Their compound eyes appear rigid - but something is happening inside, scientists report: Flies move the retinas in their eyes using two muscles in order to see moving objects clearly and to be able to better judge distances, according to experiments on the model fly Drosophila melanogaster. The system has striking similarities to the concept of moving eyes in vertebrates, the researchers say.
As you read these lines, it becomes particularly clear: Humans and many other vertebrates can move their eyes flexibly to focus on specific points in the field of vision. This enables sharp and targeted vision. But in addition to our visual principle, another is much more widespread in nature: insects and other arthropods have compound eyes that are firmly connected to the head. They are made up of many individual elements with rigid lenses that project light signals onto a layer that corresponds to our retina. The system allows good all-round vision, but appears to be rigid: it was previously thought that insects could only change their field of vision by turning their heads or moving their bodies. But the current study now shows that this assumption, at least as far as flying is concerned, has to be put into perspective.
The researchers led by Lisa Fenk from the Max Planck Institute for Biological Intelligence in Martinsried focused on previously mysterious elements that had been discovered in fly eyes: it was known from anatomical studies that house flies have two tiny muscles in their eyes that are connected to the retinas are. It has been suggested that they can change their shape. What role they actually play in the visual process, however, remains unclear. In order to follow this lead, Fenk and her colleagues targeted these structures in a fly species that offers a particularly large number of possibilities for investigation: They looked closely into the eyes of the fruit fly Drosophila melanogaster.
Fruit flies caught the eye
Thanks to the possibilities of marking and visualizing certain structures in these model animals, the researchers were initially able to clarify the basic function of the muscles in the fly's eye. The observations of the reactions showed that the two muscles can actually move the retinas under the rigid lenses of the compound eyes. As the scientists explain, the environment is imaged on the retina and the incident light is converted into nerve signals. During their investigations, it became clear that muscle movements lead to a shift in the image of the environment. "We were able to show that fruit flies use another, completely different method to adapt their visual input in addition to head and body movements," says Fenk.
The researchers then investigated how the insects use the system. To do this, they presented flies attached to a fine holder with a moving stripe pattern on a display. They used a special camera system to look at the marked net house structures on the test animals. This revealed specific movement reactions: when the stripe pattern ran past the fly's head on the left and right, the retinas moved synchronously with the stripe movement. At irregular intervals, the retinas jumped back into their original positions, so that renewed movement became possible. As the researchers explain, these reactions apparently serve to stabilize the visual impression.
Movements with visual functions
"We can move our eyes back and forth to keep moving objects in focus - flies with their fixed compound eyes cannot do that," says Lisa Fenk. However, the movable retinas of the fruit flies represent an alternative system for still being able to follow movements, explains the scientist. “The principle of image stabilization is similar to the method that the human eye also uses – only that we need six muscles and move the whole eye. It is fascinating that evolution has produced similar image stabilization strategies in these two completely different eye types,” says Fenk.
Through further experiments, the researchers were also able to prove that the mobile retinas also play a role in the locomotion of the insects. As they explain, when the flies walk, they have to decide whether they can cross a crevasse, take a detour, or fly. For this decision an estimation of the distance is necessary. As can be seen from the experiments, the movement system in the eye seems to be connected to spatial vision. The researchers had the flies run on a rotating wheel with small slits and observed the movements of the retinas. Characteristic reactions emerged that are obviously important: "We were able to observe that flies with artificially weakened retinal movements crossed the gaps in the impeller less efficiently," says Fenk. "If they can't move their retinas properly, the flies obviously have trouble judging underground cracks."
The results have now raised a few more questions: It seems unclear how widespread the system could be among the many representatives of the arthropods. Fenk and her colleagues now want to go into more detail in the case of Drosophila: They will investigate more closely how the flies process the nerve signals that are passed on to the brain by the moving retinas. "We hope to use this to further elucidate the advantages that retinal movements bring to flies for visual perception," says Fenk.
Source: Max Planck Institute for Biological Intelligence, specialist article: Nature, doi: 10.1038/s41586-022-05317-5