What is this?

On our first walk, one of the children found an interesting pod.  It looked like a dead leaf but when we shook it we could hear something inside.  However, whatever was inside didn’t seem to be moving of its own accord so I decided to cut it open.  With all of the children watching, I cut open the ‘dead leaf’ layer.  It moved!  Inside was a segmented body that was wiggling vigorously.  What is it?!?