Environmental drivers of colour and size in insects: A macroecological perspective

Climatic conditions have profound effects on the geographical distribution of species across diverse taxa and regions. However, the role of traits of species that underpin the relationship between distribution and climate is poorly understood, especially in animals. Although many physiological and local-scale studies on animals have demonstrated associations between traits and climate, to what extent such mechanistic links may impact distribution patterns of species at macroecological scales remains largely unknown. In insects, colour and size are two very important and highly multifunctional traits, which play pivotal roles in natural and sexual selection, and may hence be related to climate. The colour of insects comprises several functions, namely crypsis, aposematism, sexual selection, species recognition, UV resistance, pathogen resistance and thermoregulation. However, the relative importance of single functions is often unclear because several functions of colouration can act simultaneously. Hitherto, thermoregulation through thermal melanism is supposed to be the dominant function of colouration in insects. Dark-coloured individuals heat up faster and attain higher steady-state body temperatures compared to light-coloured individuals of similar size under equal environmetal conditions and light-coloured individuals are supposed to have advantages in warmer climates because of a reduced risk of overheating. However, it is unknown to which extent this physical mechanism influences the colour lightness of species in response to different thermal environments at macroecological scales and across continents. Moreover, thermoregulation is not the only function of colouration of insects and it is unknown to which extent other functions of colouration dominate in particular geographical areas and taxa and thereby form macroecological patterns. The size of insects affects almost all physiological rates (e.g. rate of oxygen consumption), which subsequently determine or constrain fertility, mortality and ecological processes such as competitive interactions between individuals or species. In this way, body size is ultimately linked to the spatiotemporal distribution and abundance of animals and has important implications for the impact of climate warming – from biomass production by single species to the structure and dynamics of communities.However, a synthesis and analysis of the major environmental driver for the large-scale geographical variation of insect body size is missing so far. In this thesis, I use lepidopteran and odonate species to address the following questions: i) Are assemblages of diurnal insects darker coloured in colder regions and lighter coloured in warmer regions? ii) Is crypsis, pathogen resistance or protection from UV radiation associated with large-scale geographical variation in the colouration of insects? iii) What are the major environmental drivers for the large-scale geographical variation of body size in insects? In conclusion, colour and size of insects are related to their geographical distribution even at macroecological scales. The underlying mechanisms are especially driven by environmental temperatures: for colour because of its effect on thermoregulation and immune function, and for size because of the temperature dependency of metabolic rates and voltinism. With global warming, I would thus expect that especially dark-coloured and/or large aquatic insects might shift their distribution and retreat from warmer areas. Furthermore, insect species with the ability to increase their annual number of generations should benefit and extend their distribution to higher latitudes