From the Super Mario Wiki
Three Bats fly towards Luigi.
A Bat is a minor type of ghost enemy found in Luigi's Mansion and Luigi's Mansion: Dark Moon. They come in two colors, purple and gold, and often appear in large groups. Purple Bats are found hanging from the ceiling in hallways and staircases. When Luigi walks by them, they swoop down to attack him. If one hits Luigi, he sustains five HP of damage, and the ghost disappears in a puff of smoke. If the strike is unsuccessful, the Bat returns to the ceiling and prepares for the next attack. Gold Bats also hang from ceilings, but only attack if Luigi wakes them up by turning on the Poltergust 3000 on vacuum mode (elemental mode doesn't work); strangely, they can also be awoken if Luigi is in first-person mode with the Game Boy Horror and the player presses the button. For them to attack, Luigi must be in the same room or corridor as they are. In all other respects, they are exactly the same as the purple Bats. Bats can be easily sucked up with the Poltergust 3000 or immediately destroyed with elemental powers.
They bear a passing resemblance to Swoopers, different bat enemies found in other Mario games.
In Luigi's Mansion: Dark Moon, they attack and do five HP damage to Luigi if they see him. They can be defeated by either sucking them in using the Poltergust 5000 or using the Strobulb on them. Using the Strobulb on them can make them drop a bill, a heart, or nothing. They can be hidden in furniture or windows after a curtain has been pulled. Gold Bats fly in a certain direction before disappearing; they are rarer than regular bats and yield twenty-five dollars when the Strobulb is used on them. Unlike their appearance in Luigi's Mansion, these variants are not ghosts.
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