This Lexias pardalis butterfly is perfectly split down its middle -- with male coloring on the left side of its body and wings and female coloring on the right.
It's a condition called gynandromorphism. It usually happens early in development, when cells are just beginning to split to form an embryo. One of the early cells fails to split its sex chromosomes properly (for example, an XXYY might split into an X and an XYY instead of two XY cells). These cells continue to divide and proliferate, and they're signaling for the organism to grow into two different sexes.
Sometimes the gynandromorphism is "mosaic," meaning that the cells are spread throughout the body and not noticeable outwardly. And in some species, even being split down the middle ("bilateral" gynandromorphism) isn't all that noticeable, because males and females look alike.
That's why it's most commonly noticed in the insect world -- and especially in butterflies -- who have often evolved striking differences in color and shape based on sex.
“In most cases, such specimens are ‘discovered’ in museum collections by a researcher who is carefully examining reproductive organs of insects under the microscope and stumbles across a specimen with both male and female characteristics,” Entomology Collection Manager Jason Weintraub said in a statement.
The Lexias pardalis, native to the tropical rainforests of Southeast Asia, is a particularly good example of the condition. Its males are smaller than its females, leading to a slight difference in the shape of the specimen's two wings. And while females have demure brown wings with white and yellow spots, the males have evolved iridescent black and blue wings to attract them.
Instead of letting the unusual butterfly move from the pupa hatching room into the main butterfly exhibit (where it only would have lived a week or two) the museum staff whisked it away for confirmation of its condition and preservation of the specimen. It's now on display at the museum.