Why aren’t there more green butterflies?

Matthew found this question on the BBC’s Discover Wildlife site; click on the screenshot below to the “answer”, which I reproduce below. The picture is of the green hairstreak (Callophrys rubi), Britain’s only green butterfly.

First, though, this species is not completely green—the color is only on the bottom of the wings, which it folds as shown, perhaps for camouflage. The top of the wings look like this:

Now, onto the question and “answer,” which I find lacking. Here’s Jones’s answer (indented, plain text), with my responses in bold print and flush left:

Most ‘natural’ green insect pigments (in grasshoppers and plant bugs, for example) tend to fade, since they are chemically altered by light, and there is evidence that some are derived from chlorophyll eaten by the insects.

The green of the hairstreak, though, is not a pigment, but a metallic refraction effect caused by submicroscopic parallel grooves on the wing scales, which reflect only green light. Metallic green beetles use a similar mechanism.

In contrast, melanin (the default pigment across most animals) is highly stable, as are yellow and red pigments, which occur widely. There may be an evolutionary mechanism at work here.

Or maybe not! Jones’s answer is that green pigments tend to fade compared to others, so butterflies use them less. And that might be true, but, as Jones also notes, you can achieve a green color by altering the microstructure of the wings, which gives a stable green color. Further, it’s true, as Jones says, that orthopterans like grasshoppers and katydids are quite often green. Why orthopterans and not lepidopterans? Saying that “green pigments tend to fade” is not a good answer unless you show that that fading has particularly bad consequences for butterflies compared to other insects: 

Here, for instance, are the first four rows of images given by Google for “katydid”:

And for “grasshopper”:

If Jones is going to maintain that green pigments fade is part of the answer, he has to explain this difference between insect orders. I suppose he tries to do that in the last bit of his short answer:

Sedentary butterfly (and moth) larvae tend to eat green plants, and being all the same colour – as the caterpillars of many groups are – offers them camouflage.

Many butterflies tend to sit on green plants, too, and would undoubtedly benefit from camouflage. So that consideration doesn’t explain why there are so few green butterflies. 

But the day-flying adults need to combine bright colours (for mate recognition) with muted cryptic undersides (to hide or roost), so in this case green just may not be necessary.

I’m astounded at this bit. Green coloration is a a cryptic color, and so resting with green wings folded up seems a good way to achieve camouflage. As for needing bright colors for mate recognition, well, look at the green hairstreak above. It’s fricking BROWN on top! Further, we’re not sure if butterflies evolved to be brightly colored so they can recognize mates more easily. Some, for instance, are “aposematically” colored to warn predators of their toxicity, while others are mimics of the aposematically colored ones.  And if butterflies are brightly colored to recognize mates, why aren’t katydids or grasshoppers? 

The correct answer about why so few butterflies are green is this: “We don’t know.” Jones has simply offered a speculative answer, which has problems, and it’s an answer that a lay reader will take away as the truth. Jones may be correct in part, or there may be other factors at play: for example, it may be harder for butterflies to manufacture green pigments than for other insects. But finding a good answer requires either comparative studies (Do butterflies tend to sit more often on brown trees than on green plants?) or experimental studies (Do bright colors really help butterflies recognize mates? Do orthopterans have a pigment synthesis pathway not present in lepidopterans? If so, why are caterpillars—the juvenile stages of butterflies, able to make green?)

What we have here, in fact. is a just-so story. Not only does it not make a lot of sense to me, but appears to give a definitive answer when it doesn’t. It would be much better if Jones admitted up front that the answer isn’t known, and then speculated about some possible answers, suggesting how to test them. That would show people not only that scientists can admit their ignorance, and at the same time suggesting ways to remedy it, displaying the scientific mindset. 

Now what’s your (speculative) answer?

41 Comments

  1. J Cook
    Posted May 29, 2017 at 10:09 am | Permalink

    someone snuck in a Mantid with the grasshoppers.

  2. Randy schenck
    Posted May 29, 2017 at 10:15 am | Permalink

    I have no idea but aren’t butterflies around lots of color, such as flower gardens and fields of wild flowers. Green would not be the camouflage for this setting?

    • tomh
      Posted May 29, 2017 at 10:39 am | Permalink

      Almost all of those flowers, both in gardens and in the wild, have green leaves, so I don’t know about that.

