Flying eyes: not as rigid as thought

Fruit flies move the retina to navigate their environment

Fruit flies can move their fixed compound retinas to visually track objects and successfully pass obstacles. That’s what researchers at the Max Planck Institute for Biological Intelligence (in the process of being established) and colleagues in the US discovered. The movements of the retina occur through two small muscles in the eye of the fly and are very similar to those in the human eye. Helps insects see clearly moving objects. Movements can also give them information about the distance of nearby objects.

Our eyes are in constant motion – this is how we perceive our environment as sharply and fully as possible. However, insects and many other arthropods have rigid compound eyes that are firmly attached to the head. It was previously assumed that they could only change their field of vision by turning their heads or moving their bodies. However, a new study shows that this assumption needs qualification. “We found that Drosophila use another, completely different way to adjust their visual input as well as head and body movements,” explains Lisa Fink of the Max Planck Institute for Biological Intelligence. Most of the research was conducted by Lise Meitner’s research group leader while in Gaby Maimon’s lab at Rockefeller University in New York.

Lisa Fink wanted to know the function of small muscles in fruit flies of this type Drosophila black belly connected to the retina. Other species of flies also have such muscles. However, the role they play in the visual process was previously unknown. Together with her colleagues, Lisa Fink has now been able to show that muscles can move the retina under the rigid lenses of compound eyes. The environment is imaged on the retina and the incident light is converted into nerve signals. Muscle movements alter the image of the environment without the fly having to move its head. In everyday flying, this ability brings many advantages.

The retina is in constant motion

“We can move our eyes back and forth to keep moving objects focused—flies with their fixed compound eyes can’t do that,” says Lisa Fink. “The mobile retina of fruit flies is an ingenious idea of ​​nature to be able to follow movements despite fixed eyes.” The scientists were able to observe the movements of the retina when they showed fruit flies a moving stripe pattern on the screen. When the striped pattern crossed the fly’s head left and right, the retina moved synchronously with the movement of the strip in order to stabilize the image. At irregular intervals, the retina jumped back to its original positions, so that renewed movement became possible.

“The principle of image stabilization is very similar to the way the human eye uses – we only need six muscles and we move the entire eye. It is remarkable that evolution has produced similar image stabilization strategies in these two very different types of eyes,” says Lisa Fink.

Good on foot – thanks to the retinal muscles

But the mobile retina not only plays an important role in image stabilization. As the researchers were able to show, they are also useful for movement. When fruit flies encounter a crevice in the ground as they walk, they have to decide whether to climb directly above it or take a detour. Flying might be another option, but it would cost more energy. In order to make this decision, the fly must correctly estimate the distance – thus spatial vision is required here.

With the help of a ‘running wheel’ that was developed specifically for fruit flies, the scientists were able to check if the movement of the retina could help them cross the cracks. They ran fruit flies on a spinning wheel with small slits while observing the movement of the retina. Once the flies reach the incision, the retina of the eyes always move toward each other—an important process, shown.

“We were able to observe that flies with weak retinal motions crossed the gaps in the impulse less efficiently,” explains Lisa Fink. “If they can’t move the retina properly, fruit flies obviously have trouble evaluating cracks in the ground properly.” It therefore appears that retinal movements are also important for spatial vision.

Next, Lisa Fink and her team would like to investigate how flies process nerve signals that are transmitted to the brain by the moving retina. The fly’s brain must distinguish whether the visual movement results from movements of the retina or from actual movements in the environment. The researchers now want to use the neurons involved to investigate how the fly’s brain mastered this challenge. “We hope to learn the advantages that retinal movements bring to the visual perception of flies. In this way, we may be able to better understand our own visual process,” summarizes Lisa Fink.