The ZO sex-determination system is a system that determines the sex of offspring in caddisflies and several moths. In those species, there is one sex chromosome, Z. Males have two Z chromosomes, whereas females have one Z. Males are ZZ, while females are ZO. The ancestor of Lepidoptera and Trichoptera had ZO sex determination, which became a ZW sex-determination system in the largest clade, Ditrysia, as well as in Tischerioidea. Some of the ZW species later reverted to the ZO system.