oxidation. Similarly, hoverflies (Syrphidae family, e.g., Episyrphus balteatus) exhibit flight metabolic rates around 100 times resting, underscoring the energy-intensive nature of sustained hovering for nectar feeding or prey inspection. These costs highlight the evolutionary premium on efficient power management in hovering specialists.^[37]^[38]^[39] Hovering variations include normal and inverted orientations, adapting to ecological needs like obstacle navigation. In normal hovering, typical of bees, hawkmoths, and dragonflies, wings beat in a horizontal plane with the body horizontal to produce downward lift. Dragonflies (Odonata) employ normal hovering, with wings positioned laterally and strokes in a near-horizontal plane to generate downward lift, allowing enhanced maneuverability. The induced power required for hovering, which represents the minimum aerodynamic power to sustain lift, is given by

P_\text{ind} = \frac{L^{3/2}}{\sqrt{2 \rho A}}

where L is the lift (equal to body weight), \rho is air density, and A is the effective disc area swept by the wings; this formulation illustrates how smaller insects incur disproportionately higher specific power costs due to reduced A.^[40]

Forward and Maneuvering Flight

In forward flight, insects transition from the near-horizontal stroke plane typical of hovering to a more forward-inclined orientation, which redirects aerodynamic forces to produce net thrust while sustaining lift. This adjustment enhances translational velocity by aligning the wing motion with the direction of travel, allowing insects to achieve efficient progression through the air. For instance, dragonflies (order Odonata) can reach forward speeds of up to 10 m/s, facilitated by this stroke plane reconfiguration and powerful wing beats that exploit unsteady aerodynamics like the leading-edge vortex.^[2]^[41] To minimize energy expenditure during forward motion, insects pitch their bodies downward into a streamlined, more horizontal posture, reducing the projected frontal area and thereby lowering body drag. This body alignment optimizes the balance between lift, thrust, and drag, particularly at moderate to high speeds where induced drag from hovering diminishes but parasite drag becomes prominent. In flies such as bluebottles (Calliphora spp.), forward speed correlates directly with this pitch angle, enabling sustained cruising while countering gravitational and aerodynamic loads.^[42]^[43] Maneuvering in forward flight relies on asymmetric wing kinematics to generate torques for yaw, pitch, and roll, allowing rapid directional changes without halting translation. Insects achieve yaw and roll by differentially altering stroke amplitude or deviation between contralateral wings, creating unbalanced forces that induce rotation around the body's vertical or longitudinal axes. For pitch control, symmetric shifts in stroke position relative to the body's center of mass adjust the vertical force vector. In fruit flies (Drosophila melanogaster), these asymmetries enable saccadic turns with angular velocities exceeding 1000°/s, demonstrating the precision of such mechanisms. Recent whole-body physics simulations (as of 2025) confirm the role of these angular velocities in turns and stability.^[44] Flies further stabilize maneuvers using halteres, modified hindwings that function as gyroscopes to detect and compensate for rotational perturbations during turns.^[45]^[46]^[47] During sharp turns, insects counteract centrifugal forces through dynamic wing twisting and feathering, which modulate local angles of attack to produce corrective lateral forces and enable accelerations up to 5 g. This twisting allows rapid adjustments in aerodynamic torque, preventing stall and maintaining control at high angular rates. For efficiency in prolonged forward and maneuvering flight, such as during migration, locusts (Locusta migratoria) align their bodies to minimize drag, achieving lift-to-drag ratios of approximately 1.7:1 in gliding phases interspersed with flapping, which extends range over hundreds of kilometers.^[46]^[48]

Governing Equations

The governing equation for aerodynamic lift in insect flight follows the conventional form adapted to the flapping kinematics:

