This is the larva of Ariadne ariadne pallidor (Angled Castor) - Nymphalidae/Biblidinae. I suspect you found it on Ricinus communis, its most common hostplant, which is poisonous. (Used to make the deadly chemical warfare agent, Ricin which was the substance used in the Japanese subway attack some years ago.).
Interesting story, Les!
I'm reading a bit about this Castor Oil Plant. But according to wiki (yes I know we should not always believe everything on there) this plant is not native to this region.
It made me wonder if this butterfly has been introduced (accidently?) with the introduction of these plants by people, or if it is a local species that used to feed on other host plants but has changed diet. Any ideas?
I was not aware that the plant was not native. However, it belongs to the Euphorbaceae family, many of which are native. I suspect that it may have switched its main hostplant as Ricinus communis spread. Some species of Ariadne do also use other species of this plant family. The butterfly is not an introduction, the Genus can only be found from India to China and SE Asia, as far as I can find out.
I'm no expert on plants nor insects. Your story about the plant's toxicity got me interested. Don't worry I'm not going to use it, , but I love unusual facts about plants or animals. Then I read about the plants range and made me wonder who came first, the butterfly or the plant.
I believe India is mentioned as historical range of the plant. And if the same is true for the butterflies, then it is pretty much solved, haha.