Facet

Definition

The hexagonal, externally visible corneal surface of a single ommatidium; the optical unit that forms one of the numerous small faces tiling the surface of a compound eye. Each facet corresponds to an individual visual unit and acts as a lens that focuses light onto the underlying photoreceptor cluster. Facet size, density, and curvature vary markedly across taxa and often correlate with visual ecology—nocturnal species typically possess larger facets to enhance light capture, while diurnal fast-flying insects exhibit smaller, more numerous facets for improved acuity.

Etymology

Example

The male house fly (Musca domestica) exhibits a pronounced dorsal zone of enlarged facets forming the 'acute zone,' an adaptation that facilitates detection and pursuit of females during mating flights.

Synonyms

• corneal facet

Related Terms

• Ommatidium
• compound eye
• Cornea
• fovea
• acute zone
• superposition eye
• apposition eye
• Rhabdom

Usage Notes

Specialists distinguish the external facet (the cuticular corneal lens) from the complete ommatidium, which includes underlying crystalline cone, rhabdom, and retinular cells. 'Facet' should not be used loosely for any small surface division on an insect; in entomology it specifically denotes the optical component of the compound eye. The term is sometimes qualified as 'corneal facet' to emphasize the lens cuticle versus the entire ommatidium.