Fruit flies are strongly attracted by the scent of fermenting fruit or wine. But, unlike photons or sound waves, odors do not travel in straight lines, but waft and form areas of high and low concentration. Thus it’s difficult for the fly to rely on scent alone to find that delicious rotting banana you left on the table. Since flies accomplish this efficiently and with a limited brain size, they’re also an interesting case from a computational point of view.
Homing in on a strawberry.
Credit: Floris van Bruegel
A truly beautiful investigation of this plume-tracking behavior of fruit flies appeared in this month’s Current Biology. The investigators used a miniature wind tunnel and time-lapse photography to track fruit flies as they approached a piece of fruit.
To localize an odor source, flies rely on three reflex-driven behaviors. The first two are related to in-flight adjustment. Upon detecting the scent, flies turn upwind, using their eyes to help them orient themselves; and, if they lose the trail, they exhibit a second behavior, casting about crosswind to try to pick up the scent again. Most intriguingly, all this time after sensing the odor, the flies become visually attracted to small, contrasty objects, which in the wild would be likely to be the piece of fruit emitting the odor. In a way, all three of these make intuitive sense-- think of trying to locate the coffee stand at a crowded market hall; you see people carrying cups, and you smell coffee (and you, like the flies, are probably not welcome to feed on their drinks). So you integrate cues from both senses to locate the good stuff.
What I also enjoyed about the article was the presentation of the data- the figures displayed the very complicated data set in a way that communicated the point effectively. I was happy to see that the lead author, Floris van Breugel, also maintains a nature photography blog.