I chased this butterfly along a road in Chiang Mai Zoo, last November (2013). It eventually landed on a fallen log and I was able to get this picture of it with its wing spread (Upperside).

It took me a little while to identify it though, even with the help of the excellent book on the Butterflies of Thailand by Pisuth Ek-Amnuay. Anyway, it seems that this is a very variable species, exhibiting what is called phenotypic plasticity: in other words its varies both in size and colour pattern (but its only the females that show this variation apparently). The different subspecies can also interbreed according to Pisuth Ek-Amnuay, which can produce a range of hybrids which do not fit into the neat categories beloved of taxonomists! It shows that the idea of a fixed species type is sometimes at odds with what happens out there in nature, where individuals are constantly changing and adapting to circumstances as they occur. The reasons behind such variation is not always easy for us to understand, but seasonal variation in the size and colour of this species might help it adapt to its changing environment, particularly the wet and dry seasons in the tropics (see Kemp & Jones 2001, DOI: 10.1111/j.1095-8312.2001.tb01299.x).

The butterfly eventually flew off and landed on a leaf, where I was able to get a photo of its underside.

I had previously managed to get a shot of a very ragged male (below) which is smaller than the female, and has the light purple discal patches on the wings.  This particular male lacks thew white marginal and sub-marginal lines seen on some varieities, but it might just be very worn!

There are some terrific photographs of the larvae (caterpillars) and pupae (cocoons) of this particular subspecies on an excellent website of the Butterflies of Singapore (http://butterflycircle.blogspot.co.uk/2012/09/the-life-history-of-jacintha-eggfly.html). So many great resources on the web!

Reference
DARRELL J. KEMP & RHONDDA E. JONES (2001). Phenotypic plasticity in field populations of the tropical butterfly Hypolimnas bolina (L.) (Nymphalidae). Biological Journal of the Linnean Society Volume 72, Issue 1, pages 33–45, January 2001. http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.2001.tb01299.x/abstract.