Purple crow butterfly :Sex mark

Brand : Raised area on the wing surface, circular, ovate, or elongated, which is covered with special scent scales or androconia, found in males of some species. Also called sex mark.

Butterflies in DANAINAE subfamily are fairly large in size. Most of them are marked with vary shades of brown, black and white. Some are with blue or yellow colours. The males of many have prominent sex marks at the middle of hindwings. Their flight is not rapid but rather steady. Most of them able to fly long distance and some are migration species. They do not have tails on hindwings.

 Reference:

http://www.geocities.com/brisbane_nymphs/DANAINAE.htm

http://en.wikipedia.org/wiki/Glossary_of_Lepidopteran_terms