In the modern world, biotic diversity is typically higher in low-latitude tropical regions where there is abundant insolation (light and heat) and low thermal seasonality. Because these factors broadly covary with latitude, separating their possible effects on species diversity is difficult. The Eocene was a much more equable world, however, with low temperature seasonality extending into lower-insolation higher, cooler latitudes, allowing us to test these factors by comparing insect species diversity in (1) modern, temperate, low-insolation, highly seasonal Harvard Forest, Massachusetts, U.S.A., 42°29′N; (2) modern, tropical, high-insolation, low-seasonality La Selva, Costa Rica, 10°26′N, and; (3) Eocene, temperate, low-insolation, yet low-seasonality McAbee, British Columbia, Canada, above 50°N paleolatitude. We found insect diversity at McAbee to be more similar to La Selva than to Harvard Forest, with high species richness of most groups and decreased diversity of ichneumon wasps, indicating that seasonality is key to the latitudinal diversity gradient. Further, midlatitude Eocene woody dicot diversities at McAbee, Republic (Washington, U.S.A.), and Laguna del Hunco (Argentina) are also high, similar to modern tropical samples, higher than at the modern midlatitude Harvard Forest. Modern correlations between latitude, species diversity, and seasonal climates were established some time after the Eocene.