Out on a country walk I love seeing the butterflies floating past, using their gorgeous patterned wings to perform aerial acrobatics. However, I only learned recently that a butterfly views the world very differently. Whilst I admire their colours in the visible spectrum, they see each other in the higher frequency range of ultraviolet (UV) radiation.
Humans have eyes with three colour receptors, referred to as trichromatic vision. We are sensitive to red, green and blue. By comparing how much we detect of each, known as the opponent process, humans can see colours across the visible spectrum. The butterfly on the other hand can see in some visible colours, but can also detect ultraviolet radiation. This opens up a private channel of butterfly communication denied to humans, our vision ends at violet!
The UV reflecting patterns on a butterflies’ wings help them to find mates and view the ultraviolet patterns overlaid on plants, so they can zoom in on the nectar. As sunlight reflects from the wings, the butterfly can also view different colours at different angles, a phenomena known as iridescence. Some butterflies have evolved to become tuned to specific iridescent light, seeing distinctive flickers from the movement of the wings of their own species.
The brightly coloured wings we see as humans (in the visible range) are believed to be there for survival, providing the butterflies with camouflage against predators.
Additionally, butterflies may appear to have two “eyes” but each is actually a “compound eye”, made up of clusters of photoreceptor cells called ommatidium. Overlaid with a single cornea, the compound eye provides a single image but one with a much wider field of vision. This gives the butterfly almost a 360-degree field of view; so they can seek food and keep a look out for attack.
With eyes in the back of their head and ultraviolet communication, butterflies may appear to have superhero vision. However, there is one snag – butterflies cannot see in focus. Although to be fair, they move so quickly both my eyes and camera struggled to capture one in focus too!
Interestingly, it’s the lens on our eye which is believed to block out UV. Our photoreceptors are thought to be sensitive, just UV light doesn’t reach them.