Tokyo :Scientists have found that a butterfly species from Australasia has 15 classes of light-detecting cells or photoreceptors, enabling them to see fast moving objects better than the human eye and even distinguish ultraviolet and polarized light.
Previously, no insect was known to have more than nine classes of photoreceptors, researchers said.
These photoreceptors are comparable to the rods and cones in the human eye.
Common bluebottles (Graphium sarpedon) have large eyes and use their blue-green iridescent wings for visual communication evidence that their vision must be excellent.
“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,” said lead author Kentaro Arikawa, professor at Sokendai (the Graduate University for Advanced Studies) in Japan.
“But this discovery of 15 classes in one eye was really stunning,” said Arikawa.
Have multiple classes of photoreceptors is indispensable for seeing colour. Each class is stimulated by light of some wavelengths, and less or not at all by other wavelengths.
By comparing information received from the different photoreceptor classes, the brain is able to distinguish colours.
Through physiological, anatomical and molecular experiments, researchers were able to determine that common bluebottles have 15 photoreceptor classes, one 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
Many other insects have only three classes of photoreceptor and yet have excellent colour vision. Likewise, humans have only three classes of cones, enough to distinguish millions of colours, researchers said.
The researchers believe 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.
A similar system is found in another butterfly previously studied by the same research group, the Asian swallowtail (Papilio xuthus), which has six photoreceptors.
“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,” said Arikawa.
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