Head on this male was only this purple -
As opposed to a typical fresh male which head on looks like this -
I've only noted these unpurple males there in very hot weather. Any ideas (in plain English)?
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3 comments:
Here's a slightly crackpot theory.....sexual differences are often relevant to and enhanced by sexual selection. Have noticed with Brimstones how a mating pair are often chased by a competing male rapidly beating its wings, perhaps trying to signal to the female that it is "yellower" than her partner and therefore a better mate (over time this will mean that yellower males are more successful and increase the sexual difference). Maybe at Fermyn, where there is a much higher concentration of iris, and in hot weather, when activity is greatest, males have more opportunity to do a similar thing (ie flap at mating pairs to try to lure the female away through their flashier purpler colouring). Doing this a lot might damage their wing scales and have the ultimate effect of making them less purple as they get older.
Ok it's just a idea!
I'm pretty sure that those individuals which refuse to reveal our favourite colour, irrespective of lighting conditions, do so because their wings are still damp (every such specimen I've observed is completely unblemished). These specimens also show a characteristic blotchiness on the surface of the wing. The purplish blue colour is of course 'structural', rather than pigment based, so I believe that residual pupal fluid is affecting the normal interplay between scale microstructure and light rays.
The explanation for the darker males can actually be rather easily explained, especially when considering the broader picture over recent years. As correctly implied by Neil, the darker scales (such as those present on the female wings) are ‘structural’ melalin scales. The scales present on the wings of standard iris/ilia males are however of a completely different ‘pigmented’ nature and are responsible for the refraction of light in order to give their iridescent nature. Considering the two very different types of scales, the rubbing off of one type of scale would certainly not unfortunately result in the increased amount of another. Inversely if Neil’s “scale microstructure and light rays” theory was correct, there would simply be an alteration of the normal 18° light refraction angle, which would however be present at various, albeit differing light refraction degrees which again however does not appear to be observed in the specimens presented.
Comparable with almost all known (volatile) butterfly pheromone compounds, their action is temperature dependent within a very narrow window of activity. Consistent with the “sexual selection” theory suggested by Tony, lower temperatures over recent summers would have therefore resulted in the increased reproductive success of darker (melalin rich) males who’s wing temperatures would have produced more of the active mating/seductive pheromone compounds, more rapidly at lower (decreased sun dependent) temperature ranges. Although this fact alone would account for the increased appearance of such melalin rich males this year, the sudden (and welcome) return of the sun again this season (as per other hot/sunny days) would however have also unfortunately resulted in the overheating of such males under increasingly sunny conditions and consequently resulted in their increased need to cool via the imbibing of liquid refreshment. The necessity to cool via such means would have therefore also resulted in their more frequent observations on the Fermyn rides.
For what it’s worth, that’s just my explanation….
Mark
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