This is Charlie, an unbelievably rare half-male, half-female stick insect that recently wowed scientists at the Natural History Museum, London after it was discovered Charlie is the first example of a bilateral gynandromorph seen in this species. 

Charlie is a green bean stick insect (Diapherodes gigantea) whose owner Lauren Garfield became aware of the condition after the bug molted its “skin,” revealing the unusual coloration with one side being green and the other brown, more or less right down the middle of its body.

She got into contact with researchers at the Natural History Museum who were intrigued by the individual and asked to see the specimen. Just like that, Charlie was sent to London via post to Paul Brock, a phasmid insect expert at the Museum.

“Stick-insects are popular pets, so they are sometimes posted to enthusiasts, including researchers. In this case, it was a speedy way to transfer the live insect, using next-day delivery. With food plant leaves, they travel well in the post,” Brock told IFLScience.

Brock confirmed that Charlie was the first reported case of a green bean stick insect being a gynandromorph, an organism that contains both male and female characteristics. 

"It's the first to be recorded in Diapherodes gigantea," Brock said. 

In particular, it’s an example of a bilateral gynandromorph; when the two halves of the body appear to express different sexual characteristics. Its left side is bright green and features short forewings, typical of females, while the right side is brown with longer forewings, as you'd expect to see on a male. 

Biological sex is determined by the combination of sex chromosomes. For example, in humans and some other species, males have an X and a Y chromosome, while females have two X chromosomes. (In insects, birds and some other species, sex is determined by the combination of Z and W chromosomes, but we'll stick to X and Y in this explanation for simplicity's sake). 

Many people believe the mechanisms occur during the very early stages of development. As the organism begins to grow, a male XY cell will undergo mitosis and duplicate its chromosomes, becoming XXYY, before dividing into two XY cells. In gynandromorphs, however, this process doesn’t run smoothly. Instead of dividing into two XY cells, the cell accidentally divides into an X cell and an XYY cell. If this glitch occurs in the earliest stages of development, then it can result in a large portion of the cells being both X and XYY.

It’s a rare condition, but one that has been documented in dozens of species, from birds such as the rose-breasted grosbeak and northern cardinal birds to nocturnal bees. In fact, it possibly occurs more often than thought, and we only notice it when it's really obvious, like in a species that is sexually dimorphic, where there are differences in the appearance of adult males and females.

Brock says it's even been reported in several other species of phasmid, stick insects and leaf insects also in the insect order Phasmida. However, even for a gynandromorph, it’s thought that Charlie is an especially interesting specimen.

“This individual is rather impressive, not far short of a halved gynandromorph — left side female apple green (with typical short forewings), right side male brown (with typical forewings and hindwings). Except, it has mosaic patterning (part male) on the female side of the abdomen i.e. brown blotches and the genitalia are mostly male. Some gynandromorphs are classic bilateral halved gynandromorphs," explained Brock.