This butterfly is quite literally half male and half female.
This condition is rare, but not unique. It’s been spotted in birds, insects and crustaceans all around the world. It’s unheard of in humans and it’s thought that it occurs differently in each of the groups listed above.
In insects the mechanism is fairly well understood. A fly with XX chromosomes will be a female. However, an embryo that loses a Y chromosome still develops into what looks like an adult male, although it will be sterile.
It’s thought that bilateral gynandromorphism occurs when two sperm enter an egg. One of those sperm fuses with the nucleus of the egg and a female insect develops. The other sperm develops without another set of chromosomes within the same egg. Both a male and a female insect develop within the same body.
There are several theories as to how this can occur in other animals, like birds. Some have suggested that these bilateral gynandromorphs are chimeras, where two separate embryos fuse together early in development – essentially the opposite of identical twins, where one embryo separates into two. Another hypothesis is that gynandromorphism in birds occurs when the sex chromosomes are unable to separate in the first cell division after fertilization.
Others suggest that the error occurs in the formation of the egg itself, with an egg accidentally ending up carrying two chromosomes, one of each sex, rather than the single chromosome it should possess. If an egg like this was fertilized by two sperm the resulting embryo would contain some ZZ cells and some ZW cells (female birds have ZW chromosomes and males have ZZ).
Ggynandromorphism doesn’t always look so perfect. Sometimes animals can possess a strange patchwork of different cells all across their bodies – it can even occur within a single feather. But sometimes, like in the butterfly above, bilateral gynandromorphism occurs and you see this perfect division between the two halves of the body.