Males of the butterfly species Mimeresia neavei feature blue, structurally generated color on the dorsal side of their wings, the only species in the Mimacraeina subtribe with blue coloration. Researchers at the Hungarian Natural History Museum examined M. neavei and found a form of structural coloration found in several butterfly groups but not experimentally examined within the family Lycaenidae. (Image originally published in Bálint et al 2018, Journal of Insect Science)

In sub-Saharan Africa, the butterfly species Mimeresia neavei stands alone among its subtribal clade, Mimacraeina: It is the only one among roughly 120 species in the subtribe in which males have blue, structurally generated color on the dorsal side of their wings.

Most members of the clade have orange, pigmented wings meant to mimic poisonous species in the subfamily Acraeinae. So how and why does male M. neavei exhibit its unique blue coloration? That’s a question that Zsolt Bálint of the Hungarian Natural History Museum and colleagues explored recently, and they report their findings in a new study published in May in the open-access Journal of Insect Science.

In analyzing the wing scales via multiple microscopy and spectroscopy methods, Bálint’s team found that male M. neavei borrow a form of structural coloration found in several butterfly groups but not experimentally examined within the family Lycaenidae.

All Lepidopteran scales feature two layers: the lower being flat and thin, and the upper featuring ridges and cross ribs. In some cases, a complex structure in the upper layer generates wing coloration, while in other cases pigmentation in the lower layer drives the wing appearance. A third configuration is one in which the lower layer lacks pigmentation but the upper layer is relatively simple in structure; it is this combination that Bálint and colleagues found in male M. neavei and that generates its blue coloration.

A: Mimeresia libentina with entirely closed wings hanging on a twig showing the cryptic or aposematic ventral wing surface pattern (photo credit: Adrian Hoskins). B: Mimeresia neavei resting on tree-trunk during displaying similar behavior (photo credt: Raimund Schutte). C: M. libentina, one of the congeneric species sharing habitat with M. neavei, resting on leaf surface and displaying warning coloration (photo credit: Peter Bygate). D: M. neavei perching on a tree-trunk covered by mosses and lichens exposing the dorsal wing surface structural blue coloration (photo credit: Raimund Schutte). (Images originally published in Bálint et al 2018, Journal of Insect Science)

Why M. neavei developed this form of sexual dimorphism is less clear, however, says Bálint. “The possible reason is that the imagines of the species have a different behavior than congeners all deeply involved in mimicry. So, most probably, M. neavei gave up the mimicry,” he says. But field testing would be needed to understand M. neavei behavior.

In their report, though, Bálint and colleagues note that a sexual role for the coloration would be their primary hypothesis. “Taking into account that M. neavei is the only member of the clade possessing this coloration and that the coloration appears only on the dorsal wing surfaces of the males, it seems plausible that the blue color is involved in sexual communication,” they write.

Bálint says he hopes to one day find out. “I am highly interested in genera that display structural color diversity, and I want to understand why so many well-tuned structural colors are necessary for day-flying Lepidoptera. Behind this there must be rules or a single rule, which I would like to formulate,” he says.