Butterflies may not have a human’s sharp vision, but their eyes beat us in many other ways. Their visual fields are larger, they’re better at perceiving fast-moving objects, and they can distinguish ultraviolet and polarized light.
Now, it turns out that one species of swallowtail butterfly from Australasia, the common bluebottle (Graphium sarpedon, see image), known for its conspicuous blue-green markings, is even better equipped for such visual tasks. Each of their eyes contains at least 15 different types of photoreceptors, the light-detecting cells required for color vision. These are comparable to the rods and cones found in our eyes, of which the latter are responsible for colour vision.
When light of a certain wavelength, or colour, hits them, cones transmit electrical signals to the visual cortex of the brain. There, different combinations of firing cells are cobbled together to be seen as colour.
Most humans have three types of cone – short-wavelength, medium-wavelength and long-wavelength; or red, green and blue – from which we can see millions of colours. For us to see orange, for instance, red cones fire the most, along with a little of the green, and pretty much no blue.
A small proportion of the population, however, is colour-blind. That's not to say they can't see colour – but they lack one or more types of cone (usually red). But the opposite is also possible: some poeple have a fourth type of cone. These "tetrachromats" seem to have an extra cone that peaks somewhere between red and green, allowing them to see different colours where others would see identical hues.
With one type of cone stimulated by ultraviolet, another by violet, three by varying shades of blue, one blue-green, four by green and five by red light, the common bluebottle butterfly has five times as many colour photoreceptors as we do, taking colour vision to a whole new level. Like most humans, many other insects make do with three.
Researchers believe the butterflies only use four of their photoreceptors for day-to-day colour vision, while the others 11 are used in specific environments, such as picking out objects hidden in vegetation. A similar system is found in a six-photoreceptor butterfly, the Asian swallowtail.
The brightly patterned Mantis shrimp is striking in more ways than one. Not only are they formidable hunters with a punch so fast it produces an underwater shockwave, they also possess 12 colour photoreceptors although they don't, in fact, have particularly good colour vision.
Each cone's firing range is narrow, meaning each picks up a specific colour. This means the brain doesn't have to weigh up millions of inputs to determine colour, making it less fuel-intensive and faster for the mantis shrimp to recognise different-coloured prey.
Some birds see the full colour spectrum plus ultraviolet light, bringing a new dimension to bird plumage. Where we see a brilliant palette of feather colours, birds see more. Not only do many species have better colour vision than us, but an extra ultraviolet cone picks up highlights invisible to us.
Just like humans, the good old bumblebee has three types of colour photoreceptor. But unlike us, their cones are shifted towards the ultraviolet end of the spectrum. This lets them see markings on flowering plants directing them to nectar stores, like lights along airport runways.
While they don't have much in the way of detailed colour vision, some snakes, like boas and pythons, have night vision. They can sense the long wavelengths of the infrared spectrum with a set of organs that act as infrared goggles. These "pit organs" are usually situated on the snake's face between their eyes and nostrils, and contain thousands of infrared-sensitive receptors that respond to wavelengths longer than the visible spectrum. This allows snakes to see infrared wavelengths that are invisible to human eyes, although we do feel infrared as heat.