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Color polymorphism offers rich opportunities for studying the eco‐evolutionary mechanisms that drive the adaptations of local populations to heterogeneous and changing environments. We explored the color morph diversity and composition in a Chrysomela lapponica leaf beetle across its entire distribution range to test the hypothesis that environmental and climatic variables shape spatiotemporal variation in the phenotypic structure of a polymorphic species. We obtained information on 13 617 specimens of this beetle from museums, private collections, and websites. These specimens (collected from 1830–2020) originated from 959 localities spanning 33° latitude, 178° longitude, and 4200 m altitude. We classified the beetles into five color morphs and searched for environmental factors that could explain the variation in the level of polymorphism (quantified by the Shannon diversity index) and in the relative frequencies of individual color morphs. The highest level of polymorphism was found at high latitudes and altitudes. The color morphs differed in their climatic requirements; composition of colour morphs was independent of the geographic distance that separated populations but changed with collection year, longitude, mean July temperature and between‐year temperature fluctuations. The proportion of melanic beetles, in line with the thermal melanism hypothesis, increased with increasing latitude and altitude and decreased with increasing climate seasonality. Melanic morph frequencies also declined during the past century, but only at high latitudes and altitudes where recent climate warming was especially strong. The observed patterns suggest that color polymorphism is especially advantageous for populations inhabiting unpredictable environments, presumably due to the different climatic requirements of coexisting color morphs.

Subsocial behaviour is known to occur in at least 19 insect orders and 17 families of Coleoptera. Within the leaf beetle family, Chrysomelidae, extended maternal care is reported in only 2 of 15 subfamilies: Cassidinae and Chrysomelinae. Although the emergence of subsociality in insects has received much attention, extensive analyses on the evolution of this behaviour based on phylogenetic approaches are missing. Subsociality is recorded in 33 species of tortoise beetles belonging to the tribes Mesomphaliini and Eugenysini. A molecular phylogenetic reconstruction of these tribes and the remaining five Neotropical tribes of cassidine tortoise beetles was used to investigate the evolution of maternal care and to elucidate the phylogenetic relationships among Neotropical cassidine tribes. A phylogeny was constructed using 90 species and three loci from both mitochondrial and nuclear genes (COI, CAD and 28S). Bayesian inference and maximum likelihood analyses based on a concatenated dataset recovered two independent origins, with no evidence of reversal to solitary behaviour. One origin comprises three Mesomphaliini genera tightly associated with Convolvulaceae, and the other consists of the genus Eugenysa Chevrolat (Eugenysini), a small clade embedded within a group feeding exclusively on Asteraceae. A previous hypothesis suggesting dual origins on different host plants was confirmed, whereas other hypotheses based on a phylogenetic reconstruction of Cassidinae could not be sustained. Our analysis also revealed that the tribe Mesomphaliini is a monophyletic taxon if Eugenysini is included, and for this reason, we re‐establish synonymy of both tribes. We also provide nine new records of subsociality for tortoise beetles species.

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Aspidimorpha (Aspidimorpha) wagneri sp. nov. is described from Uganda. This species belongs to the nominotypical subgenus, together with several African species that are closely related to A. mutata Boheman, 1854. Its relative position within the genus is discussed.