Some nature lovers liken it to winning the Mark Six lottery - a one-in-a-million chance of a half-male, half-female Great Mormon butterfly arriving at one's home and staying long enough to be photographed.
But that's what happened to Dr Martin Williams when the dual-sex creature - scientifically known as a gynandromorph - landed on the screen door of his home on Cheung Chau on Monday morning.
"A moth [enthusiast] friend likened it to winning Mark Six," Williams said, adding that the butterfly might still be on the island "if it is still alive".
The Great Mormon gynandromorph is about half the size of a human palm with the right male wing almost black and the left female wing white and grey with specks of orange.
Williams posted the pictures on his Facebook page and butterfly lovers have been enthusiastically circulating the news about the rare creature.
According to the London Natural History Museum, insects can become gynandromorphs if the sex chromosomes do not separate properly during the first division of a fertilised egg, leaving the insect with both male and female cells. It can also happen when an egg with two sex chromosomes is fertilised by two sperm.
The museum held a Sensational Butterflies exhibition in 2011 during which a dual-sex Great Mormon was hatched.
