Charidotella sexpunctata - Golden Tortoise Beetle

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Charles Krebs
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Charidotella sexpunctata - Golden Tortoise Beetle

Post by Charles Krebs »

If you have ever seen what appears to be a solid gold ladybug on a morning glory leaf you've found a Golden Tortoise Beetle. (A good indication they have been around is a pattern of holes eaten away in the morning glory leaves by this plant eater.)

This picture, unfortunately, does not show the brilliant gold appearance. It is quite interesting in that the insect can "turn off" the gold appearance when disturbed, and it also disappears in dead specimens. (This ones fancy feet are destined for my microscope, but I thought I would take an overall "macro" shot first.) But you can clearly see the tortoise-like "shell" (elytra) that gives it its common name.


Image

Sven Bernert
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Re: Charidotella sexpunctata - Golden Tortoise Beetle

Post by Sven Bernert »

Charles Krebs wrote:but I thought I would take an overall "macro" shot first.
Only a person who knows more of your killer microscope work is able to understand that sentence! :lol:
Charles Krebs wrote:This picture, unfortunately, does not show the brilliant gold appearance.
This is one heck of an image Charles. Even if the image doesn't justify the beauty of your subject, it is one beautiful macro image!

Best,
Sven
If you are out there shooting, things will happen for you. If you're not out there, you'll only hear about it. - Jay Maisel

DaveW
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Post by DaveW »

Charles,

Kodak's close up book years ago discussed this problem with silver coloured stripes on a beetle not reproducing in photography (gold is a similar problem). It stated that these colours act as a mirror and often reflect the colours of surrounding objects rather than what your eye sees when you try to photograph them.

Their solution in order to get the effect your eye sees when looking at them was to reflect the same colour into these mirror like stripes or wing cases. In the case of the silver striped beetle they used silver (aluminium) foil to reflect silver into the stripes. In the case of gold you would need something gold to reflect onto the wing cases.

Cheating but evidently effective!

DaveW

rjlittlefield
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Post by rjlittlefield »

There are several different issues in play here.

1. Our eyes & brain identify "shiny" by inferring reflections of the environment. Shiny silver is like "reflective light gray" -- patterns in the environment are reproduced (with some distortion if the surface is curved), the brightness is slightly diminished, and the spectrum is not changed. Shiny gold is the same thing, except that the spectrum is changed.

The effect of "shiny" is stronger when the reflections have a distinct and recognizable pattern, and they are strongest of all when the pattern being reflected can also be seen in the photo. Kodak's stripes work very well in this respect.

2. Our eyes & brain identify color by inferring a difference in spectrum between illumination and reflection. In the absence of any other information, we assume that the illumination is "white" and any deviation is due to the subject. Reflecting gold-colored stripes in a silver mirror will make the silver look gold -- unless the original stripes can also be seen. The best way to show off shiny gold is to let it reflect black and white stripes, and let the viewer see the original stripes also.

3. In any case, the problem with these gold beetles is different. It's not that the photograph fails to capture the visual effect, it's that the visual effect itself goes away when the beetle dies, dries, or just feels like changing its membrane characteristics.

Hope this helps...and I'm really looking forward to those fancy feet!

--Rik

Charles Krebs
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Post by Charles Krebs »

Thanks Sven!
Dave... this is actually something different from iridescence encountered in many subjects, and it it very interesting. Highly reflective and iridescent subjects, like the wing scales of a Sunset Moth do require some special lighting in order to effectively photograph the effect to appear as it is observed:
http://krebsmicro.com/webgal1/File005.html
http://krebsmicro.com/webgal1/File012.html

But the gold color of this beetle is different. From http://crawford.tardigrade.net/bugs/BugofMonth08.html :
"The gold color is caused by a thin layer of moisture between the cuticle and an inner layer of the elytra. Apparently the insect is able to "voluntarily" squeeze this layer, reducing its thickness and eliminating the gold color. This change also occurs involuntarily when the beetle is under moisture stress, and, of course, when it dies."

DaveW
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Post by DaveW »

Very interesting Charles and Rik, but to me your photo above seems to show coloured reflections on its back, including the green of the background, rather than just being the beetles colour itself which are being reflected from the equipment above back onto it?

The pictures in your links are great Charles! Have you a gallery of your macro shots as well as your microscope ones?

