Bats stay in their place until they spot Luigi. Once a Bat spots Luigi, it will swoop down quickly at him and bite him for 5 HP of damage. The best way for Luigi to deal with Bats is to use the Strobulb. Bats always appear in groups and Luigi can only vacuum up one of the Bats at a time leaving him open to another Bat attack. There also also Gold Bats that fly more quickly and can be strobed for a Gold Bar.