An analysis of hoverfly flight shows that wing design, not faster flapping, enables the smallest species to generate enough lift to fly.
A hoverfly feeding. Image by hyuksoo kwon from Pixabay.
The study, published today in eLife as the final Version of Record after appearing previously as a Reviewed Preprint, is described by the editors as important work. They say the authors provide convincing evidence that there is no significant link between body size and wing kinematics – the motions of the wings as they beat. Instead, changes in the shape and size (or morphology) of the wing allow smaller hoverflies to stay airborne. These findings help explain why insect wings have evolved to be so diverse in form.
The physics of flight becomes more difficult the smaller animals are. For hoverflies, hovering to hold their position in the air while feeding on nectar or scouting for potential mates is a crucial behaviour that demands precise flight control. Large hoverfly species can generate lift with relatively short wings, but smaller species face a tougher challenge as their potential to produce lift with their wingbeats decreases in relation to their body weight as their size shrinks.
“Hovering is critical for hoverflies as they feed and guard mating territory, but for the smallest insects their wings don’t scale up enough to keep pace with their body weight,” says lead author Camille Le Roy, Postdoctoral Researcher in the Experimental Zoology Group, Wageningen University, the Netherlands. “Many scientists have speculated that smaller insects overcome this by altering their wingbeat kinematics. We wanted to test whether small hoverflies really flap their wings in a different way to their larger counterparts, or if subtle changes in their wing shape enable them to hover.”
Le Roy and colleagues examined the scaling relationships between wing morphology and body mass in 28 hoverfly species ranging from 3–132 milligrams (mg). They conjointly assessed how wing morphology and wingbeat kinematics scale with body mass for eight species ranging from 5–100mg.
They measured wing span, wing area, mean chord (the average width of a wing), and the second moment of area (S2) – a measure of how wing surface is distributed along its length, which can affect how much lift is generated. They also used geometric morphometrics, a statistical method that captures differences in shape independent of size.
For the eight smaller species of hoverfly, the researchers filmed hovering sequences using three synchronised high-speed cameras. This stereoscopic setup allowed them to reconstruct 3D wing and body movements, and take down key measurements of the wingbeat including the flapping frequency and wing angles.
Across species, the team found that wingbeat kinematics did not change significantly with body size: smaller hoverflies did not flap their wings faster or with larger stroke amplitudes. Instead, their wing morphology shifted systematically with size: smaller species had proportionally longer wings and higher S2 values, meaning more of their wing area was positioned further from the hinge, giving them greater leverage.
Despite the diversity in wing shape, the pattern of vertical force was conserved across species. Each half-stroke produced a small lift peak followed by a larger one, with the backstroke typically stronger. This underscores that while wing morphology tunes how much lift is generated, the underlying wingbeat pattern broadly remains the same.
The researchers then used computer simulations combining each species’ wing shape with a common average wingbeat pattern to test how much aerodynamic lift their wing morphology alone could generate. These fluid simulations reproduced the already observed differences in lift between species, showing that most of the variation in weight support comes from their wing shape rather than flapping motion.
The authors note that a limitation of this work is the missing aspect of the interplay between wingbeat movement and muscle function (or physiology).
“Our analysis focuses on the wing-based propulsion system in hoverflies, but their wing motion is ultimately powered by the muscular motor,” says senior author Florian Muijres, Professor of Experimental Zoology in the Experimental Zoology Group, Wageningen University. “Future studies that integrate muscle physiology with aerodynamic modelling are therefore needed to complement our approach.
“For now, our results show that small hoverflies don’t solve the physics of flight by flapping their wings differently from larger ones – they solve it by being built differently. By stretching their wingspan and shifting more surface area away from the hinge, the smallest species generate the extra lift they need without having to change their specialised wingbeat.”
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