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."
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.
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.
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.