Wasp vs. fly development

A hypothesis about the evolution of the the developmental role of the even-skipped (eve) gene. The gene is present in all insects, and in most it plays a role in defining the embryonic segments. It does not play that role in some species, either because that role evolved later, or because it has been independently lost in two lineages. In one lineage, the early development is constrained because the wasp lives as a parasite within another insect, and another gene regulates segment development.

Image from Gregory A Wray and Ehab Abouheif (1998) "When is homology not homology?" Current Opinion in Genetics and Development. 8(6):675-680