Most mammals such as humans possess three types of photoreceptors in their eyes: cone cells, rod cells, and photosensitive retinal ganglion cells. Cone cells allow us to detect color vision in bright light, while rod cells help us see at low light levels.

Scientists say that some insects are known to have at least nine classes of photoreceptors in their eyes, but a remarkable new study revealed that some possess more than nine.

Experts were surprised to find out that Common Bluebottles, a species of swallowtail butterfly native in Australasia, have 15 different classes of photoreceptors in their large eyes, making them the record-holder for the most number of vision cells.

Professor Kentaro Arikawa, the study's lead author, has studied color vision in many insects for a long time. He said he and his colleagues knew that the number of photoreceptors differ from species to species.

"But this discovery of 15 classes in one eye was really stunning," said Arikawa.

Common Bluebottles use their blue-green iridescent wings for visual communication, and scientists said this is evidence of their excellent vision.

Photoreceptors are essential for seeing color, and each class of photoreceptor is sparked by light of different wavelengths.

Arikawa and his team found the following:

- one photoreceptor class is triggered by ultraviolet light

- one class is stimulated by violet

- three photoreceptors are triggered by different shades of blue lights

- one is stimulated by blue-green

- four photoreceptors are stimulated by green lights

- five photoreceptors are triggered by red lights

The Common Bluebottle's photoreceptors may seem excessive. After all, insects typically only have three and yet can see perfectly. Humans also only have three, but we can see millions of colors.

So what are the other photoreceptors for? Arikawa and his team believe that these swallowtail butterflies only use four classes of photoreceptors for routine color vision, and the other 11 can detect very specific stimuli in the environment.

For instance, Common Bluebottles can detect objects that move fast against the sky, as well as colorful objects camouflaged among vegetation.

The same system is found in the Asian swallowtail Papilio xuthus, a butterfly with six photoreceptors which was studied by the same research team.

Arikawa said butterflies have slightly lower visual sharpness than humans, but in many aspects, these insects 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," added Arikawa. "Isn't it fascinating to imagine how these butterflies see their world?"

The team's findings are featured in the journal Frontiers in Ecology and Evolution.

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