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 possess 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 and fly around him. When they are directly above Mario, they will swoop down, inflicting one point of damage on Mario if they hit him. As with other members of their species, a simple squirt with F.L.U.D.D. or a stomp will defeat them.