Ocelli
Lexikon
Besides the two large compound eyes, bees carry three tiny simple eyes on their forehead, the ocelli.
Besides the two large compound eyes, bees carry three tiny simple eyes on their forehead, the ocelli. They are barely larger than a pinhead and are arranged in a triangle on the crown of the head. Unlike the compound eye, each ocellus has only a single, strongly curved lens, behind which lie many light-sensitive cells; they therefore do not produce 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 thereby support the bee's internal clock as well as its orientation relative to the horizon, which is important for stable, balanced flight. They probably also play a role in finding the dark hive entrance leading into the interior of the hive.
In practice, the ocelli explain, among other things, why bees adjust their flight behavior under strongly changing light, for example in the shade of trees or in stormy conditions, and why the start and end of daily flight activity are closely tied to light conditions. Anyone who only knows the large compound eyes can easily overlook that, with the ocelli, bees have a second visual system that works in a completely different way, serving rough light measurement rather than actually seeing shapes.