L = \frac{1}{2} \rho U^2 S C_L

where \rho denotes air density, U the relative velocity between wing and surrounding air (typically the vector sum of flapping and induced velocities), S the effective wing area, and C_L the lift coefficient, which varies with angle of attack, Reynolds number (typically 10^3–10^4 for insects), and unsteady effects.^[2] In unsteady flow regimes prevalent during rapid wing oscillations, this equation is augmented by contributions from added mass and rotational mechanisms to account for inertial and circulatory forces. The added mass term arises from the virtual mass of air accelerated normal to the wing surface, yielding a force L_{am} \propto \rho c^2 l \dot{v}_n, where c is chord length, l spanwise element length, and \dot{v}_n the normal acceleration; this provides peak lift during stroke initiation. Rotational lift, prominent at stroke reversals, stems from bound circulation generated by wing pitching, approximated as L_{rot} = \rho \Gamma U c, with \Gamma the rotational circulation scaling with angular velocity \dot{\theta} and chord; together, these unsteady terms enable C_L values up to 2 or higher, far exceeding steady-state limits.^[29] Drag in insect flight is quantified analogously as

D = \frac{1}{2} \rho U^2 S C_D

with C_D the drag coefficient encompassing multiple components: profile drag from viscous shear and pressure differences on the wing surface, induced drag from trailing vortices associated with lift generation, and parasitic drag from bluff body effects on wings and fuselage. Profile drag predominates during slow, high-angle-of-attack translations, scaling with C_D \approx 2 \sin^2 \alpha + C_{f}, where \alpha is angle of attack and C_f skin friction; induced drag follows C_{D_i} = C_L^2 / (\pi AR e), with AR aspect ratio and e span efficiency, becoming critical in low-speed regimes like hovering. Parasitic contributions are minor but increase with body size and speed. These components collectively determine energy dissipation, with total C_D often 0.5–1.5 in flapping cycles.^[2] The mechanical power output required for sustained flight integrates these forces over the wing motion, expressed as the sum of aerodynamic power (to counter drag during translation), induced power (to sustain vortical lift), and profile power (inertial costs of flapping). A simplified form is P = P_{aero} + P_{ind} + P_{pro}, where P_{aero} = D \cdot U, P_{ind} \approx (L^{3/2} / (2 \rho A)^{1/2}) / \eta with A the swept area and \eta efficiency (typically 0.7–0.9), and P_{pro} \propto \rho f^2 S^{3/2} b^4 / m, with f stroke frequency, b mean radius, and m mass. Profile power reaches a minimum at an optimal advance ratio (forward speed over flapping speed ≈ 0.3–0.5), balancing reduced induced costs against increased translational drag; in hovering, induced power can comprise 60–80% of total, yielding specific powers of 20–100 W/kg across insect taxa.^[40] Blade element theory facilitates computation of these forces by segmenting the wing into independent annular elements along the span, assuming local two-dimensional flow without spanwise interactions. For an element at radius r with width dr, local velocity U_{loc} = \sqrt{( \omega r )^2 + V_f^2 + 2 \omega r V_f \sin \phi} (combining rotational \omega, forward V_f, and feathering angle \phi) yields incremental lift dL = \frac{1}{2} \rho U_{loc}^2 c(r) dr C_L(\alpha_{loc}) and drag dD = \frac{1}{2} \rho U_{loc}^2 c(r) dr C_D(\alpha_{loc}), where \alpha_{loc} incorporates geometric and induced angles. Quasi-steady approximations neglect time delays in vortex formation, integrating dL and dD over the span and cycle for total forces; this approach, calibrated with empirical C_L(\alpha) and C_D(\alpha) from dynamic stall data, accurately predicts mean lift within 10–20% for hover and forward flight in model insects.^[49]

