Gossamer Tapestry

Reflections on conservation, butterflies, and ecology in the nation's heartland

Wednesday, April 02, 2008

It’s all in the structure


Arid Lands Tiger Beetle (Cicindela marutha)

In response to my tiger beetle post last week, TR expressed admiration for the beautiful metallic green color of Cicindela marutha, the arid-lands tiger beetle and asked
What chemistry creates those patterns and colors?
It’s a fun question, because the beautiful reds and greens of tiger beetles aren’t created by chemistry at all. Color in the natural world can be segregated into two main varieties: pigment and structural. Pigments are chemical compounds. It’s possible to grind up, for example, monarch wings and isolate orange colors from them. In contrast, you can’t isolate a blue chemical compound from a Morpho butterfly wing.


Morpho granadensis
Photo: Jyoti

Structural color is caused by some aspect of the physical structure of, say, a wing or a shell that causes interference or diffraction of the light striking the object. Often the structure has tiny ridges or is composed of multiple layers. A nice technical presentation of structural color in Morpho butterfly scales can be found here (pdf file). If you would describe a color using words like metallic, iridescent, opalescent or even glittering you are probably describing structural color. Colors that change depending on the angle that you view them at are also structural.



Cairns Birdwing (Ornithoptera priamus). Structural green.
Photo: Jyoti

In general, greens blues or purples are structural colors. The obvious exceptions are green plants, which derive their color from the well-known pigment chlorophyll. Blue, in particular is very rare as a pigment in nature. Nearly ever time you see blue in the natural world, be it a butterfly wing, a blue jay feather, or a mussel shell, you are seeing structural color. One of my favorite butterflies in Butterfly Haven, the blue-banded shoemaker (Nessaea aglaura) is noteworthy because it contains a very rare example of a naturally occurring blue pigment. If you look at the butterfly in the photo below, you can notice that the blue band on the butterfly’s wings does not have a shiny, metallic appearance. I think it looks a bit like a stripe of tempera paint that was applied to the wings.

Blue-banded Shoemaker (Nessaea aglaura)
A very rare example of a natural blue pigment.
Photo: Jyoti

Pigments are much more likely to result in reds, oranges and yellows, as well as browns and black. These colors can also, at times, be produced structurally. Consider two tiger beetle below. The shiny metallic cast to the red colors is an indication that it is structural rather than pigmented.


White-lined Tiger Beetle (Cicindela lemniscata)
Structural red


Finally, consider white, a color that is also often the result of structure rather than pigment. This chalk-fronted corporal dragonfly has white on the body and wings. This color is the result of a colorless waxy compound that is secreted as the dragonfly ages. The color is referred to as pruinescence. The most common white associated with nature- although not biological- is also structural. If you melt snow, you don’t get a white pigment, but clear water. The white is simply light scattering off of the irregular surface.


Chalk-fronted Corporal (Libellula julia)
Structural white

Special thanks to Jyoti for letting me use her excellent photos from Butterfly Haven.

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14 Comments:

At 11:35, Blogger valown said...

Excellent explanations and very interesting. Thanks.

 
At 16:48, Blogger Dave Coulter said...

I'm certainly going to try and find some tiger beetles this summer. I read about them on wikipedia and they're pretty cool!

 
At 17:29, Blogger butterfly girl said...

Simply amazing Doug!
Although It’s possible to grind up, for example, monarch wings bothers me a hair.
Or is it hare? Dunno.

 
At 19:34, Blogger Kathiesbirds said...

Doug, what a fascinating post! I knew about the blue seen in indigo buntings being a refration of light and not a pigment but all of this is new to me. Also, I noticed you collected one of those bugs from Cochise County in AZ. Was that the word "Sunsites" beneath it? (Smile)I think we drove through there on an Audubon trip looking for birds.

 
At 21:32, Blogger Doug Taron said...

valown - Thanks

Dave - Another convert. Yes!!!

Butterfly Girl - I just realized that I can't abbreviate your name as BG, because we also have a bug cirl who comments here. Oh well. When I was in graduate scool I put a bit of monarch wing and some buffer in a tissue homogenizer. Wouldn't you know, the buffer turned a nice orange. Don't be too grossed out. Butterfly wings are dead tissue in much the same way as hair or fingernails. Broken bits don't grow back, but it causes no pain to the butterflies to break them off.

Kathie- Thanks. Indigo buntings are a great example of structural color (at least the males are). Yes, the white-striped tiger beetle is from Sunsites. I caught it at night at a gas station where it was attracted to the light. There were hundereds of them there. Cochise County is without peer in the US in terms of its tiger beetles.

 
At 21:38, Anonymous Mark H said...

You DO know, you could be charging Tuition fees for the education you're offering. THANKS for something I had never considered....what causes the coloration. THANKS, Doug. Now. WHY are most of our bugs here camouflaged instead of being brightly displayed....THERE's the question! Hah. Answer: Because it's gray, foggy, dark, and damp, most of the time. Their Camouflage IS Gray! Hah.

Hey - THANKS for old fridge idea.....I'll begin looking at Craig's list. We want some home made cheddar for next Christmas here!

 
At 23:43, Blogger Texas Travelers said...

This is a great Blog well explained and illustrated.

Sycamore Canyon had read this blog and asked me if the color of an Alpine forget-me-not in my blog was chemical or structural.

I'll send her this link:
http://tinyurl.com/cutdw

It does a good job of explaining why roses are red and violets are blue.

Again, thanks for the great blog.
I have you in my Nature Blog Roll (it's a favorite).

Troy Mullens
Retired Physical Chemist

 
At 08:19, Anonymous Jyoti said...

Excellent post! I learned something very interesting today! I loved this post.

I'm glad my pictures found a place here ... Athough it's all you and your team that has produced such a wonderful place. I just enjoy shooting there. I must have send hundreds of people to the haven. Sometimes I'm in the Orchid House [Lincoln Park] and I meet people who I direct to the Nature Museum and when I arrive to the Butterfly Haven, I see them there.

 
At 08:33, Blogger Doug Taron said...

Mark- Thanks. The last time I went insect hunting in Oregon and Washington, my experiences were very similar to what you describe in terms of color and camouflage. The bar fridge is working out well, and I'm beginning to see white mold on my camemberts.

Troy- Hi and welcome to the Tapestry. Thanks for posting the link. I remember when I was in high school staining a cloth with elderberry juice and showing it to be a pH indicator. I was a biochemist before I switched to the insect and conservation stuff.

Jyoti- Thanks. I'm beginning to think that you are our marketing department's secret weapon.

 
At 13:54, Blogger Texas Travelers said...

I forgot to mention that I love Tiger Beetles. There are quite a few here in NE Texas.

They are a great photographic subject but difficult if you use an on-camera flash. Boy, do they reflect light.

I am really enjoying going back through your older posts. This will take a while. So much to do and so little time.

 
At 21:49, Blogger Jim Lemire said...

Excellent post, Doug. Never really thought about the distinction between metallic and non-metallic colors. I'm off to check out that pdf now!

 
At 12:17, Blogger Doug Taron said...

Texas Travelers - The really metallic tiger beetles have caused me similar problems with photos. I thiank that a flash diffuser or a ring flash might help. You do have some fine examples in Texas, including a whole bunch of stuff I've never seen before.

Jim- Welcome to the Tapestry. Thanks for stopping by, and thanks for a really fine Circus this month.

 
At 23:36, Blogger Ur-spo said...

i agree; this was one of your best ones for information and clarification.

 
At 00:34, Blogger Marvin said...

Very informative. Thank you.

 

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