Why fruit flies are smarter than you think

Fruit flies, or Drosophila melanogaster, are some of the most widely studied organisms because they offer a cheap and accessible platform to research biological processes. Though they could not seem more different from humans, fruit flies share 75 percent of our disease-causing genes, which is why scientists have used them to better understand human illness.

Researchers documented long ago that these small insects carry out a deliberate strategy, known as cast and surge, to find food sources in windy environments. With this technique, a fruit fly catches a whiff of something yummy, surges upwind to follow the smell and casts side to side when it loses the scent to find it again.

Scientists explained that catching a scent in the wind does not mean the source is nearby. Rather, the breeze probably carried the smell from some distant point. As a result, a cast and surge technique is an effective way to track down the origin of a scent in the wind.

So what happens when there’s no wind?

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

The trick when there is no wind: Flies perform a sink and circle movement. The authors found that when flies encountered and then lost an odor in still air, they spiraled downward in an attempt to find the source of the smell.

This behavior may not be shocking — most cartoon depictions of flies show them circling around a stinky mound of food. Yet, this is the first time scientists have documented how fruit flies behave in still air environments, confirming long-held human intuition.

In still air, catching a scent indicates that the source is probably nearby, making a sink and circle approach more useful.

Some researchers proposed that dogs and rats exhibit similar behaviors when they sniff high and low to close in on a scent.

To conduct their study, the authors first had to figure out 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 on the study and a postdoctoral researcher in biology and engineering at the University of Nevada at Reno, described the scientists’ approach as “a virtual reality for the sense of smell.”

They used genetically modified flies that had light-activated neurons in their antennae, which is essentially a fly’s nose. As a result, the authors could trigger a fly’s sense of smell using red flashes of light instead of actual odors. The light based approach allowed researchers to bypass the difficulty associated with delivering controlled odor clouds, which are hard to locate in space.

The authors could easily turn the lights on and off to precisely deliver light as a surrogate 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. Monitoring insects in their natural, flying state is notoriously difficult, which is why a lot of research is done on walking flies, as their motion spans two dimensions instead of three.

After observing that the fruit flies moved differently based on air conditions, the authors deduced 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’s any wind or not?” asked Floris van Breugel, principal investigator of the study and assistant professor of mechanical engineering. Detecting a gentle breeze while moving through the air is hard, but fruit flies excel at it anyway.

The authors suggest that the 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 pretty complex computation that’s happening in this super tiny — supposedly simple — brain,” said Elizabeth Hong, a professor of neuroscience at the California Institute of Technology who was not involved in the study.

According to Richard Benton, a professor at the University of Lausanne with expertise in neurobiology, understanding how fruit flies track down smells can help scientists better understand harmful pests, such as mosquitoes. Scientists are especially interested in inhibiting mosquitoes’ ability to find and feed on humans to limit disease transmission.

Fly olfaction can also inform the next generation of tracking devices designed to find the source of an indoor chemical leak.

Perhaps this study can garner some respect for fruit flies. After all, they have figured out how to thrive alongside humans by tracking down scents inside our windless homes.

“They have a tiny little brain,” Benton said, “but they do so much with it.”