Function of eyespots in butterflies, part 2: peacock butterfly
(writing in progress)
...continued from https://www.inaturalist.org/journal/milewski/68246-function-of-eyespots-in-butterflies-part-1-apollo-butterflies#
In part 1 of this series, I questioned the textbook factoid about the eyespots in the Apollo butterflies.
The next ‘classic’ example of eyespots in butterflies is the peacock butterfly, Aglais io, which like Caligo is a member of the family Nymphalidae. There is a considerable literature on this butterfly and how its eyespots function, summarised by the Wikipedia excerpts below, and an abstract from a somewhat more critical thinker, Stevens.
As in the case of the owl butterfly but for different reasons, the peacock butterfly is more puzzling than it seems from the textbook factoids.
‘What’s wrong with this picture’? Well, the following:
Firstly, the other 7-odd members of the same genus, Aglais, lack the eyespots although sharing the bold colourfulness of the upper surface of the wings. This in itself should make us suspicious of the adaptive function of the eyespots in genus Aglais.
Secondly, the whole idea of eyespots against such a brightly colourful background sounds suspect. As illustrated by a moth in part 1 of this series, the startle effect of eyespots should work best against a background of crypsis or disruptive colouration.
When I look at a peacock butterfly I think: ‘make up your mind, are you pretending to be a lurking pair of disembodied eyes of some threatening vertebrate - or are you trying to look like a fruit salad?’
Thirdly, although Stevens (see abstract below and his review in Biological Reviews) is smart enough to have picked up on this inconsistency, his alternative explanation also fails to make complete sense because he seems to have come full circle to a kind of tautology: he is basically implying that black is white in the sense that he is implying that conspicuousness per se puts off predators.
How can that be, unless the peacock butterfly is yet another example of aposematism based on a noxious chemical nature? Tell us straight: is the peacock butterfly noxious, and are its bright colours themselves aposematic? I have not found a literature honestly tacking this question.
Fourthly, the caterpillars of the peacock butterfly are, in their own way, equally mixed-up and puzzling. They are a funny combination of aposematism and forms of chemical and physical defence that announce themselves more directly by ‘show and tell’.
The black with pale spots seems aposematic to me because the caterpillars are indeed conspicuously coloured in terms of tonal contrast. The spines seem to announce their own warning. The caterpillars do respond to threat by oozing noxious stuff from their bodies (misleading referred to below as ‘bleeding’).
The caterpillars live mainly on the nettle Urtica, which is the typical genus of NETTLES and defends itself by means of STINGING HAIRS, and it’s interesting to think the caterpillars in a way copy their food plant in threatening to sting with spines. But Urtica is not noxious per se; indeed I have plenty of personal experience of it as one of the most palatable and nutritious of wild herbs once the urticating hairs have been neutralised by dipping into boiling water. So why are the caterpillars of the peacock butterfly noxious? (Most noxious caterpillars derive their toxins from the defensive chemical compounds in their food plants)
The only possibility seems to be that these caterpillars manage to preserve the toxin in the urticating hairs themselves = the ‘venom’ found in all those microscopic hairs on the leaves? If so, does this toxin remain preserved in the butterfly imago despite the profound chemical transformations of metamorphosis? Is the peacock butterfly itself noxious or not, in the sense of being able to ‘sting’ its predator? Noxious enough to be aposematic?
I should perhaps have been clearer in pointing out two aspect of the behaviour of Aglais io. Firstly, it hibernates, so is a particularly long-lived butterfly. Secondly, when it spreads its wings to display the bright pattern on the upper surface, it also makes a sound which sounds like a warning. This warning sound lends support to the idea that the colouration on the upper surface of the wings somehow conveys threat.
Do readers see how, once one really starts thinking about this adaptive complex, all the simplistic explanations tend to collapse into a pile of confusion?
In summary:
The peacock butterfly Aglais io exemplifies the various butterflies that do ‘flash’ large eyespots along the lines of the ‘classic’ pattern used by certain moths, but confuse the display by being colourful in addition to the sudden show of false eyes. But on the other hand this species is not aposematic in any simple sense either.
Indeed, there is an aptness even in the name of this butterfly, because its eyespots are as puzzling as those of its namesake, the bird Pavo.
So, what exactly is going on with these eyespots, and what exactly is the defensive strategy of Aglais io, in both the larval and imago stage? Is it perhaps just sheer confusion of predators?
http://en.wikipedia.org/wiki/Aglais_io#mediaviewer/File:Peacock_butterfly_(inachis_io)_2.jpg
http://en.wikipedia.org/wiki/Aglais_io#mediaviewer/File:Tagpfauenauge_unten.jpg
http://en.wikipedia.org/wiki/Aglais_io
http://en.wikipedia.org/wiki/Aglais_io
http://beheco.oxfordjournals.org/content/19/3/525.full?sid=38a1e55b-ca10-4566-89b1-26685e34315a
http://image.shutterstock.com/display_pic_with_logo/88830/88830,1266234805,10/stock-photo-nature-caterpillar-water-nymph-inachis-io-is-very-prickly-46709812.jpg
http://www.learnaboutbutterflies.com/Lifecycle%205%20-%20larva%20survival.htm
to be continued in https://www.inaturalist.org/journal/milewski/68257-function-of-eyespots-in-butterflies-part-3-morpho-butterflies#...
(writing in progress)
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