A poppy is red to you. To a bee it is something else

The poppy is red, and bees visit it all summer. Pick one before you read on.

Pick whose eyes look. The bee's colours are computed from the petals' measured spectra, by the false-colour photography rule: ultraviolet shown as blue, blue as green, green as red.

The line shows the measured spectrum, from 300 to 700 nm. The shaded bands are the bee's three colours: ultraviolet, blue, green.

Inside each of the 5,375 facets sit nine seeing cells, and above them, on the crown, three more small eyes read the sky's light.

You share blue and green. The difference sits at the ends: you see red, the bee sees ultraviolet. Each sees a piece of the world the other never suspects.

Through a strip along its top, the eye draws compass information from the sky's polarised light. The three simple eyes on the crown see shapes poorly and sense light well.

How blurred her world is

To catch a disc's colour, the bee needs it to cover 15° of her view: a 16 cm plate held at arm's length. She glimpses its shape from 5°, about a 5 cm plate.

Her clock, though, runs faster than yours: she catches 100 flickers a second, where your film looks smooth.

The dandelion's tips and the green cup beneath them wear two different bee colours. Patterns like these guide the bee toward the flower's middle.

The flower's tips (yellow to you, ultraviolet-green to the bee) and the green cup beneath them (green to both of you) are two different signals. The difference points the way to the middle.

Many flowers wear such an ultraviolet eye: the middle absorbs ultraviolet while the petals reflect it. Look for flowers with a dark middle in ultraviolet photographs and you find them everywhere.

Ultraviolet is the bee's third colour, beside blue and green. Talk of an ultraviolet signal says more than the measurements show.

Eight statements about the bee's eyes. Some sound like stories and are true.

The sums above run on measured spectra and on papers read in full. Here is where they may go wrong:

• Romanian poppies were never measured.

  The poppy spectra come from Germany and Austria. Nobody measured Romania's, and the page says only what it knows.

• The green peak is 544 or 556.

  Studies give different values for the green receptor's peak. The page writes about 540, which covers both.

• The bee's colours are a model.

  The discs and false-colour flowers translate receptor signals into human colours. They show what the bee tells apart, not what she feels.

• One spectrum appears twice.

  Poppy records 2125 and 2184 carry nearly the same spectrum: one measurement published in two papers. The page uses it once.

• The 100 flickers come from the lab.

  The 100 Hz threshold was measured in free-flying bees, under laboratory conditions.