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 are ZZ, while females are ZO.
[1][2][3] 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.