Flies are fast flyers, great scouts and excellent Imp killers, but not that great in battle. They are natural enemies to Spiders, fighting them at every turn. As one of the three insect creatures, Flies are a key component for sacrifices, so they retain their usefulness until a Temple is built. They are weak and do not do any jobs by themselves, but love to explore and when they grow older and stronger they gain the ability of Sight which allows them the ability to see things hidden from others, such as invisible beings.
This, combined with the fact that they are extremely fast makes flies excellent scouts and patrolling dungeon guard dogs