Physiological Mechanisms

Power Generation and Output

Most advanced insects rely on asynchronous (indirect) flight muscles to achieve high wingbeat frequencies through delayed stretch activation, contrasting with synchronous muscles in basal insects like dragonflies that contract once per neural impulse for precise control. Asynchronous muscles feature specialized actin-myosin interactions where cross-bridge cycling is accelerated, with myosin dissociation rates from actin reaching up to 3,700 s⁻¹ in Drosophila indirect flight muscles, far exceeding those in vertebrate skeletal muscle.^[15] This rapid cycling allows a single action potential to sustain multiple contractions per cycle, powering oscillations at frequencies over 100 Hz in small insects.^[15] In synchronous muscles, power is generated through direct 1:1 coupling of neural impulses to contractions, enabling lower frequencies (e.g., 20–40 Hz in dragonflies) but greater control, with cross-bridge kinetics adapted for sustained force rather than oscillation. Asynchronous systems, however, dominate in derived orders, where power input to the wings is generated via work loops in these muscles, with sinusoidal strain mimicking wing motion. During the stretch phase, delayed calcium sensitivity triggers increased cross-bridge attachment, producing positive net mechanical work that exceeds energy input during shortening. For example, in beetle basalar muscle, work loops under imposed sinusoidal motion yield positive work loops with peak power outputs aligned to the stretch-activated phase, optimizing energy transfer for flight. Mechanical power output (P_mech) from the muscles to the wings is defined as the product of force and velocity, P_mech = F × v, where force arises from cross-bridge attachments and velocity from wing kinematics.^[38] In flying insects, this output achieves efficiencies of 20–40%, reflecting the conversion of chemical energy to useful aerodynamic work, with higher values in larger species due to optimized muscle mechanics.^[50] Power generation and output are measured using respirometry to estimate metabolic input combined with high-speed kinematic analysis of wing motion to compute mechanical work. For instance, in hovering Drosophila melanogaster, respirometry yields a metabolic rate supporting a mechanical power output of approximately 80 W kg⁻¹ muscle mass, sufficient for sustained flight at body weights up to 150% of normal.^[51] Wingbeat frequency, a key factor influencing power output, scales allometrically with body mass as f ∝ M^{-1/3}, reflecting geometric similarity and inertial demands in asynchronous fliers. This relation ensures smaller insects achieve higher frequencies (e.g., >200 Hz in Drosophila) to generate adequate lift, while larger ones operate at lower rates (e.g., ~20–40 Hz in moths) with greater stroke amplitudes. Allometric patterns also link muscle mass (typically 20–40% of body mass) and power density, constraining maximum body sizes in flying insects.

Elasticity and Energy Storage

Insect flight relies on elastic structures to store and release mechanical energy efficiently, minimizing the power demands on flight muscles. A key component is resilin, a rubber-like protein discovered in arthropod cuticles, particularly concentrated in wing hinges and the thoracic exoskeleton of many flying insects. Resilin enables passive energy recycling by deforming during muscle contraction and snapping back to propel wing movements, with nearly perfect elastic recovery that returns approximately 97% of stored strain energy.^[52] Its material properties include a low Young's modulus of about 1 MPa, allowing high extensibility up to 300% strain, combined with exceptional fatigue resistance capable of enduring millions of cycles without significant degradation.^[53] The primary function of resilin in flight is to facilitate energy storage and release during wing stroke reversals, particularly through mechanisms like the "click" in dipteran thoraces, where deformation stores kinetic energy from one stroke and recaptures it for the next.^[54] This elastic recapture reduces the net mechanical power required from muscles by 20-30%, enhancing overall flight efficiency in small insects with high wingbeat frequencies.^[55] In insects with indirect flight muscles, such as flies, resilin integrates with the compliant thorax to amplify these savings, though the core benefit stems from the protein's ability to buffer inertial loads without active muscular input.^[56] Representative examples illustrate resilin's role across taxa. In cicadas, which employ direct flight muscles, exocuticle regions in the thorax act as composite springs reinforced by resilin, storing energy to augment wing oscillations and sustain prolonged flight despite their large body size.^[57] Similarly, in locusts, elastic tendons containing resilin connect flight muscles to the thorax, enabling rapid energy transfer and storage that supports powerful takeoffs and sustained forward flight.^[58] These structures highlight how resilin optimizes energy use by combining with stiffer cuticular elements to form hybrid springs tailored to species-specific flight demands.^[59]

