Novel flight style and light wings boost flight performance of tiny beetles

Miniaturization in insects pushes flight into a low Reynolds number regime where viscous forces dominate and constraints differ from those at larger scales. Despite the general positive correlation between body size and flight speed, minute featherwing beetles (Ptiliidae) fly at speeds similar to larger relatives and can accelerate faster, even though their size-specific flight-muscle volume is smaller. They possess feather-like bristled wings (ptiloptery), a convergent adaptation found in several extremely small insect orders, but the functional benefits of such wings remained unclear. Prior work has focused mainly on larger millimetre-scale insects, leaving a gap in understanding the kinematics and aerodynamics of the smallest fliers. This study addresses how the morphology (bristled wings), wingbeat kinematics, and elytra dynamics enable exceptional flight performance in the tiny beetle Paratuposa placentis, and tests the hypothesis that reduced wing mass and a novel wing-motion cycle enhance performance at very low Reynolds numbers.

Previous numerical studies suggested that flow past bristled wings (idealized as cylinder lattices) reduces aerodynamic force compared with membranous wings. Physical model experiments with comb-like bristled wings indicated slightly improved lift-to-drag ratios during clap-and-fling compared to membranous wings, but did not cover a full wingbeat cycle. High-speed videography has revealed that very small insects use wingbeat cycles distinct from larger insects, relying more on drag-based force generation and specialized stroke patterns. However, the specific role of ptiloptery (bristled wings) within these novel cycles had not been explicitly considered. This work builds on and extends these findings by quantifying the complete wingbeat cycle, force decomposition, and power requirements for bristled versus membranous wings under realistic kinematics of a sub-millimetre beetle.

The authors analyzed free flight of Paratuposa placentis using a combined morphological–kinematical–dynamical approach. Morphology: three-dimensional reconstructions based on light, confocal, and scanning electron microscopy quantified body size, wing architecture (petiole, narrow blade, bristle fringe with secondary outgrowths), setal coverage, and dimensions; wing mass and inertia were estimated numerically from these reconstructions. Kinematics: synchronized high-speed videography recorded wing and elytra motion to extract Euler angles (stroke positional, stroke deviation, and wing pitch), angles of attack, wingtip trajectories, body pitch angle, and stroke plane orientation over normalized wingbeat time (t/T). The wingbeat cycle phases were identified as two power half strokes and two recovery (clap) half strokes above and below the body; Reynolds numbers were computed based on mean speed at the radius of gyration. Dynamics and aerodynamics: computational methods of solid and fluid mechanics simulated airflow and forces over the measured wing kinematics. Aerodynamic forces were decomposed into lift and drag components contributing to vertical force. Vortex structures were visualized using iso-surfaces of vorticity magnitude. The stabilizing effect of elytra motion was analyzed by computing pitching torques about the center of mass due to wing aerodynamics and elytral inertia, comparing body-pitch oscillations with and without elytra motion. Comparative modeling: bristled wings were compared to hypothetical membranous wings sharing the same planform but with different thicknesses. Wing mass and inertia matrices were computed for both wing types; power requirements (aerodynamic, inertial, and total) were calculated over the wingbeat cycle, including added mass effects. Allometric analysis and supplementary computations supported conclusions about wing mass scaling and the effect of bristle secondary outgrowths.

- Paratuposa placentis is extremely small (body length 395 ± 21 µm; mass 2.43 ± 0.19 µg) with bristled wings 493 ± 18 µm long; setae occupy 95.1 ± 0.3% of the aerodynamically effective wing area. - The wingbeat cycle consists of two power half strokes and two recovery half strokes, with claps above and below the body; wingtip paths form a pronounced figure-of-eight loop with large stroke amplitudes. - Angles of attack are high and similar in magnitude between strokes: ~73° during downstroke and ~85° during upstroke. Cycle-averaged Reynolds number is ~9 (peaking ~20 during power strokes). - Vertical force production is asymmetric: peaks occur when geometrical AOA and wing velocity are maximal during power strokes. Near-clap motions reduce downward force during recovery. - Force decomposition shows approximately 32% of cycle-averaged vertical force arises from drag and 68% from lift; airflow exhibits strong vortex rings indicative of drag-based mechanisms during power strokes. - Mean aerodynamic lift supports ~2.7 µg (vs. body mass ~2.4 µg), enabling slight positive vertical acceleration (~1.0 m s⁻²). Net vertical contributions of body and elytra are negligible. - Elytra act as inertial brakes: opening/closing movements (amplitude up to 52°) counter wing-induced pitching torques and reduce body pitch oscillation amplitude by ~50% compared with no-elytra motion. - Bristled wing mass is ~0.024 µg (~1% of body mass). Membranous wings of the same outline would weigh ~0.13–0.19 µg depending on thickness; corresponding inertia matrix maxima are much larger (bristled: ~1,600 µg µm² vs. membranous: ~13,800–20,800 µg µm²). - Secondary outgrowths on bristles uniquely found in Ptiliidae reduce wing mass by ~44% relative to smooth cylindrical bristles at similar drag. - Despite lower instantaneous forces, bristled wings generate ~68% of the mean vertical force of equivalent membranous wings at low Re. - Power requirements: for bristled wings, total mechanical power remains positive throughout the cycle due to low wing inertia and high viscous damping; mean mass-specific power ~28 W kg⁻¹ with peaks up to ~110 W kg⁻¹. - For membranous wings, inertial and aerodynamic power peaks are similar; achieving minimum mean mechanical power (~37 W kg⁻¹) requires near-perfect elastic energy storage, with peak power demands ~180–210 W kg⁻¹. - At Re ~10, the slight aerodynamic advantage of membranous wings is outweighed by the substantial reduction in inertial torques and power from lighter bristled wings with sufficient low leakiness using few slender bristles.

The findings demonstrate that miniature featherwing beetles achieve high flight performance through a combination of kinematic and structural adaptations suited to low Reynolds numbers. The novel wingbeat cycle—with two distinct power and recovery half strokes and claps above and below the body—maximizes aerodynamic asymmetry, synchronizing high angle of attack and wing velocity to boost upward force during power strokes while minimizing downward force during recovery. Bristled wings, by drastically reducing wing mass and inertia without a prohibitive loss of aerodynamic effectiveness at Re ~10, enable continuous positive mechanical power transfer to the flow, eliminating the need for elastic energy storage typical for heavier membranous wings. The elytra provide inertial braking that stabilizes body pitch despite large wing excursions and torques, improving posture control without adding aerodynamic force. Together, these features explain how extremely small insects can preserve or even enhance aerial capabilities during miniaturization, reconciling small muscle volumes with the demands of agile flight.

This study reveals that Paratuposa placentis employs a unique wing-motion cycle and ultra-light bristled wings that reduce inertial costs, maintain positive muscle power throughout the wingbeat, and leverage elytra as inertial brakes for stability. These adaptations optimize force production and power distribution at low Reynolds numbers, supporting efficient flight despite extreme miniaturization. The work suggests that such a flight style may be widespread among miniature beetles and may underlie their evolutionary success and abundance. Future research should extend comparative analyses to other microinsects with bristled wings to further elucidate the convergent evolution of ptiloptery and to map how morphological and kinematic parameters co-adapt across taxa and sizes.