Diario de swampster

Today I photographed two Eastern Tiger Swallowtails, one light/standard morph and a dark morph. I was fairly certain both were females, but I could not find any photos of male dark morphs to compare. After posing the question to the iNat Forum, I found out it's because they don't exist!

Unlike humans, females are the heterogametic sex in butterflies. Males have ZZ chromosomes while females have ZW chromosomes. The dark morph is a sex-linked trait that occurs on the W chromosome, so only females can have the trait.