Wing Coupling

Wing coupling refers to the physical and kinematic mechanisms that synchronize the motion of forewings and hindwings in many insects, enabling them to function as a single aerodynamic surface during flight to enhance stability, lift, and efficiency.^[60] These linkages minimize aerodynamic interference between the wings, allowing for coordinated flapping that reduces energy expenditure while improving performance.^[61] In insects with coupled wings, such as those in the orders Lepidoptera and Hymenoptera, the fore- and hindwings overlap or interlock mechanically, contrasting with decoupled systems where independent control provides maneuverability.^[62] Various physical mechanisms facilitate this coupling. In Lepidoptera, such as butterflies and moths, the jugal lobe—a lobe-like extension at the base of the forewing—overlaps with the humeral angle of the hindwing, creating a stable linkage that synchronizes wing motion.^[63] For instance, in the small white butterfly Pieris rapae, the forewing basal area contacts the hindwing humeral area dorsally, allowing synchronous flapping without additional hooks.^[63] In Hymenoptera, like bees and wasps, hamuli—rows of hook-like setae along the anterior margin of the hindwing—interlock with a folded posterior margin (plexus) of the forewing, forming a robust, reversible coupling that permits a wide range of wing angles during flight.^[60] This structure in honeybees (Apis mellifera), composed of sclerotized cuticle, ensures the wings act as a unified pair, improving aerodynamic performance.^[61] In Diptera, such as flies, where hindwings are modified into halteres, spanwise coupling occurs through thoracic mechanical linkages and wing-halter interactions, treating the system as coupled oscillators that maintain phase-locked motion for balance and stability.^[64] Kinematically, wing coupling often involves controlled phase differences between fore- and hindwing strokes, which can generate clap-like effects to boost lift without requiring a full wing clap-fling motion. In coupled systems, a small phase lag (e.g., 0–90°) aligns the wings to trap and expel air efficiently, increasing circulatory force and lift by recovering wake energy.^[65] For example, in butterflies, synchronous fore- and hindwing motion during the upstroke creates a partial clap, where the wings cup and collide to form a low-pressure air pocket that enhances propulsion by 28% relative to rigid-wing models. Similarly, in bees, hamuli-mediated synchronization maintains near-zero phase difference, allowing the coupled wings to produce clap-like interactions at stroke reversal, which amplifies lift through vortex interactions while minimizing drag.^[60] Adaptations in wing coupling reflect diverse flight needs, with decoupling in primitive groups like Odonata enabling independent fore- and hindwing control for agile maneuvers, such as rapid turns or hovering, at the cost of some efficiency.^[62] In contrast, advanced pterygote insects, including those with tight coupling, achieve higher efficiency by reducing slip between wings, as seen in Hymenoptera where the mechanism supports sustained forward flight and load-carrying. Elastic hinges at the wing base can briefly aid this inter-wing coordination by storing and releasing energy to fine-tune phase alignment.^[60]

