Abstract

One of the most fundamental features of the body plan of arthropods is its segmental design. There is considerable variation in segment number among arthropod groups (about 20‐fold); yet, paradoxically, the vast majority of arthropod species have a fixed number of segments, thus providing no variation in this character for natural selection to act upon. However, the 1000‐species‐strong centipede order Geophilomorpha provides an exception to the general rule of intraspecific invariance in segment number. Members of this group, and especially our favourite animal Strigamia maritima, may thus help us to understand the evolution of segment number in arthropods. Evolution must act by modifying the formation of segments during embryogenesis. So, how this developmental process operates, in a variable‐segment‐number species, is of considerable interest. Strigamia maritima turns out to be a tractable system both at the ecological level of investigating differences in mean segment number between populations and at the molecular level of studying the expression patterns of developmental genes. Here we report the current state of play in our work on this fascinating animal, including our recent finding of a double‐segment periodicity in the expression of two Strigamia segmentation genes, and its possible implications for our understanding of arthropod segmentation mechanisms in general. BioEssays 27:653–660, 2005. © 2005 Wiley Periodicals, Inc.