This pretty little molten gold beetle has been doing the rounds of the Internet lately, because not only does it look like nothing else on Earth, but it can also completely change colours. And it’s just as pretty when it does.
This is golden tortoise beetle (Charidotella sexpunctata, previously known as Metriona bicolor), a tiny, metallic North American insect that belongs to the leaf beetle family, Chrysomelidae, which includes flea beetles, asparagus beetles and longhorn beetles. Nicknamed ‘goldenbugs’, golden tortoise beetles grow to around 5.0 to 7.0 mm in length and favour foods such as sweet potato and morning glory.
More than 30 years ago, golden tortoise beetles became the first known insect species with the ability to rapidly change colour during copulation. They also do it when disturbed or agitated by predators. Since then, other species of tortoise beetle, such as the Panamanian tortoise beetle (Charidotella egregia), have been found to do the same. Reversible colour change is extremely rare, but not unheard of in insects – in Australia, male members of the dragonfly genus Austrolestes and the grasshopper genus Kosciuscola can switch from black to a brilliant blue due to changes in temperature. What sets the colour-changing tortoise beetles apart, however, is that the colour change is controlled by them, in response to specific events in their environment, such as being poked by a curious human or stumbling upon a willing mate.
Publishing in The Coleopterists Bulletin in 1979, professor of biology Edward M. Barrows from Georgetown University described the results of his investigation into the mating and colour change of the golden tortoise beetle. Barrows collected a bunch of tortoise beetles from Washington and housed them in petri dishes in his lab, feeding them, breeding them and observing their sexual habits. Not only did he find that golden tortoise beetle copulation could last anywhere between 15 to 583 minutes, but he also observed that they would change colour as quickly as two minutes into it. Those beetles that started off a brilliant gold would turn to a goldish orange with black spots and then to a brownish orange with black spots, and those that started out a duller orange would turn golden. The same changes occurred when Barrows gently applied pressure to the beetles when holding them between his fingers. Other reports have these beetles turning from golden to a shimmering red when copulating or agitated.
The golden tortoise beetle, and some other species in its subfamily, is able to change colours due to an optical illusion. Barrows noted how previous observations of golden tortoise beetle colours from other scientsists were extremely varied, ranging from brownish and purplish to bright orange or gold. “Metriona bicolor sometimes looked greenish gold in the field, and highlights of its colouration are probably related to reflected light from its substrate and other nearby objects. Also, depending on the inclination of the line of vision, the color varies from gold to green or even blue,” he wrote in his paper.
Just how this illusion could be produced was discovered in 2007 by researchers from University of Numar in Belgium, who studied the very similar gold-to-red colour change in the Panamanian tortoise beetle. Using scanning and transmission electron microscopes, they saw that the transparent shell of these beetles contains a three-tier structure, each tier made up of several tightly packed layers covered in patches of nanosized grooves. The tiers run from thickest at the bottom to thinnest at the top, and beneath them sits a layer of liquid red pigment. When the nanogrooves are filled with the red liquid, they give the layers a smooth surface, which perfectly reflects the light to give the Panamania tortoise beetle its metallic golden appearance. When the red fluid is drained from the grooves, in response to a stimulus such as agitation or copulation, it “destroys the optical properties” of the shell, leaving an “unobstructed view of the deeper-lying, pigmented red substrate”, the researchers reported in Physical Review E. It’s likely the way golden tortoise beetles manage to change colour and appear differently depending on the light they are exposed to is related to how Panamanian tortoise beetles do it.
Not much is known about why these beetles change colour the way they do, but Barrows suggested it has to do with defence and/or sexual signalling. He suggested for the beetles that changed from dull and spotty to golden, they could be signalling to the opposite sex that they are ready to mate, as beetles in his experiment that were not mature enough to produce the golden colouration did not mate. He also suggested that the metallic quality of the gold could make them more difficult for birds to see, due to the glare. Barrows also suggested that for the beetles that changed from golden to orange and spotty, they could be mimicking ladybeetles, giving them a ‘safety in numbers’ form of defence, as birds can’t tell the difference. This was even true for the golden tortoise beetles that changed from gold to orange without any spots, because in some habitats, Barrows found them living side by side with the spotless ladybeetle (Cycloneda munda). When he tested a bird in the lab that had never been in contact with either insect before, the bird found the golden tortoise beetles to be delicious, but the spotless ladybeetles to be distasteful, so perhaps this colour change is an example of the Batesian mimicry we see in Heliconius numata butterflies, where one species mimics a different, bad-tasting species to fool predators into leaving them alone.
Oh and did I mention that golden tortoise beetles, as larvae, protect themselves by sticking old skin and faecal matter to their anal forks – otherwise known as faecal parasols – to form a shield? How did something with an anal fork get to be so pretty?
For similary pretty insects, check out Ferris Jabr’s post on translucent jewel caterpillars. They’re incredible.
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