  3. Gregory Kusnick
    Posted May 29, 2017 at 10:47 am | Permalink

    It’s fricking BROWN on top!

    It’s brown to our eyes. Butterflies see UV colors that we don’t see, and (according to Wikipedia) use them for sexual signaling.

    • nicky
      Posted May 29, 2017 at 11:19 am | Permalink

      And they don’t see red…

      • W.Benson
        Posted May 29, 2017 at 3:18 pm | Permalink

        Yes they do! It’s bees (and bulls) that don’t see red.

  4. Posted May 29, 2017 at 10:57 am | Permalink

    For that matter, why are there few species of bees that are green, and I cannot think of a single green wasp.
    As mentioned above, we should consider what colors are seen by other butterflies, or by their enemies. In any case, a quick but entirely speculative reason is that they use other colors to display to each other and at times to warn of their toxicity to predators. I personally doubt that they are brightly colored to blend in with flowers.

    • Posted May 29, 2017 at 7:36 pm | Permalink

      Mark, I think there are many green bees (many euglossids for instance) and wasps, though all the ones I can think of use structural color, not pigments.

      And in fact there are very many green moth species. I find them on my windows every night. Not so many green butterflies, though.

      The green orthoptera and moth larvae don’t seem to be counter-arguments to the fading hypothesis, since they are constantly recharging with green pigments from their food. In the other hand, dead green katydids stay green for a very long time even when exposed to direct sun.

      The green moth pigments are stable for centuries when kept in the dark, but I don’t know what would happen if they were kept in the sun. They would probably fade just like all colorful Lepidoptera fade over the years when exposed to sunlight. Would green colors fade fast enough to make a difference during a butterfly’s typically short life? It would be easy to test this.

      My recollections of the bright green Malachite butterfly of subtropical and tropical America are that the colors do fade slightly with age, and doing an internet search, I find this mentioned in the literature as well. So maybe there is something to this, but the reasoning and the data– there really are many green moths— given by Jones are flawed.

  5. Posted May 29, 2017 at 11:09 am | Permalink

    Perhaps it has a metabolic reason relating to flight. For example, perhaps green wings are difficult to temperature regulate and warm up too slowly or cool down too fast (or vice-versa).

  6. Tom
    Posted May 29, 2017 at 11:21 am | Permalink

    Perhaps adult butterflies don’t stick around in one place long enough to need camouflage
    They can be as gaudy as they wish (or not) and where they do stick around for a long time it is usually in huge groups and there is protection in numbers.
    The thought also occurs to me that many predators may have evolved the instinct to hunt green prey as it is the usual colour of more sedentary insects etc.
    Why chase a brightly coloured butterfly cruising overhead when there is a tasty green meal to hand?

  7. Posted May 29, 2017 at 11:33 am | Permalink

    And another thing…why do we call them “butter”flies? Yeah, yeah, I know–try googling it.

    • Frank Bath
      Posted May 29, 2017 at 12:04 pm | Permalink

      Mispronunciation. Flutter-bys.

      • Dale Franzwa
        Posted May 29, 2017 at 11:49 pm | Permalink

        Hey, I like to call them “Flutter-bys” because that’s what they do.

  8. Bruce Lyon
    Posted May 29, 2017 at 12:20 pm | Permalink

    The points made by Mark and Michael are interesting and it does seem that it might be connected to flight for some reason. Bees and butterflies spend lots of time flying, and are are not not green. Caterpillars however are often green—same organism but just different life stage. Grasshoppers, mantids and katydids can fly but rarely do, and are green. I do not have a specific suggestion for why flight vs little flight would alter the benefits of being green.

    • Posted May 29, 2017 at 1:31 pm | Permalink

      Those are very good ideas. Now, there are metallic green flies, but those insects are wickedly hard to catch. It could be that some characters (360 degree vision, lightning reflexes) permits the fixation of other characters (flashy metallic green color).

    • Diane G.
      Posted May 30, 2017 at 3:10 am | Permalink

      “Grasshoppers, mantids and katydids can fly but rarely do, and are green. I do not have a specific suggestion for why flight vs little flight would alter the benefits of being green.”

      Because when you’re less motile, it’s more advantageous to blend in? Also, the dorsally brightly colored lepidopterans with the dull brown undersides might seem to predators to “disappear” when they land.

