Fruit flies, or Drosophila melanogaster, are some of the most widely studied organisms because they offer a cheap and accessible platform for studying biological processes. Although they may look indistinguishable from humans, fruit flies share 75 percent of our disease-causing genes, which is why scientists have used them to better understand human diseases.
Researchers have long documented that these tiny insects perform a deliberate strategy known as toss and wave to find food sources in windy environments. With this technique, the fruit fly catches a whiff of something tasty, rises upwind to follow the scent, and darts aside when it loses the scent to find it again.
The scientists explained that picking up a smell on the wind doesn’t mean the source is nearby. Rather, the breeze probably carried the smell from some distant point. As a result, the cast and wave technique is an effective way to trace the origin of scent in the wind.
So what happens when there is no wind?
“They have another trick up their sleeve,” said Markus Stensmire, associate professor of sensory biology at Lund University, who was not involved in the study.
The trick when there is no wind: The flies perform a sinking and circling motion. The authors found that when flies encounter and then lose an odor in still air, they spiral down in an attempt to find the source of the odor.
This behavior may not be shocking – most cartoon images of flies show them circling around a smelly pile of food. Still, this is the first time scientists have documented how fruit flies behave in a still air environment, confirming long-standing human intuition.
In still air, picking up an odor indicates that the source is likely nearby, making the sink and circle approach more useful.
Some researchers have suggested that dogs and rats exhibit similar behavior when they sniff high and low to get close to an odor.
To conduct their research, the authors first had to devise a way to trigger a fly’s sense of smell in a windless environment. But how do you distribute scents without wind? David Stupsky, lead author of the study and a postdoctoral fellow in biology and engineering at the University of Nevada, Reno, described the scientists approach as “virtual reality for olfaction”.
They used genetically engineered flies that have light-activated neurons in their antennae, which is essentially a fly’s nose. As a result, the authors could trigger the fly’s sense of smell using red flashes instead of actual odors. The light-based approach allowed the researchers to circumvent the difficulties associated with delivering controlled odor clouds, which are difficult to locate in space.
Authors can easily turn the lights on and off to precisely deliver light as a substitute for smell. The researchers conducted their study in a custom-built wind tunnel with 12 cameras to track a fly’s movement in three dimensions. Observing insects in their natural, flying state is extremely difficult, which is why much research is done on walking flies, as their movement spans two dimensions instead of three.
After observing that fruit flies move differently depending on air conditions, the authors concluded that fruit flies can sense both the presence and direction of wind.
“If you stick your head out the window of a car while driving, can you tell if there is wind or not?” asked Floris van Brueghel, the study’s principal investigator and assistant professor of mechanical engineering. Detecting a light breeze as you move through the air is difficult, yet fruit flies excel at this.
The authors suggest that flies slow down and turn when they encounter an odor to determine if there is wind and where it is coming from.
“It’s a pretty complex computation going on in this super tiny — supposedly simple — brain,” said Elizabeth Hong, a professor of neuroscience at Caltech who was not involved in the research.
According to Richard Benton, a professor at the University of Lausanne with a background in neurobiology, understanding how fruit flies track odors could help scientists better understand harmful pests such as mosquitoes. Scientists are particularly interested in suppressing the ability of mosquitoes to find and feed on humans to limit disease transmission.
The fly’s sense of smell could also inform the next generation of tracking devices designed to detect the source of an indoor chemical leak.
Maybe this research can garner some respect for fruit flies. After all, they’ve figured out how to thrive alongside humans by tracking scents into our windless homes.
“They have a small brain,” Benton said, “but they do so much with it.”
