Sauron the Gene-maker

[Galadriel55]

Originally Posted by Nerwen

However, it could also be an entirely passive side-effect of magical "pollution"- there are many other examples of Sauron's general corrupting influence on the surrounding natural world.

Good point. It's more in Saruman's style to breed species on purpose with his own mark on it just to show that he can. I can see how Sauron would not have created the flies on purpose, but they were still altered because of his presence.