A new study explores the patterns of pigmentation and wing scale structure in Delias butterflies that reflect both visible colors and ultraviolet light. Researchers say the classical palette of warning coloration may contain colors that humans have been overlooking all along. Here, two Delias species illustrate such patterns, with visible colors depicted on left wings and ultraviolet light on right wings. (Adapted from figure originally published in Ficarrotta et al. 2026, Journal of Insect Science)

By Vincent Ficarrotta, Ph.D.

Vincent Ficarrotta, Ph.D.

Butterflies are famous for color, but some of those colors are invisible to us. In Delias butterflies, ultraviolet-reflecting patterns occur alongside the familiar warning colors of red, yellow, and black. These ultraviolet patterns appear to be produced in a very simple way: The scales reflecting ultraviolet light may simply never be colored at all.

My colleagues and I in the Counterman Lab at Auburn University describe these findings about UV reflectance and classical warning colors in Delias butterflies in a study we published in June in the open-access Journal of Insect Science.

It began with an observation: While imaging pierid butterflies, I noticed that the ornate wing patterns of Delias included ultraviolet-reflecting regions for which I could find no scientific paper trail. Despite being the most speciose genus of butterflies (circa 250 species), few scientists have delved into Delias. These butterflies are well-known for their conspicuous and complex color patterns composed of the classical palette of aposematic, or warning, coloration. Coral snakes, poison dart frogs, and other butterflies like Heliconius use similarly conspicuous color palettes. Rather than blending into the environment, these animals advertise themselves. To a predator, the color signal can mean that this prey comes with a negative cost.

Color contrast is an important part of a warning (or aposematic) signal. Red, yellow, white, and black create striking, high-contrast patterns, as exhibited in Delias hyparete (A), Heliconius erato (B), Micrurus fulvius (C), Dasymutilla occidentalis (D), and Dendrobates pumilio (E). (Photos by Sébastien Delonglée (A), Irina Nikulina (B), Tim Colston (C), Brett Hondow (D), and Jan Sevcik (E).)

Color contrast is an important part of a warning signal. Red, yellow, white, and black create the striking patterns that humans see, but many animals do not see the world the way we do. Birds and insects can see ultraviolet, creating another possible channel for color signals. That raised the motivating questions for our work: What are these ultraviolet regions in Delias?

Butterfly wings are covered in scales, with individual scales contributing their own color to the larger wing pattern. We therefore asked whether the ultraviolet reflectance came from pigments or from the shape of the scale itself. That distinction mattered because of known iridescent ultraviolet coloration from other pierid butterflies. Beginning in the 1960s and 1970s, researchers found that some pierids use iridescent ultraviolet wing patterns during courtship. Work by Robert Silberglied, Darrell Kemp, Ronald Rutowski, and others demonstrated these butterflies’ scales have complex microscopic structures that produce the iridescent ultraviolet signal and that they play a signaling role in courtship.

Iridescent ultraviolet versus non-iridescent ultraviolet patterns in pierids. Ultraviolet photography makes UV patterns visible in greyscale photos; UV patterns appear as white. A) Demonstrates iridescence by its angle-dependent visibility. At 0 degrees, the iridescent ultraviolet pattern is visible on the Z. cesonia wing but disappears when tilted to 60 degrees. The non-iridescent ultraviolet pattern on D. dixeyi wings is not angle-dependent and therefore visible at both 0 and 60 degree angles. B) The iridescent ultraviolet patterns are due to the microscopic structures on the scale surface, colored purple, instead of due to color pigments. (Figure by Vincent Ficarrotta, Ph.D.)

The Delias scales looked different. Under scanning electron microscopy, the ultraviolet-reflecting scales had no morphology associated with iridescent ultraviolet coloration. At high magnification, they also appeared nearly colorless compared with neighboring pigmented scales. We then extracted the pterin pigments from colored scales, which are the common pigments used for pierid wing coloration. Once the pigments were removed, the scales became colorless and reflected ultraviolet light like the naturally ultraviolet-reflecting scales.

Comparison of naturally ultraviolet-reflecting scales with depigmented scales. A) D. bakeri (ultraviolet-reflecting) and D. sambawana (non-ultraviolet) species demonstrate that depigmenting D. sambawana scales reproduces the same ultraviolet reflection as naturally occurring ultraviolet reflecting scales. B) and C) magnified images of naturally occurring ultraviolet-reflecting scales and depigmented scales. (Adapted from figure originally published in Ficarrotta et al. 2026, Journal of Insect Science)

Together, these results suggest that the ultraviolet patches may not require a specialized ultraviolet pigment or an elaborate iridescent structure. Instead, these scales may develop without pigment, allowing them to reflect light broadly, including ultraviolet wavelengths. This turns the usual butterfly-color question around. Instead of asking how a scale becomes magnificently colored, we found ourselves probing the opposite direction: how scales remain unpigmented.

The ecological possibilities are the fun part. If a predator can see ultraviolet, then an ultraviolet patch placed next to red, yellow, black, or white could change the contrast of the entire warning pattern in ways that are invisible to us humans. It might make a butterfly easier to detect, make its pattern easier to learn, or add another signal available only to particular viewers. We do not yet know whether Delias use ultraviolet this way, but the possibility changes how we should think about warning coloration: The “classical” aposematic palette may contain colors that humans have been overlooking all along.

Vincent Ficarrotta, Ph.D., is a postdoctoral scientist in the Counterman Lab at the College of Sciences and Mathematics at Auburn University. Email: [email protected].