Chris Johnson, a retired chemical engineer from Swarthmore, Pennsylvania, was volunteering at the exhibit when he stumbled across the unusual sight.
Johnson noticed that the insect’s two right wings were typical of the females of its species — they were larger, and brown with yellow and white spots. But its two left wings were smaller and darker, with splashes of green, blue and purple, a pattern characteristic of males.
“I thought, somebody’s fooling with me. It’s just too perfect,” Johnson said in a statement. “Then I got goose bumps.”
Johnson spotted the creature while emptying out the butterfly exhibit’s pupa chamber, where the insects hatch from their chrysalises and cocoons. “It slowly opened up, and the wings were so dramatically different, it was immediately apparent what it was,” he said.
Johnson and his supervisor, exhibit coordinator David Schloss, isolated the butterfly and contacted Entomology Collection Manager Jason Weintraub.
A butterfly expert later confirmed that the remarkable insect was a Common Archduke butterfly (Lexias pardalis) with a rare condition called gynandromorphy, which means outwardly having both male and female characteristics. (This is distinct from hermaphroditism, in which an organism has both male and female reproductive organs, but has external characteristics of one gender.)
The condition is most commonly noticed in birds and butterflies, whose two sexes can have very different coloration, Weintraub said. It can occur when the sex chromosomes fail to separate during cell division in early development, a process known as nondisjunction. As a result of this failure, some of the animal’s cells have a female genotype, and others have a male genotype, giving rise to an animal with both male and female characteristics.