Arthropods vary enormously in segment number, from less than 20 to more than 200. This between-species variation must have originated, in evolution, through divergent selection operating in ancestral arthropod species with variable segment numbers. Although most present-day arthropod species are invariant in this respect, some are variable and so can serve as model systems. Here, we describe a study based on one such species, the coastal geophilomorph centipede Strigamia maritima. We investigate the way in which segments are formed using in situ hybridization to demonstrate the expression pattern of the engrailed gene during embryogenesis. We also analyze segment number data in mother-offspring broods and thereby demonstrate a significant heritable component of the variation. We consider how natural selection might act on this intraspecific developmental variation, and we discuss the similarities and differences in segment formation between the geophilomorphs and their phylogenetic sister-group.