The Common Palmfly has evolved to be a con artist, where males (bottom) and females (top) in some regions resemble two distinct species of toxic butterflies to evade predation.
The Common Palmfly (Elymnias hypermnestra) belongs to Nymphalidae, the largest family of butterflies, with 6000 species worldwide. Butterflies in the genus Elymnias lay their eggs on palm trees, and the caterpillars feed on palm leaves to fatten up and complete their life cycle. The underside of the Common Palmfly’s wings is rather unremarkable, while the upper parts, visible when the wings are open, can be quite striking. These patterns mimic other butterflies and help the Common Palmfly escape predation.
Mimicry, a common phenomenon among butterflies, is of two kinds. In Batesian mimicry, a species that is not toxic evolves to resemble a poisonous one, deceiving predators to not eat them. The unpalatability of the model butterflies comes from toxins stored in their body, which are obtained from the plants they eat as caterpillars. Predators, such as birds, are put off after eating these noxious butterflies and quickly learn to avoid them based on visual cues. On the other hand, in Müllerian mimicry, multiple toxic species evolve to resemble each other and present a collective and honest warning signal to predators.
The Common Palmfly is a Batesian mimic, which is harmless in itself, but copies the patterns of other toxic butterflies. The males of this species mimic a group of butterflies called crows (genus Euploea), which are dark in colour. While Common Palmflies are largely dark brown to black in colour, the upper parts of their wings have splashes of iridescent blue and the bottom edge is tinged in burnt orange. The females either look similar to the males or can be bright orange throughout, mimicking an entirely different group of butterflies called tigers (genus Danaus). The female morphs vary based on the geographic region, where the orange female morph is seen across the Indian subcontinent, Sri Lanka, Indo-China, Java and the Moluccas. A recent study used genetic tools to find that the same morph across geographic areas does not form a single close-knit evolutionary unit. Instead, the same wing pattern has evolved independently in different regions in a case of repeated evolution.
Butterflies get their colours through pigments or microscopic structures on their scales that reflect light in unique ways. There are four main types of butterfly colour pigments - melanins responsible for dark hues, flavonoids for greys and yellows, pterins for oranges, and ommochromes for reds and oranges. The specific ommochrome pigments that paint the Common Palmfly females orange differ between geographic areas, supporting the multiple independent origins of this wing pattern. Many bright shades of blue and other colours at shorter wavelengths usually have a structural origin.
Wing patterning in butterflies is regulated by four main genes - optix, cortex, aristaless and WntA, through simple genetic mechanisms. The transitions between female Common Palmfly morphs are associated with mutations in WntA, which regulates melanin production. Interestingly, these mutations influence patterns in females but not in the male butterflies that carry them. More generally, a handful of genes are the basic ingredients that offer an expansive palette of colour and pattern in butterflies, and allow seamless evolutionary transitions between.
We find the Himalayan Common Palmfly (Elymnias hypermnestra undularis) subspecies on campus, which is close to its southern range limit in peninsular India. It is replaced by the Tailed Palmfly (Elymnias caudata) further south.
