Swallowtail butterflies are large, colourful, fork-tailed butterflies that compose the family Papilionidae. Globally there are over 550 species, and though the majority of them are distributed in tropical regions, species of the family are present on all continents except Antarctica. This group of butterflies is named after the bird called the swallow in the naming of the type species Papilio machaon (Common Yellow Swallowtail).
In Bhutan, 42 swallowtail butterflies are recorded. The national butterfly, Bhutanitis ludlowi (Bhutan Ludlow's Swallowtail), is endemic to this country, yet this species of pride is endangered.
Swallowtails differ from all other butterflies in a number of anatomical characters. Most notably, their caterpillars possess a unique organ behind their heads, called the osmeterium, which is a forked retractable defense organ. When the caterpillar is disturbed or threatened, the organ emits smelly secretions containing terpenes.