DaveW

rjlittlefield
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Post by rjlittlefield »

Ah yes, a great question -- how to interpret this picture?

Here's the way I see it... (The term "see" here is a bit misleading. Most of what I'll say is definitely a result of conscious analysis, not immediate perception.)

The illumination appears to be from partly in back, partly in front. The backlighting portion is a gradient from light yellowish green at upper right, to a darker lime green at lower left. The frontlighting portion is not clear. It seems to be coming from just above the optical axis, but the color is a mystery. Could be white light reflecting off a bluish surface; could be bluish light reflecting off a colorless surface. The foreground lighting could also include some component of the background color, particularly if some of the photo equipment is reflective. However, the green areas on the elytra surfaces that are facing us appear to be a different color from the background, and also the areas around the green spots seem to all grade in the same way -- from purple, to cyan, to green, and sometimes to yellow. That spectral ordering suggests interference colors, like in a soap bubble or oil film, so overall I'm inclined toward thinking that the front lighting is more neutral, with the colors coming from the beetle's surface.

The outer margins of the elytra and pronotum appear to be transparent and refractive, with some texture. Transparency of the margins is evident by things that can be seen through them -- notably the legs, head, and antennae. Refraction is implied by overall appearance (which is similar to things that I have previously determined to be refractive through personal interaction) and is confirmed by their effect on the appearance of known structures seen through them, particularly the middle leg at bottom of picture. Because the margins are so transparent, most of their appearance must be determined by light simply coming through them. Because the backlighting is graded and the margins are transparent but refractive, details in the margins can appear either brighter or darker than the background, depending on how the surfaces are oriented. On average, margin seen against brighter background should appear darker than that background; margin seen against darker background should appear lighter than that background, and yep, that's what appears in this photo.

I can infer from this picture that the margins are not strongly colored. They could be weakly colored, and that fact would be masked by the strong color of the background. (If Charlie wanted to show off the color cast of the margins, or lack thereof, he could have used a neutral gray background. The resulting picture would be highly informative but aesthetically boring -- different techniques for different goals.)

The body of the beetle (as I see it) must really be brown, because I can't think of any other way to get that color given yellow-green backlighting and any color of frontlighting that would produce that bluish sheen.

Do I see anything else right now? Ummm, nope!

So how'd I do, Charlie?

--Rik

Charles Krebs
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Post by Charles Krebs »

The background was a leaf, angled to get a gradient of tonality. and lit rather strongly to show the translucence and texture of the edges of the elytra. The frontal light was from three flash units into a 1/2 whiffle ball enveloping the subject.

The elytra is translucent and slightly straw colored and very shiny. The insect body is a deep red-brown with six black spots ("sexpunctata" in the name). As mentioned previously the impressive deep gold is gone, but there is a certain amount of natural iridescence caused by the shell which reflects the white illuminating surface in slight blue/green/magenta hues. (But this is separate and different from the coloration that earned it it's name).

(Here's a shot of a live one that shows the "gold")
http://bugguide.net/node/view/10175

DaveW
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Post by DaveW »

Charles's latest link also shows a reflection on the insect (the whitish one). What has to be remembered is that the taking lenses multicoating also reflects certain wavelengths of light, as is its function to cut down the transmission of certain colours to the sensor or film. If the light from the brightly lit leaf is reflecting from the front element of the lens that in turn will reflect back on the mirror like back of the insect will it not?

DaveW

rjlittlefield
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Post by rjlittlefield »

DaveW wrote:Charles's latest link also shows a reflection on the insect (the whitish one). What has to be remembered is that the taking lenses multicoating also reflects certain wavelengths of light, as is its function to cut down the transmission of certain colours to the sensor or film. If the light from the brightly lit leaf is reflecting from the front element of the lens that in turn will reflect back on the mirror like back of the insect will it not?
It will. Any reflection acts just like a virtual light source. However, the function of coatings in general is primarily to increase transmission, by reducing the energy lost to reflection. They work by "impedance matching" between media (typically air and glass). See discussion in Wikipedia. The residual reflections are often strongly colored due to interference effects, but they're usually quite dim in absolute terms.

--Rik

beetleman
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Post by beetleman »

I love the pattern on the transparent edge. I have never noticed that before. an amazing picture Charles.
Take Nothing but Pictures--Leave Nothing but Footprints.
Doug Breda

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