Because the geographic scope of this book is extensive, the regions covered by it vary substantially in numerous ways of significance to dragonflies: temperature, rainfall, seasonality, physiography, terrestrial vegetation cover, amount of human settlement, and regional knowledge of Odonata. Dragonflies are sun lovers, and temperature varies by latitude as well as by altitude, so not surprisingly, odonate diversity decreases with latitude and altitude in the West. This latitudinal effect, interestingly, is not as strong in dragonflies as it is in many other groups of plants and animals. Apparently, dragonflies do relatively well in cooler climates as long as they have some warm summer days to sustain activity. Dragonflies need wetlands, so precipitation should be of importance to them. The West is generally dry and perhaps because of that supports fewer species of odonates than the East; for example, Rhode Island has a much larger species list than any western state. The wettest part of the West (the Pacific coast between Alaska and northern California), in fact, is relatively poor for dragonflies, perhaps because it combines high precipitation with cool, cloudy summers. Away from the Pacific coast, rainfall is highest in the West on the eastern edge of the region, especially in Texas, and there the diversity of dragonflies is high. A complex physiography and fairly wet summers may play a part in making southeastern Arizona another region of high odonate diversity.