Wing-beat frequency and the degree of asymmetry in wing motion were more si
milar among mimics than among sister species of passion-vine butterflies in
the genus Heliconius. Asymmetry in wing motion is not attributed to lift p
roduction, and serves as the first clear example of a mimetic behavioural s
ignal for a flying organism. Because the similarities in wing motion are to
o subtle for humans to observe with the naked eye, they serve as a previous
ly unexplored mimetic signal.