Why Fruit Flies Are Smarter Than You Think

Fruit flies, or Black-bellied fruit flyFruit flies are one of the most widely studied organisms because they provide a cheap and accessible platform for research into biological processes. Although they look no 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 documented that these tiny insects employ a deliberate strategy, known as “search and dash,” to find food sources in windy environments. Using this technique, a fruit fly will pick up the scent of something tasty, then dash upwind to follow the scent and then dash from side to side when it loses the scent to find it again.

Scientists have shown that picking up a scent on the wind doesn't mean the source is nearby. Rather, the breeze may have carried the scent from a distant point. As a result, the toss-and-dash technique is an effective way to trace the origin of a scent on the wind.

So what happens when there is no wind?

“They have another trick up their sleeve,” said Markus Stensmeier, an associate professor of sensory biology at Lund University who was not involved in the study.

The trick when there's no wind: Flies make a downward, circular motion. The authors found that when flies encounter an odor and then lose it in the still air, they head downward in an attempt to find the source of the odor.

This behavior may not come as a shock—most cartoons of flies show them circling a stinky pile of food. However, this is the first time scientists have documented how fruit flies behave in still-air environments, confirming long-standing human intuition.

In still air, picking up a scent indicates that the source may be nearby, making the sink and circle approach more useful.

Some researchers have suggested that dogs and rats exhibit similar behaviors when they sniff high and low to approach an odor.

To conduct their study, the authors first had to figure out a way to stimulate the fly’s sense of smell in a windless environment. But how do you deliver scents without wind? David Stupsky, lead author of the study and a postdoctoral researcher in biology and engineering at the University of Nevada, Reno, described the scientists’ approach: Approach as a “virtual reality of smell”.

The researchers used genetically modified flies that had light-activated neurons in their antennae, which are essentially the fly’s nose. As a result, the researchers were able to stimulate the fly’s sense of smell using red flashes of light rather than actual odors. The light-based approach allowed the researchers to bypass the difficulty of delivering controlled odor clouds, which are difficult to locate in space.

The researchers were able to easily turn the lights on and off to precisely deliver light as a proxy for scent. The researchers conducted their study in a specially designed wind tunnel with 12 cameras to track the flies’ movement in three dimensions. Observing insects in their natural state while they fly is extremely difficult, which is why much of the research is done on walking flies, whose movement spans two dimensions rather than three.

After noticing that fruit flies move differently depending on air conditions, the authors concluded that fruit flies can sense the presence and direction of wind.

“If you stick your head out the car window while driving, can you tell if there’s wind?” asked Floris van Breugel, the study’s lead author and an assistant professor of mechanical engineering. Detecting a gentle breeze while moving through the air is difficult, but fruit flies excel at it anyway.

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.

“This is a very complex computational process happening in this very small — and 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 track odors could help scientists better understand harmful pests, such as mosquitoes, according to Richard Benton, a professor at the University of Lausanne and an expert in neuroscience. Scientists are particularly interested in preventing mosquitoes from finding and feeding on humans to reduce the transmission of diseases.

The sense of smell could also provide information for the next generation of tracers designed to find the source of chemical leaks indoors.

Maybe this study could earn some respect for fruit flies. After all, they’ve figured out how to thrive alongside humans by tracking down scents inside our drafty homes.

“They have a small brain, but they do a lot of things with it,” Benton said.