The human eye has three different types of cone cells, known as photoreceptors, that allow us to see millions of different colors. Animals have varying numbers of these receptors, which allows some species, such as insects, to see wavelengths of light invisible to us. To date, the largest number of different photoreceptors ever found in an insect’s eye was nine, which is impressive enough. But researchers from Japan have just reported that they’ve discovered a species of butterfly that has at least 15 different types.
This allows the butterflies to see not just the wavelengths, and therefore colors, we can see, but also ultraviolet and polarized light. But while you might think this would mean that the butterflies are able to see shades of color unimaginable to us, the researchers don’t actually think that it works that way. Rather, the additional photoreceptors allow them to detect very specific stimuli in the environment.
These butterflies have the highest number of photoreceptor types known for any insect. Kazuo Unno
The study, published in Frontiers in Ecology and Evolution, examined the visual system of the common bluebottle (Graphium sarpedon), a butterfly native to Australasia and known for its blue-green iridescent wings. The researchers were interested in how different photoreceptors evolved, as the ancestors to all insects are only thought to have three.
Looking at the bluebottle, they found 15 different types of photoreceptors, with varying wavelengths stimulating different receptors. One was found to be reactive to ultraviolet, another to violet, three to different wavelengths of blue, one to blue-green, four to green, and five to red light. Yet the researchers think that the insects only use four of these receptors for everyday color vision, which raises the question of the need for the other 11. The additional receptors are thought to help the butterflies see other stimuli, such as fast-flying fellow butterflies against the sky (due to their ability to discern small variations in the blue-green spectrum), or colorful objects hidden in vegetation.
The mantis shrimp currently holds the record, with 16 different photoreceptor types, but this doesn't mean it sees more colors. Richard Witcombe/Shutterstock
The discovery that the common blueblottle has 15 receptors almost puts it on par with the species that currently holds the record – the coral-dwelling mantis shrimp, which has an impressive 16 different photoreceptors in their eyes. But again, a study looking into the ability of mantis shrimp to distinguish between different colors found that the crustacean wasn’t as good at the task as was previously expected, and that the idea the shrimp had some sort of “super color vision" was incorrect.
“Butterflies may have a slightly lower visual acuity than ourselves, but in many respects they enjoy a clear advantage over us: they have a very large visual field, a superior ability to pursue fast-moving objects and can even distinguish ultraviolet and polarized light,” says Kentaro Arikawa, the lead author of the study. “Isn't it fascinating to imagine how these butterflies see their world?”