Ever wonder how a fruit fly finds that rotten banana in your kitchen? Scientists at the University of Nevada, Reno have some answers.
While it may seem like flies are aimlessly flitting around your house just to annoy you, a study published Friday in the journal Current Biology found that fruit flies actually use intentional movements to search for the source of a delicious smell.
Fruit flies, or Drosophila melanogasterFruit flies are among the most studied organisms because they provide a cheap and accessible platform to study biological processes. While they couldn’t seem more different from humans, fruit flies share 75% of our disease-causing genes, which is why scientists have used them to better understand human diseases.
Researchers have long demonstrated that these tiny insects use a deliberate strategy, called “cast and surge,” to find food sources in windy environments. In this technique, a fruit fly picks up a tasty scent, rides upwind to follow the scent, and swings from side to side when it loses the scent to find it again.
Scientists explained that picking up a scent in the wind does not mean that the source is nearby. In fact, the breeze probably carried the scent from a distant point. Therefore, the diffusion and push technique is an effective way to trace the origin of a scent in the wind.
And what happens when there is no wind?
“They have another trick up their sleeve,” said Marcus Stensmyr, associate professor of sensory biology at Lund University, who was not involved in the study.
The trick when there’s no wind: Flies perform a downward and rotating motion. The authors found that when flies encountered and then lost a scent in still air, they spiraled downward in an attempt to find the source of the scent.
This behavior may not be shocking: Most cartoons show flies circling a mound of smelly food. Yet this is the first time scientists have documented fruit flies’ behavior in still-air environments, confirming a long-held human intuition.
In still air, detecting an odor indicates that the source is likely nearby, making the “sink and turn” approach more useful.
Some researchers have suggested that dogs and rats exhibit similar behaviors when sniffing high and low to get closer to a scent.
To conduct their study, the authors first had to find a way to trigger a fly’s sense of smell in a windless environment. But how do you deliver odors without wind? David Stupski, lead author of the study and a postdoctoral researcher in biology and engineering at the University of Nevada, Reno, described the scientists approach as “a virtual reality for smell”.
They used genetically modified flies with light-activated neurons in their antennae, which are like a fly’s nose. The authors were able to trigger a fly’s sense of smell using flashes of red light instead of actual odors. The light-based approach allowed the researchers to circumvent the difficulty associated with releasing controlled odor clouds, which are difficult to localize in space.
The authors could easily turn the lights on and off to accurately provide light as a proxy for smell. The researchers conducted their study in a custom-built wind tunnel equipped with 12 cameras to track a fly’s movements in three dimensions. It’s notoriously difficult to monitor insects in their natural flying state, which is why much research is done on walking flies, since their movements span two dimensions instead of three.
After observing that fruit flies moved differently depending on atmospheric conditions, the authors deduced that fruit flies can detect both the presence and direction of wind.
“If you stick your head out of a car window while you’re driving, can you tell if it’s windy or not?” asked Floris van Breugel, the study’s lead researcher and an assistant professor of mechanical engineering. Detecting a light breeze by moving through the air is difficult, but fruit flies are good at it anyway.
The authors suggest that flies slow down and turn around when they encounter an odor to determine whether there is wind and where it is coming from.
“This is a pretty complex computation going on in this tiny, supposedly simple brain,” said Elizabeth Hong, a professor of neuroscience at the California Institute of Technology, who was not involved in the study.
Understanding how fruit flies detect odors can help scientists better understand pests, such as mosquitoes, says Richard Benton, a professor at the University of Lausanne and a neurobiologist. Scientists are particularly interested in inhibiting mosquitoes’ ability to find and feed on humans to limit disease transmission.
The flies’ sense of smell may also inform the next generation of tracking devices designed to find the source of a chemical leak indoors.
Perhaps this study could inspire some respect for fruit flies. After all, they’ve figured out how to thrive alongside humans by tracking down odors inside our windless homes.
“They have tiny brains,” Benton said, “but they do so much with them.”
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