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Karner blue butterflies have a wing span of 2.5 cm, with males being slightly smaller than females. Males' wings are a violet-blue color with black margins and white fringed edges on the top. Females' wings are predominantly dark gray-brown or black, with the violet color occurring on the central part of the wing. Orange crescents are typically present on the hind wings of females. The underside or ventral side of both male and female wings are gray with black spots and have orange crescents and metallic spots. Eggs are small, slightly flattened, and pale green.
Karner blue butterflies can only survive where wild lupine plants are found, typically in the sandy soils of pine barrens and oak savannas.
Over the last 100 years, this species has declined by 99% and was federally listed as endangered in 1992. The United States Fish and Wildlife Service currently has remaining populations listed as stable. Threats include habitat loss, fragmentation, and modifications such as fire suppression or other disturbances.