Many butterfly species within the subfamily Satyrinae (Nymphalidae) have been informally reported to possess a conspicuous "inflated" or "swollen" subcostal vein on each forewing. However, the function and taxonomic diversity of these structures is unknown. This thesis comprises both experimental and comparative approaches to test hypotheses on the function and evolution of these inflated veins. A laser vibrometry study showed that ears in the common wood nymph, Cercyonis pegala, are tuned to sounds between 1-5 kHz and the inflated subcostal vein enhances sensitivity to these sounds. A comparative study showed that all species with inflated veins possess ears, but not all species with ears possess inflated veins. Further, inflated veins were better developed in smaller butterflies. This thesis provides the first evidence for the function of inflated wing veins in butterflies and supports the hypothesis that they function as aids to low frequency hearing.