Bergmann’s rule is based on the observation that closely related species within a genus, or populations within a species, tend to be smaller on average at high temperatures and larger at cold temperatures. The distribution of body sizes has important implications for food web structure and the flow of energy to higher trophic levels. Bergmann’s rule has wide support in the literature for endothermic animals, but support for ectotherms, such as insects, is equivocal. Therefore, we analyzed macroinvertebrate body size data from the NEON stream sites to assess if average body size of closely related taxa changes consistently across a 26°C gradient of environmental temperature. We hypothesize that average body size will decrease in related taxa with increasing environmental temperature. The findings of this study will provide important implications for understanding how aquatic macroinvertebrates adapt to their local thermal conditions at the continental scale, and may provide insight into how body sizes at the population- and community-scale may change in response to a rapidly warming climate. This information will help us understand how climate change is affecting the structure and function of aquatic communities.