Butterflies of Mexico, USA & Canada
Western Tiger Swallowtail
Pterourus rutulus  LUCAS, 1852
subfamily - PAPILIONINAE
Pterourus rutulus, Sierra Nevada, California, USA  Marijn van den Brink
Back in the 18th century when Linnaeus created the System Naturae, the word Papilio was used as the genus name for every known species of butterfly in the world. Since then much has been learnt about the relationships between different species. Consequently most have been reassigned to new genera, and only about 215 of the 17600 currently known species are retained in Papilio.
There are 30 Papilio species in the Australian region, 60 in the Oriental region, 40 in the Holarctic region and 54 in Africa. The taxonomy of North and South American 'Papilio' species has recently been revised, with the effect that almost all South American species and several North American species have been transferred to Heraclides, or in the case of rutulus, to Pterourus.
Pterourus rutulus was once considered to be a subspecies of glaucus. Females of glaucus however are earthy brown in colour, with no trace of dark bands on the upperside, whereas rutulus females are identical in colour and pattern to the males.
P. rutulus is restricted to the western states of the USA.
This species is found in open woodland, at altitudes between 0-1500m.
The eggs are greenish-yellow. They are laid singly on either surface of the leaves of the foodplants which include willows Salix and poplars Populus ( Salicaceae ), and various other trees and shrubs.
The caterpillars when small are dark brown, with grey prolegs, and are marked with a white dorsal saddle. When at rest on the upperside of leaves they greatly resemble a bird dropping.
When fully grown the plump larva is mid-green in colour with small pale blue dorsal and mid-dorsal spots, and a narrow cream band behind the thorax. On the 3rd thoracic segment there is a pair of cream ocelli containing a blue-centred black spot.  If the larva is molested it hunches up, causing the ocelli to expand, giving the larva a threatening appearance which probably deters avian and mammalian predators. If molested further, the larva produces from behind it's head an eversible fleshy orange forked structure called an osmaterium, which emits pungent chemicals. Experiments with captive birds have demonstrated that the chemicals are ineffective against them, but they are capable of deterring ants, and predatory / parasitic wasps and flies.
The pupa is light brown mottled with olive, and has a brown lateral stripe and a pale dorsal stripe. It is attached by the cremaster and a silken girdle to a woody stem or tree trunk.
Adult behaviour
Males patrol back and forth along roadsides and woodland rides in search of females, and in hot weather aggregate in groups to imbibe mineralised moisture from damp ground. They also feed at carrion and dung. Both sexes visit a wide range of herbaceous and bushy plants for nectar.


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