Study finds fruit flies’ visual navigation tactics differ depending on environment

Fruit flies that hover around the apples on your counter must navigate a cluttered environment to find food, from the built environment and vegetation around your home to the objects in your kitchen. Desert fruit flies, not so much.

They fly across a mostly arid landscape with far fewer, relatively predictable obstacles to find cactus fruits. However, it has long been thought that other fruit fly species navigate and maintain balance in much the same way as urban fruit flies, regardless of their habitat.

A new study contradicts this belief by finding that the fly species living in the two distinct visual habitats have very different navigation tactics. The researchers made the discovery by placing fruit flies in a device that allows them to interact with virtual objects in a way similar to the holodeck in the television series “Star Trek: The Next Generation.”

The work is published in the journal Current Biology.

Flying fruit flies have two jobs: to orient themselves toward interesting objects, but also to remain stable during flight. As in humans, vision helps them move in a straight line and remain stable while orienting themselves toward attractive objects. These two tasks are not complementary. Looking at interesting things interrupts the vision needed to remain stable. But the brain manages to process visual signals in such a way that flies can accomplish both tasks.

“Visual stability is important. Try standing on one foot with your eyes closed,” says Mark Frye, corresponding author of the study and professor of integrative biology and physiology at UCLA. “If you want to see interesting things, you have to briefly ignore the part of your vision that controls balance, and you risk falling.”

The urban fruit fly, Drosophila melanogaster, lives in a rich visual environment and can stabilize the background surrounding the objects it needs to navigate. In the desert, the few objects that the Mojave fruit fly (Drosophila mojavensis) encounters are likely to be ones of interest, such as cacti, but they also provide the scenery it needs to remain stable.

To test the effect of different environments on the balance between navigation and stability in the two species, the researchers designed a system that allowed flies to interact with virtual objects while tracking their body and eye movements with a camera. They glued a small steel pin to the fly’s back and connected it to a magnet suspended from the top of a round, drum-shaped device. Another magnet placed at the bottom of the drum created a magnetic field that held the pin exactly in place, while allowing the fly to rotate horizontally inside the drum.

The walls of the drum were completely covered in LED lights, and a computer created different moving shapes of varying sizes and orientations using the lights. The fly could choose how to interact with these virtual objects. One of the objects was a vertical bar that resembled a natural feature, such as a tree trunk. This was designed to test how the fly would use the bar and the background to move around.

“We know exactly where she’s looking because we’re tracking her body and head with a video camera. We also control exactly what the LED visual display is doing so we can recreate the fly’s view of the experiment,” Frye said.

Desert flies ignored the background and steered to smoothly follow the bar, centering it on their visual midline, whereas urban flies did not. Both desert flies and urban flies made rapid eye movements, called saccades, to track objects that moved along a continuous path. Desert flies, in contrast, relied more on a smooth fixation on the vertical bar, followed by a burst of saccades to catch up with the rapidly moving bars.

This pattern is similar to how our own eyes track moving objects.

“When our eyes follow a fast-moving object, like a cow passing us after a boring train ride, we tend to see smooth, continuous eye movements, but only if the object is moving slowly relative to us. When we accelerate faster, our eyes switch to rapid saccades to keep up,” said Martha Rimniceanu, a UCLA doctoral student and lead author of the study.

We were excited to discover different uses of smooth eye movements and saccades in closely related fly species, which we believe are adaptations to the structure of their native visual environment.

Desert flies responded to the bar moving across a stationary background by employing a “fixation and saccade” strategy to smoothly follow the bar. Urban flies smoothly fixed on the background, not the bar, and oriented toward the bar with saccades that negated the smooth stability.

“Basically, desert flies attach to the bar for balance and stability while orienting toward it as an object of interest. Urban flies attach smoothly to the background for balance and then use rapid jerks exclusively to navigate toward the bar,” Frye said.

Research has shown that whether two species are closely related or not, their visual environment determines their visual navigation tactics.

Fruit flies are often used in experiments to probe visual perception and processing. The discovery that not all fruit fly species move through their environments in the same way expands what scientists can learn. Desert fruit flies’ visual navigation is more similar to that of humans than urban fruit flies, for example, and they could serve as a model for learning more about human vision. The information could also be used to help develop autonomous vehicles.

“We can tailor the research to the questions we have, rather than being limited to what the fly will give us,” Frye said.