Drosophila melanogaster, or the fruitfly, is one of the most rigorously-studied bugs in history. Because they breed quickly, and have a relatively small but diverse genome, they are incredibly popular with geneticists. And these geneticists have carefully enumerated (and induced) all the crazy mutations you can imagine in these little insects. How crazy, you ask? That's where the Drosophila Viewer comes in.
The Drosophila Viewer is a teaching tool for students learning genetics, but it's also a visual encyclopedia of all the ways DNA can go wrong. There are twisted flies, giant flies, flies with eyes of every weird color as well as no eyes at all, flies with crumply, tiny, and double wings, flies with extra joints in their legs, and more. There are even flies with legs growing out of their mouths (see image above). Elsewhere, scientists have even figured out how to induce homosexual behavior in fruitflies (yes, you can do that with flies, but it's not so simple in humans so don't get any big ideas).
I can't stop looking at the flies in the Drosophila Viewer. If you want to know what genetic mutants are really like, you'll want to page through the viewer instead of watching C.H.U.D. or Hell Comes to Frogtown.
At the top, the illustration by T.C. Kaufman and colleagues from Indiana University shows how when the ‘pb’ (proboscipedia) gene in the fruit fly is mutated, it causes a transformation of the fruit fly’s mouth parts (slide A) into legs (slide B).