For Our Eyes Only?

The first lesser celandines are coming into flower around here and it's a delight to see their glossy yellow flowers. One of the fascinating things about the flowers that we find so aethetically appealing is that the natural selective forces that have produced their colours throughout their evolutionary history have nothing to do with human vision and everything to do with the way that insects see the world. Many insects can perceive short wavelength ultraviolet radiation, that is invisible to us, but can't see as far as us into the long wavelength, red end of the spectrum. To grasp the implications of this it's useful to consider the colour purple, which is a combination of the extreme ends of the colour spectrum that we can see - blue and red. For an insect that can see shortwave ultraviolet radiation but can only see as far as yellow into the longer wavelength end of the spectrum the combination of these extremes is a colour that we can only guess at. What is certain is that a yellow lesser celandine doesn't look yellow to many insects.

While we can't know exactly what colours an insect's eyes and nervous system perceives, we can get some idea of how flowers look at the ultra-violet end of the spectrum that bees and some butterflies can see. This is a celandine flower photographed in monochrome using old fashioned silver halide black-and-white film (remember that?), using white light (sunlight).
 
This is the same flower using a filter that only passes ultra-violet light that's reflected from the petal surface. Notice how the centre of the flower - the location of the stamens and stigmas - strongly absorbs UV and how the petals themselves reflect it. This hidden pattern of UV reflectance acts as a target, zeroing the insect in on the important parts of the flower once it picks up the reflected UV light from the petal tips. Photographing floweres using UV filters reveals all sorts of cryptic UV absorbing and reflecting patterns that insects can see but which are invisible to us.

You can see more examples of flowers viewed in a way that insects might see them here