From the Super Mario Wiki
Artwork of a Skeleton Bee.
A Skeleton Bee is an enemy in Super Mario Land 2: 6 Golden Coins that appears in Area 4 of the Tree Zone, and like the other Bee enemies in this game, it can only be found in this particular stage. The Skeleton Bee is tougher than their non-skeletal counterparts, as they can come back to life after Mario defeats it, much like Dry Bones. Doing a Spin Jump or using a Star are the only ways to defeat them permanently. Also like their counterparts they appear out of the hives in the background and move very slowly. Like the Little Skull Mouser, they may just be wearing a mask, as in official artwork, they appear to have a normal, non-skeletal body. However, in gameplay, the bottom half is able to be seen through as if they are completely composed of bones.
Skeleton Bees flying in an "O" formation.
A "defeated" Skeleton Bee.
The ? Block can be seen through the Skeleton Bee's body.
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