Fruit flies buzzing around apples on your counter have to navigate a messy environment to find that food, from the built environment and vegetation surrounding your home to the objects in your kitchen. Desert fruit flies, not so much. They fly across a mostly arid landscape dotted with far fewer, relatively predictable obstacles to find cactus fruit. But other fruit fly species have long been thought to navigate and balance in much the same way as urban fruit flies, regardless of habitat.
A new study contradicts that belief with the discovery that fly species living in the two distinct visual habitats have significantly different navigation tactics. Researchers made the discovery by placing fruit flies in a contraption that allowed them to interact with virtual objects in a manner similar to the game room in the television show “Star Trek: The Next Generation.”
Fruit flies have two jobs: to steer toward interesting objects, but also to stay stable during flight. Just like in humans, vision helps them move in a straight line and to stay stable while orienting toward attractive objects. These two tasks are not complementary. Looking at interesting things disrupts the vision needed to stay stable. But the brain manages to process visual signals in such a way that flies can perform both tasks.
“Visual stability is important: try standing on one foot with your eyes closed,” said Mark Frye, corresponding author of a paper published in Current biology and professor of integrative biology and physiology at UCLA. “If you want to look at interesting things, you have to briefly ignore the part of your vision that governs balance and you risk falling over.”
Urban fruit flies, Drosophila melanogasterThey live in a rich visual environment and can stabilize the background surrounding the objects around which they need to orient themselves. But in the desert, the few objects that Mojave fruit flies can see are Drosophila mojavensisThe encounters are likely to be those that interest them, like cacti, but they also make up the stage they need to use to stay stable.
To test how different environments affected the balance between orientation and stability in both species, the researchers devised a system that allowed flies to interact with virtual objects while tracking their body and eye movements with a camera. They taped 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 at the bottom of the drum created a magnetic field that held the pin precisely in position while allowing the fly to rotate in the horizontal plane within the drum.
The walls of the drum were completely covered in LED lights and a computer created different moving shapes of various 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 looked like a natural feature, such as a tree trunk. This was designed to test how the fly would use the bar and background to navigate.
“We know exactly where it’s looking because we’re tracking its body and head with a video camera. We also control exactly what the LED screen does so we can recreate the vision of the experiment from the fly’s point of view,” Frye said.
Desert flies ignored the background and oriented themselves to smoothly follow the bar, centering it on their visual midline, whereas urban flies did not. Both desert and urban flies made rapid eye movements, called saccades, to follow objects moving along a continuous path. Desert flies, however, relied more heavily on smooth fixation of the vertical bar, followed by a series of saccades to reach the rapidly moving bars.
This pattern is similar to how our own eyes track moving objects.
“When our eyes follow a fast-moving object, such as a cow passing by 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. As we pass by faster, our eyes switch to rapid saccades to keep up,” said Martha Rimniceanu, a PhD candidate at UCLA and lead author of the study. “We were excited to find a different use of smooth eye movements and saccades in closely related fly species. We think this is an adaptation to the structure of their native visual environment.”
Desert flies responded to the motion of the bar over a stationary background by employing a “fixate and jerk” strategy to follow the bar smoothly. Urban flies gently fixated on the background, not the bar, and oriented toward the bar with jerks that overrode the smooth stability.
“Essentially, desert flies fixate on the bar to maintain balance and stability, while also orienting toward it as an interesting object. Urban flies gently fixate on the background to maintain balance and then use rapid saccades exclusively to navigate toward the bar,” Frye said.
The research showed that regardless of whether two species are closely related or not, their visual environment determines their visual navigation tactics.
Fruit flies are often used in experiments investigating visual perception and processing. The finding that not all fruit fly species navigate their environment in the same way expands the possibilities of what scientists can learn. The desert fruit fly’s visual navigation more closely resembles that of humans than that of the urban fruit fly, for example, and could be used as a model to learn more about vision in humans. The information could also be used to aid the development of autonomous vehicles.
“We can tailor the research to the questions we have, rather than being limited to what the fly can give us,” Frye said.