We test the hypothesis that the latitudinal gradient of species diversity is more a function of difference in temperature seasonality than mean annual temperature. We compare insect species diversity in cool mean annual temperature, highly seasonal Harvard Forest, Massachusetts, USA; high mean annual temperature, seasonally equable La Selva, Costa Rica; and cool mean annual temperature, seasonally equable Eocene McAbee, British Columbia, Canada. Insect diversity in the Eocene sample is more similar to that of La Selva than Harvard Forest in increased species richness of most groups and in decreased diversity of Ichneumonidae. This implies that latitudinal patterns of species diversity, including high tropical species richness, are associated with differential temperature seasonality rather than mean annual temperature, a climatic regime established after the Eocene.