Last night I found a large larvae of some type on my fireplace. I was totally freaked out by it and then I found 3 more tiny thin ones also in the same place. I knew they were far too big to be maggots, and a search on Google has just provided me with an answer. They are mealworms, the larvae of the mealworm beetle. Apparently the beetles can fly and are particularly fond of old bird nests in chimneys, which would explain where mine were found. The fireplace has a gas fir, but isn't fully sealed up, so they must have dropped down the chimney from an old birds nest (I know there's an old nest up there)
But what's really turning my stomach is that the beetle typically lays 500 EGGS! So I may be looking at either 500 maggoty worms or 500 beetles dropping into my living room. I am not happy!
Here's a lovely picture that's identical to what I found:
http://www.whatsthatbug.com/2007/03/01/mealworm/