Posted by: Rob Viens | May 15, 2012

Jewels, Scarabs and Other Beetle Gems

May 15th – social calls done for now and Darwin is back where he longs to be – out walking and collecting specimens around Botafogo Bay.  We writes:

“Went out collecting & had a most delightful walk:—It is now full moon. I do not know whether the clear outline of the view seen by night is most admirable, or when lighted up by the gorgeous colours of a Tropical sun.” (May 15)

Speaking of “gorgeous colours”, let’s take a look at some of the more colorful families of beetles today as we continue with:

Beetle Mania logo

In his Zoological Notebook Darwin mentions finding a few specimens of the Burprestidae family (Jewel Beetles) and the Scarabaeidae family (Scarab Beetles). Both are members of the Polyphaga suborder of Coleoptera.

beetle phylogeny

Jewel Beetles have an elongate shape and are best known for their bright metallic colors. It is not pigments that give Jewel Beetles their spectacular colors.  Instead it has to do with how the cuticle that makes up the elytra reflect and absorb light. As one writer puts it, it is the same concept that causes the surface of a CD or DVD to appear iridescent. There are over 15,000 species of Jewel Beetles.

Temognatha attenuata – Jewel Beetle from Australia (from Wikipedia Commons):

 Temognatha attenuata.

Jewel Beetles from Fauna Germanica: Die Käfer des deutschen Reiches by Edmund Reitter (1911):

 Temognatha attenuata.
(see source and legend)

Scarab Beetles are “roundish” beetles with short antennae and (frequently) metallic colored shells. People are probably most familiar with Japanese beetles and dung beetles (revered in ancient Egypt – after all it was a dung beetle that rolled the sun across the sky every day). These beetles tend to feed on animal waste or decaying animal carcases.  (One of the ways Darwin would collect rare beetles that ate carrion was to bury dead animals and come back a week or two later to see what beetles showed up for a meal.) There are over 35,000 identified species of Scarab Beetles found around the world.

Coprophanaeus lancifer – giant Amazonian carrion scarab beetle from the Amazon basin (from Natural History Museum,/a>)

Coprophanaeus lancifer

Chrysina resplendens – Golden Scarab Beetle (from DK images
Golden Scarab Beetle

One of the really cool thing about scarab beetles is that some of their shells (elytra) actually reflect circular polarized light. “Huh” – you say? Polarization is the alignment of the electromagnetic waves of light in a particular direction, and circular polarization is a relatively complex way of polarizing light in a circular pattern (see the animation below).  Many sunglasses are “polarized” in order to reduce the light that gets through the lens (only light waves oriented in a particular direction can get through, the rest are filtered out.)

Like so many other things, beetles figured out how to do this before people did (and we think we are so smart)! The Golden Scarab Beetle, for example, has a helical arrangement of molecules within the cuticles that make up the beetle’s shell that has the ability to polarize light as it passes through (and is reflected back out of) the elytra surface. How cool is that!

Animation of light being circularly polarized – the arrow represents the orientation of the light waves after they pass through the polarizer. This shows “left-hand” polarization (notice the counterclockwise rotation of the light beam as it passes through the polarizer), which is what is found in Golden Scarab Beetles. (from Wikipedia Commons)

circular polarization

The grubs (larva stage) of Scarab Beetles, called curl grubs or white grubs, are a common agricultural pest – preferring to dine on the roots of ornamental plants and grass (which it turns out, can cause the plants to wilt and/or die).

While we are on the subject, it is worth noting that all beetles undergo “complete metamorphism”. This means that they experience a complete change in form as they grow from the juvenile stage (larva/grubs) into the adult beetle stage.  The most well-know example of complete metamorphism is the change from a caterpillar to a butterfly. Just like butterflies, beetles go through a pupa stage as they change from one form to another. In both cases, the adult (beetle or butterfly) looks different from the juvenile (grub or caterpillar).

Life cycle of the stag beetle (from Wikipedia Commons). (The “instars” are the stages of development for the beetle larva. – in many cases the grub just gets larger as it moves through the instars (as shown below), but for some beetles the larva change dramatically between instars)

Beetle life cycle

For more info on these little poo-eating, light polarizing gems (and/or to try to ID a sample you may have found), see the New World Guide to Scarab Beetles. (RJV)

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