Fly’s Flight Muscles, As Seen From Inside, Make 3-D Film Debut
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The flight muscles of flies in motion have been filmed for the first time, a British team of zoologists and bioengineers said Tuesday, making it the first time that any insect has been X-rayed during flight, let alone in 3-D. The only animals in action who have ever been X-rayed in 3-D before are slower organisms such as mice. The imaging method called, "in vivo time-resolved micro-tomography" has helped reveal how a blowfly, Calliphora vicina, a close relative of the house fly, can control each 150-per-second wing beat with miniscule steering muscles as thin as a strand of hair and only one fraction of its entire flight musculature. Their findings could inspire the development of new micro-air vehicles and other micromechanical devices, say the entomologists, whose work appears in Tuesday's issue of the open access journal, PLOS Biology.
"There is no flying animal that achieves such a high degree of maneuverability as a few species of flies - including the blowfly," co-author and bioengineer from Imperial College London, Holger G. Krapp, told the International Science Times. "What amazes me the most is the way the whole system is designed," he said, noting that beating its wings will generate lift and allow a fly to zoom in a straight line; and stretching its power muscles generates the ability to stay aloft; but only its steering muscles allow it to modify the straight path with asymmetric muscle action.
How could the fly's tiny steering muscles, which comprise less than three percent of its total flight muscle mass, influence the output of the more massive muscles which power its flight? Dr. Krapp and team members wondered. To find out, they made use of in vivo time-resolved micro-tomography to reveal the flies' flight muscles in action. The imaging method creates the appearance of continuous movement of a fly's whole body reconstructed from scans taken at different viewer angles. The bioengineers used a periodic trigger signal to select all the scans taken at given phases of the wing beat cycle, a 3-D reconstruction method known as retrospective gating. That gave them a box seat to view the fly's wing hinge, which took 300 million years to refine and is considered by some zoologists the most complex joint nature has to offer. Unlike those of birds or bats, insect wings contain no muscles themselves: Wing motion is driven from muscles inside the thorax. The wing hinge allows for an astonishingly wide range of wing motion and control and also allows for the wing to be disengaged and folded back over the body when it is not in use.
In insects with lower wing beat frequency, each wing beat is a voluntary motion. However all flies, wasps and bees turn their power muscles, which drive the wing beat 'on' or 'off' for several wing beats at a time, allowing them to have very high wing beat frequencies; upwards of 800 times per second in some mosquitoes. Thus, the power muscles are powerless to provide wing beat-by-beat control. The power muscles are equally useless when it comes to generating turns: They affect both wings symmetrically. Instead, they fly requires its small steering muscles to act on a wing beat-by-beat basis, allowing it to exercise amazing control during rapid flight.
The fly's flight control begins with four power muscles that subtly alter, or "deform" its exoskeleton, or thorax. The fly's wing hinge then transmits those thorax deformations to the wing. Thirteen steering muscles, attached directly onto the fly's wing hinge, fine-tune the wing's motion. The steering muscles are attached to pieces miniscule pieces of hardened cuticle that form complex, interlocking structures, which in turn attach onto the wing. "We still don't know exactly what some of these muscles do," co-lead author Simon Walker, a zoologist at the University of Oxford told the International Science Times. "Trying to understand how these bits of cuticle rotate relative to each other and change the position of the wing is itself incredibly difficult, and has been confusing scientists for decades," he said.
The imaging of the flies in action revealed that as the flies altered the deformation of their thoraxes, their steering muscles switched between different modes of oscillations, moving the wing accordingly. "By having deformations built into its structure, we showed that another muscle can cope with a large range of motion of the wing, despite only having a very small range of motion itself," Dr. Walker said. They also showed that flies are able to turn and brake by letting their muscles absorb excess energy as needed. "Since most flying insects contain a similar arrangement of muscles, it is likely that they, too, will also make use of this muscle to absorb energy during turns," Walker said. "This the first time it's been shown for insects flying and offers a new way to think about the function of the steering muscles."
Despite what Walker and Krapp have shown, they still consider the miniscule hinge of the wing of the fly to be an enigma. "It's surprising that the steering muscles are so much smaller than the power muscles, yet, are able to create large changes in the motion of the wings," Walker said. "There is still a huge amount that we don't know about how the insect flight motor works, and we will continue to study the flight muscles and the mechanics of the wing hinge." Krapp is hoping to delve even deeper into the relationship of the fly's flying ability with its sensory system, particularly its vision or "gaze" and plans to take what he's learned from the X-rays to develop biomechanical models.
"Those results are quite interesting in the context of the control of autonomous micro-air vehicles or other robotic applications," Dr. Krapp told the International Science Times. "So far, flying micro-air vehicles controlled by means of conventional engineering designs are by far less maneuverable the any blowfly. But we have some ideas, inspired from the research presented and the understanding of the neural processes in the fly brain, that should change this in the foreseeable future.
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