Thanks for posting everyone! I also posted this on reddit.com under /r/whatsthisbug and they said Dermistid beetle larvae as well. After looking at some pictures I'm pretty sure it's a Dermestes lardarius larvae, as I've seen the adult beetles around the house from time to time. I make my own FF cultures so haven't bought any in a long time, but I do buy crickets for my milk frogs, so I'm guessing they hitch hiked in with them. A couple weeks ago this culture did have a tiny unexplained hole in the lid, which I guess has now been explained...