From the Super Mario Wiki
Swaphoppers are bug-like enemies that only appear in areas where gravity flips such as Rightside Down Galaxy and Upside Dizzy Galaxy, though they also make a minor appearance in Battle Belt Galaxy. Swaphoppers have one blue, hard-shelled side with a spike on top, meaning Mario can't Spin or Jump on it. On the other side is a soft, tan underbelly. When gravity flips and exposes the Swaphopper's tan side, Mario can jump on it and defeat the Swaphopper. However, in the Battle Belt Galaxy, one of the Swaphoppers behaves oppositely; it has its tan underbelly vulnerable when the gravity is normal, and the spike on top when upside down. They attack by simply hopping towards Mario. Aside from what is described, they are roughly oval shaped, have two bored-looking eyes in between their two sides, and a pair of short legs. When they are stopped by the Co-Star Luma they will not be affected by gravity changes, making them easier to hit.
A sub-species of Swaphoppers appear in Super Mario 3D Land called Flophoppers, their appearance is slightly different and they do not possess the gravity tactics of Swaphoppers, instead attacking like a Bramball.
Their name is a pun on "swap", referring to the constant gravity changes in the places they inhabit, and "grasshopper".
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