Pellucid hawk moth’s wings like invisibility cloak
MEET THE PELLUCID hawk moth (Cephonodes hylas), a gorgeous cross between a moth, a cicada, and a glasswinged butterfly.
At home in an array of habitats across Africa, India, Southeast Asia, and in Queensland, Australia, this strange little species starts off as a bright green caterpillar, feeding upon some of the finer things in life – coffee and pomegranate plants – wherever it can find them.
Locally, these caterpillars will snack on our native butterfly (Pavetta australiensis) and banana bushes (Ervatamia angustisepala), and the leaves of our introduced gardenia plants until they’re fat enough to construct a cocoon and undergo the rather gooey process of metamorphosis.
When a pellucid hawk moth first emerges from its cocoon, the scales obscuring its stunning pair of transparent wings will begin to drop off, eventually revealing the species’ most distinctive trait.
Together with their green, yellow, or orange abdomens, banded in black, these clear, wispy wings give them the appearance of a great big bumblebee – hence the nickname of their genus, ‘the bee hawks’. The largest of the species will develop a wingspan of over 7cm.
Glass wings act like invisibility cloak
Just a handful of species in order Lepidoptera, which includes all butterflies and moths, have scaleless, transparent wings. So, you might be wondering what the point of them is, when coloured wings can serve so many functions, including communication, defence, thermoregulation, feeding, and waterproofing.
Having observed them at a nanoscale, researchers suspect the point is to achieve ‘antireflection‘ – minimising the shine produced by a surface when the light hits it.
In 2006, a team led by Akihiro Yoshida from the Yuhki Science School in Kyoto, Japan, discovered that the transparent wings of the pellucid hawk moth contained on their surface “a highly ordered array of nanosized protuberences“, which they compared to what’s known as a ‘corneal nipple array‘ – the countless bumps on the surface of an insect’s eye that reduce reflectance. Being so similar physically, could these two surfaces serve the same function?
To test this theory, Yoshida’s team created a smooth pellucid hawk moth wing by wiping its protuberances away without damaging its delicate structure, and compared how it reflected light beside a real hawk moth wing.
Publishing in the journal Zoological Science, they found that, compared with the fake moth wing, the real transparent beauties decreased the light reflectance by almost 50 percent across a broad range of light spectra. This, they suggest, would make the wings nearly imperceptible to other insect and vertebrate predators and parasites, which make them the ultimate invisibility cloaks.