The Thoas Swallowtail Butterfly, also called the King Swallowtail, is native to southern United States, Mexico and Central and South America.
It is a large butterfly with a 120-130mm wingspan.
The attractive underside is pale yellow with pale blue and dark red markings. Both male and female have similar colours and patterns.
The caterpillars look like spiky yellowish-olive bird droppings for protection against predators. They eat mainly peppers and citrus.
The Thoas Swallowtail is similar to the Giant Swallowtail, Papilio cresphontes, also called the Orange Dog after the caterpillars which are a pest of orange groves. The Giant Swallowtail (not shown here)
is the largest butterfly in North America with a wingspan up
to 160cm and, ranging from Canada to Central America,
overlaps the Thoas range. The caterpillars are almost
identical and adult butterflies hard to distinguish in flight. (The Giant African Swallowtail is a totally different species.) The Ornythion Swallowtail, P. ornythion, is another large, yellow and brown butterfly of the Central America and, rarely, southern U.S. which looks similar, but is easier to distinguish since the two yellow bands on the upperside don't merge but continue out to the wingtip.
There are several sources with different instructions on how to tell the difference between the Thoas/King and Giant/Orange Dog butterflies. The colour variation is down to individuals and both species have the large yellow spot with a brown, off-centre spot inside and the brown-bordered, yellow tails. Above left: the Thoas is said to have more regular-shaped spots than the Giant.
Above centre: the Thoas is said to have 4 significant-sized, crescent-shaped spots against the 3 (plus a small blob sometimes) of the Giant -- this doesn't always seem to be so.
Above right: this surely is the easiest to determine and so far seems to hold true -- the Thoas has an extra yellow spot above the top row on each forewing!
P.S. The photo of a Thoas on Wikipedia is a Thoas but the photo Wikipedia shows for a Giant Swallowtail is also a Thoas (using all three methods above).