The wings of butterflies gain lift because their scales point away from the leading edge of the wing, helping air flow smoothly over the wing.

References

"The scales all point away from the leading edge of the wing, to help the air flow smoothly over the wings when the insect is in flight. (It has been calculated that scales provide 15 per cent more 'lift' to the butterfly and also improve its gliding performance." (Foy and Oxford Scientific Films 1982:99)

Book
The grand design: Form and colour in animalsBLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, LondonJanuary 1, 1970
Sally Foy

Living System/s