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Tympanal organs are widespread in Nymphalidae butterflies, with a great deal of variability in the morphology of these ears. How this variation reflects differences in hearing physiology is not currently understood. This study provides the first examination of the hearing organs of the crepuscular owl butterfly, Caligo eurilochus. We hypothesize that (1) its hearing may function to detect the high-frequency calls of bats, or (2) like its diurnal relatives it may function to detect avian predators, or (3) it may have lost auditory sensitivity as a result of the lack of selective pressures. To test these hypotheses we examined the tuning and sensitivity of the C. eurilochus Vogel’s organ using laser Doppler vibrometry and extracellular neurophysiology. We show that the C. eurilochus ear responds to sound and is most sensitive to frequencies between 1-4 kHz, as confirmed by both the vibration of the tympanal membrane and the physiological response of the associated nerve branches. In comparison to the hearing of its diurnally active relative, Morpho peleides, C. eurilochus has a narrower frequency range with higher auditory thresholds. We conclude that hearing in this butterfly is partially-regressed, and may reflect a trade-off between hearing and vision for survival in low light conditions.

SUMMARY Insects active in dim light, such as moths and many beetles, normally have superposition compound eyes to increase photon capture. But there are nocturnal and crepuscular insects – such as some species of bees, wasps and butterflies – that have apposition compound eyes. These are likely to have adaptations – including large eye and facet size and coarsened spatial and temporal resolution – that improve their sensitivity and thus their visual reliability. Is this also true for crepuscular insects that are active at intermediate intensities? To test this hypothesis, the visual performance of two closely related butterflies, the diurnal blue morpho Morpho peleides and the crepuscular owl butterfly Caligo memnon, were compared. Compared to the diurnal M. peleides, the crepuscular C. memnon does not appear to be adapted to a nocturnal lifestyle in terms of spatial resolution: the interommatidial angleΔϕ is similar in both species, and acceptance angles, Δρ,are only marginally larger in C. memnon. Moreover, temporal resolution is only a little coarser in C. memnon compared to M. peleides. Using a model for sensitivity, we found that the eyes of C. memnon are about four times as light-sensitive as those of M. peleides in the frontal visual field, much of this difference being due to the larger facet diameters found in C. memnon. In summary, greater visual sensitivity has evolved in C. memnon than in M. peleides, showing that adaptations that improve sensitivity can be found not only in nocturnal apposition eyes, but also on a smaller scale in crepuscular apposition eyes.