The Eastern Tiger Swallowtail
State Butterfly of Five American States
Some adult females of this species mimic the poisonous Pipevine Swallowtail, but the caterpillars rely on their camouflage and smelly secretions for survival.
The Eastern Tiger Swallowtail (Papilio glaucus) is found throughout Eastern America as far north as Vermont. It has three different ways of avoiding being eaten by predators. The caterpillars try hiding and can emit a nasty smell, the chrysalis looks like a twig, and some adult female butterflies mimic a poisonous relation.
Camouflage
The eggs are green and laid on leaves where they are difficult to spot, but the caterpillar does not camouflage itself in the usual way - it looks very much like bird droppings and this is useful since not many predators relish that! The chrysalis, like the egg, has a conventional form of camouflage and resembles a twig. It attaches itself to the branch of a tree and is very difficult to find.
Smelly Secretions
The caterpillars of all Swallowtail Butterflies have a unique organ, the osmeterium, just behind their head. This structure is normally hidden, but if the caterpillar is threatened in any way it can be displayed and gives off a nasty smell. Presumably, if you look like faeces it reinforces the illusion if you also smell bad!
Mimicry
Where the range of the Eastern Swallowtail overlaps with that of the Pipevine Swallowtail (Battus philenor) the females are often dark coloured. This makes them look similar to their poisonous relative which, presumably, predators have learnt to avoid. The interesting thing is that the males do not adopt this strategy and the females are usually ‘normal coloured’ in areas where there are no Pipevine Swallowtails.
The Pipevine Swallowtail
The caterpillars of the Pipevine Swallowtail feed on plants which make them poisonous to most predators. They are almost black with reddish tubercles along the back – typical ‘warning colouration’. The adults retain this poison (rather like Monarch Butterflies) and they are also black with red spots like their caterpillars. In areas where the Pipevine Swallowtail is found predators learn to avoid dark butterflies and the dark form of the female Eastern Swallowtail has a distinct advantage over its ‘normal’ sisters. (Why the males not also adopted this strategy remains a mystery.) In areas where there are no Pipevine Swallowtails the dark female form has no advantage over the ‘normal’ and they are rarely found.
State Butterfly
The Eastern Swallowtail has been declared State Butterfly of Delaware, Alabama, Georgia, South Carolina and Virginia (where it is also the State Insect). It is obviously a well-known and popular butterfly rivaled only by the Monarch (which is also a very popular State Insect and State Butterfly).
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