Chris Johnson, a volunteer at the Butterflies! exhibit at the Academy of Natural Sciences of Drexel University in Philadelphia, had seen his share of exotic butterflies. But this one stood out.
"It slowly opened up, and the wings were so dramatically different, it was immediately apparent what it was," said Johnson, who recognized in an instant that this butterfly was half-male and half-female.
Soon after it emerged from its chrysalis, the butterfly spread its wings to reveal that its two right wings were brown with yellow and white spots - characteristic of a female - while the two left wings had the dark green, blue and purple colorings of the males.
"I thought: 'Somebody's fooling with me. It's just too perfect,'" Johnson said in a statement. "Then I got goose bumps."
Once it was identified, the butterfly, which is found in the tropical rain forests of Southeast Asia, was isolated, and Entomology Collection Manager Jason Weintraub was called in to investigate. Weintraub immediately confirmed Johnson's suspicion. The butterfly was Lexias pardalis, and it had an unusual condition called bilateral gynandromorphy.
"Gynandromorphism is most frequently noticed in bird and butterfly species where the two sexes have very different coloration," Weintraub said. "It can result from non-disjunction of sex chromosomes, an error that sometimes occurs during the division of chromosomes at a very early stage of development."
Such differences in the sexes are the result of what Charles Darwin called sexual selection. They evolved over many thousands of generations as a result of "choosy" females. These butterflies use color and wing pattern as signals during courtship. The mates they select pass their traits on to the next generation.
The butterfly will not be a part of the live exhibit, but it will be put on public display at the Academy from Jan. 17 to Feb. 16.