The difference in appearance between a species’ males and females is called sexual dimorphism. The term implies that there’s a bisecting line between sexes, a clear divide. But in the animal kingdom, a lot of creatures straddle it.
The natural world is replete with hermaphrodites, animals that may outwardly appear male or female but have the reproductive organs of both. Their less common cousins are gynandromorphs, animals that are a mosaic of male and female traits—say, the size and coloring of one with the genitalia of the other.
Rarer yet is the bilateral gynandromorph, an animal that’s half him and half her, split at the midline. The phenomenon has been documented in birds, crustaceans—and butterflies.