In birds, the alula is a set of small feathers attached to the thumb at the end of the anterior limb, used to stabilise flight.
In certain insects, the alula is a small fragment of the wing located near the body and on the trailing edge. Using this lobe, some insects are able to place their wings along their bodies. In syrphids, the alula is used to change direction and speed during flight.
The alula is a fragment of the wing, very useful in flight. © Shyamal, Wikipédia, cc by sa 3.0 - adaptation Toony