What's special about a mosquito is that it drinks blood.

Phil originally said this:

My point was that vampires were by definition not real - or at least, not understandable - because any time we found something real and understandable that met the definition of a vampire, we would change the definition to exclude it.

Note Phil's use of the word "because" here. Phil is claiming that if vampires weren't unreal-by-definition, then the audience would not have changed their definition whenever provided with a real example of a vampire as defined. It follows that the original definition would have been acceptable had it been augmented with the "not-real" requirement, and so this is the claim I was responding to with the unreal mosquito example.