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I would guess that the parasite is a horsehair worm (phylum Nematomorpha). As far as I know, they are the only known non-arthropod parasitoids. I haven't heard of them in Diptera before, so this is cool!

In most zoology classes, what you learn about Nematomorphans is their killing behaviour: The adults mate in fresh water, and thus mature specimens routinely cause drowning in their hosts (e.g. http://www.youtube.com/watch?v=8ylDNMNNwaE).