homonomous

Homonomous is a term used in biology to describe a body plan in which the body is divided into a series of segments that are uniform in form and function along its length. In homonomous organisms, successive segments resemble each other and there is little regional differentiation among segments. This contrasts with heteronomous segmentation, where segments become specialized for different roles, leading to distinct regions such as a head, thorax, and abdomen or specialized appendages.

Etymology and usage: The word derives from Greek roots meaning “same law” or “same arrangement,” reflecting the sameness of the segments. The term is commonly applied in comparative morphology and developmental biology to characterize the degree of segmental specialization in various phyla.

Examples and context: Homonomous segmentation is characteristic of many annelids, including some oligochaetes and polychaetes, where each metamere contains a similar set of organs and structures. Some arthropods, such as certain millipedes and centipedes, also exhibit relatively uniform trunk segments. In contrast, many groups show heteronomous segmentation, with pronounced regional differentiation (for example, tagmosis in crustaceans or the distinct head–thorax–abdomen regions in insects).

Implications: The presence of homonomous segmentation often indicates a more uniform developmental pattern of segment identity, whereas heteronomous segmentation reflects regionalized gene expression and morphological diversification that supports specialized functions in different body regions.

See also: Metamerism, Tagmosis, Segmental organization, Heteronomy.