Compound eye

A visual organ composed of many light-sensitive units called ommatidia. Compound eyes occur in insects and many other arthropods.

Ommatidium: A single optical and sensory unit within a compound eye, containing photoreceptors and associated structures. Each ommatidium samples light from a limited direction, and many samples contribute to the eye’s image.
Arthropod: An invertebrate animal with a segmented body, jointed appendages, and an exoskeleton. Compound eyes are widespread across this animal group, though their forms vary.
Motion detection: The process of detecting changes in an object’s position over time. Many compound eyes detect rapid movement effectively, supporting escape and pursuit behaviors.
Camera-type eye: An eye that forms an image through a single main aperture and lens onto a continuous light-sensitive surface. It contrasts with the compound eye’s many separate optical channels.
Apposition eye: A compound-eye design in which each ommatidium primarily receives light from its own narrow field of view. This design lets many daytime insects form images with limited overlap between neighboring ommatidia.
Cuticle: A nonliving external layer secreted by arthropods, forming much of their exoskeleton. The transparent cuticle forms the corneal lens over each ommatidium.
Optical flow: The apparent pattern of motion of objects and surfaces across an observer’s visual field. Insects use changing visual patterns to estimate self-motion during flight.
Simple eye: A visual organ with one lens or light-sensitive opening rather than many ommatidia. Some arthropods possess simple eyes alongside compound eyes, with different visual roles.
Superposition eye: A compound-eye design that combines light entering through multiple facets at one retinal point. This design gathers more light than apposition eyes, aiding vision in dim conditions.
Cornea: A transparent outer structure that refracts light into an eye. Each facet’s corneal lens directs light toward the sensory structures beneath it.

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