The Thermal Adaptation hypothesis posits that the warmer, less seasonal tropics generates populations with higher and narrower thermal limits. It has largely been tested among populations across latitudes. However, considerable thermal heterogeneity exists within ecosystems: across 31 tropical trees in a Panama rainforest, surfaces exposed to sun were 8 °C warmer and varied more in temperature than surfaces in the litter below. Tiny ectotherms are confined to surfaces and are variously submerged in these superheated boundary layer environments. We quantified the thermal performance curves of 88 ant species from this forest that ranged in average mass from 0.01 to 57 mg. This rainforest’s thermal environments generate a range of thermal maxima subsuming 74% of those previously recorded for ant populations worldwide. Canopy ants, living in a hotter, more variable thermal environment had higher thermal tolerances, but traded this durability with lower peak velocity than litter ants. The Thermal Adaptation hypothesis can be a powerful tool in predicting diversity of thermal limits within communities. Boundary layer environments are likely key to predicting the future of Earth’s tiny terrestrial ectotherm populations