From the Super Mario Wiki
Melon Bugs are little ant-like bugs that can roll into balls in Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3. They're called Melon Bugs because when rolled up the streaks across their body makes them look like a melon.
They bob up and down but when Yoshi gets too close, they hide. Stay in the air when the bugs touch the ground and they'll keep jumping. Eat one and then spit it out so it will roll into and defeat any enemy. They cannot harm Yoshi; in fact, they help him. Rolling one along the ground or spitting one into an enemy will instantly defeat it.
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