title: Force balance in the take-off of a pierid butterfly: relative importance and timing of leg impulsion and aerodynamic forces.
abstract: Up to now, the take-off stage has remained an elusive phase of insect flight that was relatively poorly explored compared with other maneuvers. An overall assessment of the different mechanisms involved in force production during take-off has never been explored. Focusing on the first downstroke, we have addressed this problem from a force balance perspective in butterflies taking off from the ground. In order to determine whether the sole aerodynamic wing force could explain the observed motion of the insect, we have firstly compared a simple analytical model of the wing force with the acceleration of the insect's center of mass estimated from video tracking of the wing and body motions. Secondly, wing kinematics were also used for numerical simulations of the aerodynamic flow field. Similar wing aerodynamic forces were obtained by the two methods. However, neither are sufficient, nor is the inclusion of the ground effect, to predict faithfully the body acceleration. We have to resort to the leg forces to obtain a model that best fits the data. We show that the median and hind legs display an active extension responsible for the initiation of the upward motion of the insect's body, occurring before the onset of the wing downstroke. We estimate that legs generate, at various times, an upward force that can be much larger than all other forces applied to the insect's body. The relative timing of leg and wing forces explains the large variability of trajectories observed during the maneuvers.
authors.affiliation: Institut de Recherche sur la Biologie de l'Insecte IRBI, CNRS UMR 7261, UFR Sciences et Techniques, Université François Rabelais, 37200 Tours, France. gaelle.bimbard@etu.univ-tours.fr
authors.forename: Gaëlle
authors.initials: G
authors.lastname: Bimbard
authors.forename: Dmitry
authors.initials: D
authors.lastname: Kolomenskiy
authors.forename: Olivier
authors.initials: O
authors.lastname: Bouteleux
authors.forename: Jérôme
authors.initials: J
authors.lastname: Casas
authors.forename: Ramiro
authors.initials: R
authors.lastname: Godoy-Diana
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology; D002080:Butterflies / Q000502:physiology*; D003198:Computer Simulation; D005426:Flight, Animal / Q000502:physiology*; D035002:Lower Extremity / Q000502:physiology*; D009038:Motion; D013997:Time Factors; D014741:Video Recording; D014921:Wings, Animal / Q000502:physiology*
keywords: aerodynamic force; butterflies; flapping flight; insect flight; leg force; take-off; variability