Typically, tortoise beetle larvae carry a fecal shield (a shield made of poop) which is attached to those two prong-like protuberances. This helps hide the beetle larva as well as make it (hopefully) unpalatable to predators. This species feeds almost exclusively on thistle, particularly the non-native but-oddly named, Canada thistle which is pretty invasive.

To see the adult: http://www.pbase.com/laroseforest/image/152082479
To see the larva with a fecal shield: http://www.pbase.com/image/156274554