Yes, ducks really do eat duckweed. Notice the shed skin of a mayfly (Ephemeroptera) in the lower left corner? Mayflies are unique in undergoing a molt once they have achieved the winged stage. Sometimes you can see what looks like hundreds of dead mayflies on the water's surface, but really, you are seeing the shed skins. There is also a small aquatic fly in the photo. Duckweed is tiny, so you can imagine how small the mayfly and fly are!