These are such unique looking caterpillars, small (about 1 inch). They are also called Filament Bearer Caterpillars. Bug Guide has this to say about the caterpillars:

"Strange! Eversible tentacles extend from the dorsal surface of A2 and A3. ...when the caterpillar is alarmed these tentacles can be extended to 2x their resting length. These same structures probably occur in other species in the genus; but otherwise I believe they may be unique."

In this photo, the tentacles are extended half way because mosquitoes were bothering the caterpillar.