From the Super Mario Wiki
A Soarin' Stu is a type of Stu appearing in the game Super Mario Sunshine. They resemble a Strollin' Stu without feet wearing a blue pair of pants and having a set of insect-like wings on their backs that allows them to fly. They will often hover in one place, but when they notice Mario, they will start to approach him. When they are directly above Mario, they will swoop down, inflicting one point of damage on Mario. As with other members of their species, a simple squirt with F.L.U.D.D. and a stomp will defeat them.
Soarin' Stus are only found in the Pinna Park level on the beach in Episode 3, as well as the Hotel Delfino's secret level in Sirena Beach, in which Mario must jump and bounce off their heads to reach the next platform. The Soarin' Stu's in this level have bat-like wings instead of insect wings.