The clue was “A butterfly’s wings” and answer was “alae”. Had to look this one up. Dictionary says:

1. a wing.
2. a winglike part, process, or expansion, as of a bone, shell, seed, or stem.
3. either of the two side petals of a flower in the legume family, as the pea.
4. (in an ancient Roman house) a small room, as an alcove, opening into a larger room or courtyard.