Another little known fact about London, or more specifically, it's Underground rail network, there's a type of mosquito down there that's unique to the underground system, apparently they went down there when it was being dug and stayed down there, adapting to their surroundings until it's not got to the point where it's virtually impossible to mate the underground mosquito with the aboveground one.
More info here:
http://www.answersingenesis.org/crea.../i2/biters.asp
I did hear murmurs of a different species of rat down there too, but I can't find any links on it.