From the Super Mario Wiki
Twirlips are a species of enemy appearing in the game Super Mario Galaxy 2. Their name is a spoof off of tulip. At first glance, they appear as white bud shaped creatures with two beady eyes much like an onion but without the beady eyes. To attack, they turn into a pink flower with a yellow center and quickly spin around the area like a drill. They do this from time to time.
In their flower form, Twirlips can only be damaged by stomping on their heads. In their bud form, Mario or Luigi can use any attack method to defeat them. Yoshi can also eat a Twirlip but only while it's in bud form. They appear in a few galaxies, most notably in the Spin-Dig Galaxy. They are also fought on some Bonus Planets.
A Twirlip in its flower form can actually defeat another Twirlip when the latter is stopped by a Co-Star Luma and letting the former ran into it.
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