Tim Melling – Purple Emperors

Tim writes:

As a child I was puzzled why Purple Emperors were invariably illustrated with asymmetrical colours; one side purple, the other side black. This is because the colours are iridescent so only shine when viewed from the right angle. So the iridescence can bounce from side to side as the angle of view moves. This one show the right side fully illuminated but the left side largely dull.

The colours on this Purple Emperor are stunningly beautiful but the iridescence is formed by wing structure rather than pigments. There is a microscopically thin translucent coating on the surface of each wing scale and light reflects off both the front and the back of this layer. This layer also causes refraction (light travels slightly slower through it compared with air). But because the layer is so thin the light waves interfere with each other causing distortion of the colours, albeit pretty distortion.

The same principle applies when a microscopically thin layer of oil floats on water, and you get a spectrum of colours. If the layer were just a few millionths of a millimetre thicker, the butterfly would appear a different colour, just as the swirling colours of an oil sheen are caused by miniscule differences in the thickness of the oil layer. Incidentally the word iridescent comes from Iris, who was a Greek Goddess, and the personification of the rainbow. That is why the word iris is used to describe colourful things like eyes and flowers.

It is rare that I get an opportunity to photograph Purple Emperors as it is a long journey from Yorkshire. But I was down south for my son’s graduation and it did not seem too much of a detour. This (below) freshly-emerged male was imbibing salts from the forest track. This is a habit that is peculiar to males (though I have once observed a female Purple Emperor taking salts). Research has shown that the caterpillar’s diet is low in Sodium, but if males take in Sodium it aids their reproductive success. The Sodium ions can be transferred to the female during mating and helps egg survival. Research has also shown that Monarch butterfly caterpillars that feed in the salt splash zone of roads develop stronger flight muscles than butterflies reared away from roads with lower Sodium levels.

 

Mark writes: rather better than my effort on my ‘phone!  Thanks Tim!

 

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