Butterflies fly using efficient propulsive clap mechanism owing to flexible wings.

L C Johansson,P Henningsson

doi:10.1098/rsif.2020.0854

Abstract

Butterflies look like no other flying animal, with unusually short, broad and large wings relative to their body size. Previous studies have suggested butterflies use several unsteady aerodynamic mechanisms to boost force production with upstroke wing clap being a prominent feature. When the wings clap together at the end of upstroke the air between the wings is pressed out, creating a jet, pushing the animal in the opposite direction. Although viewed, for the last 50 years, as a crucial mechanism in insect flight, quantitative aerodynamic measurements of the clap in freely flying animals are lacking. Using quantitative flow measurements behind freely flying butterflies during take-off and a mechanical clapper, we provide aerodynamic performance estimates for the wing clap. We show that flexible butterfly wings, forming a cupped shape during the upstroke and clap, thrust the butterfly forwards, while the downstroke is used for weight support. We further show that flexible wings dramatically increase the useful impulse (+22%) and efficiency (+28%) of the clap compared to rigid wings. Combined, our results suggest butterflies evolved a highly effective clap, which provides a mechanistic hypothesis for their unique wing morphology. Furthermore, our findings could aid the design of man-made flapping drones, boosting propulsive performance.

Highlights

The fluttery flight of butterflies over a sunny meadow instils fascination, yet the flight of butterflies remains somewhat a mystery
During take-off, we found that the butterflies primarily used the downstroke for generating weight support and the upstroke for generating thrust
The sideways tilt comes from that the butterflies were typically turning as they were taking off, which meant there was a lateral force component corresponding to the radius of the turn

Summary

Introduction

The fluttery flight of butterflies over a sunny meadow instils fascination, yet the flight of butterflies remains somewhat a mystery. The few flight mechanistic studies performed so far on butterflies have triggered suggestions that they use a variety of unsteady aerodynamic mechanisms for their force production [1,2] Among these mechanisms, the upstroke wing clap, first described by Weis-Fogh for insects already in the early 1970s [3], is one repeatedly reported as used by butterflies [1,4,5,6,7]. Despite the importance of this mechanism, as far as we know, quantitative measurements of the aerodynamics of the wing clap in freely flying animals are still lacking Apart from their characteristic fluttery flight [7,8], butterflies perform highly directed and sustained flights as, for example, seen in migratory species [9,10], in unpalatable species [8] and during take-off [11,12]. The aim of this study was to determine the aerodynamics of the iconic wing clap in a freely flying animal and to determine the function and contribution of the upstroke with wing clap to the flight of butterflies

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Conclusion