diplosegment

Diplosegment is a term used in arthropod anatomy to describe a body segment formed by the fusion of two adjacent embryonic segments (somites). It is most prominently used for the trunk organization of millipedes (class Diplopoda). In millipedes, each diplosegment typically carries two pairs of legs, one pair on each of the two original segments that have fused. Consequently, the diplopod body shows many segments that are built from a smaller number of diplosomeres, yielding the characteristic double-legged units.

Structurally, a diplosegment often includes dorsal and ventral plates representing the fused somites, with limb insertions near the ventral margin. The fusion reduces the apparent segment count compared with the organism’s embryonic segmentation and is a key aspect of diplopod tagmosis.

In broader comparative morphology, the term is used to denote similar fused-segment units in fossil or other arthropods, though it is most diagnostic for millipede anatomy. The concept helps explain the distinctive segmentation and locomotor pattern of diplopods, and it contrasts with the single-segment plan seen in centipedes, where each body segment bears one pair of legs. Diplosegments thus underlie the characteristic body plan of diplopods and reflect their evolutionary adaptation of limb-bearing units through segment fusion.