Some arthropods have simple eyes, also called ocelli. Each ocellus has up to 1,000 sensory cells behind a single lens. Simple eyes convergently evolved at least 7 times.
Ocelli often do not form images but are sensitive to light direction and intensity. Flying insects typically have 3 ocelli on the top of their head, as well as a pair of compound eyes.
Auxiliary ocelli help maintain flight stability. Flying insects with compound and simple eyes have 2 physiologically distinct but mentally integrated vision processing systems: the most complex of any animal.