You're talking about (or more specifically to) a specific fly, so it's still in the definite even though you don't have the article.
I don't get it. Does it mean that you can basically use indefinite form of the noun when speaking of something definite?
Look at it this way. Let's say we're talking about a really filthy person. You could say e.g.
- That pig!
- The pig!
Though the second is a bit less idiomatic, the point is that it's in the definite because you're not talking to it but rather about it.
However, if you were to speak to that person directly:
- You pig!
It's no longer in the definite because you're addressing the person.
English makes this less obvious than Swedish, but it's the same general principle.
Albeit a bit ridiculous, is it possible to say "Den fulla fula flugan flög"?
Yes! Flyg, fula fluga, flyg. Och den fula flugan flög. 'Fly, ugly fly, fly! And the ugly fly flew'.
Apparently this is a song sung by Swedish children? Sort of like a nursery rhyme?
A flea and a fly got lost in a flue The flea said let us fly The fly said let us flee And the both flew out of the flue
En lopp och en fluga gick vilse i en rökgrop Loppan sa att vi skulle flyga Flygan sa att låt oss fly Och de båda flög ut iv rökröret
It took me ages to translate that into swedish.