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 behaviour. 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 behavioural responses is a diverse range of looming-sensitive descending neurons that project information from the optic lobes and central brain to the motor control centres in the thoracic ganglia. We show here that these neurons are predominantly sensitive to looming stimuli located in the ventral visual field. To investigate if this is matched by behaviour, 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 behavioural responses than dorsally displayed stimuli. The behavioural data thus not only match the receptive fields of the neurons likely to support the behaviour but also highlight that behavioural output is not entirely reflexive but is strongly modulated by stimulus speed and elevation.