Rhabdom

A rhabdom is the light-sensitive portion of the ommatidium in many arthropod eyes. It is formed by the rhabdomeres, the microvilli of photoreceptor cells that are tightly organized to create a central light-guiding element called the rhabdom. The rhabdom sits within the xenic structure of the ommatidium and is separated from neighboring units by pigment cells, which help isolate light and reduce stray reflections.

Photoreceptor biology in rhabdom-based eyes involves rhabdomeric photoreceptors that house photopigments in the membranes of the microvilli. When light is absorbed by the opsins, a G-protein–coupled cascade—typically involving Gq and phospholipase C—triggers a phototransduction response. This contrasts with vertebrate eyes, where photoreceptors are ciliary and use a different transduction cascade.

Rhabdom morphology varies among species. Some insects possess a single, fused rhabdom per ommatidium, while others have multiple layers of rhabdomeres that can form complex, layered light-sensitive structures. The arrangement of the rhabdom affects optical properties such as light sensitivity, spectral responsiveness, and acuity of the eye.

Evolutionarily, rhabdomeric photoreceptors represent one major lineage of visual cells in invertebrates, distinguishing them from the ciliary photoreceptors of vertebrates. The study of rhabdoms provides insight into the diversity of visual systems and the evolution of light perception across animal groups.