In a w+ background the Wow homozygotes have a discernibly visible reduction in eye pigmentation, which is nevertheless still near the wild type colour. Wow increases pigment levels in the Malpighian tubules and testes. This increase is more marked for Wow homozygotes than for Wow heterozygotes. The wings of Wow mutant homozygotes are reduced and sometimes do not fully expand. Males and females are sterile. In some backgrounds mutants are only semi-viable, with the semi-lethality being more pronounced for males than females.