The Inertial Cause of Wing Rotation in Diptera

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Abstract
The cause of the changes in wing pitch at stroke reversal in Diptera has been investigated. The high compliance of the wing base makes it seem unlikely that pitch changes are caused by active torsion at the wing articulation. The centre of mass of insect wings tends to be behind the centre of torsion of the wing, and it is proposed that wing inertia about the torsional axis alone is responsible for pitch changes as the wing is accelerated at stroke reversal. A simplified inertial model is developed to calculate the angular velocity about the torsional axis that would be caused by wing inertia. The mass distribution and the torsional axis of the wings of two species of flies was found and it was shown that in these animals inertial causes alone could develop the angular velocity in the pitching plane that is observed at stroke reversal. Analysis of the movement of individual regions of the wing shows further that inertial effects will produce the tip to base ‘torsion wave’ seen in the wing at stroke reversal.