Further Observations on Dipteran Flight: Details of the Mechanism

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Abstract
We took high-intensity flash photographs of the wing base during tethered flight of Lucilia sericata. These show that the radial stop and the pleural wing process separate during the upper part of the wing beat and then mate together on the downstroke. Using tungsten needle probes attached to wire strain gauges, we measured the movements of the parascutal shelf (PSS), of the lateral and dorsal scutum, and of the scutellum during tethered flight. We made a detailed study of the wing base sclerites and associated muscles in order to answer criticisms of our original model (Miyan & Ewing, 1985a,b). The PSS rotates upwards about its hinge with the lateral scutum at the start of the downstroke. 4. The dorsal scutum and medial edge (at the hinge) of the PSS are held down, presumably as a result of dorsoventral muscle activity. As the downstroke progresses, the whole of the PSS and dorsal scutum are lifted together suggesting the action of a locking mechanism. At the bottom of the downstroke there is an opposite, downward rotation of the shelf about its hinge that follows the start of downward scutal movement at the beginning of the upstroke. This is followed by downward movement of the whole PSS and scutum. Movements of the lateral scutum exactly follow scuteliar and lateral PSS movements and are probably dictated by the articulation. Scanning electron micrographs illustrate the probable components of the wing base-PSS locking mechanism. Rotation of the first axillary sclerite, brought about by the rising scutellar lever arm, results in mating of its medial arm with a recess in the PSS. This prevents further rotation of the PSS which is now held at two points by the sclerite and is lifted by further movement of the lever. There is no evidence that the third axillary muscles act as wing pronators. Scanning electron micrographs show a mechanism that maintains the line of action of the muscles on the posterior edge of the wing.