Gynandromorphic Swallowtail

This butterfly is an example of a very rare phenomenon called bilateral gynandromorphy. This happens when an animal is half he and half her, split at the midline, like this eastern tiger swallowtail: the yellow side is male and the dark side is female. How does this happen? Butterflies’ sex chromosomes are the reverse of humans’-males have two alike (ZZ), and females two different (ZW). A female’s egg sometimes has two nuclei, a Z and a W. When they are “double fertilized” by a male’s Z sperm, the resulting embryo is half each sex.
In his autobiography, the writer and lepidopterist Vladimir Nabokov recalled one he’d caught as a child in Russia.
(Photo credit: James K. Adams)
Source: National Geographic Magazine and Evolution: A Visual Record by Robert Clark and Joseph Wallace