New publication! Wing scale ultrastructure underlying convergent and divergent iridescent colours in Heliconius

Our paper describing the structures responsible for producing iridescent blue colour in five species of Heliconius butterfly is out in print today in the Journal of the Royal Society Interface.

Heliconius, also know as Passion-vine butterflies, from rainforests in Ecuador have bright wing colours to warn predators that they are toxic. Different species have evolved similar colours, helping predators learn to avoid them. Some of their colours are iridescent, changing with angle. These colours are produced, not by light absorbing pigments, but by nano-structures. Colours of this type are known as structural colours.

Mimetic butterflies, Heliconius erato cyrbia and Heliconius melpomene cythera, with structurally produced blue colour. Credit: Melanie Brien

In our paper we have identified the nano-structures that produce these colours: layered ridges on hairs-width-sized scales covering the wing, which cause constructive interference of blue light.

Part of a Heliconius eleuchia scale, showing the scale ultrastructures. Blue iridescent colour is produced by the layered ridge lamellae.

Similar structures are used by different species, but the colour and brightness of the species differs because of slight differences in the nano-structural features. It seems that some features can evolve faster than others, suggesting that these are more easily modified. There are noticeable differences, particularly in brightness, between the mimetic species pairs, H. erato cyrbia, H. melpomene cythera and H. eleuchia, H. cydno. These differences in brightness come from features of the ridges, including how flat they are and how many layers are present, as well as how densely packed the ridges are on the scale. We find that the packing of the ridges appears to evolve relatively quickly and is fairly similar between co-mimics, while the features of the ridges themselves appear to evolve more slowly, explaining the lack of perfect mimicry.

The evolution of iridescent blue colour in Heliconius. This colour is found in just a few species (blue branches) but has evolved multiple times. The labelled species, accompanied by wing photographs, are those we investigated in our paper.

The project was a collaborative effort with colleagues in the Physics Department, in particular Andrew Parnell, as well as using data we collected at the ESRF, and with key input from Pete Vukusic in Exeter. The butterflies were collected in and around the Mashpi Reserve in Ecuador, providing a unique ecological viewpoint by allowing us to investigate butterflies from the same community.