Silver and Green

by Joseph DeSisto

Sunlight in the western U.S. can be intense, and all the more so when it catches the shimmering green carapace of a jewel scarab:

A Beyer's jewel scarab from Arizona. Photo by Joseph DeSisto.

A Beyer’s jewel scarab from Arizona. Photo by Joseph DeSisto.

The regal purple legs of this beetle identify it as Beyer’s jewel scarab (Chrysina beyeri). Beyer’s jewel scarabs spend most of their lives as white grubs in rotting logs, where they slowly eat their way through the wood. When they finally emerge as adult beetles, they eat oak leaves.

Although nearly a hundred jewel scarab species are found in Mexico and Central America, only four are known from the U.S.A., all in the southeastern portion of the country. Probably the most visually stunning is the so-called glorious jewel scarab (Chrysina gloriosa), with a lime-green exoskeleton sporting thick stripes of metallic silver.

The glorious jewel scarab, also from Arizona. Photo by Joseph DeSisto.

The glorious jewel scarab, also from Arizona. Photo by Joseph DeSisto.

There is something truly awe-inspiring about a beetle in which you can see your own reflection. We know that, of course, the glorious jewel scarab is not beautiful for our own gratification, but what adaptive purpose could metallic stripes possibly fulfill?

There are likely two. First, the glorious jewel scarab feeds on the leaves of juniper trees, which are common in the beetle’s high elevation habitats in Arizona and Texas. Juniper trees have tiny leaves along narrow branches, and thin strips of sun can create a shimmering effect at the right time of day. The beetle’s shining armor may actually serve as camouflage, by mimicking the narrow rays of sunlight that flicker through the trees.

A glorious jewel scarab on its host, juniper. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

A glorious jewel scarab on its host, juniper. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

Another Arizona insect uses the same trick. The royal moth’s caterpillar (Sphingicampa) is large and green, with a red stripe on each side and metallic spines jutting out of its back. Yet on their host plants, these caterpillars are well camouflaged. How? Because the plants they live on (locust, acacia, and others) all have small leaves with tiny spaces between them. The spines on royal caterpillars mimic the spaces between the leaves, and the light that flows through them.

A Sphingicampa caterpillar, showing off its metallic spines. Photo by Joseph DeSisto.

A Sphingicampa caterpillar, showing off its metallic spines. Photo by Joseph DeSisto.

The other reason for metallic strips is less obvious. It turns out that jewel and a few other scarab beetles can be so shiny because their exoskeletons reflect a unique kind of light, called circularly polarized light. To understand that, we need to understand a bit of physics.

What we call light is actually the product of tiny packets of energy called photons, travelling through space at the speed of … light. Photons travel in waves, undulating up and down or side to side. Normally, each photon has a wave pointed in its own direction, but sometimes, all the waves are oriented the same way. In other words, all the waves are on the same plane:

When light is polarized, all photons have wavelengths with the same orientation. Figure in public domain.

When light is polarized, all photons have wavelengths with the same orientation. The red wave shows the path of photons in an example. Figure in public domain.

If all the waves are on the same plane, the light is said to be polarized. Polarized light is fairly common in nature — although humans cannot identify polarized light, many insects such as bees use it to orient themselves with respect to the sun. This allows them to navigate between flowers, and to and from their hives.

Circularly polarized light, on the other hand, is extremely rare. In this situation, waves are still on the same plane as each other, but the plane rotates. If you could see the photons as they traveled directly towards you, it would look like a single photon is moving in a spiral, but in fact many photons are moving together, each at slightly an angle to its neighbor:

In circularly polarized light, the plane on which the waves are travelling rotates. Figure in public domain.

In circularly polarized light, the plane on which the waves are travelling rotates. Figure in public domain.

I won’t belabor this — physics is not my strong suit and I would rather omit details than risk getting things wrong. The point is, it takes a very special and rare kind of surface to reflect light in this way — the surface of a jewel scarab.

That’s pretty cool by itself, but it also happens that the same beetles are one of the few animals that can see circularly polarized light. There are lenses that allow us to see the same light, and if we look at a glorious jewel scarab through one of those lenses, it will appear not green and silver, but black (Sharma et al. 2009).

This can help otherwise well-camouflaged jewel scarabs find mates. Experiments with glorious jewel scarabs have shown that they are attracted to and will fly towards circular polarized light (Brady and Cummings 2010). Closely related, but less flashy beetles show no such preference. So while a predator looks at a beetle-filled juniper tree and sees nothing but leaves and shimmering sunlight, beetles can easily spot each other as big, black, radiant spots in a green world.

The Beyer's jewel scarab. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

The Beyer’s jewel scarab. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

Cited:

Brady P. and M. Cummings. 2010. Differential response to circularly polarized light by the jewel scarab beetle Chrysina gloriosa. The American Naturalist 175(5): 614-620.

Vivek S., M. Crne, J.O. Park, and M. Srinivasarao. 2009. Structural origin of circularly polarized iridescence in jeweled beetles. Science 325: 449-451.

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