Data from: 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 especially under a high level of incident radiation but also at relatively high temperatures, but independently of photoperiod. Melanic larvae had higher growth rates and shorter development times than the non-melanic ones when both temperature and the level of incident radiation were high. 4. Our results that melanism is induced and advantageous in warm desiccating conditions contradict the thermal melanism hypothesis for this species. Neither has melanism evolved to compensate time constraints due to forth-coming autumn. Instead, larvae solve seasonal variation in the time available for growth by an elevated growth rate and a shortened larval period in the face of autumnal photoperiods. The phenotypic response to the level of incident radiation and a lack of adaptive adjustment of larval growth trajectories in univoltine populations underpin the role of deterministic environmental variation in the evolution of irreversible adaptive plasticity and seasonal polyphenism.1. 黑化（melanism）程度的时空变异通常被纳入热适应的研究范畴，因为在非最优热环境下，黑化性状具备适应优势。但除此之外，尚有多种非互斥的解释可供探讨。结合地理格局分析与形态诱导机制的室内实验，有助于揭示特定多型现象（polyphenism）案例的适应价值。 2. 本研究以热黑化假说（thermal melanism hypothesis）与季节适应为研究背景，针对兼性二化性的网尺蛾（Chiasmia clathrata，鳞翅目: 尺蛾科）不同纬度种群，探讨了一系列可能影响非黑化与黑化幼虫表型表达及生长表现的环境因子。 3. 幼虫体色的地理变异与种群野外所经历的平均温度并无关联。但相较于湿润生境，干燥生境中黑化幼虫表型的占比更高。室内实验结果显示，黑化幼虫表型的诱导主要受高强度入射辐射（incident radiation）与相对较高的温度调控，且不受光周期（photoperiod）的影响。当温度与入射辐射水平均处于较高水平时，黑化幼虫的生长速率显著高于非黑化幼虫，发育时长则更短。 4. 本研究结果显示，在温暖干燥的脱水环境下，黑化表型会被诱导且具备适应优势，这与该物种的热黑化假说相矛盾。同时，黑化性状的演化也并非为了补偿即将到来的秋季所带来的时间限制。相反，面对秋季光周期时，幼虫通过提升生长速率、缩短幼虫期来应对生长可用时间的季节变异。幼虫对入射辐射水平的表型响应，以及一化性（univoltine）种群中幼虫生长轨迹缺乏适应性调整的现象，共同支撑了确定性环境变异在不可逆适应性可塑性（adaptive plasticity）与季节多型现象演化中的作用。