How bees see

The eyes

A worker honey bee has two compound eyes of roughly five thousand facets each, plus three simple eyes — ocelli — on the top of the head. Each facet feeds a separate optical unit, so the image is assembled from thousands of samples rather than focused onto a retina. That construction gives an enormous field of view and poor acuity: a bee has to be within centimetres of a flower to resolve its shape the way we can from across a room.

Drones are built differently. Their eyes are much larger, with the facets pointing forward and upward, because a drone's entire adult purpose is to spot a queen against open sky during a mating flight.

Colour, shifted

Bees are trichromatic like us, and the three receptor types sit at different wavelengths: ultraviolet, blue and green rather than blue, green and red. The whole window is shifted towards the short end. The consequence is that bees see ultraviolet as a colour and do not see red as one — a pure red flower reads as dark unless it also reflects ultraviolet or blue.

The shift also produces a colour we have no experience of. Ultraviolet and yellow together stimulate the bee's receptors in a combination with no equivalent in human vision, conventionally called bee purple. It is a real perceptual category for the animal and an unimaginable one for us.

Nectar guides

Many flowers carry patterns that are invisible to us and unmistakable to a bee: bullseyes, radiating lines, darkened centres, produced by pigments that absorb ultraviolet. A plain yellow silverweed or evening primrose photographed under ultraviolet shows a dark heart and pale rim. The markings direct an arriving bee to the nectary, which shortens the visit and makes pollen transfer more reliable — a benefit to the plant rather than to the bee.

Motion, and the flicker rate

Bees resolve flicker at several times the human rate. A light source that looks steady to us is visibly flickering to a bee. This is not a curiosity: an animal flying at several metres a second through vegetation needs to update its visual field fast enough to avoid hitting things, and the high temporal resolution is bought at the cost of spatial resolution.

The same motion sensitivity underlies how bees measure distance. Rather than tracking energy spent, a bee integrates how much the image of the world slides past as she flies. Flying bees through a narrow patterned tunnel makes the world slide past faster, and the bees then report a longer distance in their dances than they actually flew.

Polarised light

Sunlight scattered by the atmosphere is polarised in a pattern that radiates around the sun's position, and a specialised region at the top of each compound eye is built to read it. A bee that can see one patch of blue sky can therefore work out where the sun is, which is why an overcast day does not disable navigation. It is one of the classic results in animal orientation and it has held up for eighty years.

What bee vision is bad at

• Fine detail. Acuity is roughly a hundredth of ours, which is why flower shape is learned close up.
• Red as a colour, which reads as dark rather than as a hue.
• Low light. Honey bees are poor in dim conditions, though some tropical bees have adapted to forage at dusk and by moonlight.
• Distinguishing a printed image from the thing it depicts, which is what makes photographs usable as experimental stimuli in the first place.