Butterflies use humidity as a cue for wing-pattern and life history trait plasticity when temperature is unreliable.

Many butterflies have wet and dry season morphs with large and small wing eyespots respectively. Eyespot size plasticity is adaptive because butterflies develop large, conspicuous eyespots in the wet season and small, inconspicuous eyespots in the dry season, reducing predation risk in each season. Eyespot size has been shown to be regulated by rearing temperature in many species. However, temperature is an unreliable cue in some regions because it poorly predicts seasons, and other cues such as humidity may be more reliable. We investigated inter-population differences in cue use for eyespot plasticity in the butterfly Melanitis leda. We reared butterflies from three Indian populations under combinations of temperature and humidity designed to capture the differing seasonal reliability of these cues across the three regions. Butterflies from a population (Coimbatore) where temperature has the highest intra-annual variation, responded only to temperature. Butterflies from a population (Tirunelveli) where temperature and humidity are both unreliable, responded only to humidity. Butterflies from another population (Vithura) where humidity differentiates seasons but temperature does not, responded only to humidity. This suggests inter-population differences in cue use, possibly driven by local adaptation. Life-history traits also differed among populations, with the two populations from more arid regions developing faster and attaining larger body sizes than the one from the humid region. Fast development may be adaptive in dry regions where suitable host plants are available only briefly, while large body size may confer desiccation resistance. We show for the first time that humidity can regulate eyespot size, and that responses to temperature and humidity vary across populations, fitting to regional climates.