In the middle of July 2018, I saw what I thought was an enormous moth. Turns out it was a Copper Underwing (Amphipyra pyramidea) one of the many Noctuidae, or owlet moths, and apparently not nearly as big as hundreds of other species, but impressive to my eye nevertheless. I took a photograph, although never saw its copper-colored rear wings which were covered the whole time by its mottled and patterned forewings.
I mentioned the specimen to a friend, who I knew had built his own scientific, Robinson-type, moth trap many years ago and had photographed quite a few that had been attracted to its actinic light and caught within until he released them in the morning. He offered to lend me the trap, but did a trial run in his garden and had caught a wide and wild variety of species overnight when I went to collect the equipment in the morning: a large, leafy-looking Poplar Hawk-moth, a Rustic, a Brown-tail, a Buff Ermine, and many others with equally bizarre and sometimes quite mundane names such as the Setaceous Hebrew Character and the Least Carpet moth. The latter is not the moth that eats carpets though.
Incidentally, it is estimated that there are some 160,000 species of moth around the world and about 20000 species of their fellow lepidopterans, the butterflies. Butterflies evolved from moth ancestors some 55 million years ago whereas the fossil evidence of moths stretches back to 250 million years ago. And, for the record, the only real distinguishing feature of moths and butterflies is that butterflies have "club-like" antennae and almost all moths do not. Of course, there are subtle and not-so-subtle differences between species and families and most people can discern the majority of moths as moths, and butterflies as butterflies based on behavior, the way these insects hold their wings, and other factors. Nevertheless, some moths act and look more like butterflies and vice versa.
One of the more eye-catching of the moths we found in the trap that sunny morning goes by the name of Burnished Brass (Diachrysia chrysitis). This is also an owlet moth, but what makes it stand out is that, as its name would suggest, it looks metallic. It shimmered in the sunlight and as it began to warm, it set its wings aquiver to speed up the process, revving its engines, as it were, before flitting away into the garden shrubbery to vanish from sight.
Now, I suspect that my description of the Burnished Brass moth may have set the biomimetic antennae of some readers twitching. Natural iridescence, metallic sheens, and such phenomena in the world of organic substances are often an interesting and fruitful avenue of investigation. After all, the shimmering scales on butterfly wings have long been a focus of such research that seeks out natural photonic crystals and such. The bright blue of the neotropical Morpho butterflies is perhaps one of the most well-known areas of research.
I was curious as to whether anyone had taken a look at the Burnished Brass. To my eye, it really does look like a shiny chunk of metal and perhaps it has evolved simply to dazzle predators. A paper in the journal Soft Matter turned up with a quick PubMed search as a first port of call: "Golden moth-inspired structures with a synergistic effect of interference, absorption and scattering" [Savic-Ševic S. et al., Soft Matter (2018) 14(27):5595-5603. DOI: 10.1039/c8sm00683k]. That sounded almost like the archetypal paper in materials science with a biomimetic bent.
The researchers describe a new type of photonic material inspired by D. chrysitis. The moth's wings, the team points out, have a nano-structured surface that gives them a prominent golden (brassy) color as a result of interference, scattering, and absorption of light. This, they add, is in the form of a layered photonic structure with a large refractive index contrast, whose alternating layers are rough at the nanoscale level. "Theoretical analysis shows that the scattering and interference interact to enhance the local field within the layers and increase the absorption of the material, particularly in the ultraviolet-blue part of the spectrum," the team explains.
Their theoretical work on the wings of D. chrysitis thus led them to verifying the concept using holographically manufactured Bragg gratings made from dichromated-pullulan (DCP). Pullulan is a polymerized polysaccharide material made by the yeast-like fungus Aureobasidium pullulans. The alternating air and pullulan layers doped with chromium are held in place by sparsely separated nano-pillars and the voids filled with 20-100 nanometer diameter spherical nanoparticles which act as scatterers. "The resulting spectrum is wide and closely resembles spectra of metallic gold and D. chrysitis moth wings," the team reports. They add that, "Such materials, with a high refractive index contrast and nano-scale scatterers, are important for achieving large reflectance and a broad spectrum, with scattering as an additional mechanism for spectral control."
I was at first simply pleased to see D. chrysitis in the moth trap but happier still to discover that this beautiful creature has been a biological inspiration to materials scientists. Over the last few weeks I have continued to "light up" every couple of nights and have captured, identified, and subsequently released, a wide variety of moths. Actually, I've identified a mere 160 of the 2500 species found in the UK in the trap, but it is a start. While species like the Canary-shouldered Thorn which looks like an alien "leaf-form", The Vapourer, with its enormous antennae, the bespectacled Spectacle, and lurid pink and green Elephant Hawk-moth, have captured my imagination, I am yet to see another species as shimmery, shiny as the Burnished Brass.
You can check out my fast-growing Lepidoptera photo gallery here:
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.