While looking over my Antherea polyphemus cocoons, I came across several that didn't sound quite right when shaken. I found them to be parasitized by Enicospilus (I'm guessing americanus, based on it's size and my location...we'll find out for sure when they eclose). The Enicospilus cocoon was inside the polyphemus cocoon (see other photo). All that remained of the polyphemus larva was the shed skin.