Biochemical Processes

Insect flight muscles exhibit specialized biochemical pathways to sustain the extraordinarily high rates of ATP hydrolysis required for oscillatory contractions during flight. The primary energy sources are carbohydrates, primarily trehalose from hemolymph and glycogen stores within the muscle, which undergo aerobic glycolysis to produce pyruvate for mitochondrial oxidation via the tricarboxylic acid cycle and oxidative phosphorylation. In many species, such as honeybees and flies, flight is fueled almost exclusively by carbohydrate oxidation, with enzymes like hexokinase, phosphofructokinase, and pyruvate dehydrogenase exhibiting high activities to support rapid flux.^[66] Fatty acid oxidation plays a secondary role in short-burst fliers but becomes prominent in long-distance migrants like locusts, where beta-oxidation enzymes in flight muscle mitochondria enable sustained energy provision from lipid reserves mobilized from the fat body.^[67] Although lactate dehydrogenase activity is minimal in most flight muscles, preventing significant lactate accumulation, a lactate shuttle mechanism has been observed in certain contexts, such as under hypoxic stress in Drosophila, where lactate produced in the cytosol is transported to mitochondria or other tissues for oxidation, facilitating metabolic flexibility.^[68] Myosin isoforms in insect flight muscles are adapted for fast-twitch, high-frequency performance, featuring heavy chain variants with altered lever arm structures that enhance shortening velocity and power output during stretch-activated contractions. In Drosophila, alternative splicing of the myosin heavy chain gene generates at least 15 isoforms, including embryonic types that promote rapid cross-bridge cycling and delayed tension development essential for asynchronous flight.^[69] Calcium ion (Ca^{2+}) regulation of these interactions occurs via the troponin-tropomyosin complex on thin filaments, where Ca^{2+} binding to troponin C induces a conformational change that exposes myosin-binding sites on actin, enabling contraction at low Ca^{2+} concentrations typical of sustained flight. Specialized troponin isoforms, such as those with enhanced mechanical sensitivity, further optimize this regulation for oscillatory demands.^[25] Mitochondria, often termed sarcosomes in insect flight muscle, achieve exceptional densities to match the aerobic demands, comprising up to 40% of the myofibrillar volume in mature asynchronous muscles of species like blowflies and bees.^[70] This high packing density supports mass-specific ATP production rates exceeding those in vertebrate muscles by orders of magnitude, with cristae-rich membranes housing dense electron transport chains for efficient oxidative phosphorylation.^[38] Hormonal regulation mobilizes these biochemical resources, with octopamine serving as the principal "fight-or-flight" neurotransmitter in insects, rapidly elevating cyclic AMP levels in flight muscles to activate phosphorylase and trehalase for glycogen and trehalose breakdown during takeoff.^[71] In locusts, octopamine injection prior to flight increases carbohydrate oxidation rates by 50-100%, enhancing readiness through allosteric modulation of key enzymes like glycogen phosphorylase, which shifts to an active conformation via phosphorylation cascades.^[72] Adrenaline-like effects of octopamine also promote lipolysis in the fat body, supplying fatty acids for extended flight, with feedback via allosteric inhibition of glycolytic enzymes to balance substrate use.^[73]

Sensory and Neural Feedback

Insect flight control depends on intricate sensory systems that detect environmental and self-motion cues, feeding into neural circuits for instantaneous adjustments. In Dipteran insects like flies, halteres—modified hindwings—serve as specialized mechanoreceptors that sense Coriolis forces generated by body rotations during flight, enabling gyroscopic stabilization. These forces deflect the halteres from their beating plane, with campaniform sensilla and chordotonal organs at the base transducing the signals into neural impulses. Compound eyes provide visual feedback through optic flow, where the apparent motion of the visual field across the retina informs the insect about translational velocity, altitude, and obstacles, facilitating course corrections in cluttered environments. Additionally, wind-sensitive hairs (trichoid sensilla) distributed on the head, antennae, and body detect aerodynamic perturbations and airflow, contributing to the perception of wind direction and body attitude. Neural processing occurs primarily in the thoracic ganglia, where central pattern generators (CPGs) establish the rhythmic motor output for wingbeats, typically at frequencies of 10–200 Hz depending on the species. These CPGs, composed of interconnected interneurons and motoneurons, generate alternating patterns for antagonistic flight muscles even in isolated preparations, but rely on sensory modulation for fine-tuning amplitude and phase. Feedback loops integrate inputs from multiple modalities via reflex arcs, with latencies as short as 5–10 ms, allowing corrections at rates matching wingbeat frequencies, such as approximately 200 Hz in Drosophila. For instance, proprioceptive stretch receptors in the wing hinges and thorax monitor wing position and deformation, providing closed-loop control to synchronize contralateral wings and prevent desynchronization during perturbations. In insects without halteres, such as hawkmoths, antennal mechanoreceptors including Johnston's organ function as vestibulo-lateral sensors, detecting rotational accelerations and airflow to stabilize posture during hovering and maneuvering. A classic example is the optomotor response in Drosophila, where visual optic flow elicits compensatory yaw torques by modulating wingbeat amplitude, with the response gain peaking at spatial frequencies matching natural environments. Experimental ablation of halteres in flies results in immediate loss of rotational stability, causing tumbling and inability to maintain controlled flight, underscoring the indispensability of this mechanosensory input for equilibrium. These systems collectively ensure robust stability, with brief integration into maneuvering behaviors like turns, where sensory fusion prioritizes rapid threat avoidance.

