Summary

1. 1. The mechanism of dipteran flight has been investigated in a comparative study involving thorax manipulation, analysis of high-speed films and direct observation of tethered flies under stroboscopic illumination.

2. 2. The click action observed in CCl₄-anaesthetized Calliphora was found to be due to an interaction between the radial stop and the pleural wing process at the top of the upstroke. The movements occurring during unanaesthetized tethered flight were quite different as these structures were vertically separated except towards the bottom of the downstroke (Miyan & Ewing, 1985a,b).

3. 3. Results of observations on tethered insect flight and on morphology did not give full support to either the click mechanism (Boettiger & Furshpan, 1952) or the model of Miyan & Ewing.

4. 4. A novel model for the wingbeat is proposed. Distortion of the thorax brought about by the flight muscles results in upward and outward movement of the lateral scutum during the downstroke and inward and downward movement during the upstroke. In more advanced flies flexion lines result in a differentiated scutellar lever and parascutal shelf. Distortion is thereby limited largely to the posterior scutum. The parascutal shelf moves as a part of the scutal distortion, not as an independent element in the articulation, and the system is not bistable.

5. 5. The automatic changes in angle of attack are caused by inertial and aerodynamic forces acting around the torsional axis of the wing during the beat, which twist the compliant wing base.