The Xerces blue butterfly was last seen flapping its iridescent periwinkle wings in San Francisco in the early 1940s. It's generally accepted to be extinct, the first American insect species destroyed by urban development, but there are lingering questions about whether it was really a species to begin with, or just a sub-population of another common butterfly. In a new study in Biology Letters, researchers analyzed the DNA of a 93-year-old Xerces blue specimen in museum collections, and they found that its DNA is unique enough to merit being considered a species.