Xerces Blue

The Xerces Blue (Glaucopsyche xerces) is an extinct species of butterfly in the gossamer-winged butterfly family, Lycaenidae. The species lived in coastal sand dunes of the Sunset District of San Francisco. The Xerces Blue is believed to be the first American butterfly species to become extinct as a result of loss of habitat caused by urban development. The last Xerces Blue was seen in either 1941 or 1943 on land that is part of Golden Gate National Recreation Area. The Xerces Blue is one of the few butterflies species that start with the letter X.

The species was first described and documented in 1852. It was characterized by blue wings with white spots. The butterfly's food plants belonged to the genera Lotus and Lupinus. Xerces probably associated with ants in its larval stages as part of a symbiotic relationship. One of the theories regarding the extinction of the species is that with human settlement in the Bay Area, new species of ant were introduced which slowly replaced the existing ant species within the butterfly's super habitat.

Efforts are on to reestablish related butterflies in the Xerces Blue's former habitat. The Palos Verdes Blue (Glaucopsyche lygdamus palosverdesensis) which is considered a Los Angeles cousin of the Xerces, is being reared in labs. A new Xerces-like subspecies of the Silvery Blue (Glaucopsyche lygdamus) has been discovered as well.

An endangered invertebrate conservation group known as the Xerces Society is named after the Xerces Blue. The specific name derives from the French spelling of "Xerxes", the name of Persian kings Xerxes I and Xerxes II of the fifth century BC.

Xerces is also a group of XML software packages named after this butterfly.