The "neotenous" tag implies that this is not a larval stage, but rather a sexually mature adult that retains larval characteristics. So,to second spoco2's comment, it's either a beetle larva, or an adult that looks like one. "Trilobite" is just it's common name.. kinda like how a "tiger beetle" isn't actually a tiger.

Now, what weirds me out is that I found something that looks very similar to this a few months back, but it didn't have a head! I had an entomologist look at the pictures. Here's and excerpt from her response:
"think it is a Lycidae larvae (coleoptera), they also called them trilobite larvae (stupid name!)".