Sporar Klinge, K. | bioRxiv Science

bioRxiv2026

Hoverfly responses to looming stimuli depend on elevation and speed

An object on immediate collision course generates a rapidly expanding visual stimulus on the retina, which will typically trigger a fast reaction, such as an evasive behavior. In hoverflies, for example, visual looming stimuli may be generated if the insect is about to collide with a stationary object in the surround, by an approaching predator, or by conspecifics during territorial interactions. Supporting these behavioral responses is a diverse range of looming sensitive descending neurons that project information from the optic lobes and central brain to the motor control centers in the thoracic ganglia. We here show that the looming sensitive descending neurons are predominantly sensitive to looming stimuli located in the ventral visual field. To investigate if this is matched by behavior, we recorded how tethered hoverflies responded to looming stimuli presented dorsally or ventrally on a visual monitor, at four different speeds (l/|v| of 10 - 667 ms), covering a naturalistic range. We found that ventral stimuli, especially at intermediate speeds (l/|v| = 50 or 200 ms), triggered much stronger behavioral responses than dorsally displayed stimuli. The behavioral data thus not only match the receptive fields of the neurons likely to support the behavior, but also highlight that behavioral output is not entirely reflexive but is strongly modulated by stimulus speed and elevation. Significance StatementIf someone throws a ball at you, this generates a rapidly expanding object across your visual field, which will make you react before you have even had time to think. You may for example duck, dip or dive to avoid the ball, or bring your hands up to grab it. Similarly, many insects respond to rapidly approaching objects. We here show that hoverfly reactions to such looming stimuli depend on stimulus speed and elevation, with the strongest response to stimuli approaching from below. We further demonstrate that the neurons likely supporting these behaviors show highest sensitivity in the ventral visual field, suggesting a close match between neural tuning and behavioral output.

animal behavior and cognition↗

bioRxiv2025

Fast adaptation in invertebrate looming-sensitive descending neurons

Motion vision plays a crucial role in guiding dynamic behaviors, such as determining when to escape from predators, pursue prey, navigate obstacles, or adjust flight patterns during migration. Adaptation to repetitive motion stimuli is a crucial aspect of this process, allowing animals to efficiently process new stimuli while avoiding sensory overload. This helps animals remain responsive to novel or important stimuli, ensuring appropriate behavioral reactions. Adaptation to looming stimuli, which often signal an approaching threat through the rapid expansion of an objects image on the retina, allows animals to distinguish harmless from harmful stimuli. While neural adaptation has been extensively studied in the flys optic lobes, less is known about how descending neurons, which link the optic lobes to the motor centers in the thoracic ganglia, adapt. To address this gap, we investigate adaptation in looming-sensitive descending neurons in the hoverfly Eristalis tenax. Using intracellular recordings, we show that these descending neurons adapt to looming stimuli with inter-stimulus intervals of 1-3 s. We show that the level of adaptation depends on the ISI, with shorter intervals leading to greater adaptation. Specifically, we find that adaptation leads to decreased response duration, with a pronounced delayed response onset. We identified descending neurons that responded to looming stimuli either unilaterally or bilaterally and used this to show that most of the adaptation takes place within the neuron itself, rather than its pre-synaptic inputs. Finally, we found that the wing beat amplitude of tethered hoverflies did not appear to adapt to repetitive looming stimuli. Significance StatementRapidly expanding looming objects often signal approaching threats, and appropriate detection is therefore critical for survival across species. Adaptation to repetitive looming cues allows animals to filter out irrelevant information, ensuring they respond only to significant threats. While adaptation has been well studied in the brain, how descending neurons, which connect the brain with the body, adapt to looming stimuli remains poorly understood. Here, we investigate looming-sensitive descending neurons in the hoverfly Eristalis tenax and show that they adapt strongly when looming stimuli are repeated in quick succession. We show that most of the adaptation takes place within the descending neuron itself. Our research establishes a foundation for future exploration of the neural mechanisms underlying behavior in constantly changing environments.

neuroscience↗