Large-scale assembly of trait data from community science field images reveals inter- and intraspecific climate–phenology drivers of dragonfly body melanism

Insect coloration evolves under multiple selective pressures, and in dragonflies (Odonata: Anisoptera) body color likely plays roles in visual signaling and thermoregulation. Melanin-based dark body coloration in particular may aid muscle warming for flight under the thermal melanism hypothesis (TMH) and enhance immune protection in humid environments (Gloger’s rule). Recent work has shown phenology and climate jointly impact melanism at the assemblage level, but understanding processes that generate these patterns requires explicit consideration of intraspecific variation. Here we use a computer-vision pipeline to quantify threshold-based darkness metrics from ~ 75,000 iNaturalist images, representing 159 species. We also introduce new methods to account for lighting bias, key for best use of field images. Focusing on the conterminous United States, we test whether temperature, solar radiation, and humidity interact with phenology to shape melanism within and between species. We find phenology strongly mediates temperature effects: the darkest individuals are among the earliest fliers in the coldest climates. We also find support for a solar-radiation component of TMH and a nonlinear humidity–melanism relationship consistent with Gloger’s rule. Our results demonstrate that dragonfly body coloration reflects multiple interacting ecological pressures. We also provide scalable tools and methods broadly applicable in studies of color biology.