Insects have evolved a diversity of hearing organs specialized to detect sounds critical for survival. We report on a unique structure on butterfly wings that enhances hearing. The Satyrini are a diverse group of butterflies occurring throughout the world. One of their distinguishing features is a conspicuous swelling of their forewing vein, but the functional significance of this structure is unknown. Here we show that wing vein inflations function in hearing. Using the Common Wood-Nymph, Cercyonis pegala, as a model, we show that (1) these butterflies have ears on their forewings that are most sensitive to low frequency sounds (<5 kHz); (2) inflated wing veins are directly connected to the ears; (3) when vein inflations are ablated, sensitivity to low frequency sounds is impaired. We propose that inflated veins contribute to low frequency hearing by impedance matching.