Contrasting mechanisms for using humidity as cue for seasonal polyphenism in two tropical butterflies

Many tropical butterflies show wing pattern plasticity across dry and wet seasons. Dry-season morphs of satyrines (Nymphalidae: Satyrinae) typically are cryptic with small or no eyespots on the ventral wing surface. In contrast, wet-season morphs have large eyespots on their wing margins. These eyespots deflect predator attacks away from vital body parts. Extensive studies have shown that eyespot size is influenced by the temperature experienced during larval development. Temperature-mediated eyespot size plasticity is adaptive in regions where temperature increases with the onset of the wet season. However, many tropical regions lack a reliable temperature-rainfall correlation, and thus, temperature may not always be a reliable predictor of the upcoming adult environment. A potential environmental cue that correlates strongly with seasonality is relative humidity (hereafter, humidity). With the approach of the wet season, humidity increases, and then declines with the onset of the dry season. Here, we tested whether humidity modulates eyespot-size plasticity in two sympatric butterflies that experience similar ecological pressures: Mycalesis mineus and Melanitis leda. We found that humidity directly influenced eyespot size in My. mineus; individuals reared at high humidity developed into adults with larger eyespots than those from low humidity. In contrast, eyespot size in Me. leda showed a weak direct effect of humidity. Instead, humidity modulated eyespot size in Me. leda indirectly through induced changes in the host plant quality. Since the late larval and prepupal stages are temporally closer to the adult environment, selection may favour greater sensitivity to the environmental cue during these stages than during early larval stages. To determine the humidity-sensitive phase, we conducted a switching experiment where humidity was shifted from low to high, and vice versa, at different developmental stages. We demonstrated that, in My. mineus, wandering larval and prepupal stages are sensitive to humidity, whereas the early-feeding larval phases are insensitive. Contrastingly, humidity switching did not influence the eyespot size in Me. leda. This study highlights how two diverging species, evolving under similar ecological pressures, integrate environmental cues in distinct ways to modulate wing pattern plasticity.