The team of researchers from Britain's University of Bristol, Carleton University in Canada and Scotland's University of Strathclyde believes the structure in Blue Morpho (Morpho peleides) butterflies might be associated with the detection of predators, in particular birds.
The Blue Morpho, native to Central and South America, famous for their wing coloration, now turn out to have ears on their wings.
The scientists said the ear is located at the base of the wing and looks like a sheet of stretched rubber. The oval-shaped tympanal membrane, with an unusual dome in the middle, is attached directly to sensory organs and is responsible for converting sound waves into signals that can be picked up by nerve cells.
Using a tiny laser beam, lead researcher Katie Lucas scanned the surface of the membrane while it was in action, and found lower pitch sounds cause vibrations only in a part of the outer membrane, while higher pitch sounds caused the entire membrane to vibrate.
The study appears in the Journal of Experimental Biology.