Biologists say they have solved the riddle of how a tiny bacterium senses light and moves towards it: the entire organism acts like an eyeball.
In a single-celled pond slime, they observed how incoming rays are bent by the bug's spherical surface and focused in a spot on the far side of the cell.
By shuffling along in the opposite direction to that bright spot, the microbe then moves towards the light.
Other scientists were surprised and impressed by this "elegant" discovery.
Despite being just three micrometres (0.003mm) in diameter, the bacteria in the study use the same physical principles as the eye of a camera or a human.
This makes them "probably the world's smallest and oldest example" of such a lens, the researchers write in the journal eLife.
