Ocelli
Lexikon
In addition to the two large compound eyes, bees have three tiny simple eyes on the forehead, the ocelli.
In addition to the two large compound eyes, bees have three tiny simple eyes on the forehead, the ocelli. They are hardly bigger than a pinhead and are arranged in a triangle on the top of the head. Unlike the compound eye, each ocellus has only a single, strongly curved lens, behind which many light-sensitive cells lie, so they do not provide a sharp image.
The strength of the ocelli lies instead in a very high sensitivity to light. They mainly measure the general brightness of the surroundings and thus support the bee's internal clock and her orientation to the horizon, which is important for stable, balanced flight. They probably also play a role in orientation in the dark hive and in finding the bright entrance from inside.
In practice, the ocelli explain among other things why bees adapt their flight behavior in strongly changing light, for example in the shade of trees or when a thunderstorm is brewing, and why the start and end of daily flight activity are closely linked to the light conditions. Anyone who knows only the large compound eyes easily overlooks the fact that with the ocelli bees have a second visual system that works in a completely different way and serves rough light measurement rather than the actual seeing of shapes.