The African Citrus Swallowtail Butterfly, also called just the Citrus or African Lime Swallowtail, is native to sub-Saharan Africa. Its caterpillars feed on citrus trees. It doesn't have a swallow tail and from the topside (above left) looks very similar to the related Asian Lime Swallowtail. From the underside (above right), however, it looks totally different.
Male and female look similar; females are usually larger. Individual patterns vary. The dark underside is the key difference between the African and Asian species (see link below).
Like the Asian Lime, which has spread to Australasia and
Central America, the African Citrus is a strong flier. The
tattered individual shown above centre/right was in my
garden in North London -- probably an escapee but it was
many miles from any tropical butterfly house. It was
interested in the lobelia (liking small blue/purple flowers)
but flew up to the roses when camera-shy.
Although the topsides of the Asian and the African Lime/Citrus swallowtails look very similar, there are subtle differences in addition to the obvious pale vs dark underside (see link below). The red "eye-spot" or "tornal spot" differs: the round "pupil" in the Asian (demoleus) is black with a blue "eyebrow"; the "pupil" is blue in the African
(demodocus) with a black under curve.