Evolutionary Origins and Adaptations

Overview of Evolutionary Hypotheses

The evolution of insect flight represents a pivotal adaptation in arthropod history, likely originating around 400 million years ago during the Early Devonian period based on molecular clock analyses of phylogenomic data from diverse insect lineages.^[74] The earliest definitive fossil evidence of winged insects dates to the early Carboniferous, approximately 325 million years ago, with precursors such as precoxal leg exites in ancestral arthropods serving as the structural foundation for wing development.^[74] These winged precursors enabled the transition to powered flight, marking insects as the first animals to achieve this capability and facilitating their subsequent radiation across terrestrial ecosystems.^[75] Multiple lines of evidence support this timeline and origin. The fossil record, particularly from Carboniferous deposits, includes early pterygote groups like the Palaeodictyoptera, which display primitive wing venation and articulations indicative of flapping flight capabilities. Comparative anatomy highlights homologies between insect wings and proximal leg segments in crustacean relatives, suggesting a shared exite-based origin for these structures.^[76] Genetic markers further illuminate this process; for instance, the vestigial gene, a key regulator of wing patterning, is expressed in presumptive wing fields of apterous insects such as silverfish (order Zygentoma), revealing conserved developmental pathways even in wingless descendants of flying ancestors.^[77] Central debates surround the environmental context and multiplicity of flight's emergence among pterygotes—the winged insect clade. Proponents of aquatic origins argue for a "water-up" scenario, where flight structures derived from gill-like appendages in amphibious ancestors, while terrestrial "trees-down" models posit gliding from elevated arboreal positions as the precursor to active flight. Another key contention is whether wings evolved once (monophyly) or independently in separate pterygote lineages (dual or polyphyletic origins), with recent evo-devo and genomic studies favoring a single derivation from thoracic leg bases.^[78] These discussions underscore the challenges in reconciling fossil sparsity with molecular and morphological data. Flight conferred profound adaptive benefits, enabling rapid escape from predators, long-distance dispersal to exploit patchy resources, and aerial mating displays that enhanced reproductive success across insect orders.^[75] Yet, these advantages came at substantial costs, including elevated metabolic rates—up to 100 times resting levels during sustained flight—and the energetic demands of wing maintenance, which have driven secondary flightlessness in over 20% of extant species in stable or insular habitats.^[1]

