Animals that are active at night are thought to rely on senses other than vision, but a new study led by researchers from the University of Western Australia (UWA) and the NCCR Bio-Inspired Materials at the Adolphe Merkle Institute (AMI), has discovered a nocturnal moth with wing patterns that produce some highly unusual optical effects.

Lead author Dr. Jennifer Kelley, of UWA’s School of Biological Sciences, said that the study was based on a serendipitous discovery at the Western Australian Museum.

Iridescent colours – those that change with viewing angle – are commonly observed in day-flying butterflies and hummingbirds that use these colours for signalling, e.g. during courtship. This study is the first to find angle-dependent coloration in a nocturnal species, the Dot-underwing moth (Eudocima materna).

Males of this moth have three dark patches on each forewing that change in size and darkness depending on the viewing angle, causing a shape-shifting effect. In females, the whole surface of the forewing darkens with changing angle.

Sex differences in colouration typically evolve when males try to attract females using their flamboyant colours and courtship rituals. However, visual courtship behaviours are not known in nocturnal moths, which are considered to rely on pheromones for communication.

The team of biologists and physicists was able to reveal that these optical special effects are produced using specialised scales in the wings – a clever directional reflection coating straight from nature’s toolbox of biological nanoengineered materials. When viewed directly from above, these scales behave like mirrors, giving the wings a sparkly appearance. However, when the wing is viewed at an angle, an underlying layer of darker scales becomes visible, producing the dark wing patches in males.

“The power of optical nanostructures is that they can bend light at will. This often results in optical effects that cannot be achieved with pigments. Evolution has provided a diversity of mechanisms in insects, highlighting nature’s ability to bioengineer nanostructures. The moth’s nanostructures are first of a kind to be described in night-flying insects, and create this stunning visual effect”, says NCCR researcher Dr. Bodo Wilts, a group leader at AMI.

Dr. Kelley adds that “we know that some moths rapidly beat their wings before approaching the female. If this occurs in the Dot-underwing moth, the male’s wing patches would appear to flash on and off, which could be highly attractive to females. However, we know very little about this species of moth, so the next step is to study its courtship rituals”.

As humans, we have tended to assume that other animals sense the world in the same way that we do. However, we now know that many animals have visual systems, including night vision, that far surpass our own. This raises questions about how animal visual signals are optimised for transmission and reception in the dark. Future studies on visual signalling in the dark will further understanding of the evolution of vision, with the potential to also inspire novel optical technologies.

The research is a collaboration between UWA, the Western Australian Museum, Murdoch University, Deakin University and the Adolphe Merkle Institute . The results have been published in the leading journal Current Biology.

Reference: Kelley et al., A Dynamic Optical Signal in a Nocturnal Moth, Current Biology (2019)

Photo: Nikolai Tatarnic, Western Australian Museum