Tokyo, March 9: Japanese researchers have discovered that a species of swallowtail butterfly from Australasia has at least 15 different classes of light-detecting cells comparable to the rods and cones in the human eye. Having multiple classes of photoreceptors is vital for seeing colour. Common Bluebottles butterflies are a butterfly species with large eyes that use their blue-green iridescent wings for visual communication, proving that their vision is excellent. However, the researchers from the Graduate University for Advanced Studies (SOKENDAI) in Hayama, Japan, through physiological, anatomical and molecular experiments, determined that Common Bluebottles butterfly have 15 photoreceptor classes. "We have studied colour vision in many insects for many years, and we knew that the number of photoreceptors varies greatly from species to species. But this discovery of 15 classes in one eye was really stunning," said lead author Kentaro Arikawa, professor at the SOKENDAI. The findings, detailed in the open-access journal Frontiers in Ecology and Evolution, revealed that Common Bluebottles use only four classes of photoreceptor for routine colour vision, and use the other eleven to detect very specific stimuli in the environment, for example fast-moving objects against the sky or colourful objects hidden among vegetation. One class of the photoreceptor was stimulated by ultraviolet light, another by violet, three stimulated by slightly different blue lights, one by blue-green, four by green lights, and five by red lights, the researchers explained. "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 polarised light,” Arikawa added.