Common Bluebottles, a species of swallowtail butterfly from Australasia, hold the record for having the largest number of different vision cells in its eyes, compared to any other insect.

Researchers studying these large-eyed butterflies discovered the species, Graphium sarpedon, has at least 15 classes of light-detecting vision cells, known as photoreceptors. Until now, no insect was thought to have more than nine.

Photoreceptors are comparable to the rods and cones found in the human eye. Common Bluebottles have remarkably large eyes and blue-green iridescent wings used visual communication, which suggests they have incredible vision. Even so, researchers did not expect to find such a diverse array of photoreceptors.

"We have studied color vision in many insects for many years, and we knew that the number of photoreceptors varies greatly from species to species," Kentaro Arikawa, lead author of the study and a biology professor from the Graduate University for Advanced Studies in Japan, said. "But this discovery of 15 classes in one eye was really stunning."

Researchers explained that the butterflies use all of their 15 photoreceptors at the same time in order to sense color, brightness, movement and shape. Comparatively, humans have only four classes of photoreceptors: three types of cones used for color vision and rods used to distinguish shape, movement and changes in light and dark.

"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," Arikawa added. "Isn't it fascinating to imagine how these butterflies see their world?"

Using a series of physiological, anatomical and molecular experiments, researchers discovered a particular subspecies of the common bluebottle butterfly - G. s. nipponum - has seven different cells for identifying color alone, including ultraviolet, blue, red and green. They then tested the genetic material expressed by these cells, revealing that each color photoreceptor produces a pigment that is stimulated by some wavelengths of light, and less, or not at all, by others.

While Common Bluebottles are certainly equipped with a record number of photoreceptors, researchers believe the butterflies use no more than four classes for routine color vision. The other 11 would therefore be used to detect very specific stimuli in the environment, such as colorful objects hidden among vegetation, for example.

Furthermore, dye tests used to mark the location of the cells revealed most of the color photoreceptors were on the side of the eye that faces downwards, suggesting they are used to locate nectar-rich flowers and pick up on the wing patterns of potential mates. And other cells, locatedmainly on the upper side of the eye, were sensitive to green light and especially tuned for detecting rapid movement, which would be useful for spotting predatory birds or fellow butterflies.

However, researchers note that further study is needed before all of the photoreceptors can be assigned a specific function.

Their findings were recently published in the journal Frontiers in Ecology and Evolution.

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