This is the caterpillar of the Anise Sallowtail butterfly climbing an anise plant in Southern California. (The leaves in the photo are from an adjacent plant..the leaves and buds on the Anise look rather like dill) Fennel is a host plant for many species of insects.
It will become a most excellent pale yellow butterfly.
Species: P. zelicaon
Top Ten Winner: Redemption
Top Ten: A Goofy House
Top Ten: Bicolor Beauties
Top Ten: Super Macro
Top Ten: Absolute Clarity
Top Ten: Bugs or Animals on Flowers
Featured: Closeups in Nature
Featured: Bugs and Animals on Flowrs & Foliage