Environmental drivers of voltinism and body size in insect assemblages across Europe

AimGeneral geographical patterns of insect body size are still a matter of considerable debate, mainly because the annual number of generations (voltinism) and its relationship with body size have largely been ignored. We present the first analyses of voltinism and body size of insect assemblages at a continental scale using lepidopteran and odonate species. We hypothesize that voltinism is strongly driven by environmental conditions and constrains body size on macroecological scales. LocationEurope. MethodsWe compiled the distribution, voltinism and body size of 943 lepidopteran and odonate species within a 50 km x 50 km grid system, thereby presenting a novel method for estimating the body volume of species from digital images. Regressions and structural equation modelling were applied to distinguish the effects of temperature, productivity and season length on mean voltinism and body size within grid cells. We accounted for spatial autocorrelation with autoregressive models and analysed the possible effect of species richness and intraspecific variability. ResultsVoltinism consistently decreased with latitude for both lepidopterans (r(2)=0.76) and odonates (r(2)=0.86), with species having on average fewer generations per year in northern Europe and more generations per year in southern Europe. The effects of temperature, productivity and season length on body size contrasted in sign between lepidopterans and odonates, leading to opposing geographical patterns across Europe. Main conclusionsVoltinism in insect assemblages is strongly driven by environmental temperature, and trade-offs between voltinism and body size influence the occurrence of species at macroecological scales. Insects with the ability to extend their generation time over multiple years can overcome this constraint, allowing for a relatively large body size in cold areas. Our results furthermore support the idea that body sizes of terrestrial and aquatic insects form contrasting geographical patterns because they are differently affected by temperature and resource constraints.