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, evasive behavior. In hoverflies, for example, such 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. Thus, similar looming cues can evoke distinct behavioral outputs depending on their source. Supporting this diverse range of appropriate behavioral responses are a multitude of different looming sensitive descending neurons that project information from the head to the thoracic ganglia. We here show that the looming receptive fields of looming sensitive descending neurons are predominantly located in the ventral visual field. To investigate if this is matched by behavior, we recorded how tethered hoverflies responded to looming stimuli displayed either in the dorsal or ventral part of 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 - 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.