The concentric circles（同心圆状斑点）, also known as eyespots on butterfly, not only look like real eyes but may also appear to glare directly at predators（捕食者）from many directions, scientists have found. This visual illusion, called the "Mona Lisa effect", could scare would-be attackers and buy the insects enough time to escape.

Scientists suspect that eyespots, with dark "pupils（瞳孔）" in the center look like real eyes to predators. Hannah Rowland, an ecologist at the Max Planck Institute wanted to see if the direction of this fake gaze contributed to the effect.

First, Rowland and her co-author trained chicks to attack a worm hidden behind a paper printout of two eyespots at the end of a path. When the eyespots' pupils were specifically pointed in the chicks' direction, the birds repeatedly ran toward the paper and then backed away, and they waited a few minutes before attacking. But when the pupils instead appeared to look away from the direction of the chicks' approach, the birds attacked in seconds. Centrally located pupils, though not as effective as ones that stared directly at the chicks, resulted in longer delays than pupils that looked the other way.

"This suggests that they really are paying attention to the direction of the pupils in the eyespots and are perceiving them as eyelike stimulation. The concentric eyespots found most often in the insect world may seem to the chicks like a pair of eyes that follow them regardless of approach angle," says Rowland.

National University of Singapore evolutionary biologist Antonia Monteiro, who was not involved in the research, says the study is a "cool" demonstration of an evolutionary theory for eyespots. "These butterflies can be encountered from all angles, so having the pupil centrally located ends up being pretty good," Monteiro says. Still, she says, the eyespots used in the study were several millimeters larger than even the largest commonly found in nature, raising the possibility that the chicks may have been extra frightened by the size of the paper eyes.