ommatidial

Ommatidial describes the unit of a compound eye, the repeating optical channel that forms the basic element of many arthropod visual systems, notably insects and crustaceans. Each ommatidium contributes a single sampling element to the overall image, and the eye’s resolution and sensitivity arise from the array as a whole.

An ommatidium typically includes a corneal facet on the eye surface, a lens or crystalline cone that helps focus light, a set of photoreceptor cells (retinula cells, commonly labeled R1–R8), and pigment cells that isolate adjacent ommatidia. The photoreceptors contain rhabdomeres with microvilli where phototransduction occurs. Signals from the photoreceptors are transmitted to the brain via dedicated optic neuropils, such as the lamina and medulla in many insects.

Optical organization varies among species. In apposition eyes, each ommatidium has its own optical path, providing good spatial resolution under bright light. In superposition or reflecting eyes, light from multiple facets contributes to a single rhabdom, increasing sensitivity in dim conditions. Interommatidial pigment and crystalline lenses help prevent stray light from one ommatidium from leaking into neighbors, preserving image contrast.

The term ommatidial is used to describe features related to these units, including their development, arrangement, and function. The organization and number of ommatidia can range from hundreds to tens of thousands in different species, contributing to diverse visual capabilities such as motion detection, horizon scanning, and color perception.