Ommatidium

Definition

The individual optical unit of a compound eye in arthropods, consisting of a corneal lens, crystalline cone, photoreceptor cells (retina), and associated pigment and support cells, each functioning as an independent pixel that contributes to a mosaic image processed by the brain.

Etymology

From Greek ommatidion, diminutive of omma 'eye'

Example

A large dragonfly may possess 30,000 ommatidia per compound eye, enabling acute motion detection, whereas the Antarctic isopod Glyptonotus antarcticus survives with only five ommatidia per eye in its dim habitat.

Synonyms

• eyelet
• ommatidial unit

Related Terms

• compound eye
• Rhabdom
• Cornea
• Facet
• retinula cell
• apposition eye
• superposition eye
• dioptric apparatus
• acuity

Usage Notes

Used primarily in arthropod vision studies; contrast with simple eye (ocellus). The number, size, and optical design of ommatidia correlate with habitat light levels and behavioral needs—smaller ommatidia improve resolution but reduce sensitivity. In apposition eyes, each ommatidium isolates light from a narrow angle; in superposition eyes, adjacent ommatidia cooperate to capture more photons in dim conditions.