Synthetic wings are a fairy-designed technology, not an organ or limb of the creature. They are a pair of synthetic wings which are designed by either Foaly or Opal Koboi. Opal designed wings as a part of the Koboi DoubleDex wing series. For wings in general, see Wings.
Each model can be worn on the back of a fairy or attached to suits such as a Recon officer's suit. Different wings have differing speeds and designs. They generally have solar or nuclear motors and come in different designs, such as the Hummingbird Z7, which includes a satellite bounced solar battery that can fly a fairy twice around the world.
Wings are a primary source of transport in less built up areas (such as where fairies go on their wilderness holidays). From images in the books it appears these synthetic wings look like wings that might be found on a fly; each wing has an ovular shape and flaps up and down. Many different power sources are used by wings including, but not limited to, solar batteries, petrol engines, and nuclear batteries.