Why "metallic" colours on some insects?

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Beatsy
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Why "metallic" colours on some insects?

Post by Beatsy »

Sitting in the garden, watching insects go by, I noticed a metallic green flower beetle land nearby. That got me thinking as to why it "needed" to be a metallic sheen. Why not just green pigment? That's an easy colour to make in nature, but perhaps not on carapace, hence interference colours(?) In contrast, blue is a rare pigment in insects, so I can see why interference colours might evolve for that - but still have no idea why they might need to. What's the selection pressure mandating sparkly blue bits?

Also, if you're low down the food chain, you'd think "glinting" would be a the worst way to ensure your genes make it to the next generation. Better blend-in with leaf, wood and dirt colours than stand out as a shiny, sparkly, mirror-ball. You'd think. Mate selection pressure from the females perhaps? If you survived long enough to mate, despite wearing "come-and-eat-me" flags all the time, etc etc. Like the peacock's tail.

I suppose the question is pretty much the same as asking "why are insects coloured" - or is it? I just thunk myself into confused loops on that one. So I've floated it here to see what the hive-mind thinks... :D

PeteM
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Re: Why "metallic" colours on some insects?

Post by PeteM »

As you've suggested, I'd guess much of this might be due to thin film interference effects - much like the colors of an oil slick. If so, the evolutionary question could be as much "why do insects have a thin film (protective?) coating" as "why did they evolve these cool colors"?

Someone who actually knows something about insects is sure to chime in . . .

CharlesT
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Re: Why "metallic" colours on some insects?

Post by CharlesT »

Maybe they are green to avoid being thermally 'overloaded' . Overloading issues have been suggested as a possible reason why plants are green:
https://www.quantamagazine.org/why-are- ... -20200730/
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Troels
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Re: Why "metallic" colours on some insects?

Post by Troels »

It is important not to be eaten. But surviving is of little advantage if you are unable to mate because you cant find a partner.
Mammals and a lot of moths locate partners by the smell from long distances. Birds use their voices and bright colors too. They can recognize color patterns from a long distance.
Many insects live in vegetation looking like a confusing mess of mainly green, brown and yellow colors. And as far as I know most insects are only able to focus at short distances. But they might be able to catch a flash of metallic reflection from much farther away. In that case a well defined color wavelength (and perhaps polarization?) could be important for identifying a potential partner.
Structural colors could be sources of bright narrow band reflections for this purpose.
This does not exclude other origins or functions like termal regulation.
Just thinking loud.
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MarkSturtevant
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Re: Why "metallic" colours on some insects?

Post by MarkSturtevant »

My reply is along the line of what Troels already said. Where structural metallic colors appear in arthropods (not just insects), and also in birds, its seems to be about being recognized and assessed as a mate. The colors used are commonly metallic blue, which I understand is the basic structural color, and metallic green, which is really metallic blue plus melanin pigment. Melanin pigment is the the most common pigment used in the animal kingdom, and different amounts result in the black and brown colors that abound in animals. So metallic green winds up being really common since animals tend to already have melanin.
Anyway metallic colors are one strategy for being recognized during sexual selection -- selection for being sexy to the opposite sex.
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NikonUser
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Re: Why "metallic" colours on some insects?

Post by NikonUser »

Day-flying insects may use metallic reflections for sexual recognition.
However, several night-flying moths also have such colouring. There is always some light, even at night, so metallic reflections still may 'work',
but most moths rely on chemical communication so colouring seems to be more important for crypsis.

Metallic moths in my garden include the: Silver-spotted Fern Moth, image attached. The white spots are silver!
and the Green-patched Looper (I have no images, but one from BG referenced))
https://bugguide.net/node/view/1266966
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9633 Calopistria cordata.jpg
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MarkSturtevant
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Re: Why "metallic" colours on some insects?

Post by MarkSturtevant »

That is a lovely moth. I've seen it once. Night flying moths do frequently have iridescent white markings. These will likely be strongly reflective in the UV, so putting a black light on one out to be quite showy.
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Beatsy
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Re: Why "metallic" colours on some insects?

Post by Beatsy »

Thanks for the comments folks. Interesting stuff, as always.

I didn't know about metallic blue and melanin making metallic green - something learned there. Myriad questions arising too - but another time...

That silver moth is nice, I've never seen any like that in the flesh. I presume it's "silver" like silver paint rather than polished mirror-like metal, but thinking about structural colour (intereference) and *white* light is befuddling. How does that work then? Are there any extreme close-ups of the scales (I found none via Google or the duck)?

As to "why coloured" in general - concensus seems to be female mate preference is a major selection pressure there, amongst other things. I wondered, at first, why that doesn't just run away to extinction. It sure is an arms race, peacocks tail still being a stand-out (up) example! Why aren't they longer every year, until all the male peacocks can't move at all and get eaten? But when you think about it, being too picky would eventually deselect a female's genes from the pool too. She rejects too many chances to mate with the "lesser" males, and thus doesn't mate at all.

It all balances out, given sufficient time and a stable environment to do it in...

NikonUser
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Re: Why "metallic" colours on some insects?

