Possible magical abilities of flies:
• danger sense; they can see into the future so as to avoid swats even at extremely high speeds.
• psychic invisibility; if you ever saw a fly you want to kill and started watching it flying around for when it lands, you will notice that it will start zig-zagging and then it will disappear! I believe that this is not actual transparent invisibility but the fly is making itself in some etheric energy that makes it hard to notice things, like a ninja masking its presence.
• psychic vampirism; flies often take actions just to annoy you, like divebombing your head or buzzing around your ears. This is done to create emotional outburts that they feed off of. In horses and cattle they like to swarm around the eyes. Why do they do this? There are no nutrients in tears, it would be more efficient to eat the dead skin and such from the animals' bodies. It is because they want to annoy the animals to feed off of their energy.