'lecieć' is perfective, for one-time actions, also for 'right now'.
"Motyl leci" (The butterfly is flying - right now), "On leci do Londynu w czwartek" (He's flying to London on Thursday)
"latać" is imperfective, general.
"Motyl lata" (The butterfly flies, it's generally capable of flying), "On lata do Londynu w każdy czwartek" (He flies to London every Thursday)
Although things like a butterfly flying by itself are so general, that you can use "latać": Siedzę na fotelu, czytam książkę, motyl lata koło mojej głowy... (I'm sitting in an armchair, reading a book, a butterfly is flying/flies near my head...)