Among the most prominent,large-scale pattes of species richness are the increases in richness with decreasing latitude and with increasing habitat heterogeneity.Using the stream-dwelling larval and pupal stages of North American black flies (Diptera:Simuliidae),we address 3 broad questions about species richness:(i) Does a significant latitude-richness relationship exist? (ii) How does habitat heterogeneity influence gamma diversity? (iii) What is the sign (positive or negative) of the latitude richness and the heterogeneity-richness relationships? We found no evidence that habitat heterogeneity influences gamma diversity.The estimated peak species richness for black flies in North America was at 50-53°N,which also corresponds with peak generic richness.All plesiomorphic,extant lineages of the Simuliidae in the Weste Hemisphere are found in cool mountainous environments of North America,suggesting that peak richness at 50-53°N might be a signature of this phylogenetic patte and a reflection of underlying historical processes.