Rhabdom

Definition

The rod-shaped, light-guiding structure formed by the fused microvilli of photoreceptor cells (retinula cells) within an ommatidium of a compound eye. The rhabdom acts as an optical waveguide, channeling photons to the photopigment molecules embedded in its membrane, thereby converting light into neural signals. Its morphology—whether cylindrical, tapered, or divided into rhabdomeres—varies among arthropod taxa and correlates with visual ecology, such as sensitivity versus acuity trade-offs.

Etymology

From Greek rhabdos, rod, referring to its cylindrical shape.

Example

In the superposition eyes of nocturnal moths, the rhabdoms of adjacent ommatidia are optically coupled, allowing light from many facets to converge on a single photoreceptor and boost sensitivity under dim conditions; in contrast, apposition eyes of diurnal dragonflies maintain isolated rhabdoms for sharper spatial resolution.

Synonyms

• rhabdomere (when referring to unfused microvilli contributions)

Related Terms

• Ommatidium
• retinula cell
• rhabdomere
• Cornea
• crystalline cone
• tapetum
• superposition eye
• apposition eye
• photoreceptor

Usage Notes

Specialists distinguish the fused rhabdom (common in most insects) from the open rhabdom of some Diptera, where individual rhabdomeres remain separated and project to different neural cartridges, enabling neural superposition. The term is sometimes loosely used for the photosensitive portion of any arthropod visual cell, but strictly refers to the microvillar structure within ommatidia. Not to be confused with rhabdomere, which denotes the microvillar contribution of a single retinula cell in open rhabdom systems.