During flight, insects beat their wings hundreds of times per second, faster than would be possible if their flight muscles worked by contraction, like vertebrate muscles do. How they do this is a matter of some debate, but new data from an x-ray scattering study suggest that insect flight muscle activation relies on a mechanism that is shared with vertebrate muscle.

In vertebrates, muscle contraction is based on calcium: calcium ions are released in response to a nerve signal and then captured by troponins attached to actin filaments, causing the filaments to rotate and expose myosin binding sites. Myosin binds to actin, then gets bent, pulling the actin filament and causing muscle to contract. This process burns energy—far too much to support the rapid beating of an insect's wings—and requires calcium pumping, which cannot occur fast enough to sustain flight. Instead of contracting, then, insect muscles respond to nerve signals by entering a state of self-sustained oscillation induced by 'stretch activation,' a mechanism in which the force generated by the flight muscles increases as the muscles are extended. It isn't clear, however, what triggers stretch activation in insect flight muscles. Early work suggested that extension of the muscles exposed more myosin binding sites, but a more recent study proposed that insect flight muscles expressed a unique, calcium-independent form of troponin that enabled the muscle oscillations. Hiroyuki Imamoto, a biophysicist at Japan Synchrotron Radiation Research Institute (Hyogo), told Nature News, “It has long been known that many insects don't move their flight muscles in the way vertebrates do. The big question is whether the difference is unique to insect flight muscles or exploits a property common to all muscle proteins.”

To address this question, Imamoto and his colleague Naoto Yagi placed female bumblebees (Bombus ignitus) in the path of an x-ray beam and analyzed the beam scatter produced by their muscles during flight along with synchronized video footage, captured at 40 frames per wing beat, or 5,000 frames per second.

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Imamoto and Yagi found that myosin twisted as the bees' flight muscles extended, enabling stronger binding to actin (Science published online 22 August 2013; doi:10.1126/science.1237266). This observation indicates that stretch activation is based on the same fundamental myosin–actin interaction that underlies vertebrate muscle contraction. Instead of developing a new mechanism for the rapid muscle activation needed to fly, insects refined an existing mechanism for muscle force generation.