How These Nocturnal Moths Sparkle at Night

The nocturnal insect might flash its reflective spots at a potential mate

Dot-underwing moth
Iridescent spots found on the dot-underwing moth suggest that even nocturnal insects might rely on visual cues Mullookkaaran via Wikimedia Commons under CC BY-SA 3.0

Darkness won’t stop the dot-underwing moth from sparkling. Researchers at the University of Western Australia found that males of the nocturnal moth species have iridescent scales that make their black spots shape-shift depending on what angle they’re viewed at.

Iridescence is common in related species that are active during the day. But researchers thought that creatures that are active in the dark would have to rely on non-visual communication, like chemicals or vibrations. The study, published in September in the journal Current Biology, provides evidence to the contrary.

The first clue came while looking at museum moth specimens for a different research project. When viewed straight-on, the male moth’s upper wings are fully grey, reflecting light back at the viewer. But when the wings are seen at about a 30-degree angle, three dark spots appear. In females, the entire wing darkens at the same angle.

“As soon as we figured the effect was angle-dependent, we knew that to understand how it works, we had to understand the underlying optical physics,” Jennifer Kelley, an ecologist and the study’s first author, tells Harini Barath at Scientific American.

The effects come from the nano-sized scales covering the moths’ wings. The scales are tilted so that at some angles, they reflect light, and at others, let the light pass through to reveal the darker scales underneath. It’s an effect that can’t be matched by pigments alone, explains Bodo Wilts, an expert of nanophotonics at the Adolphe Merkle Institute in Switzerland, in a statement.

It’s also the first time that iridescent cues have been identified in nocturnal insects. Because the shape-shifting details only appear on the male moths, the research team suggests that they might flutter their wings at females to make their spots flicker attractively. And because the flickering happens at a specific angle, the moths may also be able to target exactly who can see it.

“Their signal is very obvious from one direction but invisible from others,” Elizabeth Tibbetts, a behavioral ecologist at the University of Michigan who was not part of the study, tells Barath. That allows “males to advertise their sexiness to females without predators noticing,” she says.

Although other moths have been observed flapping rapidly while approaching a potential mate, the dot-underwing moth’s courtship rituals haven’t been studied closely yet.