Post by NikonUser »

You have a beauty in the UK
Burnished Brass
https://ukmoths.org.uk/species/diachrysia-chrysitis/
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student of entomology
Quote – Holmes on ‘Entomology’
” I suppose you are an entomologist ? “
” Not quite so ambitious as that, sir. I should like to put my eyes on the individual entitled to that name.
No man can be truly called an entomologist,
sir; the subject is too vast for any single human intelligence to grasp.”
Oliver Wendell Holmes, Sr
The Poet at the Breakfast Table.

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Scarodactyl
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Re: Why "metallic" colours on some insects?

Post by Scarodactyl »

Thicker layers about the same or thicker than a wavelength of light could result in Mie scattering (like a traditional moonstone) which can be blue or white.

MarkSturtevant
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Re: Why "metallic" colours on some insects?

Post by MarkSturtevant »

As to peacocks, I can make some generalizations since I teach about them in my Evolution class. There has been quite a lot of research on peafowl, in large part b/c they do well in captivity.
>Pea hens are extraordinarily discerning about the # of eye spots that they see on peacock tails. If you or I are presented with two peacocks and we were asked: 'Which one has more eye spots?', we may be unable to see the difference. They could both seem equally spectacular. But a pea hen can identify who has a few more eyespots and she will be right.
>If you artificially add or remove feathers from a peacocks tail, that will change their mating success accordingly. So peahens choose their mates in that way.
>Males that have more eye spots sire chicks that grow faster, and they have stronger immune systems. So well endowed males are telling the truth about their quality as a mate! That part always amuses students, since we humans know well that show--offy human males don't necessarily tell the truth about their quality as a mate.
> So there is strong sexual selection for larger and larger peacock tails. There is similar selection for elaborate male traits in lots of birds, fishes, mammals, insects, and so on. So why doesn't selection force males to "just keep getting more and more sexy"? The hypothesis is that it's a matter of balancing benefits versus cost. The more elaborate displays of fitness also cost more, energy-wise, and there is a limit to how much energy can be spent. Peacocks are native to southern Asia, the land of tigers, wild dogs, and other predators. They can still fly, but with difficulty.

I do wonder if structural colors are a clever way to put on a good display at lower cost. One must invest energy in developing the elaborate micro-structures that produce the metallic structural colors, but that is confined to inert chitin in the cuticle of arthropods and to the dead keratin of bird feathers. So it does not need to be maintained so much. This is unlike pigments which I suppose will need regular replacement.
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Troels
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Re: Why "metallic" colours on some insects?

Post by Troels »

Interesting dicussion.
I woul like to add a few comments in addition to Mark's interesting informations.
Because we are still left with the question: what is the evolutionary reason for the females to prefer the more 'impressive' males?

Studies in sexual preferences in swallows show that males with longer tail feathers are preferred by females.
But it also shows that the longer feathers mean more energy spent in flying because of the greater air resistance.
The evolutionary mechanism could be that males with longer tails simply have to be stronger and more clever food finders to survive the handicap connected with the 'beaty'.
Thus females preferring males with short tails have reduced chances of getting ofspring with strong wings and efficient food search techniqes.
And females preferring the longtailed males get an evolutionary advantage since their ofspring get a bigger chance for surviving.

The same type of argument could be used concerning the antlers of deer, and indeed the big, heavy feathers of the peacock.
So the costs for the impressive mates are not only in terms of energy used for building and creating the nice bodies, but also for carrying around the inconvenience of the beaty and survive in spite of it.
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Lou Jost
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Re: Why "metallic" colours on some insects?

Post by Lou Jost »

Structural colors have a big advantage over pigment colors in terms of color purity. Pigment colors, especially those made from multiple pigments mixed to produce a new color, are a little bit "muddy". Structural color can be nearly monochromatic. Also, structural color evolves very quickly and easily, even over the course of just a few generations, involving (at least in some cases) structures that are already present to some degree even in species without conspicuous structural colors. It's just a matter of changing the spacing or thickness of layers, sometimes.

This has been proven experimentally by lab breeding of the Buckeye butterfly to obtain blue structural color on the wings:

https://elifesciences.org/articles/52187

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Re: Why "metallic" colours on some insects?

Post by Troels »

Lou Jost wrote:
Sun Aug 08, 2021 9:54 am
... Also, structural color evolves very quickly and easily, even over the course of just a few generations, involving (at least in some cases) structures that are already present to some degree even in species without conspicuous structural colors. It's just a matter of changing the spacing or thickness of layers, sometimes.
Interesting. In such cases the survival costs for the males (setting the upper limits for developing more of the same) is not energy spent in making the structures or carrying them around. It is probably the survival cost of beeing spectacular end easy spotted by raptors. Then the females chosing them have better chances for getting offspring with high capabilities of hiding, escaping or fighting hungry carnivors.
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Lou Jost
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Re: Why "metallic" colours on some insects?

Post by Lou Jost »

Troels, yes, as yo usay, there is virtually no energy cost, and there may even be a savings, since structural color doesn't require the metabolic energy needed to produce fancy pigments. Maybe energy economy is another reason for the prevalence of structural colors.

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