Do you have any photos of the whole plant, so we can get a better idea of what it looks like? The only clear part of your photo is the bloom, and though that flower could be a giveaway to someone, it's not ringing any bells with me. Beautiful sulpher butterfly, by the way.

Yes - partial views like this often aren't adequate

If it was growing in water or on a mudflat, maybe Justicia Americana.

Justicia does seem to fit the photo...good call, and it's quite butterfly attractive too.

Justicia americana for sure. Don't think I've ever seen it in person, but always have wanted to grow it. Don't have any moist areas to grow it properly.

It's in the shallow rivers here...so that almost the whole course of the river is purplish and has butterflies flickering around everywhere.

Hi All

Thanks a lot weighing in - it sure is Justicia americana!

I got only that 1 photo having spotted it as I was leaving. It was growing through the concrete banks of the Channel - quite sure the roots would be in water. Over the last two weeks we had a lot of rain and the water overflowed the banks so it was submerged for short periods of time. It is a dainty plant & flower - I will get better pics on my next visit.

Thanks again for sharing your knowledge!

Shyamala