Epicoxal Hypothesis

The epicoxal hypothesis proposes that insect wings originated from gill-like exites attached to the epicoxa, the proximal segment of the ancestral arthropod leg, particularly on abdominal coxae in aquatic environments. According to this model, these exites functioned initially as respiratory and locomotor flaps in water, later adapting for aerial flapping upon the transition to land. Jarmila Kukalová-Peck introduced this idea in her 1983 analysis of arthropod limb morphology and fossil evidence, arguing that the pro-wing formed from an epicoxal exite mobilized by leg-derived musculature, with the epicoxa incorporating into the pleural body wall to form the wing's articular base.^[79] Supporting evidence draws from embryological and morphological homologies, especially in Ephemeroptera (mayflies), where abdominal gills exhibit structural and developmental parallels to wings. In mayfly nymphs, these gills evaginate from the pleural region as movable, muscled appendages, mirroring the embryonic formation of wing buds that separate from leg discs rather than dorsal terga. Furthermore, coxal muscle similarities reinforce this link, as the intrinsic pleural muscles actuating mayfly gills are homologous to the direct flight muscles in adult wings, both tracing to epicoxal leg origins in the arthropod ground plan. Fossil nymphal wing pads from Paleozoic Ephemeroptera, such as those in Protereismatidae, show articulated structures consistent with serial exites derived from abdominal segments.^[79]^[80]^[81] Criticisms of the epicoxal hypothesis center on its limited fossil corroboration and overreliance on an aquatic ancestral phase. Direct transitional fossils depicting gill flaps evolving into thoracic wings are absent, with Paleozoic records showing winged insects appearing abruptly without clear intermediates from abdominal exites. Additionally, the model is challenged for implying complex wing articulations evolved de novo from simple exites, a process genetically and mechanically demanding, and for emphasizing aquatic larvae despite many insect lineages being fully terrestrial. While it aligns with exopterygote development, where wings form externally in nymphs, it contrasts with pleural theories by prioritizing exite appendages over body wall extensions.^[82]^[83]^[84] The hypothesis carries significant implications for understanding exopterygote flight origins, suggesting that powered flight arose from serial modifications of larval aquatic gills into adult thoracic wings during the Devonian-Carboniferous transition. This serial homology posits wings on multiple abdominal segments primitively, with thoracic specialization enabling aerial locomotion while abdominal remnants persisted as gills in basal groups like mayflies. Overall, it supports a monophyletic pterygote origin tied to arthropod limb evolution, influencing interpretations of insect diversification in early terrestrial ecosystems.^[79]^[85]

Paranotal Hypothesis

The paranotal hypothesis posits that insect wings originated as lateral extensions, or paranotal lobes, of the dorsal thoracic tergum in early terrestrial arthropods, initially serving as passive gliding structures before evolving into actively flapping appendages capable of powered flight.^[86] This theory, first articulated in the 1870s and widely endorsed by mid-20th-century entomologists, suggests these lobes provided an adaptive advantage for controlled descent from heights, such as trees or elevated vegetation, in a terrestrial environment.^[87] Sir Vincent Wigglesworth reinforced this view in 1973, emphasizing the transition from gliding to flapping as a gradual evolutionary process driven by selection for improved aerial maneuverability.^[86] Supporting evidence draws from both fossil and extant forms. Fossil arthropods like Arthropleura, a Carboniferous millipede relative, exhibit pronounced tergal expansions interpreted as paranotal lobes that could have facilitated gliding.^[87] In modern apterygote hexapods, such as Collembola (springtails), small vestigial dorsal lobes on the thorax are seen as remnants of these ancestral structures, consistent with a tergal origin restricted to thoracic segments.^[88] Biomechanical models further bolster the hypothesis by demonstrating that flapping paranotal lobes could generate sufficient lift for short-distance flight. Simulations by Kingsolver and Koehl (1985) analyzed aerodynamic performance across wing size scales, showing that small, flap-like extensions produce positive lift and stability during descent, aligning with the proposed gliding-to-flapping progression.^[89] Despite its strengths, the paranotal hypothesis faces criticisms for inadequately accounting for key wing features. It struggles to explain the pleural articulation that enables wing movement in modern insects, as tergal extensions would lack the necessary basal joints for rotation and folding.^[87] Similarly, the complex venation patterns in fossil and extant wings, which resemble branched limb structures rather than simple dermal outgrowths, remain difficult to derive from dorsal lobes alone.^[78]