  9. Virgil Reese
    Posted May 29, 2017 at 12:23 pm | Permalink

    I’ve long wondered, similarly, why not more green birds? Or, for that matter – why no green arboreal mammals, i.e. monkeys, squirrels etc? Sloths can grow a crop of greenish algae on their coat, and this is clearly a form of mutualism, since the sloths secrete nutrients that nourish the algae. However, sloths then gain vital nutrients from eating the algae, thus it is possible that camouflage plays little role in this symbiosis.

    • nicky
      Posted May 29, 2017 at 12:43 pm | Permalink

      I’m not sure about the rarity of these green birds, there are quite a few of them here in SA, from Malachite Sunbird to Cape White Eye and many parrots and wood cuckoos. Of course, there are many more LBJ’s (little brown jobs) but green birds are definitely not rare.

    • Posted May 29, 2017 at 7:38 pm | Permalink

      Nicky’s comments apply to South America as well, where many birds are green, especially females.

      • Posted May 29, 2017 at 7:41 pm | Permalink

        I should add that green color in birds is usually structural blue overlaying pigment yellow. I think Turacos are the only significant group of birds with green pigment (in this case, a stable copper compound).

  10. nicky
    Posted May 29, 2017 at 12:34 pm | Permalink

    Of course, the fact that insects (and birds) have a different colour perception from us does not explain anything by itself, but should be something to be kept in mind when trying to find answers.
    IIRC butterflies rely on pigments as well as diffraction & reflection for their colours. Are green butterflies green because of pigments? We don’t even know for sure…
    As a general rule: the brighter and shinier the colour, the less generated by pigments (refelction does not absorb light, like pigments do. Pigment colours also tend to be stable with a change of angle of observation, contrary to diffraction & reflection colours.

  11. Jonathan Wallace
    Posted May 29, 2017 at 1:03 pm | Permalink

    It is not strictly correct to describe the Green Hairstreak as Britain’s only green butterfly. Both the Green-veined White (Pieris napi) and the Orange-tip (Anthocharis cardamines) have green colouring on their undersides (in the former case, as the name implies, along the veins and in the case of the OT as a rather pretty mottled pattern which has the effect of making them very difficult to spot when the wings are folded and the butterfly is at rest amongst the vegetation. This suggests that camouflage as an anti predator defence may be at least part of the explanation for the coloration of the Orange-tip. I can testify that the Green Hairstreak – which always sits with its wings folded is also extremely difficult to spot when settled on vegetation – especially Bilberry (Vaccinium myrtillus) which is one of its larval foodplants so there is evidence that – contrary to what Jones suggests the greenness of the folded wings does play a significant role as camouflage.
    In the case of the GV White and the Orange-tip the green colour is not produced by green pigment but rather as a result of the mixture of yellow and black scales.
    A number of British moths (which of course are also lepidopterans) are vivid green, including the various emerald moths, Green Carpet (Colostygia pectinataria, Green Oak Tortrix (Tortrix vidana) and the Green Silver Lines (Pseudoips prasinana). As these are nocturnal insects one can speculate that the green colour in these species serves a primarily camouflage function.

    • Jonathan Wallace
      Posted May 29, 2017 at 1:05 pm | Permalink

      typo: Tortrix viridana.

  12. Michael Fisher
    Posted May 29, 2017 at 1:08 pm | Permalink

    “…the green hairstreak [Callophrys rubi], Britain’s only green butterfly.”

    Is it? I googled & found the brimstone
    [Gonepteryx rhamni] which is British:
    https://www.britishbutterflies.co.uk/species-info.asp?vernacular=Brimstone

    Perhaps someone could compare the two to identify any commonalities?

    • Jonathan Wallace
      Posted May 31, 2017 at 5:04 pm | Permalink

      I think most people would describe the Brimstone as yellow (bright yellow in the males).

  13. Paul Dymnicki
    Posted May 29, 2017 at 1:19 pm | Permalink

    As a lay reader I thought your answer is just what we need to promote scientific thinking and not just reaching out for an answer when there is not one yet.

  14. ThyroidPlanet
    Posted May 29, 2017 at 6:19 pm | Permalink

    Sub

  15. ThyroidPlanet
    Posted May 29, 2017 at 6:23 pm | Permalink

    This post is exciting personally because I just went to a museum and they had green butterflies – a rough guess is that they are 5-10% of the displayed specimens.

