How does a housefly see the world?

What the research shows

Hardie (1985) laid out how the fly retina is organised. The six outer photoreceptors of each ommatidium, R1–6, are broadband, peaking around 480–490 nm, and carry a UV-sensitising pigment that adds a second peak near 350 nm. Two central cells, R7 and R8, come in subtypes tuned to ultraviolet, blue and green, which is what gives the fly its ultraviolet-blue-green colour sense with little response to red. The eye is wired by neural superposition, combining signals from receptors in neighbouring ommatidia that look at the same point in space, and this supports a degree of hyperacuity that partly offsets the coarse optics.

Land (1997), reviewing acuity across insects, ties the fly's limit to the angle between neighbouring ommatidia. Land & Eckert (1985) mapped that angle and found it smallest, about 1.75 degrees, in the acute zone of the male eye, which sets a sampling limit near 0.29 cpd; averaged over the eye the figure is nearer 0.17 cpd. The page's 0.3 cpd, roughly 20/2000, is an estimate derived from the acute zone, not a behavioural measurement. Temporal resolution is the fly's strength, with flicker fusion of about 200–300 Hz; the page shows 250 Hz. Ultraviolet and polarisation cues can neither be recorded by a phone camera nor emitted by a display, and no image shown at human frame rates can convey sampling the world that much faster.