The Common Tiger (Danaus genutia genutia)
The flowers, stems and pods of the Rattlebox Pea (Crotalaria retusa) often attract species of the Danainae sub-family to feed on them. Whilst there are no apparent "food source" for the butterflies to feed on, these Danainae actually use their sharp tarsal "claws" on their legs to scratch the surface of the plant, flower or pod which then exudes a fluid that the butterflies feed upon. There must be some chemical compound that the butterflies are attracted to. At times, several butterflies can be found feeding on the same plant.
This male Common Tiger is feeding on the secretions of the Rattlebox Pea flower bud after scratching the surface of the bud. This shot was taken by ButterflyCircle member Simon Sng at Pulau Ubin's Butterfly Hill, where the Rattlebox Pea plants are cultivated along the roadside.