    • mikeyc
      Posted May 29, 2017 at 6:26 pm | Permalink

      What percent do you reckon each other color might be?

      • ThyroidPlanet
        Posted May 29, 2017 at 7:24 pm | Permalink

        “Rough guess” was putting it mildly. A big problem is the arrangement is more aesthetic than scientific.

        Perhaps mostly orange, yellows. Lots of dark colors. Then some that have green have another color. Few blues, maybe the same as greens….

        I’ll pay more attention next time. 😦

    • Diane G.
      Posted May 30, 2017 at 3:16 am | Permalink

      FWIW, I’m having more difficulty trying to think of red lepidopterans. But then, I don’t live in the tropics…

  16. Sarah
    Posted May 29, 2017 at 7:05 pm | Permalink

    Something that could be interesting to analyse is the life span of animals vs their tendency towards camouflage. Butterflies tend to have a short life span (there are some exceptions to the rule) and are already sexually mature. Speaking from an evolutionary standpoint, there’s not much point in camouflage for a butterfly. They don’t need to focus on survival; they’ve already survived long enough to reach sexual maturity, and they’ll die soon anyway. They only need to reproduce. Their colouration at that stage in their life is probably heavily influenced by sexual selection.

    • Gary
      Posted May 29, 2017 at 11:11 pm | Permalink

      Great question. I’m not an expert, but aren’t many Lepidopteran larvae green? And don’t butterfly larvae generally eat leaves and stems and other green parts, while the adults don’t feed on green parts but rather flowers, or not feed at all?

      • Sarah
        Posted May 29, 2017 at 11:37 pm | Permalink

        I’m no expert either, but I think you’re right about the larvae. Caterpillars seem to blend in better. So do the pupae (some species look remarkably leaf-like in chrysalis form). The adults usually drink flower nectar, yes.

      • Diane G.
        Posted May 30, 2017 at 3:17 am | Permalink

        Yes. 🙂

    • Diane G.
      Posted May 30, 2017 at 3:17 am | Permalink

      I like that hypothesis, Sarah.

    • W.Benson
      Posted May 30, 2017 at 11:05 am | Permalink

      But Sara:
      1) Butterflies often live much longer than you might suspect, several weeks or months for most species, and up to 6 months for a few.
      2) Most butterflies carefully search out host plants and must lay eggs over many days to exhaust their supply.
      3) Male butterflies can mate practically their entire adult life-span, and males that live long will, over all, have more mating and leave more longevity genes.
      4) It is true that adult moths often die in a few days (some lack functional mouths), but this has not stopped them from evolving good camouflage.

      • Sarah
        Posted May 30, 2017 at 6:47 pm | Permalink

        Thank you for such an informative reply! We get monarchs that live for up to 9 months where I’m from (our swan plants overflow in January/ February). Please correct me if I’m wrong, but don’t the long-lived species tend to either be on the move or hibernate in a sheltered spot? I can see why camouflage would be useful for the migratory species, but sheltering yourself overcomes the need for built-in camouflage.
        The moth point is very interesting. Moths seem to use pheromones to attract mates, while at least some butterflies use UV patterns or polarised light. I’m not willing to extrapolate the data from a few studied species to apply to all of them, but that could account for some of the lack of camouflage.
        Its rareness is very interesting. It could just be a matter of luck – butterflies are just making the best of what their genes dealt them, as are we all. Their designs are not necessarily optimal, but they work well enough.

        • Posted June 1, 2017 at 9:26 am | Permalink

          The mouthless, short-lived Saturniid moths like the Cecropia moth are exquisitely camouflaged when their wings are closed. Many species have clear windows in their wings to break up their silhouette, and they often have fantastic detailed false eyespots and sometimes false “snake heads” on their wings. These must have evolved under strong selection pressure by predators in spite of the short lives of these moths (often only three or four days).

          Interestingly, many of them are bright green, and it is widely known that the green color fades faster than the other colors in mounted specimens exposed to sunlight.

          • Sarah
            Posted June 1, 2017 at 10:23 am | Permalink

            Those moths are so beautiful! Very convincing camouflage. So, adult life span is not the most important factor. This isn’t my field – do you know if moths attract mates using their wing patterns?


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