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

Many tropical butterflies exhibit seasonal wing pattern plasticity, producing cryptic dry-season morphs with small or no ventral eyespots and conspicuous wet-season morphs with large wing-marginal eyespots that deflect predator attacks away from vital body parts. Eyespot size is influenced by temperature during larval development. However, many tropical regions lack a predictable temperature-rainfall correlation, limiting temperature's reliability as a cue. Relative humidity (humidity), which increases with wet season's approach, presents a potential environmental cue. We tested whether humidity modulates eyespot-size plasticity in two sympatric butterflies experiencing similar ecological pressures. High humidity directly induced larger eyespots in Mycalesis mineus, compared with those from low humidity. In contrast, Melanitis leda showed no direct effect; instead, humidity modulated eyespot size indirectly through induced changes in host plant quality. Since the late larval and prepupal stages are temporally closer to the adult environment, selection may favour greater sensitivity during these stages. Using a humidity switching experiment at different developmental stages, we demonstrated that, in M. mineus, wandering larval and prepupal stages are sensitive to humidity, while M. leda remained insensitive. This study highlights how two diverging species, evolving under similar ecological pressures, integrate environmental cues in distinct ways to modulate wing pattern plasticity.