Byasa mencius[1] is a butterfly from the family Papilionidae (swallowtails).

The wings are dark brown with chain of large red spots on the hindwings.

The underside of the body is covered in red hairs.

is stated to be "geographica mutatio" [synonym of] P. alcinous Klug var.

Their specimens were from the Chinese coastal trading city of Ningpo.