This is a morbid question, but how to bugs just appear on a corpse (asked because of Casey Anthony case)?

Flies land on a body and lay there eggs which later turn into maggots. Other insects probably smell there way there or just happen to find it, I honestly don't now, but that is probably right. Yes, a body in a casket can still get bugs in there its just how they help decay the organism and continue the proses of life