swallowtail

Concise Oxford English Dictionary © 2008 Oxford University Press:
swallowtail/ˈswɒləʊteɪl/

▶noun

• 1 a deeply forked tail.

• 2 a large brightly coloured butterfly with tail-like projections on the hindwings. [Papilio machaon and other species, family Papilionidae.]

– derivatives
swallow-tailed adjective.