Homeotic genes control the diversity of segment development, but the domains of action of homeotic genes do not obviously correspond with the major morphological subdivisions of the insect body. We suggest that this lack of correspondence is misleading, because the spatial domains defined by genetics mask fundamental differences in the roles played by individual genes in different regions. In one or more parasegments, each homeotic gene is expressed `metamerically9; that is, it is expressed from blastoderm stages onwards in all or virtually all cells of the parasegment primordium. Elsewhere, the same homeotic gene may be deployed adventitiously, only in subsets of cells and at later stages of development. We argue that the early `metameric9 domains of gene expression do correlate with the major morphological subdivisions of the fly. This suggests a relatively direct relationship between the expression of particular homeotic genes and the establishment of the `ground plan9 that characterizes segments within each major tagma of the body. This relationship allows us to suggest a scenario for the evolution of homeotic genes in relation to the evolving morphological organization of the arthropod body plan in the insect-myriapod lineage.