The tropical Paris Peacock (Papilio paris) butterfly – not to be confused with the temperate or European Peacock (Inachis io) – is widely distributed throughout India and South East Asia. This species is relatively common on the slopes of Doi Chiang Dao, a mountain in northern Thailand. These butterflies are often seen mud-puddling on sandy soils or near the banks of small streams. There is a large blue-green discal patch on the hindwing, upperside, on both males and females, although it is sometime difficult to see when the butterfly is resting on the ground (below).
I cropped some of the images to show, in detail, the small green wing scales which give the butterfly its overall green colouration. These very bright, light-reflecting scales are set on the dark background of the wings, and are found on the ventral (upper) surfaces of both the fore- and hindwings, as well as on the body (below).
The sexes are quite similar in appearance, although the females is slightly larger and but somewhat paler and duller.
Paris Peacock (Papilio paris) showing wing scales
The iridescence of the butterfly scales is produced by microscopic ridges and facets on the surface of the scale. The mechanism for these so-called structural colours is thought to be coherent scattering of light by the nanostructures on the scale itself (1). The butterflies invest a considerable amount of metabolic energy into producing these iridescent wing scales. We can imagine that they are physiologically costly to produce, and therefore important features (ornaments) for the butterflies, presumably in terms of courtship and mating. Perhaps the shiniest, most flashy males do best with the females?
The type specimen – the individual which first received this scientific name, and which can be used for comparisons – is in the Natural History Museum in London, and was named by the great Swedish naturalist Carl Linnaeus in 1758 (2).
Reference and Link
1) Richard O. Prum, Tim Quinn and Rodolfo H. Torres (2006). Anatomically diverse butterfly scales all produce structural colours by coherent scattering. The Journal of Experimental Biology 209, 748-765. http://jeb.biologists.org/content/209/4/748.long
Ray Cannon's nature notes 
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