What do ommatidia refer to in insects?

Ommatidia are indeed best described as the individual units that make up compound eyes in insects. A compound eye is composed of many tiny lenses, or ommatidia, each providing a separate field of vision. This structure allows insects to have a wide field of view and detect motion effectively.

The organization of ommatidia enables insects to perceive images with lower resolution than that of vertebrates, but they excel in detecting rapid movements and polarized light, which is critical for their survival. The multitude of ommatidia contributes to the compound eye's ability to detect changes in the environment quickly, which is advantageous for navigation and predator evasion.

The other options focus on specific characteristics of eyes or visual systems that do not accurately describe ommatidia. For instance, simple eyes with one lens and exclusive night vision do not capture the multifaceted nature of compound eyes formed by ommatidia. Additionally, while some insects can see ultraviolet light, this capability is not directly tied to the definition of ommatidia. Thus, acknowledging ommatidia as components of compound eyes highlights their unique structure and function in the insect visual system.