Function of eyespots in butterflies, part 4: owl butterfly and lycaenids

 (writing in progress)
 
...continued from https://www.inaturalist.org/journal/milewski/68257-function-of-eyespots-in-butterflies-part-3-morpho-butterflies#

 Next, I focus on the owl butterflies (Nymphalidae: Caligo) of Mexico, central America and South America.
 
If one flips casually through butterfly books, the owl butterflies come up as what seems to be a typical example of eyespots, functioning in an uncomplicated way to startle potential predators by means of a deceptive impression that some large animal is starting out from where the butterfly was last seen.
 
The trouble is that most such illustrations are misleading. The photographer usually takes the butterfly from a perspective which would not be likely in nature: a full ventral view of both wings at once = a view of the butterfly upside down.
 
In reality, because the eyespots are on the underside of the wings, they would only be seen on the vertically folded wings, one eyespot at a time. The resemblance to the face of an owl is thus less than usually implied.
 
By comparison, certain large moths do present a two-eyed ‘face’ because their large eyespots are on the upper sides of their wings and the forewings can slide forward to reveal the eyespots on the hindwings.
 
I illustrate these facts below.
 
Bottom line: in our examination of eyespots in butterflies, the conventional view (which is poorly articulated but extensively illustrated in a cherry-picking way in most books) runs into trouble immediately, because the best example of a startling eyespot, mimicking the generalised appearance of a disembodied vertebrate eye, can actually only be seen one eye at a time.
 
In the photo below, I show a good example of a large moth with classic eyespots, which function just as expected. Because the eyespots occur on the upper surface of the wings, because the overall colouration is inconspicuous, and because the larger of the eyespots are revealed by sliding forward the cover of the forewings, it seems perfectly plausible that these eyespots mimic a generalised disembodied stare by a vertebrate large enough to threaten any would-be attacker of the moth. Nothing too complicated or contentious about this, but can anyone name a single butterfly that conforms to this pattern?
http://www.deviantart.com/art/Moth-Stock-280680523

Now look at the reality for the owl butterfly Caligo memnon. The upper surface of the wings is – in contrast to the moth above – completely lacking in eyespots. The eyespots are instead on the lower surface of the wings...
http://www.gaianicaragua.org/Caligomemnon01.JPG
 
...like this. No predator is likely to view any owl butterfly in this way because a) this view is upside down and b) the butterfly actually perches with its wings folded vertically in the typical pose of most butterflies,...
http://www.gaianicaragua.org/Caligomemnon02.JPG
 
...like this. The eyespot is all very well but it is not seen as a pair, it is seen as just one eye. Would this still work in startling a potential attacker?
http://th08.deviantart.net/fs70/200H/i/2012/079/a/8/owl_butterfly_ventral_view_by_death_pengwin-d4tfvnx.jpg

Convincing examples of decoy eyespots in typical lycaenid butterflies:
  
It is well-known that hundreds, if not thousands, of species of Lycaenidae, use a decoy tactic against their predators, by virtue of a ‘false head’ on the hind end of the wings.

This often involves ‘false antennae’. However, it also involves ‘eyespots’, the function of which is no doubt to make the ‘false head’ look more convincing. To me this is a clear case of colouration in which the false eyes depicted by the butterfly are mimics of its own eyes, having similar size to the real eyes on the real head.

This is quite different from

• startling the would-be predator by presenting a disproportionately large, disembodied eye, or
• mimicking some abstract general eye or array of eyes.

I thought the photos below illustrate nicely the use of ‘eyespots’ by lycaenids, because the first photo really shows how similar the eyespots are to the real eyes of the same butterfly, while the second photo shows how the whole design of the wings, in shape, orientation, and more general colouration, supports the deception of a false head.
 
But this is my original point, which I have not seen made anywhere in the literature before.
 
Eyespots, in butterflies and other animals, often have a concentric pattern with pale on the outer and dark on the inner, to mimic the white sclera and dark iris/pupil of a vertebrate eye. Often there is even the semblance of a highlight (a pale spot in the dark centre of the eyespot).

But this concentric arrangement can extend to mimicry of the butterfly’s own eye despite the fact that the insect eye has quite different design, being a compound eye with no sclera, iris, or pupil. If one look at the first photo below, one will see that the real compound eye of the butterfly is a dark object with a crisply defined pale surround (the facial scales).

This pattern has been copied in the false eyes: something concentric, with pale outer and dark inner. I don’t think these lycaenids are trying to copy a vertebrate eye at all, i.e. the existence of scleras, irides, and pupils is completely irrelevant to these butterflies. It is sheer coincidence that both butterfly eyes and vertebrate eyes can have a concentric pattern of pale outer and dark centre, and it complicates biological interpretation because the functions differ more than the designs.
  
Lycaenidae: Nacaduba biocellata
http://www.sabutterflies.org.au/lyca/biocellata.htm
 
http://en.wikipedia.org/wiki/Pseudalmenus#mediaviewer/File:Pseudalmenus_chlorinda.jpg

(writing in progress)

Posted on 16 de julho de 2022, 10:06 AM by milewski