Plain Tiger, Danaus chrysippus chrysippus, wingspan 60-75mm. This is a male (see sex brand on underside of hindwing - a black patch with a white centre).

It is widespread in dry zone coastal areas of Sri Lanka. The larva feeds on plants of the Apocynaceae many of which have toxic chemicals, hence its (and the adult's) poisonous nature. The main food plant is Calotropis gigantea, but, according to the Butterfly Fauna of Sri Lanka (van der Poorten & van der Poorten, 2016), in dry coastal areas it also feeds on Pergularia daemia and Pentatropis capensis as often as on C. gigantea. See my other observations on C. gigantea and P. daemia.

This butterfly is probing a Cardiospermum halicacabum flower (see second uncropped image).