Function of eyespots in butterflies, part 3: morpho butterflies
(writing in progress)
...continued from https://www.inaturalist.org/journal/milewski/68255-function-of-eyespots-in-butterflies-part-2-peacock-butterfly#
The morphos (genus Morpho) are nymphalids, just like the butterflies that featured in other parts of this series of Posts. Here, the situation just gets curioser and curioser, as I will explain.
The sexes differ on the upper surface of the wings, but both sexes and all species have a series of clearly formed eyespots on the lower surfaces of all wings.
The pattern is essentially similar to that in the related, sympatric owl butterfly, but the upper surface of the wings is brighter while the eyespots have proliferated from the one large eyespot of the owl butterfly to so many in the morphos that the function cannot possibly be to startle a predator into seeing a threatening disembodied eye.
Please see the photos below: it just doesn’t make sense, does it?
The best known morpho is M. menelaus, but all species present essentially the same puzzle of adaptive colouration. Does anyone have a coherent explanation for colouration in this genus? If so, I’ve yet to see it.
Even within the wikipedia site for morphos, the authors (presumably at least two) flat-out contradict each other, one stating that morphos are palatable and the other stating that they are toxic. The same ambivalence can be seen in the caterpillars: to me they look disruptively coloured (which would indicate palatability) but they give off nasty smells (suggesting defensive chemicals more compatible with aposematism).
One explanation for the bright colours of morphos is that they are aposematic (I tend to doubt this because of the sexual differences on the upper surface, which suggest that morphos are bright for much the same reasons as so many other brightly-coloured tropical organisms. Someone has even come up with a rather far-fetched ‘pursuit aposematism’ in morphos, arguing that since these aerobatically adept, fast-flying butterflies are so hard to catch, they advertise this difficulty by means of their bright upper surfaces.
Sounds dubious to me; even springbok don’t advertise themselves for such reasons, although they do stot when threatened to demonstrate individual fitness. I don’t buy either the concept of ‘pursuit aposematism’ or the name (‘aposematism is quite incorrect here, because it properly advertises a defensive capability, not simply a capacity to flee effectively).
At the bottom of this whole conceptual tangle, of puzzles within puzzles, lies the function of the many, perfectly formed eyespots on the lower surfaces of all the wings of all morphos.
We can rule out intraspecific (sexual) attraction because the least sexually dimorphic part of the colouration seems to be the eyespots. As the screenshots below show, even within Morpho menelaus the lower surface of the wings differs a lot in detailed pattern and the eyespots do vary considerably in size, but the eyespots are always prominent.
The most plausible explanation (see screenshots below) is that these spots distract attack away from the head and body, towards more expendable parts of the anatomy. However, even this hardly makes sense. What sane bird is going to look at a choice of a dozen eyespots on a single animal and think ‘ok, that particular eyespot is obviously the head of this thing’. It’s all fanciful and incoherent and nobody really has a clue, do they?
To summarise ‘what’s wrong with this picture’, when it comes to the eyespots of morphos:
Firstly, there are just too many eyespots for the pattern to function in terms of startling a potential predator with the semblance of disembodied eyes. I pointed out in part 1 that the problem in the case of the owl butterfly is that only one large eyespot can be seen at a time.
If morphos had simply added another similar eyespot, to give the semblance of a pair of staring eyes within the same wing surface, that might have restored the effect. Instead, the ridiculous reality is that several have been added, which is even more self-defeating than the pattern in the owl butterfly.
Secondly, the extreme brightness (including in ultraviolet) of the upper surface of the wings just doesn’t seem compatible with an essentially deceptive lower surface of the same wings, does it? I can’t say whether morphos advertise themselves for mainly sexual (intraspecific) purposes or as a warning to predators (I suspect the former), but either way that commitment to conspicuousness just doesn’t seem to jive with what is effectively a denial (‘I’m not a butterfly’) on the lower surface of the same wings.
So, I would go as far as to say that in the case of morphos, ‘nobody seems to have a clue’ as to the real function of the eyespots.
http://en.wikipedia.org/wiki/Morpho
Male and female of Morpho menelaus:
https://slm-assets0.secondlife.com/assets/5075365/lightbox/Morpho_menelaus_menelaus-PAIR.jpg?1328994973
Male Morpho menelaus upper and lower views:
http://en.wikipedia.org/wiki/Morpho_menelaus#mediaviewer/File:Morpho_menelaus_huebneri_MHNT_Male.jpg
Female Morpho menelaus:
http://en.wikipedia.org/wiki/Morpho_menelaus#mediaviewer/File:Blue_Morpho.jpg
Morpho menelaus lower view:
http://us.123rf.com/400wm/400/400/carlosphotos/carlosphotos1011/carlosphotos101100097/8166493-butterfly-morpho-menelaus-bottom-view-isolated-against-a-blue-background.jpg
Morpho menelaus view of perched animal with wings folded, hiding the bright upper surface but showing the eyespots on one side (left or right):
http://www.danaida.ru/sem10/29.jpg
Morpho menelaus, showing subspecific or individual variation in eyespot pattern:
http://static.panoramio.com/photos/large/20938010.jpg
Ditto:
http://www.fotonaturaleza.cl/details.php?image_id=18593
Comparison of Morpho (probably menelaus) on left and owl butterfly on right, showing the essentially similar pattern but difference in number of eyespots:
https://c1.staticflickr.com/5/4096/4939800983_b76f3b795d_z.jpg
Caterpillars of Morpho menelaus:
http://static.squarespace.com/static/502d2cede4b0ab396711e089/t/52cac7f1e4b090304a156946/1389021172579/
to be continued in https://www.inaturalist.org/journal/milewski/68259-function-of-eyespots-in-butterflies-part-4-owl-butterfly-and-lycaenids#...
(writing in progress)
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