A member of the Somatochlora genus, otherwise known as the "striped emeralds." (1) Brighter yellow thoracic side stripes, similar in shape to Brush-tipped, Ski-tailed, and Clamp-tipped Emeralds (S. tenebrosa), fade over time and thus are dull to non-existent in adults. (2) The abdomen is long with small pale spots on some mid-segments. (3) Male claspers are ski-shaped, straight in top view, and somewhat hairy, and females have a long, sharply-pointed, T-type egg spout (perpendicular to abdomen). Brush-tipped male has shorter abdomen and wider claspers in top view. Female Clamp-tippeds are very similar but they lack the pale abdominal spots of Williamson's. This species is fairly widespread in Wisconsin, preferring shady forest streams and often flying later in summer (into August) than other emeralds. I saw just a few individuals in 2008.