Endite-Exite Hypothesis

The Endite-Exite Hypothesis proposes that insect wings evolved through the fusion of an anterior endite lobe and a posterior exite lobe originating from the pleural region of the thorax, rather than from dorsal tergal expansions. This model, developed as a variant by Kukalová-Peck in her 1987 analysis of Paleozoic insect morphology, interprets the primitive wing as a biflagellate structure derived from proximal leg podomeres that were incorporated into the body wall and flattened for aerodynamic function. These lobes, movable and articulated, preadapted the structure for flapping flight by leveraging existing arthropod limb mechanics. Supporting evidence stems from comparative morphology across arthropods, particularly the segmental homologies in crustacean biramous limbs, where the inner endopodite aligns with endite components and the outer exopodite with exites. In insects, pleural sclerites such as the episternum and epimeron represent fused remnants of these subcoxal and epicoxal elements, with ontogenetic studies in primitive orders like Ephemeroptera revealing wing buds emerging from the pleuron. Fossil specimens from Carboniferous pterygotes further corroborate this by showing articulated wing bases integrated with pleural structures, suggesting a lateral origin tied to leg-derived appendages. The hypothesis accounts for wing articulation through the derivation of key basal structures from endite-exite joints: the subcostal sulcus forms from the anterior-posterior boundary between lobes, while axillary sclerites (e.g., 1Ax and 2Ax in neopterans) arise from fused proximal podomere hinges, enabling pivoting and muscle attachment for powered flight. This setup contrasts with simpler fixed articulations in other models and aligns with the multi-sclerite band observed in early pterygote fossils. Criticisms of the Endite-Exite Hypothesis center on its structural complexity, which posits multiple podomere migrations and fusions that are difficult to verify phylogenetically without intermediate fossils or molecular markers confirming homologies. While it integrates elements of epicoxal origins—such as endite-exite lobes potentially stemming from epicoxal podomeres—the model's reliance on detailed arthropod groundplan reconstructions has led to debates over its testability compared to more parsimonious alternatives.

Other Hypotheses and Dual Origin

In addition to the primary hypotheses, alternative theories propose that insect wings originated from gill-like structures or thoracic limb derivatives. The gill-wing hypothesis suggests that wings evolved from movable gill plates on the abdominal segments of aquatic arthropod ancestors, which initially served respiratory functions before adapting for aerial locomotion. A 2024 study analyzed fossil larvae from Carboniferous deposits, proposing that thoracic and abdominal outgrowths functioned as gills and represent serial homologs to wings, supporting an aquatic origin.^[90] This view is supported by fossil evidence of Carboniferous larvae exhibiting flattened thoracic and abdominal projections resembling gills, indicating a shared developmental origin for breathing and flight organs.^[91] In contrast, the leg-branchial hypothesis posits that wings derived from proximal segments of ancestral crustacean legs, where outgrowths or lobes on thoracic limbs were incorporated into the body wall and later modified for flight. Experimental gene knockouts in crustaceans demonstrate that patterning genes for these leg segments correspond to wing-forming regions in insects, reinforcing this thoracic limb origin. The dual origin model integrates elements from multiple theories by proposing that wings evolved from the fusion of tergal (dorsal body wall) and pleural (lateral, leg-derived) tissues in pterygotes. This is evidenced by distinct Hox gene expression patterns, where Ultrabithorax (Ubx) represses wing development in the hindwing imaginal discs of basal insects but promotes it in derived lineages, allowing differential evolution of wing pairs. Activation of an ancient Wnt signaling pathway in crustacean epipodites—gill-like leg outgrowths—further supports the fusion of dorsal and lateral tissues into a unified wing structure.^[78] Recent phylogenomic analyses from the 2020s reveal multiple instances of flight loss and regain, challenging strict monophyly of wing evolution and highlighting evolutionary flexibility. For example, in stick and leaf insects (Phasmatodea), genomic data indicate at least three independent regains of wings from flightless ancestors, driven by shifts in developmental gene networks.^[92] Fossil records from the Devonian align with aquatic-to-terrestrial transitions.^[92] Hybrid models synthesize these insights by combining pleural and paranotal elements, positing that wings arose through the integration of ancestral leg-derived and dorsal body wall structures. This approach reconciles competing hypotheses, with developmental studies showing how gene co-option enabled such fusions to produce functional flight appendages.^[93]

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