How Diptera move their wings: a re-examination of the wing base articulation and muscle systems concerned with flight

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Abstract
The accepted mechanical model for dipteran wing movements is not compatible with the observations of unanaesthetized flies in tethered flight. In this paper a new model is proposed together with previously undescribed morphological features in support of it. A direct coupling is demonstrated between the indirect flight muscles, consisting of a wing base stop mechanism and a locking of the first axillary sclerite onto the parascutal shelf during the downstroke. We propose that wingbeat amplitude is controlled by axillary muscles which alter the downward force on the wing. We also describe the anatomical basis of the automatic changes in the wing stroke during the wingbeat. These are shown to be a direct consequence of the attachment between the fourth axillary sclerite and the scutellar lever arm. By virtue of their attachments to the sclerite, the fourth axillary muscles are shown to have potential control over the tonic characteristics of these automatic changes. The calypters are connected to the third and fourth axillary sclerites and this suggests an aerodynamic function for these structures which has not previously been proposed. A detailed analysis of the anatomy of the non-fibrillar muscles has allowed a complete functional characterization to be made which is consistent with our new model. The functions given to many of the muscles in this analysis are supported by physiological evidence from other studies. Muscular control of a switch mechanism comprising the radial stop and pleural wing process is discussed, as is the control of a wing base proprioceptor.