The butterfly has one antenna longer than the other and male and female reproductive organs fused down the middle. Photograph: Kevin Webb/Natural History Museum
A stunning butterfly that emerged from its chrysalis as male on one side, female on the other, has astounded curators at the Natural History Museum in London.

The peculiar nature of the freshly hatched great mormon butterfly was spotted when staff noticed a striking difference between its wing colourings and other features.

The insect, which has a 10cm wingspan, is almost black on its male side, but the female side is much paler, with clearly visible flecks of blue, red and tortoiseshell.

A closer inspection revealed the insect to have one antenna longer than the other, a single male clasp on its abdomen, and male and female reproductive organs that had fused down the middle.

The butterfly hatched two weeks ago during the Sensational Butterflies exhibition that runs at the museum until September.

"It's an amazing butterfly. The split is purely bilateral – even the colour of one side of its body is slightly different," said Luke Brown, manager of Sensational Butterflies. "It has half-male, half-female sexual organs welded together. So they don't work, it is infertile."

Insects can be born gynandromorphs – with male and female cells – when sex chromosomes fail properly to separate when the fertilised egg divides. Around one in ten thousand butterflies is a gynandromorph. Many dual-sex butterflies probably go unnoticed, because the males and females look alike.

Brown, who has seen only two other gynandromorphs in his career, said the butterfly was feeding and flying well, and was expected to have a normal life expectancy of around one month. The specimen will become part of the museum's lepidoptera collection.

In 2008, a half-male, half-female moth emerged at the museum. The insect had one bright yellow wing and another that looked brown and dusty. Crabs and lobsters can also be gynandromorphs.

Blanca Huertas, curator of butterflies at the museum, said: "The gynandromorph butterfly is a fascinating scientific phenomenon, and is the product of complex evolutionary processes. It is fantastic to have discovered one hatching on museum grounds, particularly as they are so rare.'