Butterfly Holds Record Number of Vision Cells in Its Eyes

The common bluebottle has no fewer than 15 classes of cells known as photoreceptors.

A species of swallowtail butterfly found in Australasia holds the record for having the largest number of different vision cells in its eyes for any insect, scientists say.

The team, reporting today in the journal Frontiers in Ecology and Evolution, said a butterfly known as the common bluebottle (Graphium sarpendon), has no less than 15 classes of these vision cells, known as photoreceptors.

"To date, the highest number of photoreceptor classes in any one insect was nine," said Professor Kentaro Arikawa of Sokendai (the Graduate University of Advanced Studies) in Hayama, Japan, who is an expert in the neuroscience behind insect vision.

"Fifteen is the record."

"All the photoreceptors are used at the same time, sensing colour, brightness, movement and shape."

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Humans have four classes of photoreceptors, including three types for colour vision (cones) and those specialised for seeing shape, movement and changes in light and dark (rods).

The large compound eyes possessed by butterflies cannot see as clearly as human eyes, but are better at seeing a wide panorama, fast movement, polarisation and grades of colour.

Professor Arikawa and colleagues discovered that a particular subspecies of the common bluebottle butterfly (G. s. nipponum) has seven different cells for identifying colour alone - including ultraviolet, blue, red and two types of cells optimised for picking up green.

By studying the genetic material expressed by these cells, the researchers found that each colour photoreceptor produced one or more of five opsin pigments. Each pigment is stimulated by some wavelengths, and less, or not at all, by other wavelengths.

"This is one mechanism to make a variety of photoreceptors," said Professor Arikawa.

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The team used dye to mark the location of the cells, and found that most of the color photoreceptors were on the side of the eye that faced downwards, and they believe this is likely useful for detecting nectar-rich flowers and the wing patterns of potential mates.

He and colleagues found other types of vision cells, mainly on the upper side of the eye that were sensitive to green light and especially tuned for detecting rapid movement.

This would be useful for detection of predatory birds or other butterflies, the insect might need to chase away.

"This butterfly is quite territorial. It sits on a leaf of a high tree and regards the surroundings as its own territory," said Professor Arikawa.

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He said some of the photoreceptors found by the team could not be assigned a specific function at this stage.

"They must be important for picking up some specific signals important for this particular species," said Professor Arikawa.

For example, he said, when butterflies of the same or related species gather at a water source on the ground they flap their wings and flying butterflies may be able to detect this wing movement using such specialised photoreceptors.

"I would like to investigate this in the future."

Article first appeared on ABC Science Online.

The common bluebottle (

Between September and November, North American monarch butterflies make a staggering fall migration that may take as many as 3,000 miles to complete. In appreciation of that arduous journey, let's enjoy a few pictures of these winged beauties and learn a bit about their amazing trip.

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During the summer, it's breeding season, and ordinary monarchs will live about 2-5 weeks, during which they'll mate and lay eggs. Those eggs will become the next generation, each successive generation flying a bit further north than the last. All told, there are four generations of monarchs per year. It's the fourth, and last, generation that's special. Born in the fall, they are hard-wired to migrate. As such, they don't become reproductive right away. (They'll do that later, on the return flight north, having lived for as long as nine months!) The migration kicks in once temperatures drop. Monarchs don't handle cold weather very well. Too deep a chill affects their ability to fly. So when cooler air and shorter days come in late summer and early fall, it's time for them to pack their bags and head south.

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Generally, monarchs west of the Rocky Mountains will overwinter in central and southern California, while those east of them will head for Mexico's Michoacan Mountains. They use the same overwinter sites year after year. Sometimes even the same tress, notes

Monarch Watch

, a site devoted to all things monarch butterfly. The precision of their navigation is still a bit of a mystery. It's thought they find their way using the sun and a kind of solar compass in the brain. But last summer

we learned

that they may also be using Earth's magnetic field to continue the journey on cloudy days.

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As they continue southward, only flying during the day, the butterflies will make pit stops to rest and to nectar. Astonishingly, they gain weight along the way, even though the journey is unspeakably long and draining for something so small and delicate.

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These amazing navigators make the trip on wings just 3 to 4 inches across. While they can fly quickly if something startles them, they more typically pilot along as if sailing on the winds.

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Along the way, adult monarchs will dine on the nectar of a great many plants. Wild carrot, lilac, and alfalfa to name just a few. And, as we can see, when it's time to crash, they don't mind living in close quarters.

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The eastern U.S. monarchs that head to Mexico will overwhelmingly make their way to the Monarch Butterfly Biosphere Reserve, a World Heritage site that is home to millions of monarchs from October through March. These monarchs have reached their destination and are just kicking back in Mexico.

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Once February and March arrive, spring is near. And -- guess what? -- it's time for these well-traveled monarchs to gear up and make a return flight north, as temperatures there warm up. Only now do they become reproductive. As they head back north, they will lay eggs on milkweed to seed a new generation. Their descendants will return to the same overwintering sites from which they returned. Ain't nature grand?

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