Specifically, the Chrysina beetles from Central America. They are
[f]ound in primary pine, juniper, and pine-oak forests between 50-3800m. Adults are frequently attracted to lights. Larvae live in rotten logs. Species feed on the foliage (adults) or rotting logs (larvae) of various trees including species in the genera Abies, Alnus, Arbutus, Heliocarpus, Juglans, Juniperus, Liquidambar, Pinus, Platanus, Quercus, and Turpinia.
Source: The Generic Guide to New World Scarab Beetles
What’s so cool about beetles? It’s the way they shine! Mostly in various shades of green – more spectularly, in gold or silver:
No wonder they’re “frequently attracted to lights”.
Since they don’t plate themselves in actual gold or silver, how do they keep that shiny covering? Apparently it all comes down to layers of chitin. Chitin is a big molecule that’s found all over the natural world – mostly in insects; it’s the common building block of insect exoskeletons all over the place. (It’s why they “crunch” when stepped on.) Most insects just use a regular layer of the stuff as a tough coat. The Chrysina beetles – at least these two species, and (likely) others – have a more complicated structure, that was recently worked out in detail by scientists at the University of Costa Rica:
To interpret the cause of this metallic look, Vargas and his team assumed that a sequence of layers of chitin appears through the cuticle, with successive layers having slightly different refractive indices.. In these beetles, the cuticle, which is just 10 millionths of a meter deep, has some 70 separate layers of chitin—a nitrogen-containing complex sugar that creates the hard outer skeletons of insects, crabs, shrimps, and lobsters. The chitin layers become progressively thinner with depth, forming a so-called “chirped” structure.
“Because the layers have different refractive indices,” Vargas says, “light propagates through them at different speeds. The light is refracted through—and reflected by—each interface giving, in particular, phase differences in the emerging reflected rays. For several wavelengths in the visible range, there are many reflected rays whose phase differences allow for constructive interference. This leads to the metallic appearance of the beetles.”
This isn’t the first “layered” structure produced by living things which gives a really impressive optical effect. Seashells get that interesting “rainbow sparkly” effect from a similar layered design. In this case, though, the layers are of different sizes, and they’re arranged with the thinnest layer on one side and the thickest layer on the other side. The layers themselves range (according to the model developed and reported in the paper) from 85 to 170 nm (for the gold beetles; about 65 to 190 nm for the silver ones), and the whole stack of 68 layers is only 8.1 microns thick – about one-tenth the thickness of a human hair.
The effect depends on a difference in the refractive index of the chitin layers, also – the index is the speed at which light moves through the layer. Chitin has a very interesting refractive index (as reported in the paper) – it’s about 1.65 at 400 nm, increasing to nearly 2.0 at 800 nm (anomalous dispersion). By contrast, materials such as glass typically have a higher refractive index on the low end of the visible range, and the difference is not nearly so great. This dispersion – the change in the refractive index with wavelength – for chitin is substantially higher than that of diamond or cubic zirconia… I wonder if anyone’s managed to make a “gemstone” out of chitin?!
Speaking of “who made thee”: The paper doesn’t say whether anyone has examined the beetle’s carapace directly to see whether this model is correct, which would seem like an obvious next step. Nor do they speculate on the biological mechanism for such control over the layers of chitin. The knee-jerk reaction is that “of course it has some evolutionary value, therefore natural selection favors the shiny beetles over their less visible cousins/ancestors”. Of course, they haven’t managed to out-compete the other Chrysina beetles, which are merely an attractive shiny green, and some of whom live in the same Costa Rican rainforests. Presumably they all occupy slightly different niches. The comments on the PhysOrg article, predictably, include the “small steps” idea, and the surprising notion that “nobody would kill a solid gold beetle”. (Right, ’cause nobody would ever want to capture the “gold bug” and melt it down into gold bars.)
(An alternate explanation: Because the gold and silver effects are so complicated, the “metallic green” may well be due to a simplified or partially “broken” form of the same mechanism. That is, the gold and silver beetles are the more “complex” original forms, and the rest show a degenerate form of the same pattern. “Simplest form first” is important only to an evolutionist!)
