Glaucopsyche xerces, better known as the Xerces blue butterfly or simply xerces blue is a recently extinct species of butterfly in the gossamer-winged butterfly family, Lycaenidae. The species was first described and documented in 1852, and was named after the French spelling of "Xerxes", the Greek name of the Persian kings Xerxes I and Xerxes II of the fifth century BC. Reaching around .7 to 1.18inches (18 to 30mm) in wingspan length, the xerces blue is a small, brightly colored butterfly characterized by iridescent blue on the upper wing surfaces of males, and pale spots below. It was endemic to the coastal sand dunes of the upper San Francisco Peninsula where the Xerces fed on vegetation belonging to the genus Lotus and Lupinus. The loss of the Lotus plant that the butterfly fed on while in its larval stages is believed to be the main reason for the extinction of the Xerces blue. As growing urban development resulted in extensive disturbance and loss of habitat of which the lotus plant couldn’t survive. Lupinus, the Xerces blue's other main adult food source, was not suitable for the larval stages. By the early 1940s the Xerces Blue was driven to extinction, becoming one of the first and most well-known butterflies in the United States lost due to human impact, with the last confirmed sighting of a xerces blue occurring in 1943 on land that is now part of the Golden Gate National Recreation Area. The butterfly’s extinction inspired the foundation of the Xerces Society for Invertebrate Conservation in 1971, as well as ushering the need for insect and invertebrate conservation into the public mindset. Today there are ongoing efforts to reestablish related butterflies in the Xerces blue's former habitat such as the silvery blue and the Palos Verdes blue. Also the possibility of reviving the xerces blue via de-extinction is being explored.
