As I discussed in the previous post, the forewings and halteres of flies are coupled by structures that keep the beating in rhythm. This is efficient for flying in a straight line, but not for maneuvers. During maneuvers, flies will beat one wing with a lower amplitude or briefly stop moving it. This turns the fly like a boat that is paddled only on one side. If the wings are coupled, how can they move at different speeds?
Deora and colleagues* describe a model in which the movement of a wing can be coupled and decoupled from the vibration of the thorax similar to the way that a clutch engages and disengages an automobile engine from the drive train. Part of the wing called the radial stop can contact the pleural wing process (a sclerite that vibrates with the thorax) in four discrete conformations. In “Position 0”, the radial stop is moved posterior to the pleural wing process, the wing is decoupled from the movement of the thorax and the wing is at rest even when the thorax vibrates. In Position 3, the wing base is moved anteriorly so the radial stop is fully engaged with the pleural wing process and beats at maximum amplitude.
There are two positions between 0 and 3. The pleural wing process contains two grooves that the radial stop may contact. Engaging the radial stop with either of these grooves connects the wing to the vibration of the thorax but the wing beat amplitude is less than maximal. Thus, a fly can fully engage both wings by moving the radial stop to position 3; the radial stop can be shifted posteriorly to position 2 or 1 and beat with lower amplitude than the opposite wing or the radial stop can be shifted to the extreme posterior position, disengage and stop movement. This gear mechanism provides the basic structure that allows a fly to perform elegant maneuvers.
*Tanvi Deora, Amit Kumar Singh, and Sanjay P. Sane. Biomechanical basis of wing and haltere coordination in flies. PNAS February 3, 2015. 112, no. 5 pp. 1481–1486.