December 8, 2003 | David F. Coppedge

The Fruit Fly in the Flight Simulator

The simplest things can be the most extraordinary.  If you like finding amazing wonders in everyday things, you’ll be fascinated to read about the common fly in the cover story of Caltech’s magazine E&S (Engineering and Science).1  Michael Dickinson, a zoologist turned engineer, has described his Caltech team’s work trying to reverse-engineer the flight systems of the common fruit fly, Drosophila melanogaster.
    Part of the fascination of this article is the team’s cleverness in experiments.  Dickinson and his students have built elaborate flight simulators for the tiny insects.  Imagine taking a fly, not much bigger than a large speck of dust, and putting it into a custom arena in which the scenery is computer controlled, and every response of the fly’s wings and muscles can be measured.  Imagine fastening a tiny fly with a tether and monitoring its every movement.  (This is reminiscent of the monarch butterfly flight simulator – see 07/09/2002 headline – only more elaborate.)  Dickinson’s team measured the “swatting reflex,” to see how the fly changes its angle when a large unknown object approaches.  They studied flight motion with high-speed cameras, and even built “RoboFly”, a computer-controlled set of wings fed the exact motions of a real fly, to study the aerodynamic forces on the wings.  Next, they are taking on the ambitious project of building a housefly-sized robotic insect that might be able to hover like the real thing.
    All this pales in comparison, however, to the profuse praise Dickinson lavishes on the engineering capabilities of the real live insect.  Listen to what he says, and you will take his concluding statement to heart, “I hope you will think before you swat.”  Here are some samples from his 10-page, illustrated article (emphasis added):

  • CPU:  Describing the fly’s ability to adjust flight processing in less than 30 milliseconds, “This is extraordinarily fast processing, and illustrates why the flight system of flies represents the gold standard for flying machines.” (p. 12)
  • Brain:  “Insects have quite sophisticated visual systems, and approximately two-thirds of their brain (about 200,000 neurons) is dedicated specifically to processing visual information.”
  • Eyes:  “Fruit fly’s eyes… have excellent temporal resolution and can resolve flashing lights at frequencies up to 10 times faster than our own eyes can.  This means if you take a fly on a date to the movies it will think you brought it to a slide show.”
  • Systems-level analysis of fly mechanics:  “Here things get rather humbling, because it’s the mechanical component of this biological system that we, as engineers, are the furthest away from being able to replicate.”
  • Materials science:  “Flies don’t have an internal skeleton consisting of individual bones or cartilage.  Instead, they’re surrounded by an external skeleton, the cuticle—a single, topologically continuous sheet composed of proteins, lipids, and the polysaccharide chitin.  During development, complex interactions of genes and signalling molecules spatially regulate the composition, density, and orientation of protein and chitin molecules.  Temporal regulation of protein synthesis and deposition allows the construction of elaborate, layered cuticles with the toughness of composite materials.  The result of such precise spatial and temporal regulation is a complex, continuous exoskeleton separated into functional zones.;  for instance, limbs consist of tough, rigid tubes of ‘molecular plywood’ connected by complex joints made of hard junctures separated by rubbery membranes.”
  • Joints and Hinges:  “Perhaps the most elaborate example of an arthropod joint, indeed one of the most complex skeletal structures known, is the wing hinge of insects–the morphological centerpiece of flight behavior.”  (He describes how the parts function.)  “Although the material properties of the elements within the hinge are indeed remarkable (resilin is one of the most resilient substances known), it is as much the structural complexity as the material properties that endows the origami-like wing hinge with its astonishing properties.
  • Flight mechanics:  “By controlling the mechanics of the wing hinge, the steering muscles act as a tiny transmission system that can make the wing beat differently from one stroke to the next.  Electrophysiological studies indicate this is a phase-control system.”  He describes how the steering muscles can actually alter the stiffness of the wing in flight.  “The fly uses the steering muscles as phase-control springs to alter the way the large strains produced by the power muscles are transformed into wing motion.”
  • Timing:  “During each wingbeat, sensory cells on the wings and halteres send timing signals into the brain that are used to tune the firing of the muscles.”  Considering the speed at which their wings beat, this is certainly a rapid-response system.
  • Gyroscopes:  “The information coming from the haltere,” (a drumstick-shaped organ behind each wing) “is particularly important because it is essential in stabilizing reflexes.  Beating antiphase to the wings, the halteres function as gyroscopes during flight.”  (He describes how these organs respond to Coriolis forces with appropriate compensatory reflexes.)  “The animal detects these rotations with its halteres and responds with compensatory changes in wing stroke.  These reflexes are extraordinarily robust…. The halteres are essential elements of the fly’s control system.  Cut them off, and a fly rapidly corkscrews to the ground.”
  • Computation:  “Because of the complexity of fly aerodynamics, understanding wing motion does not necessarily translate into an understanding of flight forces.  It is a common myth that an engineer once proved a bumblebee couldn’t fly, and while the true story is really much kinder to the engineer, it underscores the difficulties of studying fly aerodynamics.  At present, even brute-force mathematical computations on supercomputers cannot accurately predict the forces created by a flapping wing.

Wow.  All this in a tiny fly!  Wrapping up this amazing journey into miniaturized ultrasophisticated engineering, Dickinson puts his work into perspective:

In the end, it’s just a fly.  Is such an insignificant organism really worth all this effort?  The natural world is filled with complex things, like immune cells, the human brain, and ecosystems.  Although we’re made great progress in deconstructing life into its constituent parts such as genes and proteins, we have a ways to go before we have a deeper understanding of how elemental components function collectively to create rich behavior.  The integrative approach that we are using to study fly flight is an attempt to move beyond reductionism and gain a formal understanding of the workings of a complex entity.  The fly seems a reasonable place to start, and if successful, I hope such work will stimulate similar attempts throughout biology.  The lessons learned along the way may provide useful insight for engineers and biologists alike.  Even if you don’t buy such grand visions, I hope you will at least think before you swat.

1Michael H. Dickinson, “Come Fly With Me,” Engineering and Science, Volume LXVI, No. 3, 2003 (Caltech), pp. 10-19.

Thank you, Dr. Dickinson, for a wonderful glimpse into one of nature’s miniature engineering marvels.  We feel like we were sitting behind you on the fly’s back, soaring on a thrill ride, like your first picture humorously illustrates.  Thank you, also for reminding us that the world is filled with wonders like this, from bacteria to blue whales.  Wow.  Who would have suspected such wonders exist in a tiny fly?  Certainly not Charles Darwin.  Which reminds us, we were about to award you Story of the Month for this outstanding article, but you included this one statement which acts like the proverbial fly in the soup: “The information coming from the haltere, a hindwing modified by evolution and resembling a very small chicken drumstick, is particularly important because it is essential in stabilizing reflexes.”  Since even the FDA tolerates a certain threshold of vermin residue in food, we can overlook this one tiny slip in an otherwise excellent piece of design-based scientific research and writing.

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