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Papilio polyxenes asterius
Spotted on Jul 23, 2013
Submitted on Jul 23, 2013
1 Comment
I think this is more likely a Black Swallowtail (Papilio polyxenes asterius)caterpillar. Papilio machaon does occur in Western North America.
http://bugguide.net/node/view/298231
http://bugguide.net/node/view/2636
That being said, the caterpillars for both species are very similar.