Researchers may have finally solved the mystery of how insects flap their wings so fast. Snapshots of airborne fruit flies reveal that a protein in their wings acts like a spring, conserving energy when stretched before quickly rebounding.
Fruit flies beat their wings with two layers of muscles. When one layer contracts, it stretches the other, which itself is then primed to contract. Because this cycle proceeds at an incredibly rapid rate, it cannot be controlled by nerve impulses and thus depends on the energy stored in the muscles themselves.
To determine how the muscle cells convert energy stored in a stretch into a contraction, biophysicist Thomas Irving of the Illinois Institute of Technology, Chicago, and colleagues monitored wing muscle proteins in flapping fruit flies. The team tethered the flies to a box with a fine wire and used air and streaming lights to make the insects think they were moving. The team then took snapshots of the proteins within flies' wings using a high intensity x-ray beam. By synchronizing the wing beat to the shutter speed of the x-ray, the team created a stop-action film of the complete beat cycle.
Muscles contract because the muscle protein myosin uses its flexible head to ratchet its long tail along the filament of another muscle protein actin, dragging the ends of muscle cells inward. After contracting, the myosin head normally detaches from the actin, and the muscle cell relaxes. In their film, the researchers saw the myosin crawl on actin in contracting muscles. But in relaxed muscles, which are being stretched by the attached layer of contracting muscle, the myosin's tail did something unexpected. Not only did the myosin head hang onto the actin filament, but its tail--thought to be stiff for pulling during contraction--stretched a bit. This suggests, say the researchers, that myosin stores elastic energy that can be used in the next contraction in the same way a stretched spring holds onto energy and releases it when it recoils. The team reports its findings in the 20 January issue of Nature.
Calling the work "phenomenal," structural biologist Michael Reedy at Duke University Medical Center, Durham, North Carolina, says the technical advancement is just as important as the biological finding. "I wouldn't have believed this possible ten years ago," he says of how well the team synchronized the wing beat with the flashes of x-rays. Seeing myosin tails getting stretched in relaxed muscle is "a complete novelty," he says. He adds that human heart muscle works in much the same way as flight muscle, and this work could eventually give insight into why hearts fail.