Learning How Flies Fly

Insects rule, says Michael Dickinson, professor of integrative biology at the University of California, Berkeley, and one of this year's MacArthur fellowship genius award winners. Insects, he contends, have reigned for half a billion years and are likely to do so for a billion more, considering their biomass, the sheer number of species, and their ecological impact. What really moves Dickinson is the insects' flying proficiency. The first organisms to evolve flight, insects still represent the most sophisticated aerial machine on the planet, he says. Flies, in particular, have unique specializations that lead to extraordinary behaviors: they can take off backwards, fly sideways, and land upside down. These singular behaviors, Dickinson says, "push the envelope of useful design, presenting a clearer picture of the structure/function of relationships than do more mundane behaviors." But understanding insect flight requires more than studying the nervous system, Dickinson told neuroscientists at their annual meeting last November. "From the mechanics of the muscle, to the biomechanics of the skeleton, and the aerodynamics of the wing, [all this] is important for understanding what was originally a neurobiological question," he said. Dickinson is homing in on a particular behavior called the saccade, a term stolen from human physiology, to describe rapid eye movements that happen when scanning a visual field. Fly-related saccades refer to rapid turns that occur as often as once per second under certain conditions, such as flying in an enclosed space. Using high-speed video cameras, Dickinson has caught flies in mid-saccade and broken down the maneuver's elements. He found that the behavior is stereotypic, which in ethologist jargon means it occurs exactly the same way once initiated. However, by altering the fly's visual field in an arena that he calls the "fly-o-rama"—a controlled environment where fly movement can be tracked in three dimensions—Dickinson obtained convincing evidence that the visual system controls the behavior. While not influencing the strength of the turn, which is hard-wired, visual cues determine the timing of the turn. Clayton RyderScience on a Small Scale: Michael Dickinson tethered flies that measure 5 millimeters in length, about the size of an eraser on a #2 pencil. But not satisfied with just that answer, Dickinson also investigated what the visual cues are that flies use to control this conduct. By constructing a virtual reality arena, where tethered flies with untethered wings are put into a visual field that fools them into thinking they are flying, Dickinson showed that the visual field's expansion was the trigger. In one of two experiments, researchers placed an expanding square into the field. As it expanded, the fly thought it was getting closer and initiated the saccade. Dickinson could tell whether they were flying straight or turning by the pattern of movements the wing made. Still curious, Dickinson researched why flies stay airborne. He designed Robo-fly, a 25 cm model of Drosophila wings, and immersed it in a two-ton vat of mineral oil to replicate the forces of air on the fly's tiny wings. Using this device to measure the force that the wings generate, Dickinson has come up with some novel mechanisms describing how flies stay in the air. One hypothesis is that flies use wing rotation to change the relative amount of drag during the upstroke and downstroke; another suggests that the fly captures wake forces to create the large amount of drag energy needed to turn so fast. Dickinson hopes to understand the internal and external elements that let a fly soar in a visually rich environment. "What we call behavior results from the resonance of an animal and its sensory/mechanical control system and the external world," he says. "You take the animal, put it in a different set of circumstances, it will behave differently. This is one of those observations that is completely trivial or profound, depending on your perspective." Laura DeFrancesco (defrancesco1@earthlink.net) is a freelance writer in Pasadena, Calif.

Learning How Flies Fly

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