But the odor plume of a food source can be very slight, subject to the vagaries of wind and weather, in a world that looms large to this diminutive bug. Yet the fly is uncanny in finding a meal. How? By using more than just its sense of smell.
Reporting in the current edition of the journal Current Biology, now online, UCLA researchers show that Drosophila, with a brain smaller than a poppy seed, combines massive amounts of information from its sense of smell and vision, then transforms these sensory signals into stable and flexible flight behavior that leads them to a food source. Understanding the integration of these sensory cues could be relevant to developing smarter robotic drones.
"Essentially, we show how vision allows flies to navigate odors," said paper co-author Mark Frye, an assistant professor in the UCLA Department of Physiological Science and a member of the UCLA Brain Research Institute. Brian Duistermars, a doctoral candidate and member of Frye's lab, was also a co-author.
The conundrum is this: Fruit flies have 700 times lower visual spatial resolution than humans and five times fewer olfactory receptor types. Yet their ability to find smelly things in visual landscapes as diverse as forests, deserts and backyard patios would suggest a behavioral performance greater than might be predicted by the sum of its sensory inputs.
Frye and Duistermars found that flies stay the odor course by combining sensory information. They designed a "virtual plume simulator" in which a fly is tethered but free to steer into and out of a plume of vinegar odor (nectar to a fruit fly). At the same time, a cylinder around the fly displays a variety of background images meant to roughly mimic the images the fly might encounter in the real world -- blades of grass, say, or twigs on a tree.
In free flight, a fly's path is normally characterized by segments of straight flight interspersed with transient "spikes" called saccades, in which they veer left and right.
On the cylinder, the researchers displayed a visual backdrop of equally spaced, high-contrast vertical stripes, and then periodically switched the vinegar plume between 0 and 180 degrees within the circular arena. Then they tracked the fly's heading. Under these conditions, the fly periodically encountered the plume by steering into it. Upon plume contact, the fly performed fewer saccades, presumably because it was honing in and following the plume. Yet the saccades were not altogether absent, Frye noted, an apparent attempt on the fly's part to "constantly seek the sweet spot -- the strongest scent -- of the plume."
To examine the visual influence on odor-tracking accuracy, the researchers changed the visual backdrop. They alternated a sequence of three visual treatments - high-contrast stripes, uniform grayscale and a second high-contrast treatment. Each fly started within the vinegar plume and was exposed to the three visual stimuli at 20 second intervals. When the striped and high-contrast panoramas appeared, the flies were able to maintain their heading into the plume. But when the flat, featureless grayscale was displayed, the flies steered out of the plume and began generating saccades. Even though they occasionally would reencounter the plume within the grayscale visual panorama, they were unable to remain there until the high-contrast pattern reappeared, at which point accurate plume-tracking resumed.
Frye said this shows the flies are using the backdrop as a physical marker to keep track, along with their olfactory sense, of where the plume is. Such crossmodal integration at the behavioral and cellular level, Frye said, represents a functional adaptation for distinguishing and responding to critically important features of a complex sensory environment.
Frye noted that a fruit fly's brain only has approximately 200,000 neurons, compared with billions in a human brain.
"Yet a fruit fly casually outperforms the most sophisticated robots that humans can engineer," he said. "The research in our lab seeks to understand the mechanisms with which a tiny nervous system controls the vastly complex biomechanics of flying. In doing so, we hope to advance our understanding of sensory neurobiology and also help to develop smarter robots."