If Danielle De Niese had two more arms, she'd look like the Hindu goddess Shiva. I didn't see The Enchanted Island, but the role of Ariel must have been written for soprano in keeping with the Baroque style. If the opera had been written in the late 17th or early 18th century, the role probably would have been written for castrato or haute-contre. And since castrato roles were later sung by women, it's in keeping with tradition to make Ariel a breeches role.