Larval melanism in a geometrid moth: promoted neither by a thermal nor seasonal adaptation but desiccating environments

1. Spatio-temporal variation in the degree of melanism is often considered in the context of thermal adaptation, melanism being advantageous under suboptimal thermal conditions. Yet, other mutually non-exclusive explanations exist. Analysis of geographical patterns combined with laboratory experiments on the mechanisms of morph induction helps to unveil the adaptive value of particular cases of polyphenism. 2. In the context of the thermal melanism hypothesis and seasonal adaptations, we explored an array of environmental factors that may affect the expression and performance of non-melanic vs. melanic larval morphs in different latitudinal populations of the facultatively bivoltine moth Chiasmia clathrata (Lepidoptera: Geometridae). 3. Geographic variation in larval coloration was independent of average temperatures experienced by the populations in the wild. The melanic morph was, however, more abundant in dry than in mesic habitats. In the laboratory, the melanic morph was induced es...